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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #60007 (https://www.gutenberg.org/ebooks/60007)
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-Project Gutenberg's Earthquakes and Other Earth Movements, by John Milne
-
-This eBook is for the use of anyone anywhere in the United States and most
-other parts of the world at no cost and with almost no restrictions
-whatsoever.  You may copy it, give it away or re-use it under the terms of
-the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org.  If you are not located in the United States, you'll have
-to check the laws of the country where you are located before using this ebook.
-
-Title: Earthquakes and Other Earth Movements
-
-Author: John Milne
-
-Release Date: July 29, 2019 [EBook #60007]
-
-Language: English
-
-Character set encoding: UTF-8
-
-*** START OF THIS PROJECT GUTENBERG EBOOK EARTHQUAKES, OTHER EARTH MOVEMENTS ***
-
-
-
-
-Produced by deaurider, Robert Tonsing, and the Online
-Distributed Proofreading Team at http://www.pgdp.net (This
-file was produced from images generously made available
-by The Internet Archive)
-
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-
-
-
-
-                  THE INTERNATIONAL SCIENTIFIC SERIES
-                               VOLUME LV
-
-
-
-
-                  THE INTERNATIONAL SCIENTIFIC SERIES
-
-
-
-
-                              EARTHQUAKES
-                                  AND
-                         OTHER EARTH MOVEMENTS
-
-
-                                  BY
-                              JOHN MILNE
-
-PROFESSOR OF MINING AND GEOLOGY IN THE IMPERIAL COLLEGE OF ENGINEERING,
-                             TOKIO, JAPAN
-
-                      _WITH THIRTY-EIGHT FIGURES_
-
-                               NEW YORK
-                        D. APPLETON AND COMPANY
-                        1, 3, AND 5 BOND STREET
-                                 1886
-
-
-
-
-                               PREFACE.
-
-                   •       •       •       •       •
-
-
-In the following pages it has been my object to give a systematic
-account of various Earth Movements.
-
-These comprise _Earthquakes_, or the sudden violent movements of the
-ground; _Earth Tremors_, or minute movements which escape our attention
-by the smallness of their amplitude; _Earth Pulsations_, or movements
-which are overlooked on account of the length of their period; and
-lastly, _Earth Oscillations_, or movements of long period and large
-amplitude which attract so much attention from their geological
-importance.
-
-It is difficult to separate these Earth Movements from each other,
-because they are phenomena which only differ in degree, and which are
-intimately associated in their occurrence and in their origin.
-
-                   •       •       •       •       •
-
-Because Earthquakes are phenomena which have attracted a universal
-attention since the earliest times, and about them so many observations
-have been made, they are treated of at considerable length.
-
-As very much of what might be said about the other Earth Movements is
-common to what is said about Earthquakes, it has been possible to make
-the description of these phenomena comparatively short.
-
-The scheme which has been adopted will be understood from the following
-table:—
-
-                         I. EARTHQUAKES.
-
-  1. Introduction.
-  2. Seismometry.
-                                     {(_a_) Theoretically.
-  3. Earthquake Motion.              {(_b_) As deduced from experiments.
-                                     {(_c_) As deduced from actual
-                                             Earthquakes.
-  4. Earthquake Effects.             {(_a_) On land.
-                                     {(_b_) In the ocean.
-  5. Determination of Earthquake origins.
-                                     {(_a_) In space.
-  6. Distribution of Earthquakes.    {(_b_) In time (geological time,
-                                     {       historical  time, annual,
-                                     {       seasonal, diurnal, &c.)
-  7. Cause of Earthquakes.
-  8. Earthquake prediction and warning.
-
-                        II. EARTH TREMORS.
-
-                       III. EARTH PULSATIONS.
-
-                        IV. EARTH OSCILLATIONS.
-
-In some instances the grouping of phenomena according to the above
-scheme may be found inaccurate, as, for example, in the chapters
-referring to the effects and causes of Earthquakes.
-
-This arises from the fact that the relationship between Earthquakes
-and other Earth phenomena are not well understood. Thus the sudden
-elevation of a coast line and an accompanying earthquake may be
-related, either as effect and cause, or _vice versâ_, or they may both
-be the effect of a third phenomenon.
-
-Much of what is said respecting Earthquake motion will show how little
-accurate knowledge we have about these disturbances. Had I been writing
-in England, and, therefore, been in a position to make references to
-libraries and persons who are authorities on subjects connected with
-Seismology, the following pages might have been made more complete,
-and inaccuracies avoided. A large proportion of the material embodied
-in the following pages is founded on experiments and observations
-made during an eight years’ residence in Japan, where I have had the
-opportunity of recording an earthquake every week.
-
-The writer to whom I am chiefly indebted is Mr. Robert Mallet. Not
-being in a position to refer to original memoirs, I have drawn many
-illustrations from the works of Professor Karl Fuchs and M. S. di
-Rossi. These, and other writers to whom reference has been made, are
-given in an appendix.
-
-For seeing these pages through the press, my thanks are due to
-Mr. Thomas Gray, who, when residing in Japan, did so much for the
-advancement of observational Seismology.
-
-For advice and assistance in devising experiments, I tender my thanks
-to my colleagues, Professor T. Alexander, Mr. T. Fujioka, and to my
-late colleague. Professor John Perry.
-
-For assistance in the actual observation of Earthquakes, I have to
-thank my friends in various parts of Japan, especially Mr. J. Bissett
-and Mr. T. Talbot, of Yokohama. For assistance in obtaining information
-from Italian sources I have to thank Dr. F. Du Bois, from German
-sources Professor C. Netto, and from Japanese sources Mr. B. H.
-Chamberlain. For help in carrying out experiments, I am indebted to
-the liberality of the British Association, the Geological Society of
-London, the Meteorological and Telegraph departments of Japan, and to
-the officers of my own institution, the Imperial College of Engineering.
-
-And, lastly, I offer my sincere thanks to those gentlemen who have
-taken part in the establishment and working of the Seismological
-Society of Japan, and to my publishers, whose liberality has enabled me
-to place the labours of residents in the Far East before the European
-public.
-
-                                                          JOHN MILNE.
-
-  TOKIO, JAPAN; _June 30, 1883_
-
-
-
-
-                               CONTENTS.
-
-                   •       •       •       •       •
-
-                              CHAPTER I.
-
-                             INTRODUCTION.
-
-                                                                   PAGE
-  Relationship of man to nature—The aspect of a country
-    is dependent on geological phenomena—Earthquakes an
-    important geological phenomenon—Relationship of seismology
-    to the sciences and arts—Earth movements other than
-    earthquakes—Seismological literature—(Writings of Perrey,
-    Mallet, Eastern writings, the Philosophical Transactions of
-    the Royal Society, the ‘Gentleman’s Magazine,’ the Bible,
-    Herodotus, Pliny, Hopkins, Von Hoff, Humboldt, Schmidt,
-    Seebach, Lasaulx, Fuchs, Palmieri, Bertelli, Seismological
-    Society of Japan)—Seismological terminology                        1
-
-
-                              CHAPTER II.
-
-                             SEISMOMETRY.
-
-  Nature of earthquake vibrations—Many instruments called
-    seismometers only seismoscopes—Eastern seismoscopes, columns,
-    projection seismometers—Vessels filled with liquid—Palmieri’s
-    mercury tubes—The ship seismoscope—The cacciatore—Pendulum
-    instruments of Kreil, Wagner, Ewing, and Gray—Bracket
-    seismographs—West’s parallel motion instrument—Gray’s conical
-    pendulums, rolling spheres, and cylinders—Verbeck’s ball and
-    plate seismograph—The principle of Perry and Ayrton—Vertical
-    motion instruments—Record receiver—Time-recording
-    apparatus—The Gray and Milne seismograph                          12
-
-
-                             CHAPTER III.
-
-              EARTHQUAKE MOTION DISCUSSED THEORETICALLY.
-
-  Ideas of the ancients (the views of Travagini, Hooke, Woodward,
-    Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves
-    and vibrations—Possible causes of disturbance in the earth’s
-    crust—The time of vibration of an earth particle—Velocity
-    and acceleration of a particle—Propagation of a disturbance
-    as determined by experiments upon the elastic moduli of
-    rocks—The intensity of an earthquake—Area of greatest
-    overturning moment—Earthquake waves—Reflexion, refraction,
-    and interference of waves—Radiation of a disturbance              41
-
-
-                              CHAPTER IV.
-
-             EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.
-
-  Experiments with falling weights—Experiments with
-    explosives—Results obtained from experiments—Relative
-    motion of two adjacent points—The effect of hills and
-    excavations upon the propagation of vibrations—The intensity
-    of artificial disturbances—Velocity with which earth
-    vibrations are propagated—Experiments of Mallet—Experiments
-    of Abbot—Experiments in Japan—Mallet’s results—Abbot’s
-    results—Results obtained in Japan                                 57
-
-
-                              CHAPTER V.
-
-             EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION
-                            ON EARTHQUAKES.
-
-  Result of feelings—The direction of motion—Instruments as
-    indicators of direction—Duration of an earthquake—Period
-    of vibration—The amplitude of earth movements—Side of
-    greatest motion—Intensity of earthquakes—Velocity and
-    acceleration of an earth particle—Absolute intensity of an
-    earthquake—Radiation of an earthquake—Velocity of propagation     67
-
-
-                              CHAPTER VI.
-
-            EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.
-
-  The destruction of buildings is not irregular—Cracks in
-    buildings—Buildings in Tokio—Relation of destruction
-    to earthquake motion—Measurement of relative motion of
-    parts of a building shaken by an earthquake—Prevention of
-    cracks—Direction of cracks—The pitch of roofs—Relative
-    position of openings in a wall—The last house in a row—The
-    swing of buildings—Principle of relative vibrational periods      96
-
-
-                             CHAPTER VII.
-
-            EFFECTS PRODUCED UPON BUILDINGS (_continued_).
-
-  Types of buildings used in earthquake countries—In Japan,
-    in Italy, in South America, in Caraccas—Typical houses
-    for earthquake countries—Destruction due to the nature of
-    underlying rocks—The swing of mountains—Want of support
-    on the face of hills—Earthquake shadows—Destruction due
-    to the interference of waves—Earthquake bridges—Examples
-    of earthquake effects—Protection of buildings—General
-    conclusions                                                      122
-
-
-                             CHAPTER VIII.
-
-                    EFFECTS OF EARTHQUAKES ON LAND.
-
-  1. Cracks and fissures—Materials discharged from
-    fissures—Explanation of fissure phenomena. 2. Disturbances
-    in lakes, rivers, springs, wells, fumaroles, &c.—Explanation
-    of these latter phenomena. 3. Permanent displacement of
-    ground—On coast lines—Level tracts—Among
-    mountains—Explanation of these movements                         146
-
-
-                              CHAPTER IX.
-
-                      DISTURBANCES IN THE OCEAN.
-
-  Sea vibrations—Cause of vibratory blows—Sea waves:
-    preceding earthquakes; succeeding earthquakes—Magnitude
-    of waves—Waves as recorded in countries distant from the
-    origin—Records on tide gauges—Waves without earthquakes—Cause
-    of waves—Phenomena difficult of explanation—Velocity
-    of propagation—Depth of the ocean—Examples of
-    calculations—Comparison of velocities of earthquake waves
-    with velocities which ought to exist from the known depth of
-    the ocean                                                        163
-
-
-                              CHAPTER X.
-
-                 DETERMINATION OF EARTHQUAKE ORIGINS.
-
-  Approximate determination of an origin—Earthquake-hunting
-    in Japan—Determinations by direction of motion—Direction
-    indicated by destruction of buildings—Direction
-    determined by rotation—Cause of rotation—The use of time
-    observations—Errors in such observations—Origin determined
-    by the method of straight lines—The method of circles, the
-    method of hyperbolas, the method of co-ordinates—Haughton’s
-    method—Difference in time between sound, earth, and water
-    waves—Method of Seebach                                          187
-
-
-                              CHAPTER XI.
-
-                  THE DEPTH OF AN EARTHQUAKE CENTRUM.
-
-  The depth of an earthquake centrum—Greatest possible depth of
-    an earthquake—Form of the focal cavity                           213
-
-
-                             CHAPTER XII.
-
-            DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.
-
-  General distribution of earthquakes—Occurrence along
-    lines—Examples of distribution—Italian earthquake of 1873—In
-    Tokio—Extension of earthquake boundaries—Seismic energy in
-    relation to geological time; to historical time—Relative
-    frequency of earthquakes—Synchronism of earthquakes—Secondary
-    earthquakes 226
-
-
-                             CHAPTER XIII.
-
-  DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_)                  234
-
-
-                             CHAPTER XIV.
-
-          DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_).
-
-  The occurrence of earthquakes in relation to the position of
-    the heavenly bodies—Earthquakes and the moon—Earthquakes
-    and the sun; and the seasons; the months—Planets and
-    meteors—Hours at which earthquakes are frequent—Earthquakes
-    and sun spots—Earthquakes and the aurora 250
-
-
-                              CHAPTER XV.
-
-  BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN
-    TEMPERATURE AND EARTHQUAKES                                      266
-
-
-                             CHAPTER XVI.
-
-              RELATION OF SEISMIC TO VOLCANIC PHENOMENA.
-
-  Want of synchronism between earthquakes and volcanic
-    eruptions—Synchronism between earthquakes and volcanic
-    eruptions—Conclusion                                             270
-
-
-                             CHAPTER XVII.
-
-                       THE CAUSE OF EARTHQUAKES.
-
-  Modern views respecting the cause of earthquakes—Earthquakes
-    due to faulting—To explosions of steam—To volcanic
-    evisceration—To chemical degradation—Attractive influence of
-    the heavenly bodies—The effect of oceanic tides—Variation in
-    atmospheric pressure—Fluctuation in temperature—Winds and
-    earthquakes—Rain and earthquakes—Conclusion                      277
-
-
-                            CHAPTER XVIII.
-
-                      PREDICTION OF EARTHQUAKES.
-
-  General nature of predictions—Prediction by the observation of
-    unusual phenomena (Alteration in the appearance and taste of
-    springs; underground noises; preliminary tremors; Earthquake
-    prophets—warnings furnished by animals, &c.)—Earthquake
-    warning                                                          297
-
-
-                             CHAPTER XIX.
-
-                            EARTH TREMORS.
-
-  Artificially produced tremors—Observations of Kater, Denman,
-    Airy, Palmer, Paul—Natural tremors—Observations of Zöllner,
-    M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With
-    seismoscopes, microphones, pendulums—Work in Italy—Bertelli,
-    Count Malvasia, M. S. di Rossi—Instruments employed in
-    Italy—Tromometers, microseismographs, microphones—Results
-    obtained in Italy—In Japan—Cause of microseismic motion          306
-
-
-                              CHAPTER XX.
-
-                           EARTH PULSATIONS.
-
-  Definition of an earth pulsation—Indications of
-    pendulums—Indications of levels—Other phenomena indicating
-    the existence of earth pulsations—Disturbances in lakes and
-    oceans—Phenomena resultant on earth pulsations—Cause of earth
-    pulsations                                                       326
-
-
-                             CHAPTER XXI.
-
-                          EARTH OSCILLATIONS.
-
-  Evidences of oscillation—Examples of oscillation—Temple of
-    Jupiter Serapis—Observations of Darwin—Causes of oscillation     344
-
-
-  APPENDIX                                                           349
-
-
-  INDEX                                                              359
-
-
-
-
-                             EARTHQUAKES.
-
-                   •       •       •       •       •
-
-
-
-
-                              CHAPTER I.
-
-                             INTRODUCTION.
-
-  Relationship of man to nature—The aspect of a country is dependent
-    on geological phenomena—Earthquakes an important geological
-    phenomenon—Relationship of seismology to the sciences and
-    arts—Earth movements other than earthquakes—Seismological
-    literature—(Writings of Perrey, Mallet, Eastern writings, the
-    Philosophical Transactions of the Royal Society, the ‘Gentleman’s
-    Magazine,’ the Bible, Herodotus, Pliny, Hopkins, Von Hoff,
-    Humboldt, Schmidt, Seebach, Lasaulx, Fuchs, Palmieri, Bertelli,
-    Seismological Society of Japan)—Seismological terminology.
-
-
-In bygone superstitious times lightning and thunder were regarded
-as supernatural visitations. But as these phenomena became better
-understood, and men learned how to avoid their destructive power, the
-superstition was gradually dispelled. Thus it is with Earthquakes:
-the more clearly they are understood, the more confident in the
-universality of law will man become, and the more will his mental
-condition be advanced.
-
-In his ‘History of Civilisation in England,’ Buckle has laid
-considerable stress upon the manner in which earthquakes, volcanoes,
-and other of the more terrible forms in which the workings of
-nature reveal themselves to us, have exerted an influence upon the
-imagination and understanding; and just as a sudden fright may affect
-the nerves of a child for the remainder of its life, we have in the
-annals of seismology records which indicate that earthquakes have not
-been without a serious influence upon the mental condition of whole
-communities.
-
-To a geologist there are perhaps no phenomena in nature more
-interesting than earthquakes, the study of which is called Seismology.
-Coming, as shocks often will, from a region of volcanoes, the study
-of these disturbances may enable us to understand something about
-the nature and working of a volcano. As an earthquake wave travels
-along from strata to strata, if we study its reflections and changing
-velocity in transit, we may often be led to the discovery of certain
-rocky structures buried deep beneath our view, about which, without the
-help of such waves, it would be hopeless ever to attain any knowledge.
-
-By studying the propagation of earthquake waves the physicist is
-enabled to confirm his speculations respecting the transmission of
-disturbances in elastic media. For the physicist earthquakes are
-gigantic experiments which tell him the elastic moduli of rocks as
-they exist in nature, and when properly interpreted may lead him to
-the proper comprehension of many ill-understood phenomena. It is not
-impossible that seismological investigation may teach us something
-about the earth’s magnetism, and the connection between earthquakes and
-the ‘earth currents’ which appear in our telegraph wires. These and
-numerous other kindred problems fall within the domain of the physicist.
-
-It is of interest to the meteorologist to know the connections which
-probably exist between earthquakes and the fluctuations of the
-barometer, the changes of the thermometer, the quantity of rainfall,
-and like phenomena to which he devotes his attention.
-
-Next we may turn to the more practical aims of seismology and ask
-ourselves what are the effects of earthquakes upon buildings, and
-how, in earthquake-shaken countries, the buildings are to be made to
-withstand them. Here we are face to face with problems which demand
-the attention of engineers and builders. To attain what we desire,
-observation, common sense, and subtle reasoning must be brought to bear
-upon this subject.
-
-In the investigation of the principle on which earthquake instruments
-make their records, in the analysis of the results they give, in
-problems connected with astronomy, with physics, and with construction,
-seismology offers to the mathematician new fields for investigation.
-
-A study of the effects which earthquakes produce on the lower animals
-will not fail to interest the student of natural history.
-
-A study like seismology, which leads us to a more complete knowledge
-of earth-heat and its workings, is to be regarded as one of the
-corner-stones of geology. The science of seismology invites the
-co-operation of workers and thinkers in almost every department of
-natural science.
-
-We have already referred to the influence exerted by earthquakes over
-the human mind. How to predict earthquakes, and how to escape from
-their dangers, are problems which, if they can be solved, are of
-extreme interest to the world at large.
-
-In addition to the sudden and violent movements which we call
-earthquakes, the seismologist has to investigate the smaller motions
-which we call earth tremors. From observations which have been made
-of late years, it would appear that the ground on which we dwell is
-incessantly in a state of tremulous motion.
-
-A further subject of investigation which is before the seismologist is
-the experimental verification of the existence of what may be called
-‘earth-pulsations.’ These are motions which mathematical physicists
-affirmed the existence of, but which, in consequence of the slowness of
-their period, have hitherto escaped observation.
-
-The oscillations, or slow changes in the relative positions of land and
-sea, might also be included; but this has already been taken up as a
-separate branch of geology.
-
-These four classes of movements are no doubt interdependent, and
-seismology in the widest sense might conveniently be employed to
-include them all. In succeeding chapters we will endeavour to indicate
-how far the first three of these branches have been prosecuted, and
-to point out that which remains to be accomplished. It is difficult,
-however, to form a just estimate of the amount of seismological work
-which has been done, in consequence of the scattered and uncertain
-nature of many of the records. Seismology, as a science, originated
-late, chiefly owing to the facts that centres of civilisation are
-seldom in the most disturbed regions, and that earthquake-shaken
-countries are widely separated from each other.
-
-As every portion of the habitable globe appears to have been shaken
-more or less by earthquakes, and as these phenomena are so terrible in
-their nature, we can readily understand why seismological literature is
-extensive. In the annals of almost every country which has a written
-history, references are made to seismic disturbances.
-
-An idea of the attention which earthquakes have received may be
-gathered from the fact that Professor Alexis Perrey, of Dijon, who
-has published some sixty memoirs on this subject, gave, in 1856, a
-catalogue of 1,837 works devoted to seismology.[1] In 1858 Mr. Robert
-Mallet published in the Reports of the British Association a list of
-several hundred works relating to earthquakes. Sixty-five of these
-works are to be found in the British Museum. So far as literature is
-concerned, earthquakes have received as much attention in the East as
-in the West. In China there are many works treating on earthquakes, and
-the attention which these phenomena received may be judged of from the
-fact that in A.D. 136 the Government appointed a commission
-to inquire into the subject. Even the isolated empire of Japan can
-boast of at least sixty-five works on earthquakes, seven of which
-are earthquake calendars, and twenty-three earthquake monographs.[2]
-Besides those treating especially of earthquakes, there are innumerable
-references to such disturbances in various histories, in the
-transactions of learned societies, and in periodicals. To attempt to
-give a complete catalogue of even the books which have been written
-would be to enter on a work of compilation which would occupy many
-years, and could never be satisfactorily finished.
-
-In the ‘Philosophical Transactions of the Royal Society,’ which were
-issued in the eighteenth century, there are about one hundred and
-eighty separate communications on earthquakes; and in the ‘Gentleman’s
-Magazine’ for 1755 there are no less than fifty notes and articles on
-the same subject. The great interest shown in earthquakes about the
-years 1750–60 in England, was chiefly due to the terrible calamity
-which overtook Lisbon in 1755, and to the fact that about this time
-several shocks were experienced in various parts of the British
-Islands. In 1750, which may be described as the earthquake year of
-Britain, ‘a shock was felt in Surrey on March 14; on the 18th of the
-same month the whole of the south-west of England was disturbed. On
-April 2, Chester was shaken; on June 7, Norwich was disturbed; on
-August 23, the inhabitants of Lancashire were alarmed; and on September
-30 ludicrous and alarming scenes occurred in consequence of a shock
-having been felt during the hours of Divine service, causing the
-congregations to hurry into the open air.’[3] As might be expected,
-these occurrences gave rise to many articles and notes directing
-attention to the subject of earthquakes.
-
-Seismic literature has not, however, at all times been a measure of
-seismic activity: thus, in Japan, the earthquake records for the
-twelfth and sixteenth centuries scarcely mention any shocks. At first
-sight it might be imagined that this was owing to an absence of
-earthquakes; but it is sufficiently accounted for by the fact that
-at that time the country was torn with civil war, and matters more
-urgent than the recording of natural phenomena engaged the attention of
-the inhabitants. Professor Rockwood, who has given so much attention
-to seismic disturbances in America, tells us that during the recent
-contest between Chili and Peru a similar intermission is observable. We
-see, therefore, that an absence of records does not necessarily imply
-an absence of the phenomena to be recorded.
-
-Perhaps the earliest existing records of earthquakes are those which
-occur in the Bible. The first of these, which we are told occurred in
-Palestine, was in the reign of Ahab (B.C. 918–897).[4] One of the most
-terrible earthquakes mentioned in the Bible is that which took place
-in the days of Uzziah, king of Judah (B.C. 811–759), which shook the
-ground and rent the Temple. The awful character of this, and the deep
-impression produced on men’s minds, may be learned from the fact that
-the time of its occurrence was subsequently used as an epoch from which
-to reckon dates.
-
-The writings of Herodotus, Pliny, Livy, &c., &c., show the interest
-which earthquakes attracted in early ages. These writers chiefly
-devoted themselves to references and descriptions of disastrous shocks,
-and to theories respecting the cause of earthquakes.
-
-The greater portion of the Japanese notices of earthquakes is simply
-a series of anecdotes of events which took place at the time of
-these disasters. We also find references to superstitious beliefs,
-curious occurrences, and the apparent connection between earthquake
-disturbances and other natural phenomena. In these respects the
-literature of the East closely resembles that of the West. The
-earthquake calendars of the East, however, form a class of books which
-can hardly be said to find their parallel in Europe;[5] while, on the
-other hand, the latter possesses types of books and pamphlets which do
-not appear to have a parallel elsewhere. These are the more or less
-theological works—‘Moral Reflections on Earthquakes,’ ‘Sermons’ which
-have been preached on earthquakes, ‘Prayers’ which have been appointed
-to be read.[6]
-
-Speaking generally, it may be said that the writings of the ancients,
-and those of the Middle Ages, down to the commencement of the
-nineteenth century, tended to the propagation of superstition and to
-theories based on speculations with few and imperfect facts for their
-foundation.
-
-Among the efforts which have been made in modern times to raise
-seismology to a higher level, is that of Professor Perrey, of Dijon,
-who commenced in 1840 a series of extensive catalogues embracing
-the earthquakes of the world. These catalogues enabled Perrey, and
-subsequently Mallet in his reports to the British Association, to
-discuss the periodicity of earthquakes, with reference to the seasons
-and to other phenomena, in a more general manner than it had been
-possible for previous workers to accomplish. The facts thus accumulated
-also enabled Mallet to discuss earthquakes in general, and the various
-phenomena which they present were sifted and classified for inspection.
-Another great impetus which observational seismology received was Mr.
-Mallet’s report upon the Neapolitan earthquake of 1857, in which new
-methods of seismic investigation were put forth. These have formed
-the working tools of many subsequent observers, and by them, as well
-as by his experiments on artificially produced disturbances, Mallet
-finally drew the study of earthquakes from the realms of speculation by
-showing that they, like other natural phenomena, were capable of being
-understood and investigated.
-
-In addition to Perrey and Mallet, the nineteenth century has produced
-many writers who have taken a considerable share in the advancement of
-seismology. There are the catalogues of Von Hoff, the observations of
-Humboldt, the theoretical investigations of Hopkins, the monographs of
-Schmidt, Seebach, Lasaulx, and others; the books of Fuchs, Credner,
-Vogt, Volger; the records and observations of Palmieri, Bertelli,
-Rossi, and other Italian observers. To these, which are only a few
-out of a long list of names, may be added the publications of the
-Commission appointed for the observation of earthquakes by the Natural
-History Society of Switzerland, and the volumes which have been
-published by the Seismological Society of Japan.
-
-Before concluding this chapter it will be well to define a few of the
-more ordinary terms which are used in describing earthquake phenomena.
-It may be observed that the English word _earthquake_, the German
-_erdbeben_, the French _tremblement de terre_, the Spanish _terremoto_,
-the Japanese _jishin_ &c., all mean, when literally translated,
-_earth-shaking_, and are popularly understood to mean a sudden and more
-or less violent disturbance.
-
-Seismology (σειμός an earthquake, λόγος a discourse) in its simplest
-sense means the study of earthquakes. To be consistent with a Greek
-basis for seismological terminology, some writers have thrown aside
-the familiar expression ‘earthquake,’ and substituted the awkward word
-‘seism.’
-
-The source from which an earthquake originates is called the ‘origin,’
-‘focal cavity,’ or ‘centrum.’
-
-The point or area on the surface of the ground above the origin is
-called the ‘epicentrum.’ The line joining the centrum and epicentrum is
-called the ‘seismic vertical.’
-
-The radial lines along which an earthquake may be propagated from the
-centrum are called ‘wave paths.’
-
-The angle which a wave path, where it reaches the surface of the earth,
-makes with that surface is called the ‘angle of emergence’ of the wave.
-This angle is usually denoted by the letter _e_.
-
-As the result of a simple explosion at a point in a homogeneous medium,
-we ought, theoretically, to obtain at points on the surface of the
-medium equidistant from the epicentrum, equal mechanical effects.
-These points will lie on circles called ‘isoseismic’ or ‘coseismic’
-circles. The area included between two such circles is an ‘isoseismic
-area.’ In nature, however, isoseismic lines are seldom circles.
-Elliptical or irregular curves are the common forms.
-
-The isoseismic area in which the greatest disturbance has taken place
-is called the ‘meizoseismic area.’ Seebach calls the lines enclosing
-this area ‘pleistoseists.’
-
-These last-mentioned lines are wholly due to Mallet and Seebach.
-
-Many words are used to distinguish different kinds of earthquakes from
-each other. All of these appear to be very indefinite and to depend
-upon the observer’s feelings, which, in turn, depend upon his nervous
-temperament and his situation.
-
-In South America small earthquakes, consisting of a series of rapidly
-recurring vibratory movements not sufficiently powerful to create
-damage, are spoken of as _trembelores_.
-
-The _terremotos_ of South America are earthquakes of a destructive
-nature, in which distinct shocks are perceptible. It may be observed
-that shocks which at one place would be described as _terremoto_
-would at another and more distant place probably be described as
-_trembelores_.
-
-The _succussatore_ are the shocks where there is considerable vertical
-motion. The terrible shock of Riobamba (February 4, 1797), which is
-said to have thrown corpses from their graves to a height of 100 feet,
-was an earthquake of this order.
-
-The _vorticosi_ are shocks which have a twisting or rotatory motion.
-
-Another method of describing earthquakes would be to refer to
-instrumental records. When the vibrations of the ground have only
-been along the line joining the observer and the epicentrum, the
-disturbance might be called ‘euthutropic.’ A disturbance in which the
-prominent movements are _transverse_ to the above direction might be
-called ‘diagonic.’ If motions in both of these directions occur in the
-records, the shock might be said to be ‘diastrophic.’ If there be much
-vertical movement, the shock might be said to be ‘anaseismic.’ Some
-disturbances could only be described by using two or three of these
-terms.
-
-
-
-
-                              CHAPTER II.
-
-                             SEISMOMETRY.
-
-  Nature of earthquake vibrations—Many instruments called
-    seismometers only seismoscopes—Eastern seismoscopes, columns,
-    projection seismometers—Vessels filled with liquid—Palmieri’s
-    mercury tubes—The ship seismoscope—The cacciatore—Pendulum
-    instruments of Kreil, Wagner, Ewing, and Gray—Bracket
-    seismographs—West’s parallel motion instrument—Gray’s conical
-    pendulums, rolling spheres, and cylinders—Verbeck’s ball and
-    plate seismograph—The principle of Perry and Ayrton—Vertical
-    motion instruments—Record receivers—Time-recording apparatus—The
-    Gray and Milne seismograph.
-
-
-Before we discuss the nature of earthquake motion, the determination
-of which has been the aim of modern seismological investigation, the
-reader will naturally look for an account of the various instruments
-which have been employed for recording such disturbances. A description
-of the earthquake machines which have been used even in Japan would
-form a bulky volume. All that we can do, therefore, is to describe
-briefly the more prominent features of a few of the more important of
-these instruments. In order that the relative merits of these may be
-better understood, we may state generally that modern research has
-shown a typical earthquake to consist of a series of small tremors
-succeeded by a shock, or series of shocks, separated by more or less
-irregular vibrations of the ground. The vibrations are often both
-irregular in period and in amplitude, and they have a duration of from
-a few seconds to several minutes. We will illustrate the records of
-actual earthquakes in a future chapter, but in the meantime the idea
-that an earthquake consists of a single shock must be dismissed from
-the imagination.
-
-To construct an instrument which at the time of an earthquake shall
-move and leave a record of its motion, there is but little difficulty.
-Contrivances of this order are called _seismoscopes_. If, however,
-we wish to know the period, extent, and direction of each of the
-vibrations which constitutes an earthquake, we have considerable
-difficulty. Instruments which will in this way measure or write down
-the earth’s motions are called _seismometers_ or _seismographs_.
-
-Many of the elaborate instruments supplemented with electro-magnetic
-and clockwork arrangements are, when we examine them, nothing more than
-elaborate seismoscopes which have been erroneously termed seismographs.
-
-The only approximations to true seismographs which have yet been
-invented are without doubt those which during the past few years have
-been used in Japan. It would be a somewhat arbitrary proceeding,
-however, to classify the different instruments as seismoscopes,
-seismometers, and seismographs, as the character of the record given
-by certain instruments is sometimes only seismoscopic, whilst at other
-times it is seismometric, depending on the nature of the disturbance.
-Many instruments, for instance, would record with considerable accuracy
-a single sudden movement, but would give no reliable information
-regarding a continued shaking.
-
-_Eastern Seismoscopes._—The earliest seismoscope of which we find any
-historical record is one which owes its origin to a Chinese called
-Chôko. It was invented in the year A.D. 136. A description is given
-in the Chinese history called ‘Gokanjo,’ and the translation of this
-description runs as follows:—
-
-‘In the first year of Yōka, A.D. 136, a Chinese called Chôko invented
-the seismometer shown in the accompanying drawing. This instrument
-consists of a spherically formed copper vessel, the diameter of which
-is eight feet. It is covered at its top, and in form resembles a
-wine-bottle. Its outer part is ornamented by the figures of different
-kinds of birds and animals, and old peculiar-looking letters. In the
-inner part of this instrument a column is so suspended that it can
-move in eight directions. Also, in the inside of the bottle, there is
-an arrangement by which some record of an earthquake is made according
-to the movement of the pillar. On the outside of the bottle there
-are eight dragon heads, each of which holds a ball in its mouth.
-Underneath these heads there are eight frogs so placed that they appear
-to watch the dragon’s face, so that they are ready to receive the ball
-if it should be dropped. All the arrangements which cause the pillar to
-knock the ball out of the dragon’s mouth are well hidden in the bottle.’
-
-[Illustration: FIG. 1.]
-
-‘When an earthquake occurs, and the bottle is shaken, the dragon
-instantly drops the ball, and the frog which receives it vibrates
-vigorously; any one watching this instrument can easily observe
-earthquakes.’
-
-With this arrangement, although one dragon may drop a ball, it is
-not necessary for the other seven dragons to drop their balls unless
-the movement has been in all directions; thus we can easily tell the
-direction of an earthquake.
-
-‘Once upon a time a dragon dropped its ball without any earthquake
-being observed, and the people therefore thought the instrument of
-no use, but after two or three days a notice came saying that an
-earthquake had taken place at Rōsei. Hearing of this, those who doubted
-the use of this instrument began to believe in it again. After this
-ingenious instrument had been invented by Chōko, the Chinese Government
-wisely appointed a secretary to make observations on earthquakes.’
-
-Not only is this instrument of interest on account of its antiquity,
-but it is also of interest on account of the close resemblance it bears
-to many of the instruments of modern times.
-
-Another earthquake instrument also of Eastern origin is the magnetic
-seismoscope of Japan.
-
-On the night of the destructive earthquake of 1855, which devastated
-a great portion of Tokio, the owner of a spectacle shop in Asakusa
-observed that a magnet dropped some old iron nails and keys which had
-been attached to it. From this occurrence the owner thought that the
-magnet had, in consequence of its age, lost its powers. About two hours
-afterwards, however, the great earthquake took place, after which the
-magnet was observed to have regained its powers. This occurrence led
-to the construction of the seismoscope, which is illustrated in a book
-called the ‘Ansei-Kembun-Roku,’ or a description of the earthquake of
-1855, and examples of the instrument are still to be seen in Tokio.
-These instruments consist of a piece of magnetic iron ore, which holds
-up a piece of iron like a nail. This nail is connected, by means of a
-string, with a train of clockwork communicating with an alarm. If the
-nail falls a catch is released and the clockwork set in motion, and
-warning given by the ringing of a bell. It does not appear that this
-instrument has ever acted with success.
-
-_Columns._—One of the commonest forms of seismoscope, and one which
-has been very widely used, consists of a round column of wood, metal,
-or other suitable material, placed, with its axis vertical, on a level
-plane, and surrounded by some soft material such as loose sand to
-prevent it rolling should it be overturned. The fall of such a column
-indicates that a shaking or shock has taken place. Attempts have been
-made by using a number of columns of different sizes to make these
-indications seismometric, but they seldom give reliable information
-either as to intensity or direction of shock. The indications as to
-intensity are vitiated by the fact that a long-continued gentle shaking
-may overturn a column which would stand a very considerable sudden
-shock, while the directions in which a number of columns fall seldom
-agree owing to the rotational motion imparted to them by the shaking.
-Besides, the direction of motion of the earthquake seldom remains in
-the same azimuth throughout the whole disturbance.
-
-An extremely delicate, and at the same time simple form of seismoscope
-may be made by propping up strips of glass, pins, or other easily
-overturned bodies against suitably placed supports. In this way bodies
-may be arranged, which, although they can only fall in one direction,
-nevertheless fall with far less motion than is necessary to overturn
-any column which will stand without lateral support.
-
-_Projection Seismometers._—Closely related to the seismoscopes and
-seismometers which depend on the overturning of bodies. Mallet has
-described two sets of apparatus whose indications depend on the
-distance to which a body is projected. In one of these, which consisted
-of two similar parts arranged at right angles, two metal balls rest
-one on each side of a stop at the lower part of two inclined \/ like
-troughs. In this position each of the balls completes an electric
-circuit. By a shock the balls are projected or rolled up the troughs,
-and the height to which they rise is recorded by a corresponding
-interval in the break of the circuits. The vertical component of the
-motion is measured by the compression of a spring which carries the
-table on which this arrangement rests. In the second apparatus two
-balls are successively projected, one by the forward swing, and the
-other by the backward swing of the shock. Attached to them are loose
-wires forming terminals of the circuits. They are caught in a bed
-of wet sand in a metal trough forming the other end of the circuit.
-The throw of the balls as measured in the sand, and the difference
-of time between their successive projections as indicated by special
-contrivances connected with the closing of the circuits, enables
-the observer to calculate the direction of the wave of shock, its
-velocity, and other elements connected with the disturbance. It will be
-observed that the design of this apparatus assumes the earthquake to
-consist of a distinct isolated shock.
-
-Oldham, at the end of his account of the Cachar earthquake of 1869,
-recommends the use of an instrument based on similar principles. In his
-instrument four balls like bullets are placed in notches cut in the
-corners of the upper end of a square stake driven into the ground.
-
-_Vessels filled with liquid._—Another form of simple seismoscope is
-made by partially filling a vessel with liquid. The height to which the
-liquid is washed up the side of the vessel is taken as an indication of
-the intensity of the shock, and the line joining the points on which
-maximum motion is indicated, is taken as the direction of the shock. If
-earthquakes all lasted for the same length of time, and consisted of
-vibrations of the same period, such instruments might be of service.
-These instruments have, however, been in use from an early date. In
-1742 we find that bowls of water were used to measure the earthquakes
-which in that year alarmed the inhabitants of Leghorn. About the same
-time the Rev. S. Chandler, writing about the shock at Lisbon, tells us
-that earthquakes may be measured by means of a spherical bowl about
-three or four feet in diameter, the inside of which, after being dusted
-over with Barber’s puff, is filled very gently with water. Mallet,
-Babbage, and De la Bêche have recommended the same sort of contrivance,
-but, notwithstanding, it has justly been criticised as ‘ridiculous and
-utterly impracticable.’[7]
-
-An important portion of Palmieri’s well-known instrument consists
-of horizontal tubes turned up at the ends and partially filled with
-mercury. To magnify the motion of the mercury, small floats of iron
-rest on its surface. These are attached by means of threads to a pulley
-provided with indices which move in front of a scale of degrees. We
-thus read off the intensity of an earthquake as so many degrees, which
-means so many millimetres of washing up and down of mercury in a tube.
-The direction of movement is determined by the azimuth of the tube
-which gives the maximum indication, several tubes being placed in
-different azimuths.
-
-This form of instrument appears to have been suggested by Mallet, who
-gives an account of the same in 1846. Inasmuch as the rise and fall
-of the mercury in such tubes depend on its depth and on the period
-of the earthquake together with its duration, we see that although
-the results obtained from a given instrument may give us means of
-making approximate comparisons as to the relative intensity of various
-earthquakes, it is very far from yielding any absolute measurement.
-
-Another method which has been employed to magnify and register the
-motions of liquid in a vessel has been to float upon its surface a
-raft or ship from which a tall mast projected. By a slight motion of
-the raft, the top of the mast vibrated through a considerable range.
-This motion of the mast as to direction and extent was then recorded by
-suitable contrivances attached to the top of the mast.
-
-A very simple form of liquid seismoscope consists of a circular trough
-of wood with notches cut round its side. This is filled with mercury
-to the level of the notches. At the time of an earthquake the maximum
-quantity of mercury runs over the notches in the direction of greatest
-motion. This instrument, which has long been used in Italy, is known as
-a Cacciatore, being named after its inventor. It is a prominent feature
-in the collection of apparatus forming the well-known seismograph of
-Palmieri.
-
-_Pendulum instruments._—Mallet speaks of pendulum seismoscopes and
-seismographs as ‘the oldest probably of seismometers long set up in
-Italy and southern Europe.’ In 1841 we find these being used to record
-the earthquake disturbances at Comrie in Scotland.
-
-These instruments may be divided into two classes: first, those which
-at the time of the shock are intended to swing, and thus record the
-direction of movement; and second, those which are supposed to remain
-at rest and thus provide ‘steady points.’
-
-To obtain an absolutely ‘steady point’ at the time of an earthquake,
-has been one of the chief aims of all recent seismological
-investigations.
-
-With a style or pointer projecting down from the steady point to
-a surface which is being moved backward and forward by the earth,
-such a surface has written upon it by its own motions a record of
-the ground to which it is attached. Conversely, a point projecting
-upwards from the moving earth might be caused to write a record on the
-body providing the steady point, which in the class of instruments
-now to be referred to is supposed to be the bob of a pendulum. It is
-not difficult to get a pendulum which will swing at the time of a
-moderately strong earthquake, but it is somewhat difficult to obtain
-one which will not swing at such a time. During the past few years,
-pendulums varying between forty feet in length and carrying bobs of
-eighty pounds in weight, and one-eighth of an inch in length, and
-carrying a gun-shot, have been experimented with under a great variety
-of circumstances. Sometimes the supports of these pendulums have been
-as rigid as it is possible to make a structure from brick and mortar,
-and at other times they have intentionally been made loose and
-flexible. The indices which wrote the motions of these pendulums have
-been as various as the pendulums themselves. A small needle sliding
-vertically through two small holes, and resting its lower end on a
-surface of smoked glass, has on account of its small amount of friction
-been perhaps one of the favourite forms of recording pointers.
-
-The free pendulums which have been employed, and which were intended
-to swing, have been used for two purposes: first, to determine the
-direction of motion from the direction of swing, and second, to see if
-an approximation to the period of the earth’s motion could be obtained
-by discovering the pendulum amongst a series of different lengths which
-was set in most violent motion, this probably being the one which had
-its natural period of swing the most nearly approximating to the period
-of the earthquake oscillations.
-
-Inasmuch as all pendulums when swinging have a tendency to change the
-plane of their oscillation, and also as we now know that the direction
-of motion during an earthquake is not always constant, the results
-usually obtained with these instruments respecting the direction of
-the earth’s motion have been unsatisfactory. The results which were
-obtained by series of pendulums of different lengths were, for various
-reasons, also unsatisfactory.
-
-Of pendulums intended to provide a steady point, from which the
-relative motion of a point on the earth’s surface could be recorded,
-there has been a great variety. One of the oldest forms consisted of
-a pendulum with a style projecting downwards from the bob so as to
-touch a bed of sand. Sometimes a concave surface was placed beneath the
-pendulum, on which the record was traced by means of a pencil. Probably
-the best form was that in which a needle, capable of sliding freely up
-and down, marked the relative horizontal motion of the earth and the
-pendulum bob on a smoked glass plate.
-
-It generally happens that at the time of a moderately severe earthquake
-the whole of these forms of apparatus are set in motion, due partly to
-the motion of the point of support of the pendulum, and partly to the
-friction of the writing point on the plate.
-
-Among these pendulums may be mentioned those of Cavallieri, Faura,
-Palmieri, Rossi, and numerous others. It is possible that the
-originators of some of these pendulums may have intended that
-they should record by swinging. If this is so, then so far as the
-determination of the actual nature of earthquake motion is concerned,
-they belong to a lower grade of apparatus than that in which they are
-here included.
-
-A great improvement in pendulum apparatus is due to Mr. Thomas Gray
-of Glasgow, who suggested applying so much frictional resistance to
-the free swing of a pendulum that for small displacements it became
-‘dead beat.’ By carrying out this suggestion, pendulum instruments
-were raised to the position of seismographs. The manner of applying
-the friction will be understood from the following description of a
-pendulum instrument which is also provided with an index which gives a
-magnification of the motion of the earth.
-
-B B B B is a box 113 cm. high and 30 cm. by 18 cm. square. Inside this
-box a lead ring R, 17 cm. in diameter and 3 cm. thick, is suspended as
-a pendulum from the screw S. This screw passes through a small brass
-plate P P, which can be moved horizontally over a hole in the top of
-the box. These motions in the point of suspension allow the pendulum to
-be adjusted.
-
-[Illustration: FIG. 2.]
-
-Projecting over the top of the pendulum there is a wooden arm W
-carrying two sliding pointers H H, resting on a glass plate placed on
-the top of the pendulum. These pointers are for the purpose of giving
-the frictional resistance before referred to. If this friction plate
-is smoked, the friction pointers will write upon it records of _large_
-earthquakes independently of the records given by the proper index,
-which only gives satisfactory records in the case of shocks of ordinary
-intensity. Crossing the inside of the pendulum R there is a brass bar
-perforated with a small conical hole at M. A stiff wire passes through
-M and forms the upper portion of the index I, the lower portion of
-which is a thin piece of bamboo. Fixed upon the wire there is a small
-brass ball which rests on the upper side of a second brass plate also
-perforated with a conical hole, which plate is fixed on the bar O O
-crossing the box.
-
-If at the time of an earthquake the upper part of the index I remains
-steady at M, then by the motion at O, the lower end of the index which
-carries a sliding needle at G, will magnify the motion of the earth in
-the ratios M O: O G. In this instrument O G is about 17 cm.
-
-The needle G works upon a piece of smoked glass. In order to bring
-the glass into contact with the needle without disturbance, the glass
-is carried on a strip of wood K, hinged at the back of the box, and
-propped up in front by a loose block of wood Y. When Y is removed
-the glass drops down with K out of contact with the needle. The box
-is carried on bars of wood C C, which are fixed to the ground by the
-stakes A A.
-
-The great advantage of a pendulum seismograph working on a stationary
-plate is, that the record shows at once whether the direction of motion
-has been constant, or whether it has been variable. The maximum extent
-of motion in various directions is also easily obtained.
-
-The disadvantage of the instrument is, that at the time of a large
-earthquake, owing perhaps to a slight swing in the pendulum, the
-records may be unduly magnified.
-
-On such occasions, however, fairly good records may be obtained from
-the friction pointers, provided that the plates on which they work have
-been previously smoked. It might perhaps be well to use two of these
-instruments, one having a comparatively high frictional resistance, and
-hence ‘dead beat’ for large displacements.
-
-Many attempts have been made to use a pendulum seismograph in
-conjunction with a record-receiving surface, which at the time of the
-earthquake should be kept in motion by clockwork. In this way it was
-hoped to separate the various vibrations of the earthquake, and thus
-avoid the greater or less confusion which occurs when the index of the
-pendulum writes its backward and forward motion on a stationary plate.
-Hitherto all attempts in this direction, in which a single multiplying
-index was used, have been unsuccessful because of the moving plate
-dragging the index in the direction of its motion for a short
-distance, and then allowing it to fall back towards its normal position.
-
-In connection with this subject we may mention the pendulum
-seismographs of Kreil, Wagener, Ewing, and Gray.
-
-In the bob of Kreil’s pendulum there was clockwork, which caused a disc
-on the axis of the pendulum to continuously rotate. On this continually
-revolving surface a style fixed to the earth traced an unbroken circle.
-At the time of an earthquake, by the motion of the style, the circle
-was to be broken and lines drawn. The number and length of these lines
-were to indicate the length and intensity of the disturbance.
-
-Gray’s pendulum consisted of a flat heavy disc carrying on its upper
-surface a smoked glass plate. This, which formed the bob of the
-pendulum, was supported by a pianoforte steel wire. When set ready to
-receive an earthquake, the wire was twisted and the bob held by a catch
-so arranged that at the time of the earthquake the catch was released,
-and the bob of the pendulum allowed to turn slowly by the untwisting of
-the supporting wire. Resting on the surface of this rotating disc were
-two multiplying indices arranged to write the earth’s motions as two
-components.
-
-In the instruments of Wagener and Ewing, the clockwork and moving
-surface do not form part of the pendulum, but rest independently on
-a support rigidly attached to the earth. In Wagener’s instrument one
-index only is used, while in Ewing’s two are used for writing the
-record of the motion.
-
-A difficulty which is apparent in all pendulum machines is that when
-the bob of such a pendulum is deflected it tends to fall back to its
-normal position. To make a pendulum perfect it therefore requires
-some compensating arrangement, so that the pendulum, for small
-displacements, shall be in neutral equilibrium, and the errors due to
-swinging shall be avoided.
-
-Several methods have been suggested for making the bob of an ordinary
-pendulum astatic for small displacements. One method proposed by Gray
-consists in fixing in the bob of a pendulum a circular trough of
-liquid, the curvature of this trough having a proper form. Another
-method which was suggested, was to attach a vertical spiral spring
-to a point in the axis of the pendulum a little below the point of
-suspension, and to a fixed point above it, so that when the pendulum is
-deflected it would introduce a couple.
-
-Professor Ewing has suggested an arrangement so that the bob of the
-pendulum shall be partly suspended by a stretched spiral spring, and at
-the same time shall be partly held up from below by a vertically placed
-strut, the weight carried by the strut being to the weight carried by
-the spring in the ratio of their respective lengths. As to how these
-arrangements will act when carried into practice yet remains to be seen.
-
-Another important class of instruments are _inverted pendulums_. These
-are vertical springs made of metal or wood loaded at their upper end
-with a heavy mass of metal. An arrangement of this sort, provided at
-its upper end with a pencil to write on a concave surface, was employed
-in 1841 to register the earthquakes at Comrie in Scotland. In Japan
-they were largely employed in series, each member of a series having
-a different period of vibration. The object of these arrangements
-was to determine which of the pendulums, with a given earthquake,
-recorded the greatest motion, it being assumed that the one which was
-thrown into the most violent oscillation would be the one most nearly
-approximating with the period of the earthquake. The result of these
-experiments showed that it was usually those with a slow period of
-vibration which were the most disturbed.
-
-_Bracket Seismographs._—A group of instruments of recent origin which
-have done good work, are the bracket seismographs. These instruments
-appear to have been independently invented by several investigators:
-the germ from which they originated probably being the well-known
-horizontal pendulum of Professor Zöllner. In Japan they were first
-employed by Professor W. S. Chaplin. Subsequently they were used by
-Professor Ewing and Mr. Gray. They consist essentially of a heavy
-weight supported at the extremity of a horizontal bracket which is free
-to turn on a vertical axis at its other end. When the frame carrying
-this axis is moved in any direction excepting parallel to the length
-of the gate-like bracket, the weight causes the bracket to turn round
-a line known as the instantaneous axis of the bracket corresponding to
-this motion of the fixed axis. Any point in this line may therefore be
-taken as a steady point for motions at right angles to the length of
-the supporting bracket. Two of these instruments placed at right angles
-to each other have to be employed in conjunction, and the motion of
-the ground is written down as two rectangular components. In Professor
-Ewing’s form of the instrument, light prolongations of the brackets
-form indices which give magnified representations of the motion, and
-the weights are pivoted round a vertical axis through their centre.
-
-In the accompanying sketch B is a heavy weight pivoted at the end of
-a small bracket C A K, which bracket is free to turn on a knife-edge,
-K, above, and a pivot A, below, in the stand S. At the time of an
-earthquake B remains steady, and the index P, forming a continuation
-of the bracket, magnifies the motion of the stand, in the ratio of
-A C : C N.
-
-[Illustration: FIG. 3.]
-
-In an instrument called a double-bracket seismograph, invented by Mr.
-Gray, we have two brackets hinged to each other, and one of them to a
-fixed frame. The planes of the two brackets are placed at right angles,
-so as to give to a heavy mass supported at the end of the outer bracket
-two degrees of horizontal freedom.
-
-In all bracket machines, especially those which carry a pivoted weight,
-it is doubtful whether the weight provides a truly steady point
-relatively to the plate on which the record is written for motion
-parallel to the direction of the arm.
-
-[Illustration: FIG. 4.]
-
-_Parallel motion Instrument._—A machine which writes its record as
-two components, and which promises great stability, is one suggested
-by Professor C. D. West. Like the bracket machines it consists of
-two similar parts placed at right angles to each other, and is as
-follows: A bar of iron A is suspended from both sides on pivots at C C
-by a system of light arms hinging with each other at the black dots,
-between the upper and lower parts of the rigid frame B C. The arms are
-of such a length that for small displacements parallel to the length
-of the bar, C C practically move in a straight line, and the bar is
-in neutral equilibrium. A light prolongation of the bar _d_ works the
-upper end of the light index _e_, passing as a universal joint through
-the rigid support F. A second index _e′ _ from the bar at right angles
-also passes through F. The multiplying ends of these indices are
-coupled together to write a resultant motion on a smoked glass plate S.
-
-_Conical Pendulums._—Another group of instruments which have also
-yielded valuable records are the conical pendulum seismographs. The
-idea of using the bob of a conical pendulum to give a steady point in
-an earthquake machine was first suggested and carried into practice by
-Mr. Gray. The seismograph as employed consists of a pair of conical
-pendulums hung in planes at right angles to each other. The bob of each
-of these pendulums is fixed a short distance from the end of a light
-lever, which forms the writing index, the short end resting as a strut
-against the side of a post fixed in the earth. The weight is carried by
-a thin wire or thread, the upper end of which is attached to a point
-vertically above the fixed end of the lever.
-
-_Rolling Spheres and Cylinders._—After the conical pendulum
-seismographs, which claim several important advantages over the bracket
-machines, we come to a group of instruments known as rolling sphere
-seismographs. Here, again, we have a class of instruments for the
-various forms of which we are indebted to the ingenuity of Mr. Gray.
-
-The general arrangement and principle of one of these instruments will
-be readily understood from the accompanying figure. S is a segment
-of a large sphere with a centre near C. Slightly below this centre a
-heavy weight B, which may be a lead ring, is pivoted. At the time of
-an earthquake C is steady, and the earth’s motions are magnified by
-the pointer C A N in the proportion of C A : A N. The working of this
-pointer or index is similar to that of the pointer in the pendulum.
-
-[Illustration: FIG. 5.]
-
-Closely connected with the rolling sphere seismographs, are Gray’s
-rolling cylinder seismographs.
-
-These are two cylinders resting on a surface plate with their axes at
-right angles to each other. Near to the highest point in each of these
-cylinders, this point remaining nearly steady when the surface plate
-is moved backwards and forwards, there is attached the end of a light
-index. These indices are again pivoted a short distance from their ends
-on axes connected with the surface plate. In order that the two indices
-may be brought parallel, one is cranked at the second pivot.
-
-_Ball and Plate Seismograph._—Another form of seismograph, which is
-closely related to the two forms of apparatus just described, is
-Verbeck’s ball and plate seismograph. This consists of a surface plate
-resting on three hard spheres, which in turn rests upon a second
-surface plate. When the lower plate is moved, the upper one tends to
-remain at rest, and thus may be used as a steady mass to move an index.
-
-_The Principle of Perry and Ayrton._—An instrument which is of interest
-from the scientific principle it involves is a seismograph suggested
-by Professors Perry and Ayrton, who propose to support a heavy ball on
-three springs, which shall be sufficiently stiff to have an exceedingly
-quick period of vibration. By means of pencils attached to the ball by
-levers, the motions of the ball are to be recorded on a moving band of
-paper. The result would be a record compounded of the small vibrations
-of the springs superimposed on the larger, slower, wave-like motions
-of the earthquake, and, knowing the former of these, the latter might
-be separated by analysis. Although our present knowledge of earthquake
-motion indicates that the analysis of such a record would often present
-us with insuperable difficulties, this instrument is worthy of notice
-on account of the novelty of the principle it involves, which, the
-authors truly remark, has in seismometry been a ‘neglected’ one.
-
-_Instruments to record Vertical Motion._—The instruments which have
-been devised to record vertical motion are almost as numerous as those
-which have been devised to record horizontal motion. The earliest
-form of instrument employed for this purpose was a spiral spring
-stretched by weight, which, on account of its inertia, was supposed
-at the time of a shock to remain steady. No satisfactory results have
-ever been obtained from such instruments, chiefly on account of the
-inconvenience in making a spring sufficiently long to allow of enough
-elongation to give a long period of vibration. Similar remarks may
-be applied to the horizontally placed elastic rods, one end of which
-is fixed to a wall, whilst the opposite end is loaded with a weight.
-Such contrivances, furnished with pencil on the weight to write a
-record upon a vertical surface, were used in 1842 at Comrie, and we
-see the same principle applied in a portion of Palmieri’s apparatus.
-Contrivances like these neither give us the true amplitude of the
-vertical motion, insomuch as they are readily set in a state of
-oscillation; nor do they indicate the duration of a disturbance, for,
-being once set in motion, they continue that motion in virtue of their
-inertia long after the actual earthquake has ceased. They can only be
-regarded as seismoscopes.
-
-[Illustration: FIG. 6.]
-
-The most satisfactory instrument which has yet been devised for
-recording vertical motion is Gray’s horizontal lever spring seismograph.
-
-This instrument will be better understood from the accompanying sketch.
-A vertical spring S is fixed at its upper end by means of a nut _n_,
-which rests on the top of the frame F, and serves to raise or lower
-the spring through a short distance as a last adjustment for the
-position of the cross-arm A. The arm A rests at one end on two sharp
-points, _p_, one resting in a conical hole and the other in a V-slot;
-it is supported at B by the spring S, and is weighted at C with a lead
-ring R. Over a pin at the point C a stirrup of thread is placed which
-supports a small trough, _t_. The trough _t_ is pivoted at _a_, has
-attached to it the index _i_ (which is hinged by means of a strip of
-tough paper at _h_, and rests through a fine pin on the glass plate
-_g_), and is partly filled with mercury.
-
-Another method of obtaining a steady point for vertical motion is that
-of Dr. Wagener, who employs a buoy partly immersed in a vessel of
-water. This was considerably improved upon by Mr. Gray, who suggested
-the use of a buoy, which, with the exception of a long thin style, was
-completely sunk.
-
-Among the other forms of apparatus used to record vertical motion may
-be mentioned vessels provided with india-rubber or other flexible
-bottoms, and partially filled with water or some other liquid. As the
-vessel is moved up and down, the bottom tends to remain behind and
-provides a more or less steady point. Pivoted to this is a light index,
-which is again pivoted to a rigid frame in connection with the earth.
-Instruments of this description have yielded good records.
-
-_Record Receivers._—A large number of earthquake machines having been
-referred to, it now remains to consider the apparatus on which they
-write their motions. The earlier forms of seismographs, as has already
-been indicated, recorded their movements in a bed of sand; others
-wrote their records by means of pencils on sheets of paper. Where we
-have seismographs which magnify the motion of the earth, it will be
-observed that methods like the above would involve great frictional
-resistances, tending to cause motion in the assumed steady points of
-the seismographs. One of the most perfect instruments would be obtained
-by registering photographically the motions of the recording index by
-the reflection of a ray of light. Such an instrument would, however,
-be difficult to construct and difficult to manipulate. One of the best
-practical forms of registering apparatus is one in which the record is
-written on a surface of smoked glass. This can afterwards be covered
-with a coat of photographer’s varnish, and subsequently photographed by
-the ‘blue process’ so well known to engineers.
-
-To obtain a record of all the vibrations of an earthquake it is
-necessary that the surface on which the seismograph writes should at
-the time of an earthquake be in motion. Of record-receiving machines
-there are three types. First, there are those which move continuously.
-The common form of these is a circular glass plate like an old form of
-chronograph, driven continuously by clockwork. On this the pointers of
-the seismograph rest and trace over and over again the same circles.
-At the time of an earthquake they move back and forth across the
-circles, which are theoretically fine lines, and leave a record of
-the earthquake. Instead of a circular plate, a drum covered with
-smoked paper may be used, which, after the earthquake, possesses the
-advantage, after unrolling, of presenting the record in a straight
-line, instead of a record written round the periphery of a circle, as
-is the case with the circular glass plates. Such records are easily
-preserved, but they are more difficult to photograph.
-
-The second form of apparatus is one which is set in motion at the
-time of a shock. This may be a contrivance like one of those just
-described, or a straight smoked glass plate on a carriage. By means
-of an electrical or a mechanical contrivance called a ‘starter,’ of
-which many forms have been contrived, the earthquake is caused to
-release a detent and thus set in motion the mechanism which moves the
-record receiver.
-
-The great advantage of continuously-moving machines is that the
-beginning and end of the shock can usually be got with certainty, while
-all the uncertainty as to the action of the ‘starter’ is avoided.
-Self-starting machines have, of course, the advantage of simplicity and
-cheapness, while there is no danger of the record getting obliterated
-by the subsequent motion of the plate under the index.
-
-_Time-recording Apparatus._—Of equal importance with the instruments
-which record the motion of the ground, are those instruments which
-record the time at which such motion took place. The great value of
-time records, when determining the origin from which an earthquake
-originates, will be shown farther on. The most important result which
-is required in connection with time observations, is to determine
-the interval of time taken by a disturbance in travelling from one
-point to another. On account of the great velocity with which these
-disturbances sometimes travel, it is necessary that these observations
-should be made with considerable accuracy. The old methods of adapting
-an apparatus to a clock which, when shaken, shall cause the clock to
-stop, are of little value unless the stations at which the observations
-are made are at considerable distances apart. This will be appreciated
-when we remember that the disturbance may possibly travel at the rate
-of a mile per second, that its duration at any station may often extend
-over a minute, and that one set of apparatus at one station may stop,
-perhaps, at the commencement of the disturbance, and the other near
-the end. A satisfactory time-taking apparatus will therefore require,
-not only the means for stopping a clock, but also a contrivance which,
-at the same instant that the clock is stopped, shall make a mark on a
-record which is being drawn by a seismograph. In this way we find out
-at which portion of the shock the time was taken.
-
-[Illustration: FIG. 7.]
-
-Palmieri stops a clock in his seismograph by closing an electric
-circuit. Mallet proposes to stop a clock by the falling of a column
-which is attached by a string to the pendulum of the clock. So long as
-the column is standing the string is loose and the pendulum is free
-to move; but when the column falls, the string is tightened and the
-pendulum is arrested. The difficulty which arises is to obtain a column
-that will fall with a slight disturbance. The best form of contrivance
-for causing a column to fall, and one which may also be used in drawing
-out a catch to relieve the machinery of a record receiver, is shown in
-the accompanying sketch.
-
-S is the segment of a sphere about 4·5 cm. radius, with a centre
-slightly above C. L is a disc of lead about 7 cm. in diameter resting
-upon the segment. Above this there is a light pointer, P, about 30
-cm. long. On the top of the pointer a small cylinder of iron, W, is
-balanced, and connected by a string with the catch to be relieved. When
-the table on which W P S rests is shaken, rotation takes place near
-to C, the motion of the base S is magnified at the upper end of the
-pointer, and the weight overturned. This catch may be used to relieve
-a toothed bar axled at one end, and held up above a pin projecting
-from the face of the pendulum bob. When this falls it catches the
-projecting pin and holds the pendulum.
-
-Another way of relieving the toothed bar is to hold up the opposite
-end to that at which it is axled by resting it on the extremity of a
-horizontal wire fixed to the bob of a conical pendulum—for example, one
-of the indices of a conical pendulum seismograph. The whole of this
-apparatus, which may be constructed at the cost of a few pence, can be
-made small enough to go inside an ordinary clock case.
-
-The difficulty which arises with all these clock-stopping arrangements
-is that it is difficult for observers situated at distant stations
-to re-start their clocks so that their difference in time shall
-be accurately known. Even if each observer is provided with a
-well-regulated chronometer, with which he can make comparisons, the
-rating of these instruments is for all ordinary persons an extremely
-troublesome operation.
-
-In order to avoid this difficulty the author has of late years used a
-method of obtaining the time without stopping the clock. To do this a
-clock with a central seconds hand is taken, and the hour and minute
-hands are prolonged and bent out slightly at their extremities at right
-angles to the face, the hour hand being slightly the longest. Each
-hand is then tipped with a piece of soft material like cork, which is
-smeared with a glycerine ink. A light flat ring, with divisions in it
-corresponding to those on the face of the clock, is so arranged that
-at the time of a shock it can be quickly advanced to touch the inked
-pads on the hands of the clock and then withdrawn. This is accomplished
-by suitable machinery, which is relieved either by an electro-magnet
-or some other contrivance which will withdraw a catch. In this way an
-impression in the form of three dots is received on the disc, and the
-time known without either stopping or sensibly retarding the clock.
-
-For ordinary observers, if a time-taker is not used in conjunction
-with a record receiver, as good results as those obtained by ordinary
-clock-stopping apparatus are obtainable by glancing at an ordinary
-watch. Subsequently the watch by which the observation was made should
-be compared with some good time-keeper, and the local time at which the
-shock took place is then approximately known.
-
-From what has now been said it will be seen that for a complete
-seismograph we require three distinct sets of apparatus—an apparatus to
-record horizontal motion, an apparatus to record vertical motion, and
-an apparatus to record time. The horizontal and vertical motions must
-be written on the same receiver, and if possible side by side, whilst
-the instant at which the time record is made a mark must be made on
-the edge of the diagram which is being drawn by the seismograph. Such
-a seismograph has been constructed and is now erected in Japan. It is
-illustrated in the accompanying diagram.
-
-_The Gray and Milne Seismograph._—In this apparatus two mutually
-rectangular components of the horizontal motion of the earth are
-recorded on a sheet of smoked paper wound round a drum, D, kept
-continuously in motion by clockwork, W, by means of two conical
-pendulum seismographs, C. The vertical motion is recorded on the same
-sheet of paper by means of a compensated-spring seismograph, S L M B.
-
-The time of occurrence of an earthquake is determined by causing the
-circuit of two electro-magnets to be closed by the shaking. One of
-these magnets relieves a mechanism, forming part of a time-keeper,
-which causes the dial of the timepiece to come suddenly forwards on
-the hands and then move back to its original position. The hands are
-provided with ink-pads, which mark their positions on the dial, thus
-indicating the hour, minute, and second when the circuit was closed.
-The second electro-magnet causes a pointer to make a mark on the paper
-receiving the record of the motion. This mark indicates the part of the
-earthquake at which the circuit was closed.
-
-[Illustration: FIG. 8.]
-
-The duration of the earthquake is estimated from the length of the
-record on the smoked paper and the rate of motion of the drum. The
-nature and period of the different movements are obtained from the
-curves drawn on the paper.
-
-Mr. Gray has since greatly modified this apparatus, notably by the
-introduction of a band of paper sufficiently long to take a record for
-twenty-four hours without repetition. The record is written in ink by
-means of fine siphons. In this way the instrument, which is extremely
-sensitive to change of level, can be made to show not only earthquakes,
-but the pulsations of long period which have recently occupied so much
-attention.
-
-
-
-
-                             CHAPTER III.
-
-              EARTHQUAKE MOTION DISCUSSED THEORETICALLY.
-
-  Ideas of the ancients (the views of Travagini, Hooke, Woodward,
-    Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves
-    and vibrations—Possible causes of disturbance in the Earth’s
-    crust—The time of vibration of an earth particle—Velocity and
-    acceleration of a particle—Propagation of a disturbance as
-    determined by experiments upon the elastic moduli of rocks—The
-    intensity of an earthquake—Area of greatest overturning
-    moment—Earthquake waves—Reflexion, refraction, and interference
-    of waves—Radiation of a disturbance.
-
-
-_Ideas of Early Writers._—One of the first accounts of the varieties of
-motion which may be experienced at the time of an earthquake is to be
-found in the classification of earthquakes given by Aristotle.[8] It is
-as follows:—
-
-1. Epiclintæ, or earthquakes which move the ground obliquely.
-
-2. Brastæ, with an upward vertical motion like boiling water.
-
-3. Chasmatiæ, which cause the ground to sink and form hollows.
-
-4. Rhectæ, which raise the ground and make fissures.
-
-5. Ostæ, which overthrow with one thrust.
-
-6. Palmatiæ, which shake from side to side with a sort of tremor.
-
-From the sixth group in this classification we see that this early
-writer did not regard earthquakes as necessarily isolated events, but
-that some of them consisted of a succession of backward and forward
-vibratory motions. He also distinguishes between the total duration of
-an earthquake and the length of, and intervals between, a series of
-shocks. Aristotle had, in fact, some idea of what modern writers upon
-ordinary earthquakes would term ‘modality.’
-
-The earliest writer who had the idea that an earthquake was a
-pulse-like motion propagated through solid ground appears to have been
-Francisci Travagini, who, in 1679, wrote upon an earthquake which
-in 1667 had overthrown Ragusa. The method in which the pulses were
-propagated he illustrated by experiments.
-
-Hooke, who, in 1690, delivered discourses on earthquakes before the
-Royal Society, divides these phenomena according to the geological
-effects they have produced; thus there is a _genus_ producing
-elevations, a _genus_ producing sinkings, a _genus_ producing
-conversions and transportations, and a _genus_ which produces what, in
-modern language, we should term metamorphic action.
-
-Woodward, in his ‘Natural History,’ written in 1695, speaks of
-earthquakes as being agitations and concussions produced by water in
-the interior of the earth coming in contact with internal fires.
-
-Stukeley observed that an earthquake was ‘a tremor of the earth,’ to
-be explained as a vibration in a solid. The Rev. John Mitchell, writing
-in 1760, says that the motion of the earth in earthquakes is partly
-tremulous and partly propagated by waves.
-
-From these few examples, to which might be added many more, it will
-be seen that an earthquake disturbance has usually been regarded as
-a concussion, vibration, trembling, or undulatory movement. Further,
-it can be seen in narratives of earthquakes that it had been often
-observed that these tremblings and shakings continued over a certain
-period of time. Although it had been noticed that large areas were
-almost simultaneously affected by these disturbances, no definite idea
-appears to have existed as to how earthquake motion was propagated.
-Usually it was assumed that the disturbance spread through subterranean
-channels.
-
-The first true conception of earthquake motion and the manner of its
-propagation is due to Dr. Thomas Young, who suggested that earthquake
-motion was vibratory, and it might be ‘propagated through the earth
-nearly in the same manner as a noise is conveyed through the air.’ The
-same idea was moulded into a more definite form by Gay Lussac.
-
-The first accurate definition of an earthquake is due to Mr. Robert
-Mallet, who, after collecting and examining many facts connected with
-earthquake phenomena, and reasoning on these, with the help of known
-laws connected with the production and propagation of waves of various
-descriptions, formulated his views as follows:—
-
-An earthquake is ‘_the transit of a wave or waves of elastic
-compression in any direction from vertically upwards to horizontally,
-in any azimuth, through the crust and surface of the earth, from any
-centre of impulse or from more than one, and which may be attended
-with sound and tidal waves, dependent upon the impulse and upon
-circumstances of position as to sea and land_.’
-
-In brief, so far as motion in the earth is concerned. Mallet defined
-an earthquake as being a motion due to the transit of waves of elastic
-compression. In many cases it is possible that this is strictly true,
-but in succeeding pages it will be shown that earthquake motion may
-also be due to the transit of waves of elastic distortion.
-
-To obtain a true idea of earthquake motion is a matter of cardinal
-importance, as it forms the key-stone of many investigations.
-
-If we know the nature of the motion produced by an earthquake, we are
-aided in tracking it to its origin, and in reasoning as to how it was
-produced. If our knowledge of the nature of the motion of an earthquake
-is incorrect, it will be impossible for us intelligently to construct
-buildings to withstand the effects of these disturbances. We have thus
-to consider, in this portion of seismology, a point of great scientific
-importance, and shall deal with it at some length.
-
-_Nature of Elastic Waves and Vibrations._—When it is stated that an
-earthquake consists of elastic waves of compression and distortion, the
-student of physics has a clear idea of what is meant and a knowledge
-of the mechanical laws which govern such disturbances. The ordinary
-reader, however, and the majority of the inhabitants of earthquake
-countries, who of all people have the greatest interest in this matter,
-may not have so clear a conception, and it will, therefore, not be out
-of place to give some general explanation on this point.
-
-The ordinary idea of a wave is that it is a disturbance similar to
-that which we often see in water. Waves like these must not, however,
-be confounded with elastic waves. A disturbance produced in water,
-say, for instance, by dropping a stone into a pond, is propagated
-outwards by the action of gravity. First, a ridge of water is raised
-up by the stone passing beneath the surface. As this ridge falls
-towards its normal position in virtue of its weight, it raises a second
-ridge. This second ridge raises a third ridge, and so on. The water
-moves vertically up and down, whilst the wave itself is propagated
-horizontally.
-
-To understand what is meant by elastic waves, it is first necessary
-to understand what is meant by the term elastic. In popular language
-the term elastic is confined to substances like india-rubber, and but
-seldom to rock-like materials, through which earthquake waves are
-propagated. India-rubber is called elastic because after we remove a
-compressive force it has a tendency to spring back to its original
-shape. The elastic force of the india-rubber is in this case the force
-which causes it to resist a change of form. Now, a piece of rock may,
-up to a certain point, like the india-rubber, be compressed, and when
-the compressing force is removed it will also tend to resume its
-original form. However, as the rock offers more resistance to the
-compressing force than the india-rubber offers, we say that it is the
-more elastic. It may be here observed that a substance like granite
-offers great resistance, not only to compression or a change of volume,
-but also to a change of form or shape; whereas a substance like air,
-which is also elastic, only offers resistance to compression, but not
-to a change of shape.
-
-With these ideas before us we will now proceed to consider how, after
-a body has been suddenly compressed or distorted, this disturbance
-is propagated through the mass. For the elastic body let us take a
-long spiral spring hung from the ceiling of a room and kept slightly
-stretched by a weight. If we give this weight an upward tap from below,
-say with a hammer, we shall observe a pulse-like wave which runs up the
-spring until it reaches the ceiling of the room. Here it will, so to
-speak, rebound, like a billiard ball from the end of a table, and run
-towards the weight from which it started. Whilst this is going on we
-may also observe that the weight is moving up and down.
-
-Here, then, we have two distinct things to observe—one being the
-transmission of motion up to the ceiling, which we may liken to the
-transmission of an earthquake wave between two distant localities on
-the earth’s surface, and the other being the up and down motion of our
-weight, which we may compare to the backward and forward swinging which
-we experience at the time of an earthquake.
-
-These two motions—namely, the pulse-like wave produced by the
-transmission of motion, and the backward and forward oscillation of the
-weight or of any point on the spring—must be carefully distinguished
-from each other.
-
-First, we will consider the backward and forward motion of the weight.
-The distance through which the weight moves depends upon the force
-of the blow. The number of up and down oscillations it makes, say
-in a second, depends upon the stiffness of the spring. The weight,
-supposing it to be always the same, will move more quickly at the end
-of a stiff spring than at the end of a flaccid one; that is to say, its
-velocity is quicker. As in any given spring the number of up and down
-oscillations are always the same in a given interval of time, if these
-oscillations are of great extent, the weight must move more quickly
-with large than with small oscillations.
-
-At the time of an earthquake the manner in which we are moved backward
-and forward is very similar to the manner in which the weight is moved.
-If we stand on a hard rock-like granite, we are to a great extent
-placed as if we were attached to a stiff quickly-vibrating spring.
-If, however, we are on a soft rock, it is more like being on a loose
-flaccid spring.
-
-All that has thus far been considered has been a backward and forward
-kind of motion, where there is a rectilinear _compression_ and
-_extension_ amongst the particles on which we stand.
-
-We might, however, imagine our rock, which for the moment we will
-consider to be a square column, to be twisted, and thus have its
-_shape_ altered. When the twisting force is taken off it seems evident
-that the column would endeavour to untwist itself or regain its
-original form. Now the force which a body offers against a change of
-_volume_ may be very different from that which it offers against a
-change of _form_.
-
-In disturbances which take place in the rocky crust of our earth,
-it would seem possible that we may have vibrations set up which are
-either compressions and extensions or twistings and distortions. These
-may take place separately or simultaneously, or we may have resultant
-motions due to their combination.
-
-The following are examples of possible causes which might give rise to
-these different orders of disturbance:—
-
-1. Imagine a large area stretched by elevation until it reaches the
-limit of its elasticity and cracks. After cracking, in consequence
-of its elasticity, it will fly back over the whole area like a
-broken spring, and each point in the area will oscillate round its
-new position of equilibrium. In this case there will be no waves
-of distortion excepting near the end of the crack, where waves are
-transmitted in a direction parallel to the fissure.
-
-2. The ground is broken and slips either up, down, or sideways, as
-we see to have taken place in the production of faults. Here we get
-distortion in the direction of the movement, and waves are produced
-by the elastic force of the rock, causing it to spring back from its
-distorted form. In a case like this the production of a fissure running
-north and south might give rise to north and south vibrations, which
-would be propagated end on towards the north and south, but broadside
-on towards the east and west. With disturbances of this kind, on
-account of the want of homogeneousness in the materials in which
-they are produced, we should expect to find waves of compression and
-extension.
-
-3. A truly spherical cavity is suddenly formed by the explosion of
-steam in the midst of an elastic medium. In this case all the waves
-will be those of compression, each particle moving backward and forward
-along a radius.
-
-Should the cavity, instead of being truly spherical, be irregular, it
-is evident that, in addition to the normal vibration of compression,
-transverse waves of distortion will be more or less pronounced,
-depending upon the nature of the cavity.
-
-The combination of these two sets of vibrations may cause a point in
-the earth to move in a circle, an ellipse, the form of a figure eight,
-and in other curves similar to these, which are produced by apparatus
-designed to show the combination of harmonic motion. From these
-examples it will be seen that we have therefore to consider two kinds
-of vibrations—one produced by compression or the alteration of volume,
-and the other produced by an alteration in shape.
-
-Now the resistance which a body offers, either to a change in its
-volume or in its shape, is called its elasticity, and the law which
-governs the backward and forward motion of a particle under the
-influence of this elasticity may be expressed as follows:
-
-If T be the time of vibration, or the time taken by a particle to make
-one complete backward and forward swing, D the density of the material
-of which this particle forms a part, and E the proper modulus of
-elasticity of the material, then,
-
-                                  _____
-                          T = 2π √ D/E
-
-                           _____
-From this formula, T = 2π √ D/E, we see that the time of
-vibration of the earth during an earthquake, or the rate at which we
-are shaken backwards and forwards, varies directly as the square root
-of the density of the material on which we stand, and inversely as the
-square root of a number proportional to its elasticity.
-
-_Velocity and Acceleration of an Earth Particle._—Another important
-point, which the practical seismologist has often brought to his
-notice, is the question of the velocity with which an earth particle
-                                         _____
-moves. According to the formula, T = 2π √ D/E, we should expect
-that a particle would make each semi-vibration in an equal time, and
-from a knowledge of the density and elastic moduli of a body this
-time might be calculated. Although the time of a semi-oscillation may
-be constant, we must bear in mind that, like the bob of a pendulum
-during each of its swings, the particle starts from rest, increases in
-velocity until it reaches the middle portion of its half swing, from
-which it gradually decreases in speed until it reaches zero, when it
-again commences a similar motion in the opposite direction.
-
-These pendulum-like vibrations are sometimes spoken of as simple
-harmonic motions. If we know the distance through which an earthquake
-moves in making a single swing, and the time taken in making this
-swing, on the assumption that the motion is simple harmonic we can
-easily calculate the maximum velocity with which the particle moves.
-
-Thus, if an earth particle takes one second to complete a
-semi-oscillation, half of which, or the amplitude of the motion, equals
-_a_, the maximum velocity equals π × _a_.
-
-Again, assuming the earth vibrations to be simple harmonic, the maximum
-acceleration or rate of change in velocity will come about at the ends
-of each semi-oscillation; and if V be the maximum velocity of
-the particle, and _a_ the amplitude or half semi-oscillation, then the
-                            V^2
-maximum acceleration equals ———.
-                            _a_
-
-Later on it will be shown, as the result of experiment, that certain of
-the more important earth oscillations in an earthquake are not simple
-harmonic motion. Nevertheless the above remarks will be of assistance
-in showing how the velocity and other elements connected with the
-motion of an earth particle, which are required by the practical
-seismologist, may be calculated, irrespective of assumptions as to the
-nature of the motion.
-
-_Propagation of a Disturbance._—We may next consider the manner in
-which a disturbance, in which there are both vibrations of compression
-and of distortion, is propagated. The first or normal set of vibrations
-are propagated in a manner similar to that in which sound vibrations
-are propagated. From a centre of disturbance these movements approach
-an observer at a distant station, so to speak, end on. The other
-vibrations have a direction of motion similar to that which we believe
-to exist in a ray of light. These would approach the observer broadside
-on.
-
-If the disturbance passed through a formation like a series of
-perfectly laminated slates, each of these two sets of vibration might
-be subdivided, and we should then obtain what Mallet has termed
-ordinary and extraordinary normal and transverse vibrations.
-
-In consequence of the difference in the elastic forces on which the
-propagation of these two kinds of vibration depends, the normal
-vibrations are transmitted faster than the transversal ones—that is
-to say, if an earthquake originated from a blow, the first thing that
-would be felt at a point distant from the origin of the shock would be
-a backward and forward motion in the direction to and from the origin,
-and then, a short interval afterwards, a motion transversal, or at
-right angles to this, would be experienced.
-
-From the mathematical theory of vibratory motions it is possible to
-calculate the velocity with which a disturbance is propagated. As the
-result of these investigations it has been shown that normal vibrations
-travel more quickly than transverse vibrations.
-
-Deductions from experiments on small specimens are, however,
-invalidated by the fact that the specimens used for experiments are,
-of course, nearly homogeneous, whilst the earthquake passes through
-a mass which is heterogeneous and more or less fissured. Mallet, by
-experiments ‘on the compressibility of solid cubes of these rocks,
-obtained the mean modulus of elasticity,’ with the result that ‘nearly
-seven-eighths of the full velocity of wave-transit due to the material,
-if solid and continuous, is lost by reason of the heterogeneity and
-discontinuity of the rocky masses as they are found piled together in
-nature.’ The full velocities of wave-transit, as calculated by Mallet
-from a theorem given by Poisson, were—
-
-  For slate and quartz transverse to lamination, 9,691 feet per second.
-        „         „    in line of lamination,    5,415  „         „
-
-This more rapid transmission in a direction transverse to the
-lamination, Mr. Mallet observes, may be more than counterbalanced by
-the discontinuity of the mass transverse to the same direction.
-
-_The Intensity of an Earthquake._—The intensity of an earthquake
-is best estimated by the intensity of the forces which are brought
-to bear on bodies placed on the earth’s surface. These forces are
-evidently proportional to the rate of change of velocity in the body,
-and, as the destructive effect will be proportional to the maximum
-forces, we may consistently indicate the intensity of an earthquake by
-giving the maximum acceleration to which bodies were subject during
-the disturbance. On the assumption that the motion of a point on
-the earth’s surface is simple harmonic, the maximum acceleration is
-directly as the maximum velocity and inversely as the amplitude of
-              _v_^2
-motion, or as ————— where _v_ indicates velocity and _a_ amplitude.
-               _a_
-
-The next question of importance is to determine the manner in which
-earthquake energy becomes dissipated—that is, to compare together
-the intensity of an earthquake as recorded at two or more points at
-different distances from the origin. First let us imagine the origin
-of our earthquake to be surrounded by concentric shells, each of
-which is the breadth of the vibration of a particle. Going outwards
-from the centre, each successive shell will contain a greater number
-of particles, this number increasing directly as the square of the
-distance from the origin. Let the blow have its origin at the centre,
-and give a vibratory movement to the particles in one of the shells
-near the centre.
-
-This shell may be supposed to possess a certain amount of energy,
-which will be measured by its mass and the square of the velocity of
-its particles. In transferring this energy to the neighbouring shell
-which surrounds it, because it has to set in motion a greater number
-of particles than it contains itself, the energy in any one particle
-of the second layer will be less than the energy in any one particle
-in the first layer; the total energy in the second shell, however,
-will be equal to the total energy in the first shell. Neglecting the
-energy lost during the transfer, if the energy in a particle of the
-first shell at any particular phase of the motion be K_{1},
-and the energy in a particle of the second shell K_{2}, these
-quantities are to each other inversely as the masses of the shells—that
-is, inversely as the squares of the mean radii of the shells.
-
-                  K_{2}   _r__{1}^2
-  In symbols,     ————— = —————————                   (1)
-                  K_{1}   _r__{2}^2
-
-Assuming that energy is dissipated,
-
-              K_{2}   _r__{1}^2       _r__{1}^2
-              ————— > ————————— = _f_ —————————       (2)
-              K_{1}   _r__{2}^2       _r__{2}^2
-
-where _f_ < 1 is the rate of dissipation of energy which is assumed to
-be constant.
-
-_Area of greatest Overturning Moment._—Although the rate of dissipation
-of the impulsive effects of an earthquake may follow a law like that
-just enumerated, it must be remembered that if the depth of the origin
-is comparable with the radius of the area which is shaken, the maximum
-impulsive effect as exhibited by the actual destruction on the surface
-may not be immediately above the origin where buildings have simply
-been lifted vertically up and down, but at some distance from this
-point, where the impulsive effort has been more oblique.
-
-At the _epicentrum_ we have the maximum of the true intensity as
-measured by the acceleration of a particle, or the height to which
-a body might be projected, but it will be at some distance from
-this where we shall have the maximum intensity as exhibited by an
-overturning effort.
-
-This will be rendered clear by the following diagram.
-
-In the accompanying diagram let O be the origin of a shock, and O C the
-seismic vertical equal to _r_. Let the direct or normal shock emerge at
-C, C_{1}, C_{2}, and at the angles θ_{1}, θ_{2}, &c.
-
-Assuming that the displacement of an earth particle at C equals C
-B, and at C_{1} equals _c__{1} _b__{1}, and at C_{2} equals _c__{2}
-_b__{2}, &c., and let these displacements C B, _c__{1} _b__{1}, _c__{2}
-_b__{2}, &c., for the sake of argument, vary inversely as _r_, _r__{1},
-_r__{2}, &c.
-
-[Illustration: FIG. 9.]
-
-The question is to determine where the horizontal component C A of
-these normal motions is a maximum.
-
-First observe that the triangle O C _c_ is similar to _a_, _b_, _c_.
-
-            _h_
-Also _r_ = —————, and therefore the normal component _c__{1}
-           sin θ
-
-                               sin θ
-_b__{1} at C_{1} is equal to C —————.
-                                _h_
-
-Also _c__{1} _a__{1} = _c__{1} _b__{1}, cos θ.
-
-                       sin θ cos θ    _c_    sin 2θ
-      ∴ C_{1} _a_ = C ———————————— = ———— ∙ ———————,
-                          _h_         _h_      2
-
-and sin 2θ is greatest when 2θ = 90° or θ = 45°.
-
-That is to say, the horizontal component reaches a maximum where the
-angle of emergence equals 45°.
-
-This question has been discussed on the assumption that the amplitude
-of an earth particle varies inversely as its distance from the origin
-of the shock. Should we, however, assume that this amplitude varies
-inversely as the square of the distance from the origin, we are led to
-the result that the area of greatest disturbance is nearer to the point
-where the angle of emergence is 55° 44′  9″. Both of these methods are
-referred to by Mallet, but the first is considered as probably the more
-correct.
-
-_Earthquake Waves._—Hitherto we have chiefly considered earthquake
-vibrations; now we will say a few words about earthquake waves. If
-we strike a long iron rod at one end, we can imagine that, as in the
-long spring, a pulse-like motion is transmitted. If the rod be struck
-quickly, the pulses will rapidly succeed each other, and if struck
-slowly the pulses will be at longer intervals. Each individual pulse,
-however, will travel along the rod at the same rate, and hence the
-distance between any two will remain constant; but that distance will
-depend on the interval between the blows producing these pulses being
-equal to the distance travelled by one pulse before the next blow is
-struck.
-
-From this we see that an irregular disturbance will produce an
-irregular succession of motions; some will be like long undulations in
-a wide deep ocean, whilst others will be like ripples in a shallow bay.
-Again, consider the bar to be struck one blow only, and then left to
-itself. The bar will propagate a series of pulses along its length, due
-to the out and in vibration of its end. These will succeed each other
-at regular intervals, and will be mixed up with the pulses we have
-previously considered.
-
-From this we see that in an earthquake, if it be produced by one blow,
-the motion will be isochronous in its character; but if it be due to
-a succession of blows at regular intervals, the motion will be the
-resultant of a series of isochronous motions, and will be periodical.
-If the impulses are irregular, you have a motion which is the resultant
-of a number of isochronous motions due to each impulse, but these
-compounded together in a different manner at each instant during the
-earthquake, and giving as a result a motion which is in no sense
-isochronous. This approaches more nearly to the actual motions we feel
-as earthquakes.
-
-If we can imagine the ground shaken by an earthquake, made of a
-transparent material which transmitted less light when compressed,
-and we could look down upon a long extent of this at the time of an
-earthquake, we should see a series of dark bands indicating strips of
-country which were compressed. The distances between these bands might
-be irregular. Keeping our attention on one particular band, this would
-be seen to travel forward in a direction from the source. If we kept
-our eye on one particular point, it would appear to open and shut,
-becoming light and dark alternately.
-
-As to the existence of these elastic waves in actual earthquakes we
-have no direct experimental evidence. The only kind of wave with which
-we are familiar is a true surface undulation, which, although having
-the appearance of a water-wave, may nevertheless represent a district
-of compression.
-
-
-
-
-                              CHAPTER IV.
-
-             EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.
-
-  Experiments with falling weights—Experiments with
-    explosives—Results obtained from experiments—Relative motion
-    of two adjacent points—The effect of hills and excavations
-    upon the propagation of vibrations—The intensity of artificial
-    disturbances—Velocity with which earth vibrations are
-    propagated—Experiments of Mallet—Experiments of Abbot—Experiments
-    in Japan—Mallet’s results—Abbot’s results—Results obtained in
-    Japan.
-
-
-_Experiments with Falling Weights._—A series of experiments, as the
-nature of the disturbance produced in the surface of the earth when
-a heavy weight is allowed to fall on it, was begun in November 1880
-by Mr. T. Gray and the author. These experiments were carried out at
-the Akabane Engineering Works in Tokio. The weight used was a ball
-of iron weighing about a ton, which in the different experiments was
-allowed to fall from heights varying between ten and thirty-five feet.
-The position of the place where the ball was allowed to fall was such
-that in one direction the vibrations were transmitted up the side of
-a steep hill, in another direction across a pond with perpendicular
-sides, and in another direction across a level plain the material of
-which consisted for the most part of hardened mud extending to a very
-considerable depth. The vibrations produced by the fall of the ball
-were transmitted through this hard mud with considerable intensity to a
-distance of between 300 and 400 feet.
-
-The object of the experiment was to find the nature of the vibrations
-produced in the crust of the earth by such a blow, the velocity of
-transmission through this comparatively soft material, the effect of
-hills and excavations in cutting off such disturbances, and the law
-according to which the amplitude of the vibrations diminishes with the
-distance from the source.
-
-A considerable variety of apparatus was used during these experiments,
-but the most reliable results were obtained from the records of a
-rolling sphere seismograph, which wrote the vibrations on a stationary
-plate, and from the records of two bracket seismographs, similar to
-Professor Ewing’s horizontal lever seismographs, which gave a record
-of the vibrations as two rectangular components on a moving plate of
-smoked glass.
-
-[Illustration: FIG. 10.]
-
-The general result as to the nature of the disturbance was that two
-distinct sets of vibrations were set up by the blow. In one set the
-direction of motion was along a line joining the point of observation
-with the point from which the disturbance emanated; in the other set
-the direction of motion was at right angles to that line. The nature
-of the resultant motion will be gathered from fig. 10, which is taken
-from the records drawn by the rolling sphere seismograph at a distance
-of 50 feet, 100 feet, and 200 feet respectively from the point where
-the ball struck the ground. The direct or normal vibrations reached
-the instrument first, and were followed at an interval depending on
-the distance of the instrument from the origin by the transverse
-vibrations. From the records of these two sets of vibrations as
-separated by the bracket seismographs, combined with the known rate of
-motion of the glass plate, the velocity of transmission was found to
-be, for normal vibrations 446–438 feet per second, and for transverse
-vibrations 357–353 feet per second.
-
-The effect of the hill in cutting off the disturbance seemed to be
-slight, but the direction of the vibrations which ascended the side
-was mostly transverse. The pond, on the other hand, seemed completely
-to cut off the disturbance, which, however, gradually crept round the
-side, so that only a comparatively small triangular area was in shadow.
-
-The amplitude of the vibrations diminished directly as the distance
-increased for some distance from the origin, but at greater distance
-the rate of diminution seemed to be slower. The transverse vibration
-seemed to die out less quickly than the normal vibrations.[9]
-
-These experiments were afterwards very considerably extended by the
-author. In these later experiments charges of from one to two pounds of
-dynamite were placed in bore-holes of various depths and exploded by
-means of electricity. The results obtained confirmed the conclusions
-already arrived at from the former experiments. The experiments on
-velocity, however, seemed to indicate that the higher the initial
-impulse the greater was the velocity. The velocity of propagation of
-the transverse vibrations seemed to approach more and more to that of
-the directed vibrations as the distance from the origin of disturbance
-increased. Fig. 11 shows the nature of the record obtained from the
-explosion of two pounds of dynamite at the bottom of a bore-hole eight
-feet deep. These records show the interval of time which elapsed
-between the arrival of the normal and the transverse vibrations at
-points distant 100, 250, and 400 feet from the bore-hole. In the case
-of the 100-feet station it will be observed that the motion towards the
-origin is greater than that from the origin. It is also to be noticed
-that the period of vibration becomes greater as the distance from the
-origin increases.
-
-[Illustration: FIG. 11.—Records obtained at three stations
-of the motion of the ground produced by the explosion of 2 lbs. of
-dynamite.]
-
-_The Intensity of Artificial Disturbances._—The data which we have at
-our disposal for determining the intensity of an earth particle which
-has been caused to vibrate by the explosion of a charge of dynamite are
-a series of records similar to that given on p. 60. These disturbances
-are practically surface movements, and may be compared with the
-movements of an earthquake which spreads over an area the radius of
-which is great as compared with its depth.
-
-To find the mean acceleration of an earth particle, which quantity
-has been taken to represent intensity, during any simple backward or
-forward motion of the earth, it will be first necessary to determine
-the amplitude of this motion and its maximum velocity, the mean
-                            V^2
-acceleration being equal to ———.
-                            2A
-
-[Illustration: FIG. 12.]
-
-The second and third movements in a shock invariably exhibited the
-greatest intensity, and to a distance of 400 feet from the origin,
-where about three pounds of dynamite had been exploded in a bore-hole
-about six feet deep, these intensities decreased directly as the
-distance from the origin. The less intense movements also decreased
-directly as the distance from the origin to a certain point, but after
-that they decreased more slowly. A mean result of the more prominent
-vibrations in four sets of experiments is shown in the curve, fig. 12,
-where the horizontal measurements represent distance from the origin
-in feet, and the vertical measurements mean acceleration in thousands
-of millimetres per second.
-
-This curve approximates to an equi-angular hyperbola. The area between
-the curve and its asymptotes is proportional to the whole energy of
-the shock. The area of the diagram is proportional to the energy given
-up to the ground by the explosion of three pounds of dynamite. If we
-call the unit shock the effect produced by the explosion of one pound
-of dynamite, the above artificial earthquake had an intensity equal to
-three.
-
-The only other investigations which have been made in this interesting
-branch of observational seismology are those by Mr. Robert Mallet,[10]
-and those by General Henry L. Abbot.[11]
-
-_Mallet’s Results._—The velocity with which earth vibrations were
-transmitted as deduced by Mr. Mallet were as follows:—
-
-                                                  Feet per second
-  In sand                                              824·915
-  In contorted stratified rock, quartz, and slate
-      at Holyhead                                    1,088·669
-  In discontinuous and much shattered granite        1,306·425
-  In more solid granite                              1,664·574
-
-A striking result which was obtained by Mallet in his experiments at
-Holyhead was that the transit velocity increases with an increase in
-the intensity of the initial shock. Thus with a charge of 12,000 pounds
-of powder the transit rate was 1,373 feet per second, whilst with 2,100
-pounds the transit rate was 1,099 feet per second. In these experiments
-tremors were observed as preceding and following the main shock.
-
-_Abbot’s Results._—The important results obtained by General Abbot are
-contained in the following table:—
-
-  A. No. of Observation
-  B. Distance to Station in miles
-  C. Type of Seismometer
-  D. Velocity in feet per second
-  +--+----------------+--------------------------+--------+---+-------+
-  | A|      Date      |      Cause of Shock      |   B    | C |   D   |
-  +--+----------------+--------------------------+--------+---+-------+
-  | 1| Aug.  18, 1876 | 200 lbs. of dynamite     |  5 ±   | B | 5,280 |
-  | 2| Sept. 24, 1876 | Hallet’s Point Explosion |  5·134 | A | 3,873 |
-  | 3|        „       |   „        „       „     |  8·330 | B | 8,300 |
-  | 4|        „       |   „        „       „     |  9·333 | A | 4,521 |
-  | 5|        „       |   „        „       „     | 12·769 | B | 5,309 |
-  | 6| Oct.  10, 1876 |  70 lbs. dynamite        |  1·360 | A | 1,240 |
-  | 7| Sept.  6, 1877 | 400  „      „            |  1·169 | A | 3,428 |
-  | 8|        „       |  „   „      „            |  1·169 | B | 8,814 |
-  | 9| Sept. 12, 1877 | 200  „      „            |  1·340 | A | 6,730 |
-  |10|        „       |  „   „      „            |  1·340 | B | 8,730 |
-  |11|        „       |  70  „      „            |  1·340 | A | 5,559 |
-  |12|        „       |  „   „      „            |  1·340 | B | 8,415 |
-  +--+----------------+--------------------------+--------+---+-------+
-
-A seismometer of type A means that the telescope used in observing the
-tremor produced on the surface of a vessel of mercury by the passage of
-the shock had a magnification of 6, whilst a telescope of the type B
-had a magnification of 12.
-
-The mean velocity given by six observations with type A is 4,225 feet
-per second, while that given by the same number with type B is 7,475
-feet per second.
-
-[Illustration: FIG. 13.]
-
-If we assume that the first tremor observed in the mercury is to
-determine the true rate of transmission, General Abbot tells us that we
-must reject all observations made with type A, inasmuch as they do not
-reveal the velocity of the leading tremor. However, he also tells us
-that a still higher power above 12 might have detected still earlier
-tremors.
-
-When gunpowder was the explosive, the observers noted that the
-disturbance observed in the mercury took a much longer time to reach a
-maximum than it did when dynamite was employed.
-
-It was also observed that explosions fired beneath deep water gave
-a higher velocity than similar explosions which took place beneath
-shallow water. In the latter case much of the energy was probably
-expended in throwing a jet of water into the air.
-
-Another point which was observed appears to have been that the rate
-varied with the initial shock. Thus:—
-
-                                         Feet per second
-  400 lbs. of dynamite  gave                  8,814
-  200  „         „                            8,730
-   70  „       powder (deep) gave             8,415
-
-Also it is probable that the rate of a wave diminished with its
-advance. For,
-
-                                           Feet per second
-    200 lbs. of dynamite gave for 1  mile      8,730
-     „   „         „      „       5  miles     5,250
-  50,000 „         „      „       8    „       8,300
-    „    „         „      „      13½   „       5,300
-
-General Abbot’s general conclusions are:—
-
-1. A high magnifying power of telescope is essential in seismometric
-observations.
-
-2. The more violent the initial shock the higher is the velocity of
-transmission.
-
-3. This velocity diminishes as the general wave advances.
-
-4. The movements of the earth’s crust are complex, consisting of many
-short waves first, increasing and then decreasing in amplitude; and
-with a detonating explosive the interval between the first wave and
-the maximum wave, at any station, is shorter than with a slow burning
-explosive.
-
-_Results obtained in Japan._—From some experiments made by the author
-in the grounds of the Meteorological Department in Tokio, the following
-results were obtained:—
-
-  A. Velocity in feet per second for the first 200 ft. (A to B)
-  B. Velocity in feet per second for the second 200 ft. (B to C)
-  C. Velocity in feet per second for 400 ft. (A to C)
-  D. Number of Cartridges of Dynamite (6 = 1 lb.)
-
-  +--------------------+-----+-----+-----+------+
-  |  No. of Explosion  |  A  |  B  |  C  |  D   |
-  +--------------------+-----+-----+-----+------+
-  |            {    I. | 464 | 186 | 265 |  8·3 |
-  |  Vertical  {  III. |  -- | 211 |  -- | 10·1 |
-  | vibrations {   IV. | 352 | 234 | 281 |  7·1 |
-  |            {    V. | 343 | 232 | 277 |  5·0 |
-  |  Normal    {   VI. |  -- |  -- | 407 | 10·0 |
-  | vibrations {  VII. |  -- |  -- | 516 | 12·5 |
-  | Transverse } VIII. |  -- |  -- | 344 | 12·5 |
-  | vibrations }       |     |     |     |      |
-  +--------------------+-----+-----+-----+------+
-
-The general results to be deduced from the above appear to be:—
-
-  1. For vertical motion.
-    (_a_) For the first 200 feet. The velocity depends upon the initial
-        force—the greater the charge of dynamite the greater the
-        velocity.
-    (_b_) For the second 200 feet. The above law only appears in
-        experiments IV. and V., but it must be remembered that the
-        origins of I. and III. were farther removed from A than IV.
-        and V.
-  The speed of the wave during the second 200 feet is always less
-        than during the first 200 feet.
-
-  2. For normal vibrations.
-    Here the speed between A and C is all that was measured, but we
-        again see that the greater the initial force, or the nearer
-        we are to the origin of the disturbance, the greater is the
-        velocity. This velocity is greater than the velocity of the
-        vertical or transverse vibrations.
-
-  3. For transverse vibrations.
-    If we assume that the vertical vibrations are a component of the
-        transverse motions we see the same law as before—namely, that
-        the nearer we are to the origin of the disturbance the greater
-        is the speed with which that disturbance is propagated.
-
-It will be observed that the chief law here enunciated respecting the
-decrease in speed of earth vibrations is the same as that pointed
-out by General Abbot, from which it only differs by its being in all
-cases proved without the introduction of personal errors, for the same
-explosion, along the same line of ground and for different kinds of
-vibrations.
-
-
-
-
-                              CHAPTER V.
-
-     EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION ON EARTHQUAKES.
-
-  Result of feelings—The direction of motion—Instruments as indicators
-    of direction—Duration of an earthquake—Period of vibration—The
-    amplitude of earth movements—Side of greatest motion—Intensity
-    of earthquakes—Velocity and acceleration of an earth
-    particle—Absolute intensity of an earthquake—Radiation of an
-    earthquake—Velocity of propagation.
-
-
-_Result of Feelings._—As the result of our experiences, and by
-observations upon the movements produced in various bodies, we can
-say that an ordinary earthquake consists of a number of backward and
-forward motions of the ground following each other in quick succession.
-Sometimes these commence and die out so gently that those who have
-endeavoured to time the duration of an earthquake have found it
-difficult to say when the shock commenced and when it ended. This was
-a difficulty which Mr. James Bissett in Yokohama, and the author in
-Tokio, had to contend against when, in 1878, they commenced to time
-shocks between these two places.
-
-Sometimes these motions gradually increase to a maximum and then die
-out as gradually as they commenced.
-
-Sometimes the maximum comes suddenly, and at other times during an
-earthquake our feelings distinctly tell us that there are several
-maxima.
-
-These have been the experiences of many observers, and have been
-recorded by writers since the earliest times. Mallet devotes a
-chapter to a consideration of the tremulous motion that precedes and
-follows a shock, and he tells us that a single shock is an absolute
-impossibility. In speaking of earthquakes, he says: ‘The almost
-universal succession of phenomena recorded in earthquakes is, first a
-trembling, then a severe shock, or several in quick succession, and
-then a trembling gradually but rapidly becoming insensible.’
-
-A quantitative and exact knowledge of the nature of earthquake
-motion has only been attained of late years. The chief results which
-investigators have aimed at have been the measurement of the amplitude,
-the period, the direction, and the duration of the motions which
-constitute an earthquake. Attention has also been given to the velocity
-with which a disturbance is propagated.
-
-_The Direction of Motion._—One of the most ordinary observations
-which are made about an earthquake is its direction. If we were to
-ask the inhabitants of a town which had been shaken by an earthquake
-the direction of the motion they experienced, it is not unlikely that
-their replies would include all the points of the compass. Many,
-in consequence of their alarm, have not been able to make accurate
-observations. Others have been deceived by the motion of the building
-in which they were situated. Some tell us that the motion had been
-north and south, whilst others say that it was east and west. A certain
-number have recognised several motions, and amongst the rest there will
-be a few who have felt a wriggling or twisting. Leaving out exceptional
-cases, the general result obtained from personal observation as to
-the direction of an earthquake of moderate intensity is extremely
-indefinite, and the only satisfactory information to be obtained is
-that derived from instruments or from the effects of the earthquake
-exhibited in shattered buildings and bodies which had been overturned
-or projected.
-
-By the direction in which walls, columns, and other objects had been
-overthrown or fractured, Mallet was enabled to determine the position
-of the origin of the Neapolitan earthquake. Similar phenomena have many
-times been taken advantage of by other investigators of earthquake
-phenomena. Effects produced upon structures are, however, only to be
-observed as the results of a destructive earthquake, at which time
-cities may be regarded as collections of seismometers. (_See_ chapter
-on Effects in Buildings.)
-
-To determine the direction of movement during a small earthquake, the
-most satisfactory method appears to be an appeal to instruments.
-
-_Instruments as Indicators of Direction._—The relative values of
-different kinds of instruments, such as columns, pendulums, and the
-like, as indicators of direction have already been discussed.
-
-By the use of pendulum seismographs it has been shown that during an
-earthquake the ground may move in one, two, or several directions (see
-p. 21); and it is, generally speaking, only in those cases where we
-experience a decided shock in the disturbance that we can determine
-with any confidence the direction in which the motion has been
-propagated. Such directions are usually indicated by the major axis of
-certain more or less elliptical figures which have been drawn, which
-in themselves appear to indicate the combination of two rectilinear
-movements.
-
-Results similar to those indicated by the records of pendulum
-seismographs have also been obtained upon moving plates with a double
-bracket seismograph. Thus, in the earthquake which shook Tokio at 6
-A.M. on July 5, 1881, there were indications of the following
-motions:—
-
-Near the commencement of the shock the motion was N. 112° E. One and a
-half second after this, the direction of motion appears to have been
-N. 50° E. In three-fourths of a second more it gradually changed to a
-direction N. 145° E., and after a similar interval to N. 62° E. Half a
-second after this it was N. 132° E., and four seconds later the motion
-was again in the original direction—namely, N. 112° E.
-
-These particular directions of motion have been selected because they
-were so definitely indicated.
-
-The commonest type of earthquake which is experienced in Japan, and
-probably also in other earthquake-shaken districts, is the compound or
-diastrophic form.
-
-That earthquakes often have motions compounded of two sets of
-vibrations, has also been proved by the analysis of the records
-obtained from two component seismographs. From an analysis of a record
-of this description, Professor Ewing has shown that in the earthquake
-felt in Tokio on March 11, 1881, there were approximate circular
-(somewhat spiral) movements.
-
-This leads us to the consideration of the twisting and wriggling
-motions which are said to be experienced by some observers. Motions
-like these, which by the Italians and Mexicans are called _vorticosi_,
-are usually supposed to be the cause of objects like chimneys and
-gravestones being rotated. These phenomena, it will be seen from what
-is said in the chapter upon the effects produced in buildings, can be
-more easily explained upon the supposition of a simple rectilinear
-movement.
-
-That at the time of an earthquake there may be motion in more than one
-direction has been recognised since the time of Aristotle; and it is
-possible that two sets of rectilinear motion, as, for instance, the
-normal and transverse movements, may have led observers to imagine that
-there has been a twisting motion taking place, and this especially when
-the two sets of movements have quickly succeeded each other.
-
-Persons inside flexible buildings may possibly have experienced more
-or less of a rotatory motion, although the shock was rectilinear; the
-building assuming such a motion in consequence of its construction and
-its position with regard to the direction of the shock.
-
-In the case of destructive earthquakes, especially at points situated
-practically above the origin, the universal testimony, Mallet tells
-us, is that a twisting, wriggling motion in different planes, attended
-by an up-and-down movement of greater range, is experienced. To such
-disturbances the word _sussultatore_ is sometimes applied. Mallet has
-given many elliptical and other closed curves to illustrate the nature
-of such motions.
-
-_Duration of an Earthquake._—When reading accounts of earthquakes
-it is often difficult to determine the length of time a shaking was
-continuous. In Japan, in A.D. 745, there was a shaking which is said
-to have lasted sixty hours; and in A.D. 977 there were a series of
-shakings lasting 300 days. Often we meet with records of disturbances
-which have lasted from twenty to seventy days.
-
-At San Salvador, in 1879, more than 600 shocks were felt within ten
-days; in 1850, at Honduras, there were 108 shocks in a week; in 1746,
-at Lima, 200 shocks were felt in twenty-four hours; at the island of
-St. Thomas, in 1868, 283 shocks were felt during about ten hours.
-
-Disturbances like these, which succeed each other with sufficient
-rapidity to cause an almost continual trembling in the ground, may be
-regarded as collectively forming one great seismic effort which may
-last a minute, an hour, a day, a week, or even several years. Strictly
-speaking, they are a series of separate earthquakes, the resultant
-vibrations of which more or less overlap. Whenever a large earthquake
-occurs it is generally succeeded by a large number of smaller shocks.
-
-The seismic disturbance as regards time is, as Mallet remarks, very
-often ‘like an occasional cannonade during a continuous but irregular
-rattle of musketry.’ In the New Zealand earthquake of 1848, shocks
-continued for nearly five weeks, and during a large portion of the time
-there were at least 1,000 shocks per day.[12]
-
-The earthquake of Lisbon, which in five minutes destroyed the whole
-town, was followed by a series of disturbances lasting over several
-months. After Basle had, on October 18, 1356, been laid in ruins,
-it is stated shocks followed each other for a period of a year. The
-Calabrian earthquake was continued with considerable strength for a
-year, and it is said that the earth did not come completely to rest for
-ten years. During this cannonade the heavy shocks announced, as they
-do in most earthquake countries at the present day, a series of weaker
-disturbances. In certain exceptional cases this order of events has
-been inverted, and slight shocks have announced the coming of heavy
-ones. Fuchs gives an example of this in the earthquake of Broussa, when
-the first shock was on February 28, 1855. On March 9 and 23 there were
-heavier shocks, but the heaviest did not arrive until March 28.
-
-Under certain conditions it is possible to have a sensible vibration
-produced in the ground which is practically of unlimited duration;
-thus, for instance, it has been noticed that the falling of water
-at certain large waterfalls, by its continuous rhythmical impact on
-the rocks, produces in them tremors which are to be observed at
-great distances. Of this the author convinced himself at the Falls
-of Niagara, where he observed the reflected and ever-moving image of
-the sun in a pool of water. Under favourable circumstances almost
-continual condensation of steam might take place in volcanic foci, each
-condensation giving rise to a blow sufficiently powerful to produce
-vibrations in the surrounding ground. Those who have stood near a large
-geyser, like the one in Iceland, when it makes an ineffectual effort
-to erupt, will recognise how powerful such a cause might be. Humboldt
-has remarked shocks on Vesuvius and Pichincha which were periodic,
-occurring twenty to thirty seconds before each ejection of vapour and
-ashes.
-
-Earthquakes like these may be of vast extent, gradually spreading
-further and further outwards. This spreading of earth vibrations may
-be observed at a large factory containing heavy machinery or a steam
-hammer. After the machinery comes to rest, it is probably some time
-before the ground returns to rest. Examples of disturbances of this
-nature are spoken of under the head of Earth Tremors.
-
-The record of the duration of an ordinary earthquake as observed at a
-given point is dependent upon the sensibility of our instruments.
-
-Continuous motions perceptible to our senses without the aid of
-instruments usually last from thirty seconds to about two or three
-minutes. In Japan the shocks, as timed by watches, usually last from
-twenty to forty seconds. Occasionally a continuous shaking is felt for
-more than one and a half minutes, and cases have been recorded where
-the motion has continued for as much as four minutes and thirty-three
-seconds.
-
-Seismometers having a multiplication of 6 to 12 usually indicate that
-motion continues longer than is perceptible to the senses.
-
-_Period of Vibration._—When an earthquake contains several prominent
-vibrations which might be called the _shocks_ of the disturbance, our
-feelings tell us that these have occurred at unequal intervals.
-
-About the time which is taken for the complete backward and forward
-oscillation of the ground which constitutes the shock a little has
-already been said. This was deduced from the records of disturbances as
-drawn by seismographs. From the same sources we can readily obtain the
-period of all the prominent vibrations in a disturbance.
-
-In any given earthquake there are irregularities in period, and
-different earthquakes differ from each other. About the early attempts
-to determine the period of earth vibrations something has been said in
-the chapter on Earthquake Instruments.
-
-In the earthquake of March 11 (referred to on p. 70) we find that both
-components commenced with a series of small vibrations, about five or
-six to the second; next came the shock, consisting of two complete
-vibrations executed in two seconds. In this it is to be observed that
-the motion eastwards was performed much more quickly than the motion
-westwards. Next, by reference to the east and west component, it is
-seen that there are a number of large vibrations, about one per second,
-on which a number of smaller motions are superposed. As the motion
-proceeds, these become less and less definitely pronounced and more
-irregular in their intervals, until finally the motion dies away.
-
-This earthquake, as recorded at the author’s house in Tokio, lasted
-about one and a half minute.
-
-The same earthquake, as recorded by Professor Ewing at a station
-situated about one and a half mile distant, but on flat ground, appears
-to have lasted four and a half minutes. The largest wave had a period
-of 0·7 second.
-
-In the earthquake of March 8, 1881, there were on an average 1·4
-vibrations per second. These vibrations were executed in a direction
-transverse to the line joining the observing station and the locality
-from which the disturbance must have originated as determined by time
-observations. It can, therefore, be assumed that these vibrations,
-having so slow a period, were transverse motions, this slowness or
-sluggishness being due to the fact that the modulus for distortion
-is less than the modulus which governs the propagation of normal
-vibrations.
-
-_The Amplitude of Earth Movements._—In making estimates of the
-distances through which we are moved backward and forward at the time
-of an earthquake, if we judge by our feelings, we may often be misled.
-If a person is out of doors and walking, an earthquake may take place
-sufficiently strong to cause chimneys to fall and unroof houses, which,
-so far as the actual shaking of the ground is concerned, will be passed
-by unnoticed. On the other hand, to persons indoors, especially on an
-upper story, it is impossible even for a tremor to pass by without
-creating considerable alarm by the angular movement that has been taken
-up by the building.
-
-Many observers have endeavoured to make actual measurements of the
-maximum extent through which the earth moves at the time of an
-earthquake. Among the reports of the British Association for 1841 is
-the report of a committee which had been appointed ‘for obtaining
-instruments and registers to record shocks of earthquakes in Scotland
-and Ireland’. We read that in one earthquake which had been measured
-the displacement of the ground had been half an inch, and in another
-it had been less than half an inch. The instruments used to make these
-observations depended upon the inertia of pendulums which at the time
-of the disturbance were supposed to remain at rest. Observations
-similar to these have been made in Japan. One long series were made by
-Mr. E. Knipping for Dr. Gr. Wagener. They extended from November 1878
-to April 1880, and were as follows:—
-
-   Number of     Maximum horizontal
-  Earthquakes    motion of the ground
-      10            ·0   to  0·15 mm.
-       7            ·15   „  0·5  „
-       8            ·5    „  2·5  „
-       2           2·5    „  more „
-
-With his apparatus for vertical motion Dr. Wagener also made
-observations on the absolute vertical motion. This seldom reached ·02
-mm. The greatest value was that observed for the destructive shock of
-Feb. 22, 1880, which was ·56 mm.
-
-By means of a number of instruments distributed at various localities
-round Tokio, the chief of which were pendulums with friction pointers
-to render them ‘_dead beat_,’ and with magnifying apparatus to show
-the actual motion of the ground, the author arrived at results similar
-to those obtained by Dr. Wagener—namely, that the earth’s maximum
-horizontal motion at the time of a small earthquake was usually only
-the fraction of a millimetre, and it seldom exceeded three or four
-millimetres. When we get a motion of five or six millimetres, we
-usually find that brick and stone chimneys have been shattered.
-
-The results obtained for vertical motion were also very small. In Tokio
-it is seldom that vertical motion can be detected, and when it is
-recorded it is seldom more than a millimetre.
-
-These results, which were put forward some years ago, have since
-received confirmation by the use of a variety of instruments in the
-hands of different observers.
-
-Mallet, in his account of the Neapolitan earthquake of 1857,
-approximated to the amplitude of an earth particle by observing the
-width, at the level of the centre of gravity, of fissures formed
-through and remaining in great masses of very inelastic masonry.
-
-Taking stations situated on or very nearly on the same line passing
-through the seismic vertical (_epicentrum_), Mallet observed the
-amplitude increased as some function of the distance, as will be seen
-from the following table:—
-
-  +-------------------------+------+-------+-------+---------+-------+
-  |          Station        |Polla |La Sala|Certosa|Tramutola|Sarconi|
-  +-------------------------+------+-------+-------+---------+-------+
-  | Distance from Seismic } |      |       |       |         |       |
-  |  Vertical in          } | 3·45 | 11·60 | 16·50 |  20·60  | 26·7  |
-  |  geographical miles   } |      |       |       |         |       |
-  | Amplitude in inches     | 2·5  |  3·5  |  4·0  |   4·5   |  4·75 |
-  +-------------------------+------+-------+-------+---------+-------+
-
-The possibility of a law such as this having an existence for places at
-a distance from the seismic vertical comparable with the vertical depth
-of the centrum will be shown farther on.
-
-With regard to the maximum displacement of an earth particle. Mallet
-was of opinion that there was evidence to show that it had in some
-cases been over one foot. M. Abella, in an earthquake which occurred in
-the Philippines in 1881, made a rough observation of the motion of the
-earth to a distance of about _two metres_. This, as might be expected,
-was beyond the elastic limits of the material, and caused fissures to
-be formed, which were seen to open and shut.
-
-_Intensity of Earthquakes._—In speaking of the strength of an
-earthquake, we usually employ terms like ‘weak,’ ‘strong,’ ‘violent,’
-&c. Although these expressions, accompanied by illustration of the
-effects which an earth quake has produced, convey a general idea of
-the strength of a shock as felt at some particular locality, our ideas
-nevertheless wanting in definiteness; and if we endeavour to compare
-one shock with another, as a whole, our want of exactness is augmented.
-We have seen that Palmieri’s seismograph indicates intensity by a
-certain number of degrees, which, to a certain extent, is a measure of
-the violence of the motion as indicated at a particular locality. The
-degrees, as before stated, refer to the height to which in consequence
-of the shaking, a certain quantity of mercury was washed in a tube,
-which is a function of the depth of mercury in the tube, and also of
-the duration of the disturbance.
-
-From this it seems possible that a very slow motion of small amplitude,
-continuing over a sufficient period of time, might, if it agreed with
-the period of the mercury, indicate an earthquake of many degrees of
-intensity, whilst residents in the neighbourhood might not have noticed
-the disturbance; and, on the other hand, a short but intense shock
-creating considerable destruction might have been recorded as of only a
-few degrees of intensity.
-
-Although objections like these might be raised to such a method of
-recording intensity, in practice it would appear that such results are
-not pronounced, and the indications of the instrument usually give us
-approximate indications of relative intensity.
-
-In writing about the Neapolitan earthquake of 1857, Mallet says that
-‘area alone affords no test of seismic energy.’
-
-The area over which a shock is felt will depend not only upon the
-initial force of the disturbance, but also upon the focal depth of
-a shock, the form and position of that focus, the duration of the
-disturbance, and the nature and arrangement of the materials which are
-shaken.
-
-From observations in Japan, it is clearly shown that massive mountain
-ranges exert a considerable influence upon the extension of seismal
-disturbances. On one side of a large range of mountains large cities
-might be laid in ruins, whilst on the other side the disturbance
-creating this destruction might not be noticed.
-
-_Velocity and Acceleration of an Earth Particle._—We now pass on to
-methods of determining the intensity of an earthquake which are less
-arbitrary than those which have just been discussed. These methods have
-already been discussed when speaking of artificial disturbances, where
-it was shown that the intensity of an earthquake as measured by its
-destructive effects greatly depended upon the suddenness with which the
-backward and forward motions of the ground were commenced or ended.
-
-Amongst the earlier investigators of seismic phenomena who observed
-that there existed a connection between the distance to which bodies
-had been projected during an earthquake and the suddenness or initial
-velocity with which the ground had been moved beneath them, was
-Professor Wenthrop of Cambridge, Massachusetts, who noted that bricks
-from his chimneys had, by the New England earthquake of 1755, been
-thrown thirty feet. From this and the known height of the chimney, he
-calculates that the bricks had been projected with an initial velocity
-of twenty-one feet per second.[13]
-
-The calculations made by Mallet respecting the maximum velocity of
-an earth particle at the time of the Neapolitan earthquake in 1857
-depended upon the overthrow, projection, and fracture of bodies.
-
-The principles which guided him in making the calculations will be
-understood from the following illustration.
-
-[Illustration: Fig. 14.]
-
-If a column, A B C D, receive a shock or be suddenly moved in the
-direction of the arrow, the centre of gravity, G, of this column will
-revolve round the edge, and tend to describe the path G O. If it passes
-O, the column will fall. The work done in such a case as this is equal
-to lifting the column through the height _o_ _h_.
-
-If G A = _a_, the angle G A _h_ = φ, and the weight of the body = W,
-then the above work equals
-
-                           W_a_ (1 - cos φ).
-
-This must equal the work acquired—that is to say, the kinetic energy of
-rotation of the body, or
-
-                                        W _w_^2 K^2
-                     W_a_ (1 - cos φ) = ———————————.
-                                           2 _g_
-
-Where _w_ is the angular velocity of the body at starting, K the radius
-of gyration round A, and _g_ the velocity acquired by a falling body in
-one second. Whence
-
-                    _w_^2 K^2 = 2 _ga_ (1 - cos φ),
-
-but _w_, the angular velocity, is equal to the statical couple applied,
-divided by the moment of inertia, or,
-
-                                 V_a_ cos φ
-                           _w_ = ——————————,
-                                    K^2
-
-squaring and substituting
-
-                                  K^2   1 - cos φ
-                     V^2 = 2_g_ × ——— × —————————,
-                                  _a_    cos^2 φ
-
-                                                            K^2
-and since the length of the corresponding pendulum is _l_ = ———,
-                                                            _a_
-
-                                      1 - cos φ
-                        V^2 = 2_gl_ × —————————.
-                                       cos^2 φ
-
-To apply this to any given case we must find the value of _l_ or of
-
-K^2
-———.
-_a_
-
-Mallet finds these values for the cube, solid and hollow rectangular
-parallelopipeds, solid and hollow cylinders, &c. In these formulæ we
-have a direct connection between the dimensions and form of a body and
-the velocity with which the ground must move beneath it to cause its
-overthrow.
-
-[Illustration: FIG. 15.]
-
-Not only is the case discussed for horizontal forces, but also for
-forces acting obliquely. Similar reasonings are applied to the
-productions of fractures in walls, but as there is uncertainty in our
-knowledge of the co-efficient of force necessary to produce fracture
-_through joints across_ beds of masonry, the deductions ought not to
-be applied as the measures of velocity. Where the fractures occur at
-the base or in horizontal planes, or in those of the continuous beds of
-the masonry, or through homogeneous bodies, the uncertainty is not so
-great, and for cases like these Mallet gives several illustrations. The
-distance to which bodies had been projected, as, for example, ornaments
-from the tops of pedestals, coping-stones from the edges of roofs, were
-also used as means of determining the angle at which the shock had
-emerged, or, if this be known, for determining the velocity.
-
-Thus by a shock in the direction O C, a ball, A, on the top of a
-pedestal would describe a trajectory to the point C. Let the angle
-which O C makes with the horizon be _e_, the vertical height through
-which the ball has fallen be _b_, and the horizontal distance of
-projection be _a_; then
-
-                                          _a_^2
-                   _b_ = _a_ tan _e_ + ————————————,
-                                       4H cos^2 _e_
-
-H being the height due to the velocity of projection. Whence
-
-                              ___________________
-                        2H ± √4H(H + _b_) - _a_^2
-              Tan _e_ = ——————————————————————————.
-                                  _a_
-
-                                _a_^2 _g_
-                V^2 = ———————————————————————————————.
-                      2 cos^2 _e_ (_b_ - _a_ tan _e_)
-
-For the back motion or subnormal wave in the direction C O,
-
-                              ___________________
-                        2H ± √4H(H + _b_) - _a_^2
-              Tan _e_ = ——————————————————————————.
-                                  _a_
-
-                                _a_^2 _g_
-                V^2 = ———————————————————————————————.
-                      2 cos^2 _e_ (_b_ + _a_ tan _e_)
-
-A serious error which may enter into calculations of this description
-when practically applied has been pointed out when speaking of columns
-as seismometers. It was then shown that such bodies before being
-overthrown may often be caused to rock, and therefore that their final
-overthrow may not have any direct connection with the impulse of the
-succeeding shock.
-
-Another point to which attention must be drawn respecting the above
-calculations is that if there was no friction or adherence between the
-projected body and its pedestal, in consequence of its inertia it
-would be left behind by the forward motion of the shock, and simply
-drop at the foot of its support. In the case of frictional adherence it
-would be carried forward by the velocity acquired before this adherence
-was broken, and thrown in a direction _opposite_ to that given in the
-figure—that is to say, in the direction of the shock.[14]
-
-_The Absolute Intensity of the Force exerted by an Earthquake._—No
-doubt it has occurred to many who have experienced an earthquake
-that the power which gave birth to such a disturbance must have been
-enormously great. The estimates which we shall make of the absolute
-amount of energy represented by an earthquake cannot, on account of the
-nature of the factors with which we deal, be regarded as accurate. They
-may, however, be of assistance in forming estimates of quantities about
-which we have at present no conception. One method of obtaining the
-result we seek is that which was employed by Mallet in his calculations
-respecting the Neapolitan earthquake. Although disbelieving in the
-general increment of temperature as we descend in the earth at an
-average rate of 1° F. for every fifty or sixty feet of descent, for
-want of better means. Mallet assumes this law to be true, and, knowing
-from a variety of observations the depth of various parts of the cavity
-from which the disturbance sprang, he calculates the temperatures of
-this cavity in various parts as due to its depth beneath the surface.
-Next, it is assumed that steam was suddenly admitted into this cavity,
-which might exert the greatest possible pressure due to the maximum
-temperature. This was calculated as being about 684 atmospheres.
-
-Next, he determined the column of limestone necessary to balance such a
-pressure, which is about 8,550 feet in height. As the least thickness
-of strata above this cavity was 16,700 feet, the pressure of 684
-atmospheres was not sufficient to blow away its cover, but if suddenly
-admitted or generated in the cavity it might have produced the wave of
-impulse by the sudden compression of the walls of the cavity.
-
-The pressure of 684 atmospheres is equivalent to about 4·58 tons on
-the square inch, and, as the total area of the walls of the cavity is
-calculated at twenty-seven square miles, the total accumulated pressure
-would be more than 640,528 millions of tons. Mallet, however, shows
-that it is probable that the temperature of the focal cavity was much
-greater than that due to the hypogeal increment, and that therefore the
-pressure may have been greater.
-
-The capability of producing the earthquake impulse, however, depends
-on the _suddenness_ with which the steam is flashed off. According
-to the experiments of Boutigny and others, Mallet tells us that the
-most sudden production of steam would take place at a temperature of
-500°-550° C., which is but a few degrees below that calculated for the
-mean focal depth.
-
-Assuming the above calculated pressure to be true, and knowing the
-co-efficient of compression of the materials on which it acted, the
-volume of the wave at a given moment near the instant of starting—that
-is, at the focus—can be calculated, and from this the wave amplitude on
-reaching the surface may be deduced.
-
-Proceeding backwards, if we have observed the wave amplitude,
-calculated the depth of the focus, and know the co-efficient of
-expansion, then the total compression may be calculated and the
-temperature due to the pressure producing this may be arrived at. In
-this way earthquakes may be used as a means of calculating subterranean
-temperature at depths that can never be attained experimentally.
-
-A method of proceeding which is probably more definite than that
-adopted by Mallet would be the application of the method indicated when
-speaking of the intensity of artificial disturbances.
-
-If for a given earthquake the origin of which is known we have
-determined by seismographs the mean acceleration of an earth particle
-at two or more stations at different distances from that origin, we are
-enabled to construct a curve of intensity the area between which and
-its asymptotes was shown to be a measure of the total intensity of the
-shock. Comparing this area with that of a unit disturbance produced,
-say, by the explosion of a pound of dynamite, one may approximately
-calculate in terms of this unit the initial intensity of the earthquake.
-
-_Radiation of an Earthquake._—The tremors preceding the more violent
-movements of an earthquake may be due, as Mallet has suggested,[15]
-to the free surface waves reaching a distant point before the direct
-vibrations.
-
-The fact that earth vibrations produced by striking a blow on or near
-the surface of the ground are wholly obliterated in reaching a cutting
-or valley, there being no underground waves of distortion to crop up
-on the opposite side of the valley, indicates that the disturbance is
-one that travels on the surface; the same fact is illustrated when we
-endeavour to transmit vibrations through the side of a hill into a
-tunnel.
-
-In the tunnel, although the distance may be small, no sensible effects
-are produced, whilst the same disturbance may be recorded at a long
-distance from its origin on the surface of the ground outside the
-tunnel.
-
-Lastly, we may refer to the experiences of miners underground.
-
-Occasionally it has happened that miners when deep underground, as in
-the Marienberg in the Saxon Erzgebirge, have felt shocks which have not
-been noticed on the surface. These observations are rare, and it is
-possible that they may be explained by the caving in of subterranean
-excavations.
-
-The usual experience is, that if a shock is felt underground it is also
-felt on the surface, as for example in the lead mines in Derbyshire at
-the time of the Lisbon disturbance (1755).
-
-The most frequent observation, however, is that a shock may be felt on
-the surface while it is not remarked by the miners beneath the surface,
-as at Fahlun and Presburg in November, 1823.
-
-At the Comstock Lode in Colorado about twelve years ago many
-earthquakes were felt. On one particular day twenty-four were counted.
-Superintendent Charles Foreman told the author when he visited Virginia
-City in 1882, that special observations were made to determine whether
-these shocks were felt as severely deep down in the mines as on the
-surface, where they were on the verge of being destructive. The
-universal testimony of many observers was that in most cases they were
-not felt at all underground, and when a shock was felt it was extremely
-feeble. At Takashima Colliery, in Japan, it is seldom that shocks are
-felt underground.
-
-The explanation of these latter observations appears to be either
-that, in consequence of a smaller amplitude of motion in the solid
-rocks beneath the surface as compared with the extent of motion on
-the surface, the disturbances are passed by unnoticed, or else the
-disturbance is, at a distance from its origin, practically confined to
-the surface.
-
-_Velocity of Propagation of an Earthquake._—Although many have written
-upon earthquakes and have endeavoured to give to us the velocity with
-which they were propagated, the subject is one about which we have as
-yet but little exact information.
-
-The importance of this branch of investigation is undoubtedly great.
-By knowing the velocity with which an earthquake has travelled in
-various directions we are assisted in determining the locality of
-its origin; we may possibly make important deductions respecting the
-nature of the medium through which it has passed; perhaps also we
-may learn something regarding the intensity of the disturbance which
-created the earthquake. In the Report of the British Association for
-1851 Mallet gives the table on next page, in which are placed together
-the approximate rates of transit of shocks of several earthquakes
-which he discusses. Some of these, it will be observed, are records of
-disturbances which must have passed through or across the bed of the
-ocean.
-
-In Mallet’s British Association Report for 1858, he gives data compiled
-by Mr. David Milne[16] respecting the Lisbon earthquakes of 1755 and
-1761, from which data the tables of velocities (p. 89) have been
-calculated, omitting those which Mr. Mallet has marked as uncertain.
-
-The distances are marked in degrees of seventy English miles each, and
-the time is reduced to Lisbon time.
-
-  +---------------------+-------+--------------------------+----------+
-  |                     |Approx.|                          |          |
-  |                     |rate in| Formation constituting   |          |
-  | Occasion and Place  |feet   | Range on surface as far  |Authority |
-  |                     | per   | as known or conjectured  |          |
-  |                     |second |                          |          |
-  +---------------------+-------+--------------------------+----------+
-  |Rev. John Mitchell’s |1,760  |Sea bottom, probably on   | Mitchell |
-  | guesses from the    |       | slates, secondary and    |          |
-  | Lisbon earthquakes  |       | crystalline rocks        |          |
-  |Von Humboldt’s       |1,760  |From observations in      | Humboldt |
-  | guesses from South  |  to   | various South American   |          |
-  | America             |2,464  | rocks in great part      |          |
-  |                     |       | volcanic                 |          |
-  |                     |       |                          |          |
-  | _Lisbon Earthquake  |       |                          |          |
-  |     of 1761._       |       |                          |          |
-  |Lisbon to Corunna    |1,994  |Transition, carboniferous | ‘Annual  |
-  |                     |       | and granitoid            |Register’ |
-  |Lisbon to Cork       |5,228  |Transition, carboniferous |   „      |
-  |                     |       | crystalline slates and   |          |
-  |                     |       | granitoid, probably,     |          |
-  |                     |       | under sea bottom         |          |
-  |Lisbon to Santa Cruz |3,261  |The same with many        |   „      |
-  |                     |       | alterations              |          |
-  |                     |       |                          |          |
-  |     _Antilles._     |       |                          |          |
-  |Pointe à Pitre to    |6,586  |Probably volcanic rocks   |Stier and |
-  |  Cayenne (doubtful) |       | under sea bottom         | Perrey’s |
-  |                     |       |                          | memoran- |
-  |                     |       |                          |dum, Dijon|
-  |                     |       |                          |          |
-  |      _India._       |       |                          |          |
-  |Cutch to Calcutta,   |1,173  |Alluvial, secondary,      | ‘Royal   |
-  |  1819               |       | granitoid and later      | Asiatic  |
-  |                     |       | igneous rocks            | Journal’ |
-  |India, Nepauls, and  |       |                          |          |
-  | basin of the Ganges,|       |                          |          |
-  | 1834:--             |       |                          |          |
-  |Rungpur to Arrah     |2,314 }|Deep alluvia, with        | ‘Royal   |
-  |Monghyr to Gorackpur |3,520 }| occasional transition,   | Asiatic  |
-  |Rungpur to Monghyr   |  990 }| carboniferous, granitoid,| Journal’ |
-  |Rungpur to Calcutta  |1,210 }| and later igneous rocks  |          |
-  |Ships ‘Rambler’ and  |1,056  |Sea bottom resting on     |‘Nautical |
-  | ‘Millwood,’ at sea, |       |  unknown rock            |Magazine’ |
-  | 1851; between lat.  |       |                          |          |
-  | 16° 30′ N.L., 54°   |       |                          |          |
-  | 30′ W., and lat. 23°|       |                          |          |
-  | 30′ N.L., 58° 0′ W. |       |                          |          |
-  +---------------------+-------+--------------------------+----------+
-
-              THE LISBON EARTHQUAKE ON NOVEMBER 1, 1755.
-
-  +----------------------------+---------+--------+--------+
-  |                            | Moment  |Distance|Velocity|
-  |         Localities         |observed |  from  |in feet |
-  |                            |of shock |presumed|  per   |
-  |                            |         | origin | second |
-  +----------------------------+---------+--------+--------+
-  |                            |   h. m. |   °  ′ |        |
-  | Presumed focus of shock,   | }  9 23 |    --  |   --   |
-  |   lat. 30°, long. 11° W.   | }       |        |        |
-  | A ship at sea in lat. 38°, | }  9 24 |   0 30 |  3,091 |
-  |   long. 10° 47′ W.         | }       |        |        |
-  | Colares                    |    9 30 |   1 30 |  1,325 |
-  | Lisbon                     |    9 32 |   1 30 |  1,030 |
-  | Oporto                     |    9 38 |   2 30 |  1,030 |
-  | Ayamont                    |    9 50 |   4  0 |    916 |
-  | Cadiz                      |    9 48 |   5  0 |  1,236 |
-  | Tangier and Tetuan         |    9 46 |   5 30 |  1,478 |
-  | Madrid                     |    9 43 |   6  0 |  1,855 |
-  | Funchal                    |   10  1 |   8 30 |  1,382 |
-  | Portsmouth                 |   10  3 |  12 30 |  1,431 |
-  | Havre                      |   10 23 |  13  0 |  1,339 |
-  | Reading                    |   10 27 |  13 30 |  1,304 |
-  | Yarmouth                   |   10 42 |  15  0 |  1,174 |
-  | Amsterdam                  |   10  6 |  17  0 |  2,444 |
-  | Loch Ness                  |   10 42 |  18  0 |  1,409 |
-  +----------------------------+---------+--------+--------+
-
-
-             THE LISBON EARTHQUAKE OF MARCH 31, 1761.[17]
-
-  +----------------------------+---------+--------+--------+
-  |                            | Moment  |Distance|Velocity|
-  |         Localities         |observed |  from  |in feet |
-  |                            |of shock |presumed|  per   |
-  |                            |         | origin | second |
-  +----------------------------+---------+--------+--------+
-  |                            |   h. m. |   °  ′ |        |
-  | Presumed focus, lat. 43°,  | } 11 51 |    --  |  --    |
-  |   long. 11° W.             | }       |        |        |
-  | Ship at sea in lat. 43°,   | }       |        |        |
-  |   many leagues from coast  | } 11 52 |   0 30 |  3,091 |
-  |   of Portugal              | }       |        |        |
-  | Ship in lat. 44° 8′ and    | } 11 54 |   1 45 |  3,607 |
-  |   about 80 leagues from    | }       |        |        |
-  |   coast                    | }       |        |        |
-  | Corunna                    |   11 51 |   2 30 |  2,576 |
-  | Ship lat. 44° 8′ and 80    | }       |        |        |
-  |   leagues NNW. of Cape     | } 11 58 |   3 30 |  3,091 |
-  |   Finisterre               | }       |        |        |
-  | Lisbon                     |   noon  |   4 30 |  3,091 |
-  | Madeira                    |   12  6 |  10  0 |  4,122 |
-  | Cork                       |   12 11 |   9 30 |  2,937 |
-  +----------------------------+---------+--------+--------+
-
-These tables, owing to the nature of the materials which Mallet had at
-his disposal, are but rude approximations to the truth. Two interesting
-facts are, however, observable: the first being that the velocities
-for the earthquake of 1761 are much higher than those obtained for the
-earthquake of 1755; and, secondly, that in both cases the velocities as
-determined from the observations of ships at sea closely approximate
-to each other, in all cases being nearly the same as that with which a
-sound wave would travel through water.
-
-The great differences in transit velocity obtained for different
-earthquakes is a point worthy of attention.
-
-Seebach’s velocity is a _true_ transit velocity, and its determination
-is dependent on the assumption that the shock radiated from the
-_centrum_ and not from the _epicentrum_, Seebach’s method is explained
-when speaking about the determination of origins.
-
-Some interesting observations on the velocity with which the earthquake
-of October 7, 1874, was propagated, are given by M. S. di Rossi.[18]
-
-One assumption is that the disturbance radiated from an origin
-to surrounding points of observation, whilst another is that the
-disturbance followed natural fractures, the direction of which is
-derived from the crest of certain mountain ranges. These velocities are
-as follows, Maradi being at or near the origin of the disturbance:—
-
-  +-----------------------------+-----------------------------------+
-  | Velocity in feet per second |  Velocity in feet per second by   |
-  |   with direct radiation     | propagation along mountain chains |
-  +-----------------------------+-----------------------------------+
-  | Modigliana       820        | By the Valley of Marenzo    1,080 |
-  | Bologna          656        |  „       „       Saveno     1,080 |
-  | Forli            874        |  „       „       Montone    1,080 |
-  | Modena           518        |  „       „       Panaro    {1,080 |
-  |                             |                            {  984 |
-  | Firenze          273        |  „       „       Sieve        540 |
-  | Compiobbi        328        |  „       „         „          540 |
-  +-----------------------------+-----------------------------------+
-
-Another set of interesting results are those of P. Serpieri on the
-earthquake of March 12, 1873. The curious manner in which this shock
-radiated is described in the chapter on the Geographical Distribution
-of Earthquakes (see p. 231). Two large areas appear to have been
-almost simultaneously struck, so that, there being no time for elastic
-yielding, the velocities calculated between places situated on either
-of the areas are exceedingly great.[19]
-
-  From Ragusa to  Venice the velocity was 2,734 feet per second
-   „   Spoleto     „            „         4,101   „        „
-   „   Perugia to Orvieto       „           601   „        „
-   „      „     „ Ancona        „         1,640   „        „
-   „      „     „ Rome          „    {    1,640   „        „
-                                     {or, 2,186
-
-The following are examples of approximate earthquake velocities which
-have been determined in Japan.
-
-_The Tokio Earthquake of October 25, 1881._—From records respecting
-this earthquake it appears to have been felt over the whole of Yezo
-and the northern and eastern coast of Nipon, a little farther south
-than Tokio. It was severest at Nemuro and Hakodate, and at the former
-place a little damage was done. From these facts, together with the
-indications of instruments recording direction of movement and a
-general inspection of the time records, it seems that the disturbance
-must have originated beneath the sea on the east coast of Yezo at a
-very long distance to the north-east of Tokio, from which place it
-passed in a practically direct line on to Yokohama.
-
-As the disturbance was felt at Yokohama twenty-one seconds later than
-at Tokio, and the distances between these two places is about sixteen
-geographical miles, for this portion of its course the disturbance
-must have travelled at a rate of at least 4,300 _feet per second_. If
-we assume that the shock, after having reached Hakodate, travelled on
-at the same rate as it did between Tokio and Yokohama in order to reach
-Saporo, where the shaking was felt eighteen seconds after Hakodate,
-it must have had about thirteen geographical miles to travel after
-Hakodate was shaken before Saporo felt its effect.
-
-Drawing from Hakodate a tangent to the eastern side of a circle of
-thirteen miles radius described round Saporo, the origin of the
-disturbance must be on the line bisecting this tangent at right angles.
-As it also lies on a line drawn through Tokio and Yokohama, it lies in
-a position about 41 N. lat. and 144° 15′ E. long., which is a position
-somewhat nearer to Nemuro than Hakodate, as we should anticipate. If
-this be taken as approximately indicating the origin, then the shock,
-after reaching Hakodate from the Hakodate _homoseist_, travelled about
-218 miles to reach Tokio in 128 seconds, which gives a _velocity of
-10,219 feet per second_.
-
-The method here followed is equivalent to that of the hyperbola and one
-direction (see p. 204). The hyperbola is described on the assumption
-that the velocity deduced from the time taken to travel between Tokio
-and Yokohama is correct, and also that the earth waves travelled with
-approximately the same velocity in the vicinity of Saporo as near
-Tokio. The probability, however, is that they travelled more quickly.
-If this be so, then the origin is thrown somewhat to the south-east and
-the velocity between the Hakodate homoseist and Tokio reduced. Thus,
-if the velocity in the Saporo district be double that observed in the
-Tokio district, the origin is shifted about twenty-eight miles to the
-south-west, and the last-mentioned velocity is reduced to about 9,000
-feet per second.
-
-If we work by the method of circles, and assume the velocity to have
-been constant in all directions, then this velocity must have been
-about 6,000 feet per second. If we assume that the indications of
-direction obtained from seismographs and other sources give to us by
-this intersection a proper origin, the velocity in some directions may
-have been as much as 17,000 feet per second.
-
-An origin thus determined, or even if determined by the method of
-circles, is in discord with the fact that places like Nemuro, in the
-north-east of Yezo, were nearer to the origin than any of the other
-places which have been mentioned.
-
-The conclusion which we are therefore led to with regard to this shock,
-assuming of course that the time observations are tolerably correct,
-is that the velocity of propagation was variable, being greater when
-measured between points near to the origin than between points at a
-distance. The velocities estimated vary between 4,000 and 9,000 feet
-per second.
-
-In the case of the earthquake which has just been discussed, we have
-an example of a disturbance which must have passed between Tokio and
-Yokohama in what was almost a straight line from the origin. As this
-direction ought to give the maximum time of transit if all earthquakes
-are propagated with the same velocity, the following table is given of
-the interval between the time of observation of several shocks at these
-two stations:—
-
-       FROM YOKOHAMA TO TOKIO.         FROM TOKIO TO YOKOHAMA.
-
-  1880 December 20th    36 seconds | 1882 October 25th   21 seconds
-  1881 January 7th   14–31    „    | 1883 February 6th   23    „
-   „   March  8th       60    „    |  „   March 1        53    „
-   „     „   17th.      66    „    |  „     „   „        63    „
-   „   November 15th    31    „    |  „          8th     27    „
-  1882 February 16th    22    „    |  „     „   11th     26    „
-
-As these are observations which have been made with the assistance of a
-telegraphic signal daily employed to correct and rate the clocks from
-which the observations were obtained, they may be regarded as being
-tolerably, correct.
-
-The disturbance of February 6, the two shocks of March 1, appear, like
-that of October 25, to have passed in almost a direct line from an
-origin in the N.N.E, through Tokio on to Yokohama. Their velocities of
-propagation as calculated from the above intervals are approximately
-3,900, 1,900, and 1,400 feet per second. The shock of February 16
-appears to have had its origin near to a point in Yedo Bay about eight
-miles east of Yokohama. Assuming this to be the case, the shock between
-the Yokohama homoseist and Tokio travelled at the rate of 2,454 feet
-per second, but between the Tokio homoseist and Chiba at the rate
-of 750 feet per second; that is to say, the velocity of propagation
-rapidly decreased as the disturbance spread outwards.
-
-At Yokohama it was recorded at 5.31.54, at Tokio at 5.32.16, and at
-Chiba at 5.33.48. These times are given in Tokio mean time.
-
-The shock of March 11, which was recorded at Tokio at 7.51.22 P.M.
-and at Yokohama at 7.51.33 P.M., appears, from the indications of
-instruments which were exceptionally definite in their records, to have
-originated in the N.E. corner of Yedo Bay, about nineteen miles S.S.W.
-from Chiba. This shock was rather severe, fracturing several chimneys.
-From the Tokio homoseist it appears to have travelled on to Yokohama at
-the rate of about 2,200 feet per second. Assuming these observations
-to be _approximately_ accurate, if we take them with the records of
-previous observers they lead us to the following conclusions:—
-
-1. Different earthquakes, although they may travel across the same
-country, have very variable velocities, varying between several
-hundreds and several thousands of feet per second.
-
-2. The same earthquake travels more quickly across districts near to
-its origin than it does across districts which are far removed.
-
-3. The greater the intensity of the shock the greater is the velocity.
-
-
-
-
-                              CHAPTER VI.
-
-            EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.
-
-  The destruction of buildings is not irregular—Cracks in
-    buildings—Buildings in Tokio—Relation of destruction to
-    earthquake motion—Measurement of relative motion of parts of a
-    building shaken by an earthquake—Prevention of cracks—Direction
-    of cracks—The pitch of roofs—Relative position of openings in a
-    wall—The last house in a row—The swing of buildings—Principle of
-    relative vibrational periods.
-
-
-The subject of this chapter is, from a practical point of view, one
-of the most important with which a seismologist has to deal. We
-cannot prevent the occurrence of earthquakes, and unless we avoid
-earthquake-shaken regions, we have not the means of escaping from
-them. What we can do, however, is in some degree to protect ourselves.
-By studying the effects produced by earthquakes upon buildings of
-different construction and variously situated, we are taught how to
-avoid or at least to mitigate calamities which, in certain regions of
-the world, are continually repeated. The subject is an extensive one,
-and what is here said about it must be regarded only as a contribution
-to the work of future writers who may give it the attention it
-deservedly requires.
-
-_The Destruction produced by Earthquakes is not irregular._—If we
-were suddenly placed amongst the ruins of a large city which had
-been shattered by an earthquake, it is doubtful whether we should at
-once recognise any law as to the relative position of the masses of
-_débris_ and the general destruction with which we are surrounded.
-The results of observation have, however, shown us that, amongst the
-apparently chaotic ruin produced by earthquakes, there is in many cases
-more or less law governing the position of bodies which have fallen,
-the direction and position of cracks in walls, and the various other
-phenomena which result from such destructive disturbances.
-
-Mallet, at the commencement of his first volume, describing the
-Neapolitan earthquake of 1857, discusses the general effect produced by
-various shocks upon differently constructed buildings. First he shows
-that, if we have a rectangular building, the walls at right angles to
-the shock will be more likely to be overthrown than those which are
-parallel to it. Experience teaches a similar lesson. Thus Darwin, when
-speaking of the earthquake at Concepcion in 1835,[20] tells us that
-the town was built in the usual Spanish fashion, with all the streets
-running at right angles to each other. One set ranged S.W. by W. and
-N.E. by E. and the other N.W. by N. and S.E. by S. The walls in the
-former direction certainly stood better than those in the latter. The
-undulations came from the S.W.
-
-In Caraccas it is said that every house has its _laga securo_, or safe
-side, where the inhabitants place their fragile property. This _laga
-securo_ is the north side, and it was chosen because about two out of
-every three destructive shocks traversed the city from west to east,
-so that the walls in these sides of a building have been stricken
-broadside on.[21]
-
-_Cracks in Buildings._—Results like the above come from destructive
-earthquakes rather than from movements such as those we have to deal
-with ordinarily. When a building is subjected to a slight movement
-it is assumed that the walls at right angles to the direction of the
-shock move backwards and forwards as a whole, and there is little or
-no tendency for them to be fractured at their weaker parts, these
-weaker parts being those over the various openings. The walls, however,
-which are parallel to the direction of the movement are, so to speak,
-extended and contracted along their length, and in consequence they may
-be expected to give way over the various openings. This tendency for
-extension and contraction of a wall along its length may be supposed,
-for instance, to be due to the different portions of a wall, owing to
-differences in dimensions and elasticity, having different periods of
-natural vibration, or possibly for two portions of a long line of wall
-to be simultaneously affected by portions of waves in different phases.
-
-As an illustration of the giving way of a building in the manner here
-suggested we may take the case of a large brick structure which was
-recently being erected in Tokio. This building, at the time of the
-earthquake, was only some fourteen or fifteen feet above the surface
-of the ground. The length of the building stretched from N.W. to S.E.,
-and it was intersected by many walls at right angles to this direction.
-Through all the walls of this building there were many arched openings.
-In the central part of the transverse walls, which walls were fully
-five feet in thickness, the arches which joined them together were 4
-feet 4 inches in thickness. The arches therefore formed a comparatively
-lightly constructed link between heavy masses of brickwork.
-
-On March 3, 1879, at 4.43 P.M., an earthquake was felt throughout
-Tokio, the strength of which, as judged by our feelings, was above that
-of an average shock. As registered by one of Palmieri’s instruments, it
-had a direction S.S.W. to N.N.E. and an intensity of 11°. On the same
-day there were several smaller shocks having the same direction, and
-these were succeeded by others on the 9th of the month.
-
-Immediately after these shakings it was discovered that almost every
-arch in the internal walls of the building here referred to had been
-cracked across the crown in a direction about N. 40° W. All the
-other arches of the building, of which there were a great number in
-walls at right angles to the direction of the shock, were found not
-to have sustained any injury. To this statement, however, there was
-one exception, which was subsequently proved to have been due to a
-settlement taking place.
-
-After examining these cracks the only cause to which they could be
-attributed was the series of shakings which they had just experienced.
-It seemed as if the heavy walls right and left of the arches had been
-in vibration without synchronism in their periods, and as a consequence
-the arches which connected them had been torn asunder.
-
-Although the time at which the cracks were formed and the peculiar
-positions in which they were only to be found pointed distinctly to
-their origin, to be certain that they were not due to settlement of the
-foundations, horizontal lines were ruled upon the brickwork and from
-time to time subsequently observed.
-
-The points to which the various cracks extended were also marked and
-observed. Beneath the walls as foundations there were beds of concrete
-about three feet thick and about ten feet in width. These had been
-under the pressure of the partially built walls for two years before
-the arches had been put in. As these foundations were unusually strong,
-being intended to carry so very much greater weight than that to which
-they had been subjected, if any settlement had been detected it would
-have been a matter of surprise.
-
-Some weeks after the formation of these cracks it was observed that
-they gradually closed. This was probably due to the gradual falling
-inwards of the two broken portions of the arch, their position when
-open being one of instability.
-
-[Illustration: FIG. 16.—Cracks in a corner house, Belluno, June 29,
-1873 (Bittner).]
-
-Had this building been more complete at the time of the shock, and
-the heavy walls been tied together at higher points, although the
-archways would have been points of weakness, it is quite possible
-that fracture would not have taken place. This illustration shows us
-that when a building is shaken in a definite direction there will be
-some rule as to the positions in which fractures occur. As another
-example, we may take the observations of Alexander Bittner upon the
-buildings of Belluno after the shock of June 29, 1873 (see Beiträge
-zur Kenntniss des Erdbebens von Belluno am 29 Juni, 1873, p. 40. Von
-Alexander Bittner. Aus dem LXIX. Bande der Sitzungsb. der K. Akad. der
-Wissensch., II. Abth., April-Heft. Jahrg. 1874).
-
-Speaking generally, he remarks that ‘Houses similarly situated have
-suffered in corresponding walls and corners in a similar manner. In
-Belluno there is a certain kind of damage which is repeated everywhere,
-making a peculiar system of splits in the S.W. and N.E. corners of the
-houses.’ This is well shown in the accompanying sketch, which evidently
-illustrates the effect of a shock oblique to the direction of two walls
-at right angles to each other.
-
-[Illustration: FIG. 17.—Brick buildings in Tokio, showing
-fractures.]
-
-_Buildings in Tokio._—For the purpose of finding out what has been the
-effect produced by earthquakes upon the buildings of Tokio, and at
-the same time for ascertaining whether blocks of buildings ranging in
-different directions suffered to the same extent, the author examined,
-in company with Mr. Josiah Conder, a large number of foreign-built
-houses in the district of the Ginza. The chief reason for choosing
-this was because it was the only district where a large number of
-_similar_ buildings could be found. By examining houses or buildings of
-different constructions, the effects produced upon them by earthquakes
-are very often likely to show so many differences that it becomes
-almost an impossibility to determine what the general effect has
-been—unsymmetrical construction involving unsymmetrical ruin.
-
-A number of similarly constructed buildings in one locality may be
-regarded as a number of seismographs, the effect upon any one of them
-being judged of by the average of the general effect which has been
-produced upon the whole. The general form of two of these houses
-which were examined is shown in fig. 17. In this figure the general
-character of the fractures which have been produced can also be seen.
-The houses are built of brick, and are in many cases faced with a thin
-coat of white plaster. Projecting from the level of the upper floor
-there is a balcony fronted by a low balustrade. This is supported by
-small beams which at their outer extremity are carried on a row of
-cylindrical columns. This forms a covered way in front of each row of
-houses. The roofs are covered with thick tiles. It will be observed
-that the arches of the upper windows spring _sharply_ from their
-abutments, and at their crown they carry a heavy key-stone. The lower
-openings, which have a span of 9 feet, have evidently been constructed
-in imitation of the open front of an ordinary Japanese house. These
-archways curve out _gently_ from their abutments. The outside walls
-have a thickness of 13½ inches.
-
-The results obtained from a careful examination of 174 houses in
-streets running N.E. and 156 houses in streets running N.W., all of
-these houses being similar, were as follows:—
-
-1. In the upper windows nearly all the cracks ran from the springing,
-which formed an angle with the abutment.
-
-2. In the lower arches, which _curved_ into the abutments, not a single
-crack was observed at the springway. The cracks in these arches were
-near the crown, where beams projected to carry the balcony. In many
-instances the cracks proceeded from such beams, even if there were
-no arch beneath. That cracks should occur in peculiar positions, as
-is here indicated, is shown in the illustrations which accompany the
-accounts of many earthquakes.
-
-3. The houses which were most cracked were in the streets running
-parallel to the direction in which the greater number and most powerful
-set of shocks cross the city.
-
-The results showed that, in order to avoid the effects of small shocks,
-all walls containing principal openings should be placed as nearly
-as possible at right angles to the direction in which the shocks of
-the districts usually travel. The blank walls, or those containing
-unimportant openings, would then be parallel to the direction of the
-shocks—that is, presuming our building to be made up of two sets of
-walls at right angles to each other.
-
-Another point of importance would be to build archways _curving_ into
-the supporting buttresses; the archways over doors and windows which we
-find in earthquake countries do not appear to be in any way different
-from those which are built in countries free from earthquakes. In
-the one country these structures have simply to withstand vertical
-pressures applied statically; in the other, they have to withstand more
-or less horizontal stresses, applied suddenly.
-
-_Relation of Destruction to Earthquake Motion._—The relations which
-exist between the overturning and projection of bodies and the motion
-of the ground have already been discussed. It may be interesting
-to call attention to the fact that in the formulæ showing three
-relationships, it was the _shape_ rather than the _weight_ of a body
-which determined whether it should be overturned or projected by a
-motion at its base.
-
-As an interesting proof that light bodies may be overturned as easily
-as heavy ones. Mallet refers to the overturning of several large
-haystacks as one of the results of the Neapolitan earthquake.
-
-If masses of material are displaced or fractured, then Mallet remarks
-                                       _____
-that the maximum velocity will exceed √2_gh_, where _h_ is the
-amplitude of the wave. Should the maximum velocity be less than this
-quantity, the masses which are acted upon will be simply raised and
-lowered, and there will be no relative displacements even if the
-emergence of the wave be nearly or quite vertical.
-
-When we get a vertical wave acting upon an irregular mass of masonry,
-the heavier portions of the masonry, by their inertia, tend to descend
-relatively to the remaining portions, and in this way vertical fissures
-will be produced. For this reason it would not be advisable to use
-heavy materials above archways, heavy roofs, or heavy floors. The
-vertical fissures, Mallet remarks, would have their widest opening at
-the base.
-
-In considering cases of fracture produced by earthquake motion, it
-must be remembered that these are due to stresses applied _suddenly_,
-and that if the same amount of stress had been _slowly_ applied to a
-building, fractures might not have occurred.
-
-If a disturbance is horizontal, and has a direction parallel to the
-length of a wall, the wall is carried forward at its foundations. This
-motion is opposed by the inertia of the upper portion of the wall and
-the various loads it carries. The wall being elastic, distortion takes
-place, and cracks, which are widest at the top, will be formed. In a
-uniform wall the two most prominent fissures ought to be near the ends.
-
-If the horizontal backward and forward movement has a direction
-oblique to the plane of the wall, the wall will be either overthrown,
-fractured, or have a triangular fragment thrown off towards the origin
-from the end last reached.
-
-Should the wave emerge steeply, diagonal fissures at right angles to
-the direction of transit will be formed, or else triangular pieces will
-be projected.
-
-The accompanying figures are reduced from Mallet’s ‘Account of the
-Neapolitan Earthquake of 1857.’
-
-[Illustration: FIG. 18.—Cathedral Church, Potenza (Mallet).]
-
-Taking _a_ _b_ as the general direction of the fractures in fig. 18,
-then _c_ _d_ will represent the direction in which the shock emerged,
-which is at an angle of 23°·20′ to the horizon. It might be argued that
-the direction of these fractures was due to the direction in which
-surface undulations had travelled, or to the relative strengths and
-proportions of different portions of the building. The directions of
-cracks in a building are undoubtedly due to a complexity of causes, but
-for buildings situated in the region of shock the impulsive effect of
-the shock is probably the most important function to be considered.
-The method of applying the directions of emergence, deduced from
-observations on fractures, to determine the origin of a disturbance
-will be referred to in Chapter X.
-
-[Illustration: FIG. 19.—The Cathedral, Paterno (Mallet).
-Neapolitan Earthquake of 1857.]
-
-Mallet observed that, although two ends of a building might be nearly
-the same, the fissures and joints do not occur at equal distances from
-the ends, nor are they equally opened.
-
-The end where the joints are the most opened is that which was first
-acted upon, and this phenomenon may be sufficiently well pronounced
-to indicate the direction in which we must look to find the origin
-of a disturbance. Amongst possible explanations for this disposition
-of fractures in a wall. Mallet suggests that they may be due to real
-differences in the two semiphases of the wave of shock, the second
-semiphase being described with a somewhat slower velocity than the
-first. This, it will be observed, is contrary to the indications of
-seismographs.
-
-Fig. 19, of the cathedral at Paterno, shows the effect of a subnormal
-shock striking a wall obliquely and projecting one of its corners.
-
-
-           MEASUREMENTS OF THE RELATIVE MOTION OF PARTS OF A
-                BUILDING AT THE TIME OF AN EARTHQUAKE.
-
-In 1880 a series of observations was made in Tokio to determine whether
-at the time of an earthquake the various parts of the arched openings
-which we see in many buildings synchronised in their vibrations, or,
-for want of synchronism, were caused to approach and recede from each
-other. The arches experimented on were heavy brick arches forming the
-two corridors of the Imperial College of Engineering. The direction of
-one set of these corridors is N. 40° E. and that of the other N. 50° W.
-
-The thickness of the walls in which these arches are placed is 1 ft.
-11 in. They are built of Japanese bricks bound together with ordinary
-lime. The span of the arches is 8 ft. 3 in., and the height of the arch
-from the springing-line to the crown 4 ft. 1 in. The height of the
-abutments is 7 ft. 1½ in. The voussoirs of the arch are formed of a
-light grey soft volcanic rock, and on their faces show a depth of 12
-inches. The width of the intermediate columns between the arches is 4
-ft. 6⅞ in.
-
-To determine whether at the time of an earthquake there was any
-variation in the dimensions of these arches, a light stiff deal
-rod, about 2 in. by ½ in. in cross section, was placed across the
-springing-line of the arch. One end of this was firmly fixed to the
-top of one abutment by means of a spike; on the other end, which was
-to indicate any horizontal movement if the abutments approached each
-other, there was fixed a pointer made out of a piece of steel wire.
-This rested on a piece of smoked glass fixed to the ledge on which
-the loose end of the rod was resting. If the abutments approached or
-receded from each other a line would be drawn measuring the extent of
-the motion. As a further indication of motion, a second smoked glass
-plate was fixed on the transverse rod, which plate was marked on by a
-pointer attached to a vertical rod hanging down from the crown of the
-arch.
-
-As a general result of these experiments it may be said that the
-portions of the building which were examined usually either did not
-move at all, or else they practically synchronised in their movements.
-When they did move, the extent of motion was small, and the small
-differences in movement which were observed were in every probability
-far within the elastic limits of the structure.
-
-_Observations on Cracks._—To determine whether the walls of a building
-which have once been cracked, when subjected to a series of shocks,
-similar to those which they experienced before being cracked, still
-continued to give way, the extremities of a considerable number of
-cracks in the N.E. end of the museum buildings of the Engineering
-College were marked with pencil. Although since the time of marking
-there had been many severe shocks, these cracks did not visibly extend.
-These marks were made on the outside wall of the building. On the
-inside, one of these same cracks showed itself as a fissure about ¼
-inch in width. Across this crack a horizontal steel wire pointer was
-placed. One end of this wire was fixed in the wall; the other end,
-which was pointed, rested on the surface of a smoked glass plate placed
-on the other side of the crack. After small earthquakes there was no
-indication of motion having taken place, but after a shock on February
-21, as indicated by a line upon the smoked glass plate, it was seen
-that the sides of the crack had approached and receded from each other
-through a distance of about 1/16 inch.
-
-By similar contrivances placed on cracks in a neighbouring building
-exactly similar results were obtained, namely, that during small
-earthquakes the two sides of the crack had retained their relative
-positions, but at the time of a large shock this position had been
-changed.
-
-In this building it was also observed that the cracks in many instances
-increased their length.
-
-By attaching levers to the end of the pointers to multiply any motion
-that might take place, no doubt the indications would be more frequent
-and more definite. It would also be easier to note the relative
-distances of motion in two directions, namely, how far the cracks
-had closed and how far they had opened. As to whether motion would
-occur or not, much would no doubt depend upon the direction of the
-earthquake.
-
-_Prevention of Fractures._—One conclusion which may perhaps be drawn
-from these observations is, that a cracked building at the time of an
-earthquake shows a certain amount of flexibility. Whether a building
-which had been designed with cracks or joints between those parts
-which were likely to have different periods of vibration would be more
-stable, so far as earthquake shakings are concerned, than a similar
-building put up in an ordinary manner, is a matter to be decided by
-experiment. Certainly some of the cracks which have been examined
-indicate that if they had not existed, the strain upon the portion of
-the building where they occur would have been extremely great.
-
-_Direction of Cracks._—In looking at the cracks produced by small
-earthquakes it is interesting to note the manner of their extension.
-The basements of the buildings which have been most carefully examined
-are, for a height of two or three feet, built of large rectangular
-blocks of a greyish-coloured volcanic rock. In these parts the cracks
-pass in and out between the joints of the stone, indicating that
-the stones have evidently been stronger than the mortar which bound
-them together, and as a consequence the latter had to give way.
-Above this basement when the cracks enter the brickwork, they no
-longer exclusively confine themselves to the joints, but run in an
-irregular line through all they meet with, sometimes across the bricks
-and sometimes through the mortar joints. In places where they have
-traversed the brickwork, we can say that the mortar has been stronger
-than the bricks. This traversing of the bricks rather than the joints
-is, I think, the general rule for the direction of the cracks in the
-brickwork of Tokio buildings.
-
-_The Pitch of Roofs._—From observation of the effects produced by
-earthquakes, it appears to us that the houses which lost the greater
-number of tiles appear to be those with the steepest pitch, and those
-where the tiles were simply laid upon the roof and not in any manner
-fastened down. It would seem that destruction of this sort might to a
-great extent be obviated by giving the roofs a less inclination and
-fixing the tiles with nails. It was also noticed that the greatest
-disturbance amongst the tiles was upon the ridges of the roofs.
-Destruction of this sort might be overcome by giving especial attention
-to these portions during the construction of the roof.
-
-[Illustration:  FIG. 20.      FIG.  21.]
-
-_Relative Position of Openings in Walls._—From what has been said about
-the fractures in the buildings of Tokio it will have been seen that,
-with but few exceptions, they have all taken place above openings
-like doorways and windows. If architecture demands that openings like
-arches should be placed one above another in heavy walls of this kind,
-as in fig. 17, there will be lines of weakness running through the
-openings parallel to the dotted lines. As arches are only intended
-to resist vertical thrusts, special construction must be adopted to
-make them strong enough to resist horizontal pulls. For instance, a
-flat arch would offer more resistance to horizontal pulls than an
-arch put together with ordinary voussoirs, there being in the former
-case more friction to prevent the component parts sliding over each
-other. Or again, above each arch an iron girder or wooden lintel might
-be inserted in the brick or stone arch. It was suggested to me by my
-colleague, Mr. Perry, that the best form calculated to give a wall
-uniform strength, would be to build it so that the openings of each
-tier would occupy alternate positions, that is to say, along lines
-parallel to the struts and ties of a girder. In this way we should have
-our materials so arranged that they would offer the same resistance to
-horizontal as to vertical movements. Such a wall is shown in fig. 20:
-the dotted lines running through the openings, and all similar lines
-parallel to the former, representing lines of weakness. If we compare
-this with fig. 21, we shall see that in the case of a horizontal
-movement _a_ _b_ or of a vertical movement _c_ _d_, we should rather
-expect to find fractures in a house built like fig. 21 than in one
-built like fig. 20. If, however, these two buildings were shaken by a
-shock which had an angle of emergence of about 45° in the direction _e_
-_f_, the effects might be reversed. Usually, however, and always in a
-town like Tokio which is visited by shocks originating at a distance,
-the movements are practically horizontal ones, and, therefore,
-buildings erected on the principles illustrated by fig. 20 should
-be much superior, so far as resisting earthquakes is concerned, to
-buildings constructed in the ordinary manner, as in fig. 21. Fractures
-following a vertical line of weakness are shown in the accompanying
-drawing, fig. 22, of the Church of St. Augustin, at Manilla, shattered
-by the earthquakes of 1880.
-
-_The last House in a Row._—When an earthquake shock enters a line
-of buildings, and proceeds in a direction coincident with that of
-the buildings, we should expect that the last of these houses, being
-unsupported on one side, would be in the position of the last person in
-Tyndall’s row of boys. From this it would seem that the end house in a
-row would show the greatest tendency to fly away from its neighbours.
-If the last house stood upon the edge of a deep canal or a cliff,
-there would be a layer of ground, equal in thickness to the depth of
-the canal or to the height of the cliff, as the case may be, which
-would also be in a position to be thrown forward. The effect which is
-sometimes produced upon an end building is shown in fig. 23, which is
-taken from the photograph of a house shattered in 1868 at San Francisco.
-
-[Illustration: FIG. 22.—Church of St. Augustin, Manilla.
-Earthquakes of July 18–20, 1880.]
-
-_The Swing of Buildings._—The distance through which buildings are
-moved at the time of an earthquake depends partly on their construction
-and partly on the extent, nature, and duration of the movement
-communicated to them at their foundations. By violent shocks buildings
-may be completely overthrown. In the case of small earthquakes, the
-upper portion of a house may frequently move through a much greater
-distance than the ground at its foundation. For instance, during the
-Yokohama earthquake of February, 1880, when the maximum amplitude of
-the earth’s motion was probably under ¾ of an inch, from the slow swing
-of long Japanese pictures, from three to six feet in length, which
-oscillated backwards and forwards on the wall, it is very probable
-that the extent through which the upper portion of houses moved was
-very considerable. In some instances these pictures seem to have swung
-as much as two feet, and from the manner in which they swung they
-evidently synchronised with the natural swing of the house.
-
-[Illustration: FIG. 23.—Webber House, San Francisco. Oct. 21, 1868.]
-
-From this it would seem that such a house must have rocked from side to
-side one foot out of its normal perpendicular position. That the motion
-was great is testified by nearly all who tried to stand at the time of
-the shock, it having been impossible to walk steadily across the floor
-of a room in an upper story. The houses here referred to are either
-those which are purely Japanese, or else those which are framed of wood
-and built on European models, a class of building which is very common
-in Tokio and Yokohama.
-
-Perry and Ayrton calculated the period of a complete natural vibration
-of different structures. For a square house whose outer and inner
-sections were respectively 30 and 26 feet, and whose height was 30
-feet, the period calculated would be about ·06 second.
-
-At the time of the above earthquake many houses seem to have moved like
-inverted pendulums. On the morning after the shock my neighbour, who
-was living upstairs in a tall wooden house with a tile roof, told me
-that he endeavoured to count the vibrations, and was of the impression
-that to make a complete swing it took about 2 seconds.
-
-Assuming now that the distance through which the top of a wooden house
-moved was about 1 foot, and the number of vibrations which it made per
-second was about ·5, then the greatest velocity of a point on the top
-of such a house must have been about 6 feet per second.
-
-Mallet, who made observations upon the vibrations of various
-structures, tells us that Salisbury spire moves to and fro in a gale
-more than 3 inches. A well-constructed brick and mortar wall, 40 feet
-high and 1 foot 6 inches thick, was observed to vibrate in a gale 2
-feet transversely before it fell.
-
-An octagonal chimney with a heavy granite capping, 160 feet high, was
-observed instrumentally to vibrate at the top nearly 5 inches.[22]
-
-At the time of a severe earthquake it does not seem impossible but that
-a building may be swung completely over. The accompanying illustration,
-fig. 24, taken from a photograph,[23] apparently indicates a movement
-of description.
-
-[Illustration: FIG. 24.—Stud Mill at Haywards, California. Oct. 21,
-1868.]
-
-_Principle of relative Vibrational Period._—If a lath or thin pole
-loaded at one end with a weight fixed to the ground, so as to stand
-vertically, be shaken by an earthquake it will be caused to rock to
-and fro like an inverted pendulum. The period of its swing will be
-chiefly dependent on its dimensions, its elasticity, and its load. In
-a building we have to consider the vibration of a number of parts,
-the periods of which, if they were independent of each other, would be
-different. On account of this difference in period, whilst one portion
-of a building is endeavouring to move towards the right, another
-is pulling towards the left, and, in consequence, either the bonds
-which join them or else they themselves are strained or broken. This
-was strikingly illustrated by many of the chimneys in the houses at
-Yokohama, which by the earthquake of February 20, 1880, were shorn off
-just above the roof. The chimneys were shafts of brick, and probably
-had a slower period of vibration than the roof through which they
-passed, this latter vibrating with the main portion of the house, which
-was framed of wood.
-
-A particularly instructive example of this kind which came under my
-notice is roughly sketched in fig. 25.
-
-[Illustration: FIG. 25.]
-
-This is a chimney standing alone, which, for the sake of support, was
-strapped by an iron band to an adjoining building. It would seem that
-at the time of the shock, the building moving one way and the chimney
-another, the swing of the heavy building gave the chimney a sharp jerk
-and cut it off. The upper portion, being then loose upon the lower
-part, rotated under the influence of the oscillations in manner similar
-to that in which gravestones are rotated.
-
-Mallet made observations similar to these in Italy. He tells us that a
-buttress may often not have time to transmit its stability to a wall.
-The wall and the buttress have different periods of vibration, and
-therefore they exert impulsive actions on each other. Effects like
-these were strikingly observable in many of the rural Italian churches
-where the belfry tower is built into one of the quoins of the main
-rectangular building.
-
-Not only have we to consider the relative vibrations of the various
-parts of a building amongst themselves, but we have to consider the
-relation of the natural vibrations of any one of them or the vibration
-of the building as a whole, with regard to the earth, the vibrations of
-which it must be remembered are not strictly periodic.
-
-Some of the more important results dependent upon the principle of
-‘relative vibrational periods’ may be understood from the following
-experiments:—
-
-[Illustration: FIG. 26.]
-
-In fig. 26 A, B, and C are three flat springs made out of strips of
-bamboo, and loaded at the top with pieces of lead. At the bottom they
-are fixed into a piece of board D E, and the whole rests on a table F
-G. The legs of this table being slightly loose, by placing the fingers
-on the top of it, a quick short backward and forward movement can be
-produced. The weights on A and B are the same, but they are larger than
-the weight on C. Consequently the periods of A and B are the same, but
-different to the period of C. The dimensions of these springs are as
-follows: height, 18 inches; A and B each carry weights equal to 320
-grammes, and they make one vibration per second; C has a weight of 199
-grammes, and makes 0·75 vibrations per second.
-
-_First Experiment._—It will be found that by giving the table a gentle
-backward and forward movement, the extent of which movement may be so
-small that it will be difficult to detect it with the eye, either A
-and B may be made to oscillate violently whilst C remains still; or
-_vice versâ_, C may be caused to oscillate whilst A and B remain still.
-In the one case the period of shaking will have been synchronous with
-the natural period of A and B, whilst in the latter it will have been
-synchronous with that of C. This would seem to show us that if the
-natural period of vibration of a house, or of parts of it, at any time
-agree with the period of the shock, it may be readily thrown into a
-state of oscillation which will be dangerous for its safety.
-
-_Second Experiment._—Bind A and B together with a strip of paper
-pasted between them. (The paper used was three-eighths of an inch
-broad and would carry a weight of nearly three pounds.) If the table
-be now shaken as before, A and B will always have similar movements,
-and tend to remain at the same distance apart, and as a consequence
-the strip of paper will not be broken. From this experiment it would
-seem that so long as the different portions of a building have almost
-the same periods of vibration, there will be little or no strain upon
-the tie-rods or whatever contrivance may be used in connecting the
-different parts.
-
-_Third Experiment._—Join A and C, or B and C with a strip of paper in a
-manner similar to the last experiment. If the table be now shaken with
-a period approximating either to that of A and B, or with that of C,
-the paper will be suddenly snapped.
-
-This indicates that if we have different portions of a building of
-such heights and thicknesses that their natural periods of vibration
-are different, the strain upon the portions which connect such parts
-is enormous, and it would seem, as a consequence, that either the
-vibrators themselves, or else their connections, must, of a necessity,
-give way. This was very forcibly illustrated in the Yokohama earthquake
-of February 1880 by the knocking over of chimneys. The particular case
-of the chimneys is, however, better illustrated by the next experiment.
-
-_Fourth Experiment._—Take a little block of wood three-quarters of an
-inch square and about one inch high, and place it on the top of A, B,
-or C. It will be found that, although the spring on which it stands
-is caused to swing backwards and forwards through a distance of three
-inches, the little block will retain its position.
-
-This little block we may regard as the upper part of a chimney standing
-on a vibrating stack, and we see that, so long as this upper portion is
-light, it has no tendency to fall.
-
-_Fifth Experiment._—Repeat the fourth experiment, having first placed a
-small leaden cap on the top of the block representing the chimney. (The
-cap used only weighed a few grammes.) When vibration commences it will
-be found that the block quickly falls. This would seem to indicate that
-chimneys with heavy tops are more likely to fall than light ones.
-
-_Sixth Experiment._—Bind A and B together with a strip of paper and
-stand the little block upon the top of either. It will be found that
-the block will stand as in the fourth experiment.
-
-_Seventh Experiment._—Bind A and C, or B and C together, and place
-the block upon the top of either of them. When vibration commences,
-although the paper may not be broken, the little block will quickly
-fall.
-
-_Eighth Experiment._—Take two pencils or pieces of glass tube and
-place them under the board D E. If the table F G be now shaken in the
-direction D E, it will be found that the springs will not vibrate.
-
-In a similar manner if a house or portion of a house were carried on
-balls or rollers, as has already been suggested, it would seem that the
-house might be saved from much vibration.
-
-_Ninth Experiment._—Set any of the springs in violent vibration by
-gently shaking D E instead of the table, and then suddenly cease the
-actuating motion. It will be observed that at the moment of cessation
-the board and the springs will have a sudden and very decided motion
-of translation in the same direction as that in which the springs were
-last moving, and although the springs were at the time swinging through
-a considerable arc, all motion will suddenly cease.
-
-This shows, that if a house is in a state of vibration the strain at
-the foundations must be very great.
-
-It would not be difficult to devise other experiments to illustrate
-other phenomena connected with the principle of relative vibrational
-periods, but these may perhaps be sufficient to show to those who have
-not considered this matter its great importance in the construction
-of buildings. Perhaps the greater portion of what is here said may by
-many be regarded as self-evident truisms hardly worth the trouble of
-demonstration. Their importance, however, seems to be so great that I
-hope that their discussion has not been altogether out of place.
-
-I may remark that in the rebuilding of chimneys in Yokohama the
-principles here enunciated were taken advantage of by allowing the
-chimneys to pass freely through the roofs without coming in contact
-with any of the main timbers.
-
-In putting up buildings to resist the effects of an earthquake, besides
-the idea of making everything strong because the earthquake is strong,
-there are several principles which, like the one just enunciated, might
-advantageously be followed which as yet appear to have received but
-little attention.
-
-
-
-
-                             CHAPTER VII.
-
-             EFFECTS PRODUCED UPON BUILDINGS (continued).
-
-  Types of buildings used in earthquake countries—In Japan, in Italy,
-    in South America, in Caraccas—Typical houses for earthquake
-    countries—Destruction due to the nature of underlying rocks—The
-    swing of mountains—Want of support on the face of hills—Earthquake
-    shadows—Destruction due to the interference of waves—Earthquake
-    bridges—Examples of earthquake effects—Protection of
-    buildings—General conclusions.
-
-
-_Types of buildings used in earthquake countries._—In Japan there
-are excellent opportunities of studying various types of buildings.
-The Japanese types, of course, form the majority of the buildings.
-The ordinary Japanese house consists of a light framework of 4 or
-5 inch scantling, built together without struts or ties, all the
-timbers crossing each other at right angles. The spaces are filled
-in with wattle-work of bamboo, and this is plastered over with mud.
-This construction stands on the top of a row of boulders or of square
-stones, driven into the surface soil to a distance varying from a few
-inches to a foot. The whole arrangement is so light that it is not an
-uncommon thing to see a large house rolled along from one position to
-another on wooden rollers. In buildings such as these after a series of
-small earthquake shocks, we could hardly expect to find more fractures
-than in a wicker basket.
-
-The larger buildings, such as temples and pagodas, are also built of
-timber. These are built up of such a multitude of pieces and framed
-together in such an intricate manner that they also are capable of
-yielding in all directions. The European buildings are, of course,
-made of brick and stone with mortar joints. Some of these, as the
-buildings of the Ginza in Tokio, are not designed for great strength.
-On the other hand, others have thick and massive walls and are equal in
-strength to those we find in Europe.
-
-The third type of buildings are those which are built in blocks; and
-these blocks being bound together with iron rods traversing the walls
-in various directions are especially designed to withstand earthquakes.
-A system somewhat similar to this has been patented in America, and
-examples of these so-called earthquake-proof buildings are to be found
-in San Francisco.
-
-Speaking of Japanese buildings, Mr. R. H. Brunton, who has devoted
-especial attention to them says that,[24] ‘to imagine that slight
-buildings, such as are seen here (i.e. in Japan), are the best
-calculated to withstand an earthquake shock is an error of the most
-palpable kind.’ After describing the construction of a Japanese house
-in pretty much the same terms as we have used, he says ‘that with its
-unnecessarily heavy roof and weak framework it is a structure of all
-others the worst adapted to withstand a heavy shock.’ He tells us,
-further, that these views are sustained by the truest principles of
-mechanics. In order to render buildings to some extent proof against
-earthquakes, some of the heavy roofs in Tokio have been so constructed
-that they are capable of sliding on the walls. Mr. Brunton mentions
-a design for a house, the upper part of which is to rest on balls,
-which roll on inverted cups fixed on the lower part of the building,
-which is to be firmly embedded in the earth. A similar design was,
-at the suggestion of Mallet, used to support the tables carrying the
-apparatus of some of the lighthouses erected in Japan by Mr. Brunton.
-The very existence of these designs seems to indicate that the ordinary
-European house, however solidly and strongly it may be built, is not
-sufficient to meet the conditions imposed upon it. What is required,
-is something that will give way—an approximation to the timber frame
-of a Japanese house, so strongly condemned by Mr. Brunton and others.
-The crucial test of the value of the Japanese structure, as compared
-with the modern buildings of brick and stone, is undoubtedly to be
-found by an appeal to the buildings themselves. So far as my own
-experience has gone, I must say that I have never seen any signs in
-the Japanese timber buildings which could be attributed to the effects
-of earthquakes, and His Excellency Yamao Yozo, Vice Minister of Public
-Works, who has made the study of the buildings of Japan a speciality,
-told me that none of the temples and palaces, although many of them
-are several centuries old, and although they have been shaken by small
-earthquakes and also by many severe ones, show any signs of having
-suffered. The greatest damage wrought by large earthquakes appears to
-have resulted from the influx of large waves or from fires. In every
-case where an earthquake has been accompanied by great destruction,
-by consulting the books describing the same, it can be seen, from the
-illustrations in these books portraying conflagrations, that this
-destruction was chiefly due to fire. When we remember that nearly
-all Japanese houses are constructed of materials that are readily
-inflammable, it is not hard to imagine how destruction of this kind
-has come about. To a Japanese, living as he does in a house which has
-been compared to a tinder-box, fire is one of his greatest enemies,
-and in a city like Tokio it is not at all uncommon to see during the
-winter months many fires which sweep away from 100 to 500 houses. In
-one winter I was a spectator of three fires, each of which was said to
-have destroyed upwards of 10,000 houses.
-
-Although it would appear that the smaller earthquakes of Japan produce
-no visible effect upon the native buildings, it is nevertheless
-probable that small effects may have been produced, the observation
-of which is rendered difficult by the nature of the structure. If we
-look at buildings of foreign construction, by which are meant buildings
-of brick and stone, the picture before us is quite different, and
-everywhere the effects of earthquakes are palpable even to the most
-casual observer. Of these effects numerous examples have already been
-given. Not only are these buildings damaged by the cracking of walls
-and the overturning of chimneys, but they also appear to be affected
-internally. For instance, in the timbers of the roof of the museum
-attached to the Imperial College of Engineering in Tokio, there are a
-number of diagonal faces acting as struts or ties intended to prevent
-more or less horizontal movements taking place. Those which are rigidly
-joined together with bolts and angle irons have apparently suffered
-from their rigidity, being twisted and bent into various forms. The
-buildings in Tokio, which are strongly put together, being especially
-designed to withstand earthquakes, appear to have suffered but little.
-I know only one example which at the time of the severe shock of 1880
-had several of its chimneys damaged.
-
-The ordinary houses in Italy, though built of stone and mortar, are
-but poorly put together, and, as Mallet has remarked, are in no way
-adapted to withstand the frightful shakings to which they are subjected
-from time to time.
-
-In the large towns, like Naples, Rome, and Florence, where happily
-earthquakes are of rare occurrence, although the building may be
-better than that found in the country, the height of the houses and
-the narrowness of the streets are sufficient to create a shudder, when
-we think of the possibility of the occurrence of a moderately severe
-earthquake.
-
-In South America, although many buildings are built with brick
-and stone, the ordinary houses, and even the larger edifices, are
-specially built to withstand earthquakes. In Mr. James Douglas’s
-account of a ‘Journey Along the West Coast of South America,’ we read
-the following[25]: ‘The characteristic building material of Guayaquil
-is bamboo, which grows to many inches in thickness, and which, when
-cut partially through longitudinally at distances of an inch or so,
-and once quite through, can be opened out into fine elastic boards of
-serviceable width. Houses, and even churches, of a certain primitive
-beauty are built of such reeds, so bound together with cords that few
-nails enter into the construction, and which, therefore, yield so
-readily to the contortions of the earth during an earthquake as to be
-comparatively safe.’
-
-Here we have a house, which, so far as earthquakes are concerned, is
-an exaggerated example of the principles which are followed in the
-construction of an ordinary Japanese dwelling.
-
-Another plan adopted in South America can be gathered from the same
-author’s writings upon Lima, about which he says, ‘To build high houses
-would be to erect structures for the first earthquake to make sport of,
-and, therefore, in order to obtain space, safety, and comfort, the
-houses of the wealthy surround court after court, filled with flowers,
-and cooled with fountains, connected one with another with wide
-passages which give a vista from garden to garden.’
-
-History would indicate that houses of this type have been arrived at as
-the results of experience, for it is said that when the inhabitants of
-South America first saw the Spaniards building tall houses, they told
-them they were building their own sepulchres.[26]
-
-In Jamaica, we find that even as early as 1692 experience had taught
-the Spaniards to construct low houses, which withstood shakings better
-than the tall ones.[27]
-
-In Caraccas, which has been called the city of earthquakes, it is
-said that the earthquakes cause an average yearly damage amounting to
-the equivalent of a _per capita_ tax of four dollars. To reduce this
-impost to a minimum much attention is paid to construction. ‘Projecting
-basement corners (giving the house a slightly pyramidal appearance)
-have been found better than absolutely perpendicular walls; mortised
-corner-stones and roof beams have saved many lives when the central
-walls have split from top to bottom; vaults and key-stone arches, no
-matter how massive, are more perilous than common wooden lintels, and
-there are not many isolated buildings in the city. In many streets
-broad iron girders, riveted to the wall, about a foot above the house
-door, run from house to house along the front of an entire square.
-Turret-like brick chimneys, with iron top ornaments, would expose the
-architect to the vengeance of an excited mob; the roofs are flat,
-or flat terraced; the chimney flues terminate near the eaves in a
-perforated lid.’[28]
-
-_Typical houses for earthquake countries._—From what has now been said
-about the different buildings found in earthquake countries, it will be
-seen that if we wish to put up a building able to withstand a severe
-shaking, we have before us structures of two types. One of these types
-may be compared with a steel box, which, even were it rolled down a
-high mountain, would suffer but little damage; and the other, with a
-wicker basket, which would equally withstand so severe a test. Both
-of these types may be, to some extent, protected by placing them upon
-a loose foundation, so that but little momentum enters them at their
-base. One suggestion is to place a building upon iron balls. Another
-method would be to place them upon two sets of rollers, one set resting
-upon the other set at right angles. The Japanese, we have seen, place
-their houses on round stones. The solid type of building is expensive,
-and can only be approached partially, whilst the latter is cheap, and
-can be approached closely. In the case of a solid building it would be
-a more difficult matter to support it upon a movable foundation than in
-the case of a light framework. Such a building is usually firmly fixed
-on the ground, and consequently at the time of an earthquake, as has
-already been shown by experiment, must be subjected to stresses which
-are very great. In consequence also of the greater weight of the solid
-structure, more momentum will enter it at its base than in the case of
-the light structure. Also, we must remember that the rigidity favours
-the transmission of momentum, and with rigid walls we are likely to
-have ornaments, coping-stones, and the comparatively freer portions
-forming the upper part of a building displaced; whilst, with flexible
-walls absorbing momentum in the friction of their various parts, such
-disturbances would not be so likely. Mr. T. Ronaldson, referring to
-this, says, that in 1868, at San Francisco, the ornamental stone work
-in stone and cement buildings was thrown from its position, whilst
-similar ornaments in neighbouring brick buildings stood.
-
-To reduce the top weight of a building, hollow bricks might be
-employed. To render a building more homogeneous and elastic, the
-thickness of bricks might be reduced. Inasmuch as the elasticity
-of brick and timber are so different, the two ought to be employed
-separately. For internal decorations plaster mouldings might be
-replaced by _papier mâché_ and _carton-pierre_, the elastic yielding of
-which is comparatively great.[29] Houses, whether of brick and stone,
-or of timber, ought to be broad and low, and the streets three or four
-times as wide as the houses. The flatter the roofs the better.
-
-One of the safest houses for an earthquake country would probably be a
-one-storied strongly framed timber house, with a light flattish roof
-made of shingles or sheet-iron, the whole resting on a quantity of
-small cast-iron balls carried on flat plates bedded in the foundations.
-The chimneys might be made of sheet-iron carried through holes free of
-the roof. The ornamentation ought to be of light materials.
-
-At the time of severe earthquakes many persons seek refuge from their
-houses by leaving them. In this case accidents frequently happen from
-the falling of bricks and tiles. Others rush to the doorways and
-stand beneath the lintels. Persons with whom the author has conversed
-have suggested that strongly constructed tables and bedsteads in
-their rooms would give protection. To see persons darting beneath
-tables and bedsteads would undoubtedly give rise to humiliating and
-ludicrous exhibitions. This latter idea is not without a value, and
-most certainly, if applied in houses of the type described, would be
-valuable.
-
-The great danger of fire may partially be obviated by: the use of
-‘earthquake lamps,’ which are so constructed that before they overturn
-they are extinguished. It is said that in South America some of the
-inhabitants are ready at any moment to seek refuge in the streets,
-and they have coats prepared, stocked with provisions and; other
-necessaries, which, if occasion demands, will enable them to spend the
-night in the open air. These coats, called ‘earthquake coats,’ might
-also, with properly constructed houses, be rendered unnecessary.
-
-_Destruction due to the nature of the underlying rocks._—That the
-nature of the ground on which a building stands is intimately related
-with the severity of the blow it receives is a fact which has often
-been demonstrated.
-
-One cause of destruction is due to placing a building on foundations
-which are capable of receiving the full effects of a shock, and
-transmitting it to the buildings standing on them.
-
-For instance, the reason why a soft bed might possibly make a good
-foundation, is, as has been pointed out by Messrs. Perry and Ayrton,
-because the time of transmission of momentum is increased; in fact, the
-soft bed is very like a piece of wood interposed between a nail and
-the blows of a hammer—it lengthens the duration of impact. For this
-reason we are told that a quaking bog will make a good foundation. When
-a shock enters loose materials its waves will be more crowded, and it
-is possible that a line of buildings may rest on more than one wave
-during a shock. There are many examples on record of the stability of
-buildings which rested on beds of particular material at the time of
-destructive earthquakes. As the observations which have been made
-by various writers on this subject appear to point in a contrary
-direction, I give the following examples:—
-
-In the great Jamaica earthquake of 1692, the portions of Port
-Royal which remained standing were situated on a compact limestone
-foundation; whilst those on sand and gravel were destroyed (‘Geological
-Observer,’ p. 426). Again, on p. 148 of the same work, we read,
-‘According to the observations made at Lisbon, in 1737, by Mr. Sharpe,
-the destroying effects of this earthquake were confined to the tertiary
-strata, and were most violent on the blue clay, on which the lower part
-of the city is constructed. Not a building on the secondary limestone
-or on the basalt was injured.’
-
-In the great earthquakes of Messina, those portions of the town
-situated on alluvium, near the sea, were destroyed, whilst the high
-parts of the town, on granite, did not suffer so much. Similar
-observations were made in Calabria, when districts consisting of
-gravel, sand, and clay became, by the shaking, almost unrecognisable,
-whilst the surrounding hills of slate and granite were but little
-altered. At San Francisco, in 1868, the chief destruction was in the
-alluvium and made ground.
-
-At Talcahuano, in 1835, the only houses which escaped were the
-buildings standing on rocky ground; all those resting on sandy soil
-were destroyed.
-
-From the results of observations like these, it would seem the harder
-rocks form better foundations than the softer ones. The explanation of
-this, in many cases, appears to lie in the fact that the soft strata
-were in a state of unstable equilibrium, and by shaking, they were
-caused to settle. Observations like the following, however, point out
-another reason why soft strata may sometimes afford a bad foundation.
-
-‘Humboldt observed that the Cordilleras, composed of gneiss and
-mica-slate, and the country immediately at their foot, were more shaken
-than the plains.’[30]
-
-‘Some writers have asserted that the wave-like movements (of the
-Calabrian earthquake in 1783) which were propagated through recent
-strata from west to east, became very violent when they reached the
-point of junction with the granite, as if a reaction was produced when
-the undulatory movement of the soft strata was suddenly arrested by the
-more solid rocks.’
-
-Dolomieu when speaking of this earthquake says, the usual effect ‘was
-to disconnect from the sides of the Apennines all those masses (of sand
-and clay) which either had not sufficient bases for their bulk, or
-which were supported only by lateral adherence.’
-
-These intensified actions taking place at and near to lines of
-junction between dissimilar strata is probably due to the phenomena of
-reflection and refraction.
-
-When referring to the question as to whether buildings situated on
-loose materials suffered more or less than those on solid rocks,
-Mallet, in his description of the Neapolitan earthquake of 1857,
-remarks: ‘We have in this earthquake, towns such as Saponara and
-Viggiano, situated upon solid limestone, totally prostrated; and we
-have others such as Montemarro, to a great extent based upon loose
-clays, totally levelled. We have examples of almost complete immunity
-in places on plains of deep clay as that of Viscolione, and in places
-on solid limestone, like Castelluccio, or perched on mountain tops like
-Petina.’[31]
-
-After reading the above, we see that the probable reason why, in
-several cases, beds of soft materials have not made good foundations,
-consists in the fact that they have either been of small extent or else
-have been observed only in the neighbourhood of lines which divided
-them from other formations, which lines are always those of great
-disturbances.
-
-At the end of his description of the Neapolitan earthquake of 1857,
-Mallet says that more buildings were destroyed on the rock than on the
-loose clay. This, however, he remarks, is hardly a fact from which we
-can draw any valuable deductions, because it so happened that more
-buildings were constructed on the hills than on the loose ground.[1]
-
-Professor D. S. Martin, writing on the earthquake of New England in
-1874, remarks that in Long Island the shock was felt where there was
-gneiss between the drift. Around portions to the east the observations
-were few and far between. He also remarks that generally the shocks
-were felt more strongly and frequently on rocky than on soft ground.[32]
-
-From these examples, it would appear that the hard ground, which
-usually means the hills, forms a better foundation than the softer
-ground, which is usually to be found in the valleys and plains. Other
-examples, however, point to a different conclusion. For instance, a
-civil engineer, writing about the New Zealand earthquake of 1855, when
-all the brick buildings in Wellington were overthrown, says that ‘it
-was most violent on the sides of the hills at those places, and least
-so in the centre of the alluvial plains.’[33]
-
-In this example it must be noticed that the soft alluvium here referred
-to was of large extent, and not loose material resting on the flanks of
-rocks, from which it was likely to be shaken down, as in most of the
-previous examples.
-
-The results of my own observations on this subject point as much in one
-direction as in the other. In Tokio, from instrumental observations
-upon the slopes and tops of hills, the disturbance appears to be
-very much less than it is in the plains. Thus, at my house, situated
-on the slope of a hill about 100 feet in height, for the earthquake
-of March 11, 1882, I obtained a maximum amplitude of motion of from
-three to four millimètres only, whilst Professor Ewing, with a similar
-instrument, situated on the level ground at about a mile distant,
-found a motion of fully seven millimètres. This calculation has been
-confirmed by observations on other earthquakes. Thus, for instance, in
-the destructive earthquake of 1855, when a large portion of Tokio was
-devastated, it was a fact, remarked by many, that the disturbance was
-most severe on the low ground and in the valleys, whilst on the hills
-the shock had been comparatively weak. As another illustration, I may
-mention that within three-quarters of a mile from my house in Tokio
-there is a prince’s residence which has so great a reputation for the
-severity of the shakings it receives that its marketable value has been
-considerably depreciated, and it is now untenanted.
-
-In Hakodadi, which is a town situated very similarly to Gibraltar,
-partly built on the slope of a high rocky mountain and partly on a
-level plain, from which the mountain rises, the rule is similar to
-that for Tokio, namely, that the low, flat ground is shaken more
-severely than the high ground. At Yokohama, sixteen miles south-west
-from Tokio, the rule is reversed, as was very clearly demonstrated
-by the earthquake of February 1880, when almost every house upon the
-high ground lost its chimney, whilst on the low ground there was
-scarcely any damage done; the only places on the low ground which
-suffered were those near to the base of the hills. The evidence as to
-the relative value of hard ground as compared with soft ground, for
-the foundation of a building, is very conflicting. Sometimes the hard
-ground has proved the better foundation and sometimes the softer, and
-the superiority of one over the other depends, no doubt, upon a variety
-of local circumstances.
-
-These latter observations open up the inquiry as to the extent to which
-the intensity of an earthquake may be modified by the topography of the
-disturbed area.
-
-_The swing of mountains._—If an earthquake wave is passing through
-ground the surface of which is level, so long as this ground is
-homogeneous, as the wave travels further and further we should expect
-its energy to become less and less, until, finally, it would insensibly
-die out. If, however, we have standing upon this plain a mountain,
-judging from Mallet’s remarks, this mountain would be set in a state
-of vibration much in the same way as a house is set in vibration,
-and it would tend to oscillate backward and forward with a period of
-vibration dependent upon the nature of its materials, size, and form.
-The upper portion of this mountain would, in consequence, swing through
-a greater arc than the lower portion, and buildings situated on the top
-of it would swing to and fro through a greater arc than those which
-were situated near its foot. This explanation why buildings situated
-on the top of a mountain should suffer more than those situated on
-a plain, is one which was offered by Mallet when writing of the
-Neapolitan earthquake. He tells us that towns on hills are ‘rocked as
-on the top of masts,’ and if we accept this explanation it would, in
-fact, be one reason why the houses situated on the Bluff at Yokohama
-suffered more than those situated in the settlement. This explanation
-is given on account of the great authority it claims as a consequence
-of its source. It is not clear how the statement can be supported,
-as different portions of the mountain receive momentum in opposite
-directions at the same time.
-
-_Want of support on the faces of hills._—When a wave of elastic
-compression is propagated through a medium, we see that the energy of
-motion is being continually transmitted from particle to particle of
-that medium. A particle, in moving forwards, meets with an elastic
-resistance of the particles towards which it moves, but, overcoming
-these resistances, it causes these latter particles to move, and in
-turn to transmit the energy to others further on. So long as the medium
-in which this transfer of energy is continuous, each particle has a
-limit to its extent of motion, dependent on the nature of the medium.
-When, however, the medium, which we will suppose to be the earth, is
-not continuous, but suddenly terminates with a cliff or scarp, the
-particles adjacent to this cliff or scarp, having no resistance offered
-to their forward motion, are shot forward, and, consequently, the
-ground here is subjected to more extensive vibrations than at those
-places where it was continuous. This may be illustrated by a row of
-marbles lying in a horizontal groove; a single marble rolled against
-one end of this row will give a concussion which will run through the
-chain, like the bumping of an engine against a row of railway cars, and
-as a result, the marble at the opposite end of the row, being without
-support, will fly off. Tyndall illustrates the same thing with his
-well known row of boys, each one standing with his arms stretched out
-and his hands resting upon the shoulders of the boy before him. A push
-being given to the boy at the back, the effect is to transmit a push
-to the first boy, who, being unsupported, flies forward.
-
-In the case of some earthquakes, most disastrous results have occurred
-which seem only to admit of an explanation such as this. A remarkable
-instance of this kind occurred when the great earthquake of 1857 ‘swept
-along the Alps from Geneva to the east-north-east, and its crest
-reached the edge of the deep glen between Zermatt and Visp. Then the
-upper part of the wave-movement, a thousand or two thousand feet in
-depth from the surface, came to an end; the forward pulsation acted
-like the breaker of the sea, and heavy falls of rock encumbered the
-western side of the valley.’
-
-_Earthquake shadows._—If a mountain stands upon a plain through which
-an elastic wave is passing, which is almost horizontal, the mountain
-is, so to speak, in the _shadow_ of such a wave. If we only consider
-the normal motion of this wave, we see that the only motion which the
-mountain can obtain will be a wave of elastic distortion produced
-by a shearing force along the plain of the base. Should, however,
-the wave approach the mountain from below, and emerge into it at a
-certain angle, only the portion of the mountain on the side from
-which the wave advanced could remain in shadow, whilst the portion
-on the opposite side would be thrown into a state of compression and
-extension. Portions in shadow, however, would be subject to waves
-of elastic distortion. In a manner similar to this we may imagine
-that certain portions of the bluff, so far as the advancing wave was
-concerned, were in shadow, and thus saved from the immediate influence
-of the direct shock. A hypothetical case of such a shadow is shown in
-the accompanying section, illustrating the contour of the ground at
-Yokohama. The situation which might be in the shadow of one shock,
-however, it is quite possible might not be in that of another. We
-must also remember that a place in shadow for a direct shock might be
-affected by reflected waves, and also by the transverse vibrations of
-the direct shock. These effects are over and above the effects produced
-by the waves of elastic distortion just referred to. It might be asked
-whether whole countries, like England, which are but seldom shaken, are
-in shadow.
-
-[Illustration: FIG. 27.—Hypothetical section at Yokohama.]
-
-_Destruction due to the interference of waves._—Referring to the
-section of the ground at Yokohama (Fig. 27), it will be seen that
-both the settlement and the bluff stand upon beds of gravel capping
-horizontal beds of grey tuff. The gravel of that portion of the
-settlement on the seaboard originally formed the line of a shingle
-beach. That portion of the settlement back from the sea stands upon
-ground which was originally marshy. In the central portions of the
-settlement this bed of gravel is very thick, perhaps 100 feet or so,
-but as you near the edge of the bluff it probably becomes thinner,
-until it finally dies out upon the flanks of the scarps.
-
-On the top of the bluff, the beds of gravel will, in every probability,
-be generally thinner than they are upon the lower level. The beds of
-tuff, which is a soft grey-coloured clay-like rock, produced by the
-solidification of volcanic mud, appear, when walking on the seaboard,
-to be horizontally stratified. If there is a dip inland, it is in all
-probability very slight. Here and there the beds slightly faulted.
-Taken as a whole we may consider these beds as being tolerably
-homogeneous, and an earthquake in passing through them would meet
-with but little reflection or refraction. At the junction of these
-beds with the overlying gravels, both reflection and refraction would
-comparatively be very great.
-
-On entering the gravel, as the wave would be passing into a less
-elastic medium, the direction of the wave would be bent towards the
-perpendicular to the line of junction, and the angle of emergence at
-the surface would consequently be augmented. At the surface certain
-reflection would also take place, but the chief reflections would be
-those at the junction of the tuff and the alluvium.
-
-Under the settlement it is probable that all the reflections which
-took place would be single. Thus wave fronts like A_{1} advancing in
-a direction parallel to the line _a__{1}; would be reflected in a
-direction _a__{2} and give rise to a series of reflected waves A_{2}.
-These are shown by thicker lines. Similarly all the neighbouring waves
-to the right and left of A_{1} would give rise to a series of reflected
-waves. If the lines drawn representing wave fronts are districts of
-compression, then, where two of the lines cross each other, there would
-be double energy in producing compression. Similarly, districts of
-rarefaction might accord, and, again, compression of one wave might
-meet with the rarefaction of another and a neutralisation of effect
-take place. A diagram illustrating concurrence and interference of this
-description is given in Le Conte’s ‘Elements of Geology,’ p. 115. The
-interference which has been spoken of, however, is not the greatest
-which would occur. The greatest would probably be beneath the bluff and
-the scarps which run down to join the level ground below. This would be
-the case because it is a probability that there might not only be cases
-of interference of single reflected waves, but also of waves which
-had been not only twice but perhaps thrice reflected. For example, a
-wave like B_{1} (which is parallel to A_{1} of the first supposition),
-advancing in a direction parallel to _b__{1} might be reflected along
-the line _b__{2} giving rise to waves like B_{2}, which in turn might
-be reflected along _b__{3} giving rise to waves like B_{3}. The number
-of districts where there would be concurrence and interference would,
-in consequence of the number of times waves might be reflected, be
-augmented. Here the violence of the shock would, at certain points, be
-considerably increased, but as a general result energy must be lost,
-so that even if some of the reflected waves found their way into the
-portion we have regarded as being in shadow, their intensity would not
-be so great as if they had entered it directly.
-
-The shaking down of loose materials from the sides of hills may be
-partially explained on the assumption of an increased disturbance due
-to interference.
-
-_Earthquake bridges._—In certain parts of South America there appear
-to exist tracts of ground which are practically exempt from earthquake
-shocks, whilst the whole country around is sometimes violently shaken.
-It would seem as if the shock passes beneath such a district as water
-passes beneath a bridge, and for this reason these districts have been
-christened ‘bridges.’
-
-This phenomenon appears to depend upon the nature of the underlying
-soil. When an elastic wave passes from one bed of rock to another of a
-different character, a certain portion of the wave is reflected, while
-the remainder of it is transmitted and refracted, and ‘bridges’ we
-may conceive of as occurring where the phenomenon of total reflection
-occurs.
-
-In the instances given of soft materials having proved good
-foundations, it was assumed that they had chiefly acted as absorbers
-of momentum. They have also acted as reflecting surfaces, and where no
-effects have been felt by those residing on them, this may have been
-the result of total reflection, and the soft beds thus have played the
-part of bridges.
-
-Fuchs gives an example taken from the records of the Syrian earthquake
-of 1837, where not only neighbouring villages suffered differently, but
-even neighbouring houses. In one case a house was entirely destroyed,
-whilst in the next house nothing was felt.
-
-In Japan, at a place called Choshi, about 55 miles east of the capital,
-earthquakes are but seldom felt, although the surrounding districts may
-be severely shaken.
-
-From descriptions of this place it would appear that there is a large
-basaltic boss rising in the midst of alluvial strata. The immunity from
-earthquakes in this district has probably given rise to the myth of
-the Kanam rock, which is a stone supposed to rest upon the head of a
-monstrous catfish (Namadzu), which by its writhings causes the shakings
-so often felt in this part of the world.[34]
-
-Prof. D. S. Martin, writing on the earthquake of New England in 1874,
-says that it was felt at four points; it was felt in the heart of
-Brooklyn all within a circle of half a mile across; ‘and this fact
-would suggest that a ridge of rock perhaps approaches the surface at
-that point, though none is known to appear.’[35]
-
-The subject of special districts, which are more or less protected from
-severe shakings, will be again referred to, and it will be seen that
-after a seismic survey has been made even of a country like Japan,
-where there are on the average at least two earthquakes per day, it
-is possible to choose a place to build in as free from earthquakes as
-Great Britain.
-
-_General examples of earthquake effects._—The following examples of
-earthquake effects are drawn from Mallet’s account of the Neapolitan
-earthquake of 1857.
-
-At a town called Polla there was great destruction. Judging from
-the fissures in the parts that remained standing it seemed that the
-emergence of the shock had been more vertical in the upper part of the
-town than in the lower, proving that whatever had been the angle below,
-the hill had itself vibrated, which, being horizontal, had modified the
-angle of the fissures.
-
-Diano suffered but little, partly because it was well built, and partly
-on account of its situation, which was such that before the shock
-reached it the disturbance had to pass from beds of clay into nearly
-vertically placed beds of limestone. Also a great portion of the shock
-was cut off by the Vallone del Raccio to the north and north-west of
-the town. Here the effects of the partial extinction of the wave on the
-‘free outlaying stratum’ were visible in the masses of projected rock.
-
-Castellucio did not suffer because its well buttressed knoll was end
-on to the direction of shock, and on account of a barrier of vertical
-breccia beds protecting it upon the east.
-
-Pertosa stands on a mound. The destruction was least in the southern
-part of the town. From the relation of the beds of breccia on which the
-town stands, and the direction of the wave path, it is evident that the
-southern part of the town received the force of the shock through a
-greater thickness of the breccia beds than the other parts did.
-
-Petina, standing on a level limestone spur jutting out from a mountain
-slope, suffered nothing, whilst Anletta five miles to the south-west,
-and Pertosa six miles distant, were in great part prostrated. (1) The
-terrace did not vibrate, and (2) between Petina and Anletta there is
-almost 6,000 feet of piled up limestone, so that any shock emergent at
-a steep angle had to pass up transversely through these beds.
-
-_Protection of buildings._—In addition to giving proper construction
-to our buildings, choosing proper foundations and positions for them,
-something might possibly be done to ward off the destructive effects of
-an earthquake. We read that the Temple of Diana at Ephesus was built on
-the edge of a marsh, in order to ward off the effect of earthquakes.
-Pliny tells us that the Capitol of Rome was saved by the Catacombs,
-and Elisée Reclus[36] says that the Romans and Hellenes found out that
-caverns, wells, and quarries retarded the disturbance of the earth,
-and protected edifices in their neighbourhood. The tower of Capua was
-saved by its numerous wells. Vivenzis asserts that in building the
-Capitol the Romans sunk wells to weaken the effects of terrestrial
-oscillations. Humboldt relates the same of the inhabitants of San
-Domingo.
-
-Quito is said to receive protection from the numerous cañons in the
-neighbourhood, whilst Lactacunga, fifteen miles distant, has often been
-destroyed.
-
-Similarly, it is extremely probable that many portions of Tokio have
-from time to time been protected more or less from the severe shocks of
-earthquakes by the numerous moats and deep canals which intersect it.
-
-Although we are not prepared to say how far artificial openings of this
-description are effectual in warding off the shocks of earthquakes,
-from theoretical considerations, and from the fact that their use has
-been discovered by persons who, in all probability, were without the
-means of making theoretical deductions, the suggestions which they
-offer are worthy of attention.
-
-_General conclusions._—The following are a few of the more important
-results which may be drawn from the preceding chapter:—
-
-1. In choosing a site for a house find out by the experience of others
-or experimental investigation the localities which are least disturbed.
-In some cases this will be upon the hills, in others in the valleys and
-on the plains.
-
-2. A wide open plain is less likely to be disturbed than a position on
-a hill.
-
-3. Avoid loose materials resting on harder strata.
-
-4. If the shakings are definite in direction, place the blank walls
-parallel to such directions, and the walls with many openings in them
-at right angles to such directions.
-
-5. Avoid the edges of scarps or bluffs, both above and below.
-
-6. So arrange the openings in a wall, that for horizontal stresses the
-wall shall be of equal strength for all sections at right angles.
-
-7. Place lintels over flat arches of brick or stone.
-
-8. To withstand destructive shocks either rigidly follow one or other
-of the two systems of constructing an earthquake-proof building. The
-light building on loose foundations is the cheaper and probably the
-better.
-
-9. Let all portions of a building have their natural periods of
-vibration nearly equal.
-
-10. If it is a necessity that one portion of a building should have a
-very different period of vibration to the remainder, as for instance a
-brick chimney in a wooden house, it would seem advisable either to let
-these two portions be sufficiently free to have an independent motion,
-or else they must be bound together with great strength.
-
-11. Avoid heavy topped roofs and chimneys. If the foundations were free
-the roof might be heavy.
-
-12. In brick or stone work use good cement.
-
-13. Let archways curve into their abutments.
-
-14. Let roofs have a low pitch, and the tiles, especially those upon
-the ridges, be well secured.
-
-
-
-
-                             CHAPTER VIII.
-
-                    EFFECTS OF EARTHQUAKES ON LAND,
-
-  1. Cracks and fissures—Materials discharged from
-    fissures—Explanation of fissure phenomena. 2. Disturbances in
-    lakes, rivers, springs, wells, fumaroles, &c.—Explanation of
-    these latter phenomena. 3. Permanent displacement of ground—On
-    coast lines—Level tracts—Among mountains—Explanation of these
-    movements.
-
-
-_Cracks and fissures formed in the ground._—Almost all large
-earthquakes have produced cracks in the ground. The cracks which were
-found in the ground at Yokohama (February 22, 1880) were about two or
-three inches wide, and from twenty to forty yards in length. They could
-be best seen as lines along a road running near the upper edge of some
-cliffs which overlook the sea at that place. The reason that cracks
-should have occurred in such a position rather than in others was
-probably owing to the greater motion at such a place, due to the face
-of the cliff being unsupported, and there being no resistance opposed
-to its forward motion. It often happens that earthquake cracks are many
-feet in width. At the Calabrian earthquake of 1783, one or two of the
-crevasses which were formed were more than 100 feet in width and 200
-feet in depth. Their lengths varied from half a mile to a mile.[37]
-Besides these large cracks, many smaller ones of one or two feet in
-breadth and of great length were formed. In the large fissures many
-houses were engulfed. Subsequent excavations showed that by the closing
-of the fissures these had been jammed together to form one compact
-mass. These cracks are usually more or less parallel, and at the same
-time parallel to some topographical feature, like a range of mountains.
-For example, the cracks which were formed by the Mississippi earthquake
-of 1812 ran from north-east to south-west parallel to the Alleghanies.
-By succeeding shocks these crevasses are sometimes closed and sometimes
-opened still wider. Their permanency will also depend upon the nature
-of the materials in which they are made.
-
-During an earthquake large cracks may suddenly open and shut.
-
-During the convulsions of 1692 which destroyed Port Royal, it is said
-that many of the fissures which were formed, opened and shut. In some
-of these, people were entirely swallowed up and buried. In others they
-were trapped by the middle, and even by the neck, where if not killed
-instantaneously they perished slowly. Subsequently their projecting
-parts formed food for dogs.[38]
-
-The earthquake which, July 18, 1880, shook the Philippines caused many
-fissures to be found, which in some places were so numerous that the
-ground was broken up into steps. Near to the village of San Antonio the
-soil was so disturbed that the surface of a field of sugar-canes was
-so altered that in some cases the top of one row of full grown plants
-was on a level with the roots of the next. Into one such fissure a boat
-disappeared, and into another, a child.
-
-Subsequently the child was excavated, and its body, which was found a
-short distance below the surface, was completely crushed.[39]
-
-At the time of the Riobamba earthquake, not only were men engulfed,
-but animals, like mules, also sank into the fissures which were formed.
-
-The fissures which were formed at the time of the Owen’s Valley
-earthquake in 1872 extended for miles nearly parallel to the
-neighbouring Sierras. In some places the ground between the fissures
-sank twenty or thirty feet, and at one place about three miles east of
-Independence, a portion of the road was carried eighteen feet to the
-south by a fissure.[40]
-
-Speaking generally, it may be said that all large earthquakes are
-accompanied by the formation of fissures. The Japanese have a saying
-that at the time of a large earthquake persons must run to a bamboo
-grove.
-
-The object of this is to escape the danger of being engulfed in
-fissures, the ground beneath a bamboo grove being so netted together
-with fine roots that it is almost impossible for it to be rent open.
-
-_Materials discharged from fissures._—Together with the opening of
-cracks in the earth it often has happened that water, mud, vapours,
-gases, and other materials, have been ejected.
-
-At the time of the Mississippi earthquake water, mixed with sand and
-mud, was thrown out with such violence that it spurted above the tops
-of the highest trees. In Italy such phenomena have often been repeated.
-
-From the fissures which were formed in 1692 at the time of the
-earthquakes in Sicily, water issued which in some instances was
-salt.[41]
-
-By the Cachar earthquake (January 10, 1869) numerous fissures were
-formed parallel to the banks of a river, from this water and mud were
-ejected. Dr. Oldham, who describes this earthquake, says that the
-first shot of dry mud or sand was mistaken for smoke or steam. The
-water was foul, and hotter than surface water at the time, but only
-slightly so; and the sulphurous smell was nothing more than you would
-perceive in stirring up the mud at the bottom of any stagnant pool
-which had lain undisturbed for some time.[42]
-
-In 1755, when Tauris was destroyed, boiling water issued from the
-cracks which were formed. Similar phenomena were witnessed at a place
-eight miles from La Banca in Mexico, in the year 1820. Part of this hot
-water was pure and part was muddy.
-
-Sometimes the water which has been ejected has been so muddy that the
-mud has been collected to form small hills. This was the case at the
-time of the Riobamba earthquake. The mud in this case consisted partly
-of coal, fragments of augite, and shells of infusoria.
-
-At the time of the Jamaica earthquake men who had fallen into crevices
-were in some cases thrown out again by issuing water.
-
-Sometimes, as has already been mentioned, vapour, gases, and even
-flames issue from fissures. Vapour of sulphur appears to be exceedingly
-common. Kluge says that many fish were killed in consequence of the
-sulphurous vapours which rose in the sea near to the coast of New
-Zealand in 1855.
-
-On December 14, 1797, an insupportable smell of sulphur was observed to
-have accompanied the earthquake which at that time shook Cumana, which
-was greatest when the disturbance was greatest.
-
-Sulphurous fumes which were combustible were belched out of the
-earth at the time of the Jamaica earthquake in 1692. The smell which
-accompanied this was so powerful that it caused a general sickness
-which swept away about 3,000 persons.[43]
-
-From the fissures formed at Concepcion in 1835, water, which was black
-and fœtid, issued.[44]
-
-The earthquakes of New England in 1727 were accompanied by the
-formation of fissures, from which sand and water boiled out in
-sufficient quantity to form a quagmire. In some places ash and sulphur
-are said to have been ejected.
-
-At one house the stink of sulphur accompanying the earthquake was so
-great that the family could not bear to remain in doors.[45]
-
-Emanations of gas sometimes appear to have burst out from submarine
-sources.
-
-Thus the earthquake at Lima, in March, 1865, was accompanied with great
-agitation of the water and an odour of sulphuretted and carburetted
-hydrogen. This former gas was developed to such an extent that the
-white paint of the U.S. ship ‘Lancaster’ was blackened.[46] With the
-smell, flames have sometimes been observed, as, for instance, at the
-time of the Lisbon earthquake.
-
-At the time of the earthquakes of 1811 and 1813, in the Mississippi
-valley, steam and smoke issued from some of the fissures which were
-formed.
-
-Instances are recorded where stones have been shot up from fissures
-unaccompanied by water, as, for instance, at the earthquake of Pasto
-(January, 1834). It is imagined that the propelling power must have
-been the sudden expansion of escaping gases.
-
-It has been suggested that flames seen above fissures might perhaps
-be due to the burning of materials like sulphur. Mr. D. Forbes, who
-examined the effects of the earthquakes of Mendoza, which were felt for
-a distance of 1,200 miles, says that where the hard rock came to the
-surface there were no traces of fissures, these being entirely confined
-to the alluvium. The rumours of fire and smoke having appeared at some
-of the fissures were without foundation, the presumed smoke being
-nothing but dust.[47]
-
-In addition to flames lights appear often to have been observed, the
-origin of which cannot be easily explained.
-
-The earthquake of November 22, 1751, at Genoa is said to have been
-accompanied by a light like that of a prodigious fire which seemed to
-arise out of the ground.[48]
-
-_Explanation of fissure phenomena._—The manner in which fissures are
-formed has already been explained when referring to the want of support
-in the face of hills (page 136).
-
-Similar remarks may be applied to the banks of rivers and all
-depressions, whether natural or artificial, which have a steep slope.
-At such places the wave of shock emerges on a free surface, which,
-being unsupported in the direction of its motion, tends to tear itself
-away from the material behind, and form a fissure parallel to the face
-of the free surface. The distance of the fissure from the face of the
-free surface will, theoretically, be equal to half the amplitude of the
-wave of motion, one half tending to move forwards, and the other half
-backwards. The reason that water and other materials rush forth from
-fissures has been explained by Schüler as being due to cracks having
-been opened through impervious strata, which, before the earthquake,
-by their continuity prevented the rising of subterranean water under
-hydrostatic pressure.[49]
-
-Kluge explains the coming up of the waters as being due to the same
-causes which he considers may be the origin of disturbances in the sea.
-
-The most reasonable explanations of the eruption of water, mud, sand,
-and gas through fissures are those given by Oldham and Mallet in their
-account of the Cachar earthquake.
-
-In the case of a horizontal shock passing through a bed of ooze or
-water-bearing strata, the elastic wave will tend to pack up the water
-during the forward motion to such an extent that it will flow or
-spout up through any aperture communicating with the surface. By the
-repetition of these movements causing ejections, sand or mud cones,
-like those produced by a volcanic eruption, may be formed, and by a
-similar action water may be shot violently up out of wells, as was the
-case in Jamaica in 1692.
-
-If an emergent wave acts through a water-bearing bed upon a
-superincumbent layer of impervious material, this upper layer is,
-during the upward motion, by its inertia suddenly pressed down upon the
-latter.
-
-This pressure is equal to that which would raise the upper layer to
-a height equal to the amplitude of the motion of an earth particle,
-and with a velocity at least equal to the mean velocity of the earth
-particle resolved in the vertical direction.
-
-For a moment the water-bearing strata receive an enormous squeeze, and
-the water or mud starts up through any crevice which may be formed
-leading to the surface.
-
-From this we see that liquids may rise far beyond the level due to
-hydrostatic pressure.[50]
-
-Volger has attributed the origin of lights or flames appearing above
-fissures to the friction which must take place between various rocky
-materials at the time when the fissures are opened. As confirmatory of
-this he refers to instances where similar phenomena have been observed
-at the time of landslips. At the time of these landslips the heat
-developed by friction has been sufficiently intense to convert water
-into steam, the tension of which threw mud and earth into the air like
-the explosion of a mine.[51]
-
-The gas eruptions which occasionally take place with earthquakes are
-probably due to the opening of fissures communicating with reservoirs
-or strata charged with products of natural distillation, or chemical
-action, which previously had accumulated beneath impervious strata. Of
-the existence of such gases we have abundant evidence. In coal mines we
-have fire damp which escapes in increased quantities with a lowering of
-the barometrical pressure. In volcanic regions we have many examples of
-natural springs of carbon dioxide.
-
-These various gases sometimes escape in quantity, or erupt without
-the occurrence of earthquakes. Rossi mentions an instance where a few
-years ago quantities of fish were killed by the eruption of gas in the
-Tiber, near Rome. Another instance is one which occurred at Follonica
-on April 6, 1874. On the morning of that day many of the streets and
-roads were covered with the dead bodies of rats and mice. It seemed as
-if it had rained rats. From the facts that the bodies of the creatures
-seemed healthy, that the destruction had happened suddenly, and not
-come on gradually like an epidemic, it was supposed that they had been
-destroyed by an emanation of carbon dioxide. The fact that many of them
-lay in long lines suggested the idea that they had been endeavouring
-to escape at the time of the eruption.[52] If we can suppose sudden
-developments of gas like this to have occasionally accompanied
-earthquakes, we may sometimes have the means of accounting for the
-sickness which has been felt.
-
-_Disturbances in lakes._—It has often been observed that, at the time
-of large earthquakes, lakes have been thrown into violent agitation,
-and their waters have been raised or lowered. At the time of the great
-Lisbon earthquake, not only were the waters of European lakes thrown
-into a state of oscillation, but similar effects were produced in the
-great lakes of North America. In some instances, as in the case of
-small ponds, these movements may be produced by the horizontal backward
-and forward motion of the ground. At other times they are probably due
-to an actual tipping of a portion of their basins. Movements like these
-latter will be again referred to in the chapter on Earth Pulsations.
-On January 27, 1856, there was a shock of earthquake at Bailyborough,
-Ireland, which occasioned an adjacent lough to overflow its banks
-and rush into the town with great impetuosity. In returning it swept
-away two men, leaving behind a great quantity of pike and eels of a
-prodigious growth.[53]
-
-_Disturbances in rivers._—Just as lakes have been disturbed, so also
-have there been sudden disturbances in rivers. Sometimes these have
-overflowed their banks, whilst at other times they have been suddenly
-dried up. In certain cases the reason that a portion of a river should
-have become dry has been very apparent, as, for instance, at the time
-of the Zenkoji earthquake in Japan in 1847, when the Shikuma-gawa
-became partly dry in consequence of the large masses of earth which
-had been shaken down from overhanging cliffs damming a portion of
-its course, and thus forming, first, lakes, and subsequently, new
-water-courses. As another example, out of the many which might be
-quoted, may be mentioned the sudden drying up of the river Aboat,
-a tributary of the Magat, in the Philippine Islands, on July 27,
-1881, shortly after a severe shock of earthquake. The water of this
-river ceased to flow for two hours, after which it reappeared with
-considerable increase of volume and of a reddish colour. Signor E. A.
-Casariego, who describes this, remarks that the phenomenon could easily
-be explained through the slipping down of the steep banks in narrow
-parts of its upper valley, by which means its flow had been obstructed
-until the water had time to accumulate and pass over or demolish the
-obstruction.
-
-After the earthquake of Belluno (June 29, 1873), the torrent Tesa,
-which is ordinarily limpid, became very muddy.[54] Similar phenomena
-have been observed even in Britain, as, for instance, in 1787, when, at
-the time of a shock which was felt in Glasgow, there was a temporary
-stoppage in the waters of the Clyde. Again, in 1110, there was a
-dreadful earthquake at Shrewsbury and Nottingham, and the Trent became
-so low at Nottingham that people walked over it.
-
-The earthquake of 1158, which was felt in many parts of England, was
-accompanied by the drying up of the Thames, which was so low that it
-could be crossed on foot even at London.[55]
-
-Facts analogous to these are mentioned in the accounts of many large
-earthquakes. Sometimes rivers only come muddy or change their colour.
-In an account of the Lisbon earthquake we read that some of the rivers
-near Neufchâtel suddenly became muddy.[56]
-
-At other times large waves are formed. Thus the earthquake of Kansas
-(April 24, 1867) apparently created a disturbance in the rivers at
-Manhattan, which rolled in a heavy wave from the north to the south
-bank.[57]
-
-Sometimes curious phenomena have happened with regard to rivers without
-the occurrence of earthquakes. Thus, for instance, on November 27,
-1838, there was a simultaneous stoppage of the Teviot, Clyde, and Nith.
-
-In these rivers similar phenomena have been observed in previous years.
-
-Again, on January 1, 1755, there was a sudden sinking of the river
-Frooyd, near Pontypool. This appears to have been due to the water
-sinking into chasms which were suddenly opened.[58]
-
-_Effects produced in springs, wells, fumaroles, &c._—Springs also are
-often affected by earthquakes. Sometimes the character of their waters
-change; those which were pure become muddy, whilst those which were hot
-have their temperature altered.
-
-Sometimes springs have been dried up, whilst at other times new springs
-have been formed.
-
-This latter was the case in New England (October 27, 1727). In some
-places springs were formed, whilst at other places they were either
-entirely or partly dried up.[59]
-
-At and near Lisbon, in 1755, some fountains became muddy, others
-decreased, others increased, and others dried up. At Montreux, Aigle,
-and other places, springs became turbid.
-
-The baths at Toplitz, in Bohemia, which were discovered in
-A.D. 762, were seriously affected by the same earthquake.
-Previous to the earthquake it is said that they had always given a
-constant supply of hot water. At this time, however, the chief spring
-sent up vast quantities of water and ran over. One hour before this
-it had grown turbid and flowed muddy. After this it stopped for about
-one minute, but recommenced to flow with prodigious violence, driving
-before it considerable quantities of reddish ochre. Finally, it settled
-back to its original clear state and flowed as before.[60]
-
-In 1855, at the earthquake of Wallis, many new springs burst forth, and
-some of these in Nicolai Thale were so rich in iron that they quickly
-formed a deposit of ochre.
-
-At the time of the Belluno earthquake (June 29, 1873), a hot spring, La
-Vena d’Oro, suddenly became red.[61]
-
-The following examples of like changes are taken from the writings of
-Fuchs.[62]
-
-In 1738 the hot springs of St. Euphema rose considerably in their
-temperature.
-
-During the earthquake of October, 1848, the hot springs of Ardebil,
-which usually had a temperature of from 44° to 46° C., rose so high
-that their temperature was sufficient to cause scalding.
-
-At the time of the earthquake of Wallis, in 1855, the temperature of
-hot springs rose 7°, and the quantity of water increased three times.
-
-During the earthquake of 1835 in Chili, the springs of Cauquenes fell
-from 118° to 92° F. Subsequently, however, they again rose.
-
-Fumaroles are similarly disturbed. Thus, at the time of the earthquakes
-of Martinique (September, 1875), the fumaroles there showed an abnormal
-activity.[63]
-
-Wells often appear to be acted upon in the same manner as springs.
-
-At the time of the California earthquake (April, 1855), the level of
-the water in certain wells was raised ten to twelve feet.
-
-A consequence of the earthquake at Neufchâtel, in 1749, was to fill
-some of the wells with mud.[64] At Constantinople, on September 2,
-1754, wells became dry.[65]
-
-_Explanation of the above phenomena._—That the water in springs and
-wells should be caused to rise at the time of an earthquake, admits of
-explanation on the supposition of compressions taking place similar
-to those which cause the rise of water in fissures. That the water in
-wells and springs should be rendered turbid, is partly explained on the
-supposition of more or less dislocation taking place in the earthy or
-rocky cavities in which they are contained or through which they flow.
-
-At the time of a large earthquake it is extremely probable that there
-is a general disturbance in the lines of circulation of subterranean
-waters and gases throughout the shaken area. By these disturbances, new
-waters may be brought to the surface, two or more lines of circulation
-may be united, and the flow of a spring or supply of a well be
-augmented. Fissures, through which waters reached the surface, may be
-closed, wells may become dry, or springs may cease to flow, hot springs
-may have their temperature lowered by the additions of cold water
-from another source, and, in a similar manner, waters may be altered
-in their mineralisation. An important point to be remembered in this
-consideration is the mutual dependence of various underground water
-supplies, and the area over which any given supply may circulate. A
-well in the higher part of Lincoln Heath is said to be governed by the
-river Trent, which is ten miles distant; when the river rises the well
-rises in proportion, and when the river falls the water in the well
-falls.[66]
-
-The change which is usually observed in hot springs is, that before or
-with earthquakes they increase in temperature, but afterwards sink back
-to their normal state. This increase in temperature may possibly be due
-to communication being opened with new or deeper centres of volcanic
-activity, or a temporarily increased rate of flow.
-
-That the water issuing from newly formed fissures or springs should
-be hot, might be explained on the supposition of its arising from a
-considerable depth, or from some volcanic centre. It might also be
-attributed to the heat developed by friction at the opening of the
-fissures. These changes which earthquakes produce upon the underground
-circulation of waters are phenomena deserving especial attention.
-Although we know much about the circulation of surface water, it is but
-little that we yet know about the movement of the streams hidden from
-view, from which these surface waters have their sources. Earthquakes
-may be regarded as gigantic experiments on the circulatory system of
-the earth, which, if properly interpreted, may yield information of
-scientific and utilitarian value.
-
-The sudden elevations, depressions, or lateral shifting of large tracts
-of country at the time of destructive earthquakes are phenomena
-with which all students of geology are familiar. In most cases these
-displacements have been permanent; and evidences of many of the
-movements which occurred within the memory of man, remain as witnesses
-of the terrible convulsions with which they were accompanied.
-
-_Movements on coast lines and level tracts._—At the time of the
-great earthquake of Concepcion, on February 20, 1835, much of the
-neighbouring coast line was suddenly elevated four or five feet
-above sea level. This, however, subsequently sank until it was only
-two feet. A rocky flat, off the island of Santa Maria, was lifted
-above high-water mark, and left covered with ‘gaping and putrefying
-mussel-shells, still attached to the bed on which they had lived.’ The
-northern end of the island itself was raised ten feet and the southern
-extremity eight feet.[67]
-
-By the earthquake of 1839, the island of Lemus, in the Chonos
-Archipelago, was suddenly elevated eight feet.[68]
-
-Of movements like these, especially along the western shores of South
-America, Darwin, who paid so much attention to this subject, has given
-many examples. In 1822, the shore near Valparaiso was suddenly lifted
-up, and Darwin tells us that he heard it confidently asserted ‘that a
-sentinel on duty, immediately after the shock, saw a part of a fort
-which previously was not within the line of his vision, and this would
-indicate that the uplifting was not vertical.’[69] That the large areas
-of land should be shifted permanently in horizontal directions, as well
-as vertically, we should anticipate from the observations which we are
-able to make upon large fissures which are caused by earthquakes.
-
-Another remarkable example of sudden movement in the rocky crust is
-that which took place during the earthquakes of 1811–12 in the valley
-of the Mississippi, near to the mouth of the Ohio, which was convulsed
-to such a degree, that lakes, twenty miles in extent, were formed
-in the course of an hour. This country, which is called the ‘sunk
-country,’ extends some seventy to eighty miles north and south, and
-thirty miles east and west.[70]
-
-In the ‘Gentleman’s Magazine’ we read of the little territory of
-Causa Nova, in Calabria, being sunk twenty-nine feet into the earth
-by an earthquake, without throwing down a house. The inhabitants,
-being warned by a noise, escaped into the fields, and only five were
-killed.[71]
-
-Other examples of these permanent dislocations of strata are to be
-found in almost every text-book on geology.
-
-_Geological changes produced._—Passing over the accounts of earth
-movements which are more or less fictitious, and confining our
-attention to the well authenticated facts, we see at once the important
-part which earthquakes have played as agents working geological
-changes. Even in the nineteenth century long tracts of coast, as in
-Chili and New Zealand, have been raised, whilst other areas, like the
-Delta of the Indus, have been sunk. Sir H. Bartle Frere, speaking about
-the disturbance which took place in his latter region in 1819, remarks
-that all the canals drawn from the Fullalee River ceased to run for
-about three days, probably indicating a general upheaval of the lower
-part of the canal. In consequence of the earthquakes in former times
-it is not unlikely that water-courses have ceased to flow, water has
-decreased in wells, and districts have been depopulated.[72]
-
-Sometimes these changes have taken place gradually and sometimes with
-violence. Mountains have been toppled over, valleys have been filled,
-cities have been submerged or buried.
-
-With the records of these convulsions before us, we see that seismic
-energy yet exhibits a terrible activity in changing the features of the
-globe.
-
-_Reason of these movements._—To formulate a single reason for these
-catastrophes would be difficult. Where they are of the nature of
-landslips, or materials have been dislodged from mountain sides, the
-cause is evidently the sudden movement of this ground acting upon
-strata not held together in a sufficiently stable condition. A similar
-explanation may be given for the sudden elevations or depressions of
-strata in a district removed from the centre where the disturbance had
-its origin. The seismic effort exhibits itself in a certain area round
-its origin as a sudden push, and by this push, strata are fractured and
-caused to move relatively to each other.
-
-At or near to the origin of an earthquake it might be argued that it
-was the sudden falling of rocky strata towards a position of stable
-equilibrium that caused the shaking, and in such a case the movements
-referred to may be regarded as the cause rather than the effect of an
-earthquake.
-
-A subject closely connected with the sudden dislocation of strata,
-is the production of secondary or consequent earthquakes, due to the
-disturbance of ground in a critical state (see p. 248).
-
-
-
-
-                              CHAPTER IX.
-
-                      DISTURBANCES IN THE OCEAN.
-
-  Sea vibrations—Cause of vibratory blows—Sea waves: Preceding
-    earthquakes; Succeeding earthquakes—Magnitude of waves—Waves
-    as recorded in countries distant from the origin—Records on
-    tide gauges—Waves without earthquakes—Cause of waves—Phenomena
-    difficult of explanation—Velocity of propagation—Depth of the
-    ocean—Examples of calculations—Comparison of velocities of
-    earthquake waves with velocities which ought to exist from the
-    known depth of the ocean.
-
-
-_Sea vibrations._—Whilst residing in Japan I have had many
-opportunities of conversing with persons who had experienced
-earthquakes when on board ships, and it has often happened that these
-same earthquakes have been recorded on the shore. For example, at the
-time of every moderately severe earthquake which has shaken Yokohama,
-the same disturbance has been felt on board the ships lying in the
-adjoining harbour. In some cases the effect had been as if the ship
-was grounding; in others, as if a number of sharp jerks were being
-given to the cable. The effect produced upon a man-of-war lying in the
-Yokohama harbour on the evening of March 11, 1881, was described to me
-as a ‘violent irresistible shaking.’ Vessels eighty miles at sea have
-recorded and timed shocks which were felt like sudden blows. These were
-accompanied by a noise described as a ‘dull rattle like thunder.’
-
-In none of the cases here quoted was any disturbance of the water
-observed.
-
-The great earthquake of Lisbon was felt by vessels on the Atlantic,
-fifty miles away from shore.
-
-On February 10, 1716, the vessels in the harbour of New Pisco were so
-violently shaken that both ropes and masts were broken, and yet no
-motion in the water was observed. Some have described these shocks like
-those which would be produced by the sudden dropping of large masses of
-ballast in the hold of the vessel. Other cases are known where rigging
-was damaged, and even cannon have been jerked up and down from the
-decks on which they rested.
-
-_Cause of vibratory blows._—From the rattling sound which has
-accompanied some of these submarine shocks, many of which, it may be
-remarked, have never been recorded as earthquakes upon neighbouring
-shores, it does not seem improbable that they may have been the result
-of the sudden condensation of volumes of steam produced by submarine
-volcanic eruptions.
-
-As confirmatory of this supposition we have the fact, that many of
-the marine disturbances which might be called ‘sea-quakes,’ have been
-observed in places which are close to, or in the line of, volcanic
-vents. Thus, M. Daussy, who has paid special attention to this subject,
-has collected evidence to show that a large number of shocks have been
-felt by vessels in that portion of the Atlantic between Cape Palamas,
-on the west coast of Africa, and Cape St. Roque, on the east coast of
-South America.[73]
-
-Some of the vessels only felt shocks and tremblings, but others saw
-smoke, and some even collected floating ashes. In considering the
-submarine shocks of this particular area, we must bear in mind that it
-lies in the line of Iceland, the west coast of Scotland, the Azores,
-Canaries, Cape de Verd Islands, St. Helena, and other places, all of
-which, if not at present in volcanic activity, shew evidence of having
-been so within recent times. The connection between volcanic action and
-earthquakes will be again referred to.
-
-_Sea waves._—Although in the above-mentioned instances sea waves have
-not been noticed, it is by no means uncommon to find that destructive
-earthquakes have been accompanied by waves of an enormous size, which,
-if the earthquake has originated beneath the sea, have, subsequently
-to the shaking, rolled in upon the land, to create more devastation
-than the actual earthquake. It may, however, be mentioned that a few
-exceptional cases exist when it is said that the sea wave has preceded
-the earthquake, as, for example, at Smyrna, on September 8, 1852.
-
-Again, at the earthquake in St. Thomas, in 1868, it is said that the
-water receded shortly _before_ the first shock. When it returned, after
-the second shock, it was sufficient to throw the U.S. ship ‘Monagahela’
-high and dry.[74]
-
-Another American ship, the ‘Wateree,’ was also lost in 1868 by being
-swept a quarter of a mile inland by the sea wave which inundated
-Arequippa.
-
-Much of the great destruction which occurred at the time of the great
-Lisbon earthquake was due to a series of great sea waves, thirty to
-sixty feet higher than the highest tide, which swamped the town. These
-came in about an hour after the town had been shattered by the motion
-of the ground.
-
-The first motion in the waters was their withdrawal, which was
-sufficient to completely uncover the bar at the mouth of the Tagus.
-At Cadiz, the first wave, which was the greatest, is said to have been
-sixty feet in height. Fortunately the devastating effect which this
-would have produced was partially warded off by cliffs.
-
-At the time of the Jamaica earthquake (1692) the sea drew back for a
-distance of a mile.
-
-In South America sea waves are common accompaniments of large
-earthquakes, and they are regarded with more fear than the actual
-earthquakes.
-
-On October 28, 1724, Lima was destroyed, and on the evening of that day
-the sea rose in a wave eighty feet over Callao. Out of twenty-three
-ships in the harbour, nineteen were sunk, and four others were carried
-far inland. The first movement which is usually observed is a drawing
-back of the waters, and this is so well known to precede the inrush of
-large waves, that many of the inhabitants in South America have used it
-as a timely warning to escape towards the hills, and save themselves
-from the terrible reaction which, on more than one occasion, has so
-quickly followed.
-
-At Caldera, near to Copiapo, on May 9, 1877, which was the time when
-Iquique was devastated, the first motion which was observed in the sea
-was that it silently drew back for over 200 feet, after which it rose
-as a wave over five feet high. At some places the water came in as
-waves from twenty to eighty feet in height.
-
-At Talcahuano, on the coast of Chili, in 1835, there was a repetition
-of the phenomena which accompanied the destruction of Penco in 1730
-and 1751. About forty minutes after the first shock, the sea suddenly
-retired. Soon afterwards, however, it returned in a wave twenty feet
-high, the reflex of which swept everything towards the sea. These
-phenomena were repeated three times.[75]
-
-When Callao and Lima were destroyed, in 1746, the sea first drew back,
-then came in as waves, four or five minutes after the earthquake.
-Altogether, between October 28 and February 24, 451 shocks were
-counted. At one time the sea came in eighty feet above its usual
-level. One account says that the large waves came in forty-one and a
-half hours after the first shock, and seventeen and a half hours after
-comparative tranquillity had prevailed.[76]
-
-At the eruption of Monte Nuovo, near Naples, in September 27, 1538, the
-water drew back forty feet, so that the whole gulf of Baja became dry.
-
-In 1696, at the time of the Catanian earthquake, the sea is said to
-have gone back 2,000 fathoms. Instances are recorded where the sea has
-receded several miles.
-
-The time taken for the flowing back of the sea is usually very
-different. Sometimes it has only been five or six minutes, whilst at
-other times over half an hour, and there are records where the time is
-said to have been still longer.
-
-Thus, at the earthquake of Santa (June 17, 1678), the sea is stated to
-have gone as far back as the eye could reach, and did not rise again
-for twenty-four hours, when it flooded everything.
-
-In 1690 at Pisco the sea went back two miles, and did not return for
-three hours. When it returns it does so with violence, and examples of
-the heights to which it may reach have been given. The greatest sea
-wave yet recorded, according to Fuchs, is one which, on October 6,
-1737, broke on the coast of Lupatka, 210 feet in height.
-
-There are, however, cases known where the sea has returned as gradually
-as it went out. Thus, on December 4, 1854, when Acapulco was
-destroyed, the sea is said to have returned as gently as it went out.
-
-When sea waves have travelled long distances from their origin, as,
-for instance, whenever a South American wave crosses the Pacific to
-Japan, the phenomena which are observed are like those which were
-observed at Acapulco; the sea falls and rises, at intervals of from ten
-minutes to half an hour, to heights of from six to ten feet, without
-the slightest appearance of a wave. Its phenomenon is like that of an
-unusually high tide, which repeats itself several times per hour. Even
-if we watch distant rocks with a telescope, although the surface of the
-ocean may be as smooth as the surface of a mirror, there is not the
-slightest visible evidence of what is popularly called a wave. The sea
-being once set in motion it continues to move as waves of oscillation
-for a considerable time. In 1877, as observed in Japan, the motion
-continued for nearly a whole day. The period and amplitude of the rise
-and fall were variable, usually it quickly reached a maximum, and then
-died out gradually. As observed in a self-recording tide gauge at San
-Francisco, the disturbance lasted for about four days. A diagram of
-this is here given. In its general appearance it is very similar to the
-records of other earthquake waves. The large waves represent the usual
-six hours rise and fall of the tides; usually these are fairly smooth
-curves. Superimposed on the large waves are the smaller zigzag curves
-of the earthquake disturbance, lasting with greater or less intensity
-for several days. As these curves are drawn to scale—horizontally for
-hours, and vertically one fifth inch to the foot, to show the extent of
-the rise and fall—they will be easily understood.
-
-Sometimes, as in the present example, the first movement in the
-waters is that of an incoming wave. In many instances, however, this
-observation may be due to the slow and more gentle phenomena of the
-previous drawing out of the water, which in a steep waste, or when the
-water is rough, would be difficult to observe, not having been remarked.
-
-The distance to which these sea waves have extended has usually been
-exceedingly great.
-
-[Illustration: FIG. 28.—Record of Tide Gauge at Port Point, San
-Francisco. Showing Earthquake Waves of May 1877.]
-
-The sea wave of the Iquique earthquake of May 9, 1877, like many of its
-predecessors, was felt across the basin of the whole Pacific, from New
-Zealand in the south, to Japan and Kamschatka in the north. And but for
-the intervention of the Eurasian and American continents would have
-made itself appreciable over the surface of the whole of our globe. At
-places on the South American coast, it has been stated that the height
-of the waves varied from twenty to eighty feet. At the Samoa Islands
-the heights varied from six to twelve feet. In New Zealand the sea rose
-and fell from three to twenty feet. In Australia the heights to which
-the water oscillated were similar to those observed in New Zealand. In
-Japan it rose and fell from five to ten feet. In this latter country,
-the phenomena of sea waves which follow a destructive earthquake on the
-South American coast are so well known that old residents have written
-to the local papers announcing the probability of such occurrences
-having taken place some twenty-five hours previously in South America.
-In this way news of great calamities has been anticipated, details of
-which only arrived some weeks subsequently. Just as the destructive
-earthquakes of South America have announced themselves in Japan, in
-a like manner the destructive earthquakes of Japan have announced
-themselves upon the tide gauges of California. Similarly, but not so
-frequently, disturbances shake the other oceans of the world.
-
-For example, the great earthquake of Lisbon propagated waves to the
-coasts of America, taking on their journey nine and a half hours.
-
-_Sea waves without earthquakes._—Sometimes we get great sea waves
-like abnormal tides occurring without any account of contemporaneous
-earthquakes. Although earthquakes have not been recorded, these ill
-understood phenomena are usually attributed to such movements.
-
-Several examples of these are given by Mallet. Thus, at 10 A.M. on
-March 2, 1856, the sea rose and fell for a considerable distance
-at many places on the coast of Yorkshire. At Whitby, the tide was
-described ebbing and flowing six times per hour, and this to such a
-distance that a vessel entering the harbour was alternately afloat and
-aground.
-
-In 1761, on July 17, a similar phenomena was observed at the same place.
-
-A like occurrence took place at Kilmore, in the county of Wexford, on
-September 16, 1864, when the water ebbed and flowed seven times in the
-course of two hours and a half. These tides, which appear to have taken
-about five minutes to rise and five minutes to fall, were seen by an
-observer approaching from the west as six distinct ridges of water. The
-general character of the phenomena appears to have been very similar to
-that which was produced at the same place by the Lisbon earthquake of
-1755; and the opinion of those who saw and wrote about their occurrence
-was that it was due to an earthquake disturbance. Such phenomena are
-not uncommon on the Wexford coast, where they are popularly known as
-‘death waves,’ probably in consequence of the lives which have been
-lost by these sudden inundations.
-
-They have also been observed in other parts of Ireland, the north-east
-coast of England, and in many parts of the globe. They will be again
-referred to under the head of earth pulsations.
-
-_Cause of sea waves._—Mallet, who in his report to the British
-Association in 1858, writes upon this last-mentioned occurrence at
-considerable length, whilst admitting that many may have originated
-from earthquakes, he thinks it scarcely probable that an earthquake
-blow, sufficiently powerful to have produced waves like those observed
-at Kilmore, should not have been felt generally throughout the south of
-Ireland. He, therefore, suggests that sometimes waves like the above
-might be produced by an underwater slippage of the material forming
-the face of a submarine bank, the slope of which by degradation and
-deposition, produced by currents, had reached an angle beyond the
-limits of repose of the material of which it was formed. Mallet does
-not insist upon the existence of these submarine landslips, but only
-suggests their existence as a means of explaining certain abnormal sea
-waves which do not appear to have been accompanied by earthquakes.
-
-In the generality of cases sea waves are accompanied by earthquakes,
-but it may often happen that the connection between the two is
-difficult to clearly establish. One simple explanation for the origin
-of waves occurring with earthquakes, is, that in consequence of the
-earthquake a large volume of water suddenly finds its way into cavities
-which have been opened, the disturbance produced by the inrush giving
-rise to waves.
-
-A second explanation is, that the land along a shore is caused by an
-earthquake to oscillate upwards, the water running off to regain its
-level. A supposition like this is negatived by the fact that these
-disturbances are felt far away from the chief disturbance, on small
-islands. Also, it may be added, that the whole disturbance appears to
-approach the land from the sea, and not in the opposite direction.
-Thus, in the earthquake of Oahu (February 18, 1871), it was remarked
-that the shock was first felt by the ships farthest from the land.[77]
-
-Another suggestion is that the waves are due to a sudden heaving up
-of the bottom of the ocean. If this lifting took place slowly, then
-the first result would be that the water situated over the centres of
-disturbance would flow away radially in all directions from above the
-area of disturbance.
-
-If, however, the submarine upheaval took place with great rapidity, say
-by the sudden evolution of a large volume of steam developed by the
-entry of water into a volcanic vent, as the water was heaped above the
-disturbed area, water might run in radially towards this spot.
-
-Supposing a primary wave to be formed in the ocean by any such causes,
-then the falling of this will cause a second wave to be formed,
-existing as a ring round the first one. The combined action of the
-first and second wave will form a third one, and so the disturbance,
-starting from a point, will radiate in broadening circles. During the
-up and down motion of these waves, the energy which is imparted to any
-particle of water will, on account of the work which it has to do in
-displacing its neighbours, by frictional resistance, gradually grow
-less and less, until it finally dies away. The waves which are the
-result of this motion will also grow less and less.
-
-If a series of sea waves were produced by a single disturbance, we see
-that these will be of unequal magnitude. Now, for small waves, the
-velocity with which they travel depends upon the square root of their
-lengths; but with large waves, like earthquake waves, the velocity
-depends upon the square root of the depth of water, and these latter
-travel more quickly than the former.
-
-If, therefore, we have a series of disturbances of unequal magnitude
-producing sea waves, which, from the series of shocks which have
-been felt upon shores subsequently invaded by waves, seems in all
-probability often to have been the case, it is not unlikely that the
-waves of an early disturbance may be overtaken and interfered with by a
-series which followed.
-
-These considerations help us to understand the appearance of the
-records on our tide gauges, and also the phenomena observed by those
-who have recorded tidal waves as they swept inwards upon the land. For
-instance, we understand the reason why sea waves, as observed at places
-at different distances from the origin of a disturbance, should be of
-different heights. We also see an explanation for the fact that small
-waves should sometimes appear to be interpolated between large ones,
-and that these should occur at varying intervals.
-
-The fact that whenever a wave is produced, a certain quantity of water
-must be drawn from the level which surrounds it, in order that it
-should be formed, explains the phenomena that the sea is often observed
-first to draw back. Out in the open ocean it is drawn from the hollow
-between two waves. As has been pointed out by Darwin, it is like the
-drawing of the water from the shore of a river by a passing steamer.
-
-The difference in the height of waves, as observed at places lying
-close to each other, is probably due to the configuration of the coast,
-the interference of outlying islands, reefs, &c.—causes which would
-produce similar effects in the height of tide.
-
-As a wave approaches shallow water it gradually increases in height,
-its front slope becomes steep, and its rear slope gentle, until finally
-it topples over and breaks. This increasing in height of waves is
-no doubt connected with the destruction of Talcahuano and Callao,
-which are situated at the head of shallow bays. Valparaiso, which is
-on the edge of deep water, has never been overwhelmed.[78] Another
-case tending to produce anomalies in the character of waves would be
-their reflection and mutual interference, the reflections due to the
-configuration of the ocean bed and coast lines.
-
-The complete phenomena which may accompany a violent submarine
-disturbance are as follows:—
-
-By the initial impulse of explosion or lifting of the ground, a ‘great
-sea wave’ is generated, which travels shorewards with a velocity
-dependent upon its size and the depth of the ocean. At the same
-instant, a ‘sound wave’ may be produced in the air, which travels at a
-quicker rate than the ‘great sea wave.’ A third wave which is produced,
-is an ‘earth wave,’ which will reach the shore with a velocity
-dependent on the intensity of the impulse and the elasticity of the
-rocks through which it is propagated. This latter, which travels the
-fastest, may carry on its back a small ‘forced sea wave.’ On reaching
-the shore and passing inland, this ‘earth wave’ will cause a slight
-recession of the water as the ‘forced sea wave’ slips from its back.
-
-As these ‘forced sea waves’ travel they will give blows to ships
-beneath which they may pass, being transmitted from the bottom of the
-ocean to the bottom of the ships like sound waves in water. At the
-time of small earthquakes, produced, for example, by the explosion of
-small quantities of water entering volcanic fissures, or by the sudden
-condensation of steam from such a fissure entering the ocean, aqueous
-sound waves are produced, which cause the rattling and vibrating jars
-so often noticed on board ships.
-
-_Phenomena difficult of explanation._—Although we can in this way
-explain the origin and phenomena of sea waves, we must remember, as
-Kluge has pointed out, that it is not the simple backward and forward
-movement of the ground which produces sea waves, and that the majority
-of earthquakes which have occurred in volcanic coasts have been
-unaccompanied by such phenomena. Out of 15,000 earthquakes observed on
-coast lines, only 124 were accompanied by sea waves.[79] Out of 1,098
-earthquakes catalogued by Perrey for the west coast of South America,
-only nineteen are said to have been accompanied by movements in the
-waters. According to the ‘Geographical Magazine’ (August 1877, p. 207),
-it would seem that out of seventy-one severe earthquakes which have
-occurred since the year 1500 upon the South American coast many have
-been accompanied by sea waves. Darwin also remarks, when speaking of
-South America, that almost every large earthquake has been accompanied
-by considerable agitation in the neighbouring sea.[80]
-
-On April 2, 1851, when many towns in Chili were destroyed, the sea was
-not disturbed. At the time of the great earthquake of New Zealand (June
-23, 1855), although all the shocks came from the sea, yet there was no
-flood. The small shock of February 14, however, was accompanied by a
-motion in the sea.
-
-To these examples, which have been chiefly drawn from the writings of
-Fuchs, must be added the fact that the greater number of disturbances
-which are felt in the north-eastern part of Japan, although they
-emanate from beneath the sea, do not produce any visible sea waves.
-They are, however, sufficient to cause a vibratory motion on board
-ships situated near their origin.
-
-Another point referred to by Fuchs, as difficult of explanation,
-is, that the water, when it draws back, often does so with extreme
-slowness, and farther, in some instances, it has not returned to its
-original level. That the sea might be drawn back for a period of
-fifteen or thirty minutes is intelligible, when we consider the great
-length of the waves which are formed. Cases where it has retired for
-several hours or days, and when its original level is altered, appear
-only to be explicable on the assumption of more or less permanent
-changes in the levels of the ground. For example, in the earthquake
-of 1855 which shook New Zealand, the whole southern portion of the
-northern island was raised several feet.
-
-These sudden alterations in the levels of coast lines have already
-been referred to.
-
-Other points which are difficult to understand are the occurrence
-of disturbances in the sea at the time of feeble earthquakes, and
-with earthquakes occurring in distant places. As examples of such
-occurrences, Fuchs quotes the following: ‘On May 16, 1850, at 4.28
-A.M., an earthquake took place in Pesth, and at 7.30 a motion was
-observed in the sea at Livorno. Again, at the time of the earthquake
-of December 19, 1850, which shook Heliopolis, a flood suddenly came
-in upon Cherbourg.’ May not these phenomena be the result of an earth
-pulsation, which produced an earthquake at one point, and a sea wave at
-another?
-
-Equally difficult to understand are the observations when the
-disturbance in the sea has occurred several hours after an earthquake;
-as, for instance, at Batavia, in 1852, when there was an interval of
-two hours; and to this must be added the observations where the motion
-of the sea has preceded that of the earthquake—as, for instance, in
-1852, at Smyrna. Whilst recognising the fact that it is possible to
-suggest explanations for many of these anomalies, we must also bear
-in mind that they are, generally speaking, exceptional, and, in some
-instances, may possibly be due to errors in observations.
-
-_Velocity of propagation of sea waves, and depth of the ocean._—It
-has long been known to physical science that the velocity with which
-a given wave is propagated along a trough of uniform depth, holds a
-relation to the depth of the trough.
-
-If _v_ is the velocity of the wave, and _h_ the depth of the trough,
-this relation may be expressed as follows:—
-
-                           _v_^2          (_v_)
-                     _h_ = ————— or _h_ = (———)^2
-                            _g_           (_k_)
-
-                  Where _g_ = 32·19 and _k_ = 5·671.
-
-It will be observed that these two formulæ (the first of which is
-known as Russell’s formula, and the second as Airy’s) are practically
-identical.
-
-The apparent difference is in the average value assigned to the
-constant.
-
-For large waves such as we have to deal with, it would be necessary,
-if we were desirous of great accuracy, to increase the value of _h_
-by some small fraction of itself. We might also make allowance for
-the different values of _g_, according to our position on the earth’s
-surface. With these formulæ at our disposal it is an easy matter, after
-having determined the velocity with which a wave was propagated, to
-determine the average depth of the area over which it was transmitted.
-
-In making certain earthquake investigations the reverse problem is
-sometimes useful—namely, determining the velocity with which a sea wave
-has advanced upon a shown line, from a knowledge of the depth of the
-water in which it has been propagated.
-
-Calculations of the average depths of the Pacific, dependent on the
-velocity with which earthquake waves have been propagated, have been
-made by many investigators.
-
-In most cases, however, in consequence of having assumed the wave to
-have originated on a coast line, when the evidence clearly showed it
-to have originated some distance out at sea, the calculations which
-have been made are open to criticism. The average depths which I
-obtained for various lines across the Pacific appear to be somewhat
-less than the average depth as given by actual soundings. We must,
-however, remember that the common error in actual soundings is that
-they are usually too great, it being difficult in deep-sea sounding
-to determine when the lead actually reaches the bottom. Until oceans
-have been more thoroughly surveyed with the improved forms of sounding
-apparatus, we shall not be able to verify the truth of the results
-which have been given to us by earthquake waves.
-
-
-                EXAMPLES OF CALCULATIONS ON SEA WAVES.
-
-1. _The wave of 1854._—This wave originated near Japan, and it was
-recorded on tide gauges at San Francisco, San Diego, and Astoria.
-
-On December 23, at 9.15. A.M., a strong shock was felt at Simoda in
-Japan, which, at 10 o’clock, was followed by a large wave thirty feet
-in height. The rising and falling of the water continued until noon.
-Half an hour after, the movement became more violent than before. At
-2.15 P.M. this agitation decreased, and at 3 P.M. it was comparatively
-slow. Altogether there were five large waves.
-
-On December 23 and 25, unusual waves were recorded upon the
-self-registering tide gauges at San Francisco, San Diego, and Astoria.
-
-At San Francisco three sets of waves were observed. The average time of
-oscillation of one of the first set was thirty-five minutes, whilst one
-of the second and third sets was almost thirty-one minutes.
-
-At San Diego three series of waves were also shown, but with average
-times of oscillation of from four to two minutes shorter than the waves
-at San Francisco.
-
-The San Francisco waves appear to indicate a recurrence of the same
-phenomena.
-
-The record at San Diego shows what was probably the effect of a
-series of impulses, the heights increasing to the third wave, then
-diminishing, then once more renewed, after which it died away.
-
-The result of calculations based on these data were:—
-
- +---------------+------------+------------+--------+--------------+
- |               |  Distance  |    Time    |Velocity|   Depth of   |
- |               |geographical|     of     |in feet |   ocean in   |
- |               |   miles    |transmission|per sec.|   fathoms    |
- +---- ----------+------------+------------+--------+--------------+
- |               |            |  h.  m.    |        |              |
- | Simoda to San |    4917    |  12  13    |  545   |     2100     |
- |   Diego       |            |            |        |              |
- | Simoda to San |    4527    |  12  39    |  528   | 2500 or 2230 |
- |   Francisco   |            |            |        |              |
- +---------------+------------+------------+--------+--------------+
-
-The difference for the depths in the San Francisco path depends whether
-the length of the waves is reckoned at 210 or 217 miles. The length of
-the waves on the San Diego path were 186 or 192 miles.[81]
-
-_The wave of 1868._—On August 11, 1868, a sea wave ruined many cities
-on the South American coast, and 25,000 lives were lost. This wave,
-like all the others, travelled the length and breadth of the Pacific.
-
-In Japan, at Hakodate, it was observed by Captain T. Blakiston, R.A.,
-who very kindly gave me the following account:
-
-On August 15, at 10.30 A.M., a series of bores or tidal waves
-commenced, and lasted until 3 P.M. In ten minutes there was a
-difference in the sea level of ten feet, the water rising above high
-water and falling below low water mark with great rapidity. The
-ordinary tide is only two and a half to three feet. The disturbance
-producing these waves originated between Iquique and Arica, in about
-lat. 18.28 S. at about 5 P.M. on August 13. In Greenwich time this
-would be about 13h. 9m. 40s. August 13. The arrival of the wave at
-Hakodate in Greenwich time would be about 14h. 7m. 6s. August 14:
-that is to say, the wave took about 24h. 57m. to travel about 8,700
-miles, which gives us an average rate of about 511 feet per second.
-These waves were felt all over the Pacific. At the Chatham Islands they
-rushed in with such violence that whole settlements were destroyed. At
-the Sandwich Islands the sea oscillated at intervals of ten minutes for
-three days.
-
-Comparing this wave with the one of 1877 we see that one reached
-Hakodate with a velocity of 511 feet per second, whilst the other
-travelled the same distance at 512 feet per second.
-
-An account of this earthquake wave has been given by F. von Hochstetter
-(‘Über das Erdbeben in Peru am 13. August 1868 und die dadurch
-veranlassten Fluthwellen im Pacifischen Ocean,’ Sitzungsberichte
-der Kaiserl. Akademie der Wissenschaften, Wien 58. Bd., 2. Abth.
-1868). From an epitome of this paper given in ‘Petermann’s Geograph.
-Mittheil.’ 1869, p. 222, I have drawn up the following table of the
-more important results obtained by F. von Hochstetter.
-
-The wave is assumed to have originated near Arica.
-
- +----------------+------------+----------+----------+----------+
- |                |  Distance  |   Time   | Velocity | Depth of |
- |                |  sea miles | taken by | in feet  | ocean in |
- |                | from Arica |   wave   |per second|   feet   |
- +----------------+------------+----------+----------+----------+
- |                |            |  h.  m.  |          |          |
- |Valdivia        |   1,420    |   5   0  |    479   |   7,140  |
- |Chatham Islands |   5,520    |  15  19  |    608   |  11,472  |
- |Lyttleton       |   6,120    |  19  18  |    533   |   8,838  |
- |Newcastle       |   7,380    |  22  28  |    538   |   9,006  |
- |Apia (Samoa)    |   5,760    |  16   2  |    604   |  11,346  |
- |Rapa            |   4,057    |  11  11  |    611   |  11,598  |
- |Hilo            |   5,400    |  14  25  |    555   |   9,568  |
- |Honolulu        |   5,580    |  12  37  |    746   |  17,292  |
- +----------------+------------+----------+----------+----------+
-
-Calculations on the same disturbance are also given by J. E.
-Hilgard.[82]
-
-Assuming the origin of the wave to have been at Arica, his results are
-as follows:
-
- +----------------+------------+------------+----------+----------+
- |                |            |            | Nautical |          |
- |                |  Distance  |  Time of   |   miles  |Mean depth|
- |                |from Africa |transmission| per hour | of ocean |
- +----------------+------------+------------+----------+----------+
- |                |   miles    |   h.  m.   |          |   feet   |
- |San Diego       |   4,030    |   10  55   |    369   |  12,100  |
- |Fort Point      |   4,480    |   12  56   |    348   |  10,800  |
- |Astoria         |   5,000    |   18  51   |    265   |   6,200  |
- |Kodiak          |   6,200    |   22  00   |    282   |   7,000  |
- |Rapa            |   4,057    |   10  54   |    372   |  12,200  |
- |Chatham Islands |   5,520    |   15  01   |    368   |  12,100  |
- |Hawaii          |   5,460    |   14  10   |    385   |  13,200  |
- |Honolulu        |   5,580    |   12  18   |    454   |  18,500  |
- |Samoa           |   5,760    |   15  38   |    368   |  12,100  |
- |Lyttelton       |   6,120    |   19  01   |    322   |   9,200  |
- |Newcastle       |   9,380    |   22  10   |    332   |   9,800  |
- |Sydney          |   7,440    |   23  41   |    314   |   8,800  |
- +----------------+------------+------------+----------+----------+
-
-_The wave of 1877._—Two sets of calculations have been made upon the
-wave of 1877 by Dr. E. Geinitz of Rostock.[83]
-
-The following table is taken from Dr. Geinitz’s second paper, in which
-there are several modifications of his first results. The origin of the
-disturbance is assumed to have been near Iquique.
-
- +---------------------+--------+----------+-------+--------+--------+
- |                     |Distance|          |       |Velocity|  Mean  |
- |     Observation     |  from  | Arrival  | Time  |in feet |depth of|
- |      stations       |Iquique | of wave  | taken |  per   | ocean  |
- |                     | geol.  |          |by wave|second  |  in    |
- |                     | miles  |          |       |        |fathoms |
- +---------------------+--------+----------+-------+--------+--------+
- |                     |        | h. m.    | h. m. |        |        |
- |Taiohāc (Marquesa    |        |          |       |        |        |
- |  Islands)           |  4,086 | 8 40 A.M.| 12 15 |  563·8 |  1,647 |
- |Apia (Samoa)         |  5,740 |12  0  „  | 15 30 |  610·4 |  1,930 |
- |Hilo (Sandwich       |  5,526 |10 24  „  | 14  0 |  667·9 |  2,310 |
- |  Islands)           |        |          |       |        |        |
- |Kahuliu     „        |  5,628 |10 30  „  | 14  5 |  675·2 |  2,361 |
- |Honolulu    „        |  5,712 |10 50  „  | 14 25 |  669·7 |  2,319 |
- |Wellington (New      |  5,657 | 2 40 P.M.| 18 15 |  524·2 |  1,430 |
- |  Zealand)           |        |          |       |        |        |
- |Lyttelton    „       |  5,641 | 2 48  „  | 18 23 |  519·8 |  1,400 |
- |Newcastle (Australia)|  6,800 | 2 32  „  | 18  7 |  633·0 |  2,075 |
- |Sydney         „     |  6,782 | 2 35  „  | 18 10 |  631·4 |  2,065 |
- |Kamieshi (Japan)     |  8,790 | 7 20  „  | 22 55 |  649·0 |  2,182 |
- |Hakodate    „        |  8,760 | 9 25  „  | 25  0 |  592·5 |  1,818 |
- |Kadsusa     „        |  8,939 | 9 50  „  | 25 15 |  604·9 |  1,895 |
- +---------------------+--------+----------+-------+--------+--------+
-
-The mean depths represent a mean of two sets of calculations, one
-made with the aid of Airy’s formula, and the other by Scott-Russell’s
-formula. The result of my own investigation about this disturbance, the
-origin of which, by several methods of calculation, is shown to have
-been beneath the ocean, near 71° 5′ west long., and 21° 22′ south lat.,
-are given on next page.
-
-Dr. Geinitz considers that his calculated depths of the ocean and those
-obtained by actual soundings are in accordance, a result which is
-diametrically opposed to that which I have obtained.
-
-This difference between my calculations and those of Dr. Geinitz,
-Hochstetter, and others, chiefly rests on the origin we have assigned
-for the sea waves. Dr. Geinitz, for instance, although he says that
-the origin of the 1877 earthquake was not on the continent but to the
-west in the ocean, bases all his calculations on the assumption that
-the _centrum_ was at or near to Iquique, and the time at which that
-city was disturbed was the time at which the waves commenced to spread
-across the ocean. This time is 8.25 P.M. At this time, however, it
-appears that the waves must have been more than double the distance
-between the true origin and Iquique, from Iquique on their way towards
-the opposite side of the Pacific. Introducing this element into the
-various calculations which have been made respecting the depth of the
-Pacific Ocean as derived from observations on earthquake waves—which
-element, insomuch as the waves appear to have come in to inundate the
-land some time after the shock, needs to be introduced—we reduce the
-velocity of transit of the earthquake wave and, consequently, the
-resultant depths of the ocean.
-
- Key:
-   A Distance from the origin in miles (calculated in great circles)
-   B Velocity in feet per second
- +-----------+---------+---------+-----+-----+---+-------+--------+---------+
- |           |         | Arrival |Time |     |   |Depth  |        |Interval |
- |           |         | of wave |taken|     |   |of the | Height | between |
- |           |Longitude|   in    | by  | A   | B |ocean  |  of    |waves in |
- |           |         |Greenwich|wave |     |   |in feet| waves  | minutes |
- |           |         |mean time|     |     |   |       |        |         |
- +-----------+---------+---------+-----+-----+---+-------+--------+---------+
- |           |  °  ′   |day h. m.|h. m.|     |   |       |        |         |
- |Origin     |         |         |     |     |   |       |        |         |
- |  of wave  | 71  5 W.| 9 12  59|     |     |   |       |        |         |
- |San        |         |               |     |   |       |        |         |
- |  Francisco|122 32   |10  2  28|13 29|4,578|498| 7,721 |  9 in. |         |
- |Callao     | 77 15   | 9 17   9| 4 10|  658|231| 1,657 |        |         |
- |Iquique    | 70 14½  | 9 13  21| 0 22|   87|348| 3,770 | 20 ft. |22       |
- |Cobija     | 70 21   | 9 13  19| 0 20|   80|352| 3,857 | 30 „   |         |
- |Mejillones | 70 35   | 9 13  27| 0 28|  108|339| 3,587 | 35 „   |15 or 45 |
- |Chanaral   | 71 34   | 9 15  26| 2 27|  455|272| 2,309 |        |10       |
- |Coquimbo   | 71 24   | 9 15  15| 2 16|  508|328| 3,363 |        |30       |
- |Valparaiso | 71 38   | 9 16  16| 3 17|  695|310| 3,000 |        |         |
- |Concepcion | 73  5   | 9 16  52| 3 53|  928|350| 3,824 |        |12 to 15 |
- |Honolulu   |157 55   |10  3  52|14 53|5,694|561| 9,807 |34 to   |25       |
- |           |         |         |     |     |   |       |  54 ft.|         |
- |Hilo       |155  3   |10  3   5|14  6|5,506|563|10,217 |30 or   |{3 or 15 |
- |           |         |         |     |     |   |       |   8  „ |{18 or 27|
- |Kahuliu    |156 43   |10  3  12|14 13|5,611|579|10,437 |        |         |
- |Samoa      |171 41 W.|10  3  57|14 58|5,773|566| 9,972 | 12 ft. |10       |
- |Taurauga   |176 11 E.|10  8  15|19 16|5,615|427| 5,697 |        |         |
- |Wellington |174 30   |10  7  22|18 23|5,574|445| 6,168 | 11 „   |10       |
- |Akaroa     |172 59   |10  7  28|18 29|5,542|440| 6,031 |        |         |
- |Lyttelton  |172 45   |10  7  29|18 30|5,558|441| 6,055 |        |         |
- |Kameishi   |140 50   |10 12  37|23 38|8,844|549| 9,378 |  6 „   |15       |
- |Hakodate   |140 50   |10 14   7|25  8|8,778|512| 8,169 |  7 „   |20       |
- +-----------+---------+---------+-----+-----+---+-------+--------+---------+
-
-In Dr. Geinitz’s paper there are also some slight differences in the
-times at which the earthquake phenomena were observed at various
-localities. These, however, are but of minor importance. At the end
-of the paper by Dr. Geinitz two interesting tide gauge records are
-introduced, one from Sydney and the other from Newcastle. These appear
-to show a marked difference in the periods of the sea waves at these
-two places.[84]
-
-_Comparison of velocities of wave-transit which have been actually
-observed, with velocities which ought to exist from what we know of
-the depth of the Pacific by actual soundings._—From a chart given in
-‘Petermann’s Geograph. Mittheilungen,’ Band xxiii. p. 164, 1877, it is
-possible to draw approximate sections on lines in various directions
-across the bed of the Pacific.
-
-From the origin of the shock to Japan (Kameishi) the line would be as
-follows:—
-
-  about 7,441 miles       15,000  feet deep
-        1,100   „         18,000      „
-          160   „         27,000      „
-           80   „         12,000      „
-           60   „          6,000      „
-
-On account of the Tuscarora and Belkap Deeps this would be the most
-irregular line over which the wave had to travel.
-
-From the origin to New Zealand (Wellington) the line would be
-
-  about 5,274 miles       15,000 feet deep.
-    „     300  „          12,000   „
-
-From the origin to Samoa the line would be
-
-  about 5,773 miles       15,000 feet deep.
-
-From the origin to the Sandwich Islands (Honolulu) the line would be
-
-  almost 6,634 miles      15,000 feet deep
-  and       60   „        12,000     „
-
-By Scott-Russell’s rule, or, what is almost identically the same, by
-Airy’s general formula, we can calculate how long it would take such
-waves as we have been speaking about to travel over the different
-portions of each of these lines, and by adding these times together we
-obtain the time taken to travel across any one line. I have made these
-calculations, but as I get in every case answers which are too small, I
-think it unnecessary to give them.
-
-The actual times taken to travel the distances just referred to were,
-
-  To Japan (Kameishi)               23 hr. 38 min.
-   „ New Zealand (Wellington)       18  „  23  „
-   „ Samoa                          14  „  58  „
-   „ Sandwich Islands (Honolulu)    14  „  53  „
-
-From San Francisco to Simoda the line is almost 3,567 miles, 3,000
-fathoms deep, 840 miles, 2,500 fathoms deep, and 120 miles 1,000
-fathoms deep. This gives an average depth of about 2,854 fathoms. Bache
-calculated the depth at 2,500 fathoms.
-
-If we are to consider that, because the sea wave at Simoda came in
-some time after the land shock had been felt, the origin of this
-earthquake, instead of being at Simoda, was some distance out at sea,
-this calculated depth would be reduced.
-
-
-
-
-                              CHAPTER X.
-
-                 DETERMINATION OF EARTHQUAKE ORIGINS.
-
-  Approximate determination of an Origin—Earthquake-hunting
-    in Japan—Determinations by direction of motion—Direction
-    indicated by destruction of buildings—Direction determined by
-    rotation—Cause of rotation—The use of time observations—Errors
-    in such observations—Origin determined by the method of
-    straight lines—The method of circles, the method of hyperbolas,
-    the method of co-ordinates—Haughton’s method—Difference in time
-    between sound, earth, and water waves—Method of Seebach.
-
-
-One of the most practical problems which can be suggested to the
-seismologist is the determination of the district or districts in any
-given country from which earthquake disturbances originate. With a
-map of a country before us, shaded with tints of different intensity
-to indicate the relative frequency of seismic disturbance in various
-districts, we at once see the localities where we might dwell with
-the least disturbance, and those we should seek if we wish to make
-observational seismology a study. Before erecting observatories for
-the systematic investigation of earthquakes in a country, it would
-be necessary for us, in some way or other, to examine the proposed
-country to find out the most suitable district. The special problem
-of determining _approximately_ the origin or origins of a set of
-earthquakes would be given to us. Having made this preliminary
-investigation, the next point is, by means of observatories so
-arranged that they could always work in conjunction with each other,
-to determine the origins more accurately. By knowing the origin from
-which a set of shocks spring we know the general direction in which
-we may expect the most violent disturbances, and we can arrange our
-seismometers accordingly.
-
-_Approximate determination of origins._—In 1880 I obtained a tolerably
-fair idea of the distribution of seismic energy throughout Japan, by
-compiling the facts obtained from some hundreds of communications
-received from various parts of the country respecting the number of
-earthquakes that had been felt.
-
-The communications were replies to letters sent to various residents
-in the country and to a large number of public officers. By taking
-these records, in conjunction with the records made by instruments, it
-was ascertained that in Japan alone there were certainly 1,200 shocks
-felt during the year, that is to say, three or four shocks per day.
-The greater number of these shocks were felt along the eastern coast,
-commencing at Tokio, in the south, and going northwards to the end of
-the main island. These shocks were seldom felt on the west coast. It
-appeared as if the central range of mountains formed a barrier to their
-progress. Similarly, ranges of mountains to the south-west of Tokio
-prevented the shocks from travelling southwards. Proceeding in this way
-the conclusion was arrived at that the west coast, the southern part
-of Japan, and the islands of Shikoku and Kiushiu, had their own local
-earthquakes.
-
-_Earthquake-hunting._—These preliminary enquiries having shown that
-the northern part of Japan was a better district for seismological
-observations than the southern half, the next step was to subject the
-northern half to a closer analysis. This analysis was commenced by
-sending to all the important towns, from thirty to one hundred miles
-distant from Tokio, bundles of postcards. These were entrusted to the
-local government offices with a request that each week one of these
-cards would be returned to Tokio stating the number of shocks felt.
-In this way it was quickly discovered that the majority of shakings
-emanated from the north and east, and seldom, if ever, passed a heavy
-range of mountains to the south. The barricade of postcards was then
-extended farther northwards, with the result of surrounding the origin
-of certain shocks amongst the mountains, whilst others were traced to
-the sea shore. By systematically pursuing earthquakes it was seen that
-many shocks had their origin beneath the sea—they shook all the places
-on the north-east coast, but it was seldom that they crossed through
-the mountains, forming the backbone of the island, to disturb the
-places on the west coast.
-
-The actual results obtained in three months by this method of working
-are shown in the accompanying map, which embraces the northern half of
-the main island of Nipon and part of Yezo. The shaded portion of the
-map indicates the mountainous districts, which are traversed by ranges
-varying in height from about 2,000 and 7,000 feet. The dotted lines
-show the boundaries of the more important groups of earthquakes which
-were recorded.
-
-I. is the western boundary of earthquakes, which at places to the
-eastward are usually felt somewhat severely. Some of these have been
-felt the most severely at or near Hakodadi, whilst farther south their
-effects have been weak. Occasionally the greatest effect has been near
-to Kameishi. Sometimes these earthquakes terminate along the western
-boundaries of III. or IV., not being able to pass the high range of
-mountains which separate the plain of Musashi from Kofu.
-
-[Illustration: FIG. 29.—Northern Japan. Mountainous districts shaded
-with oblique lines.]
-
-II. is the boundary of a shock confined to the plain which surrounds
-Kofu. These earthquakes are evidently quite local. Many of the
-disturbances have evidently originated beneath the ocean, having come
-in upon the land in the direction of the arrows A or B.
-
-III. This line indicates the boundary of a group of shocks which are
-often experienced in Tokio. These may come in the directions D, E,
-or F. It is probable that some of them originate to the eastward of
-Yokohama, on or near to the opposite peninsula.
-
-IV. V. and VI. The earthquakes bounded by these lines probably
-originate in the directions C or D.
-
-VII. The earthquakes bounded by this line probably come from the
-direction E.
-
-VIII. This line gives us the boundary of earthquakes which may come
-from the direction B.
-
-The above boundaries sometimes do not extend so far to the westward as
-they are shown. At other times, groups like V. and VI. extend farther
-to the south-west. These earthquake boundaries, which so clearly show
-the effects of high mountains in preventing the extension of motion,
-have been drawn up, not from single earthquakes, but from a large
-series of earthquakes which have been plotted upon blank maps, and are
-now bound together to form an atlas. To give an idea of the material
-upon which I have been working, I may state that between March 1 and
-March 10, 1882, I received records of no less than thirty-four distinct
-shocks felt in districts between Hakodate and Tokio, and for each of
-these it is quite possible to draw a map. In addition to the boundaries
-of disturbances given in the accompanying map, other boundaries might
-be drawn for shocks which were more local in their character. The
-groups which contained the greatest number of shocks are III., IV.,
-V., VI., and VII. By work of this description it was found that a very
-important group of earthquakes might be studied by a line of stations
-commencing at Saporo in the north, passing through Hakodate, down the
-east coast of the main island, to Tokio or Yokohama in the south. A
-further aid to the study of this group, together with the study of
-an important local group, might be effected with the help of a few
-additional stations properly distributed on the plain of Musashi, which
-surrounds Tokio. With this example before us it will be recognised that
-the choice of sites for a connected set of seismological observatories
-will often be more or less a special problem. If earthquake stations
-were to be placed in different directions around Tokio without
-preliminary investigation, it is quite possible that some of them might
-be so situated that they would seldom if ever work in conjunction with
-the remaining observatories, and therefore be of but little value.
-And this remark must equally apply to districts in other portions
-of the globe. The method is crude, and, so far as actual earthquake
-origins are concerned, it only yields results which are approximate.
-The crudeness and the want of absoluteness in the results is, however,
-more than counterbalanced by the certainty with which we are enabled
-to express ourselves with regard to such results as are obtained. Even
-when working with the best instruments we have at our command, unless
-we are employing some elaborate system, this method of working gives a
-most valuable check upon our instrumental records, and enables us to
-interpret them with greater confidence.
-
-_Determination of earthquake origins from the direction of motion._—If
-we assume that an earthquake is propagated from a centre as a series
-of waves, in which normal vibrations are conspicuous, and obtain at
-two localities, not in the same straight line with the origin, and
-sufficiently far separated from each other, the direction of movement
-of these normal motions, by drawing lines parallel to these directions
-through our two stations, the lines would intersect at a point above
-the required origin. If instead of two points we had three, or, better
-still, a large number, the results we should obtain ought to be
-still more certain. Unfortunately, it seems that earthquakes seldom
-originate from a given point, and, further, normal motions are not
-always (sufficiently) prominent. Sometimes, as has already been shown
-in the chapters on earthquake motion, they may be non-existent. It is
-probable, however, that difficulties of this sort are more usually
-associated with non-destructive earthquakes. Mallet regards the
-destructive effects of an earthquake as almost solely due to normal
-motions. If this be true, for destructive earthquakes, the problem is
-shorn of many of its difficulties. In cases where normal vibrations
-are not prominent, where we have only transverse vibrations, motions
-due to the interference of normal or transverse motions, or directions
-of motions due to the topographical or geological nature through which
-the disturbance has passed, the determination of the origin of an
-earthquake by observations on the direction in which the ground has
-been moving appears to be a problem which is practically without a
-solution. We will, therefore, only consider the determination of the
-origin of those earthquakes which have predominating directions in
-their movements, which directions we will consider as normal ones.
-The question which is, then, before us, is the determination of the
-direction of these normal movements. First of all we may take the
-evidence of our senses. In exceptional cases these have given results
-which closely approximate to the truth, but in the majority of cases
-such results are not to be relied upon, as the inhabitants of a town
-will, for the same shock, give directions corresponding to all points
-of the compass. Much, no doubt, depends upon the situation of the
-observer, and much, perhaps, upon his temperament. If he is sitting
-in a room alone, and is accustomed to making observations on an
-earthquake, on feeling the earthquake, if he concentrates his attention
-on the direction in which he is being moved, his observations may be
-of value. If, however, he is not so situated, and his attention is
-not thus concentrated, his opinions, unless the motion has been very
-decided in its character, are usually of but little worth.
-
-_Direction determined from destruction of buildings._—When an observer
-first sees a town that has been partially shattered by an earthquake,
-all appears to be confusion, and it is difficult to imagine that in
-such apparent chaos we are able to discover laws. If, however, we take
-a general view of this destruction and compare together similarly
-built buildings, it is possible to discover that similar and similarly
-situated structures have suffered in a similar manner. By carefully
-analysing the destruction we are enabled to infer the direction in
-which the destroying forces have acted. It was chiefly by observing the
-cracks in buildings, and the direction in which bodies were overthrown
-or projected, that Mallet determined the origin of the Neapolitan
-earthquake. From the observations given in Chapter VII. it would appear
-that, with destructive earthquakes, walls which are transverse to the
-direction of motion are most likely to be overturned, whilst, with
-small earthquakes, these walls are the least liable to be fractured.
-
-From a critical examination of the _general_ nature of the damage
-done on the buildings of a town, earthquake observers have shown that
-the direction of a shock may often be approximately determined. The
-direction in which a body having a regular form like a prismatic
-gravestone or a cylindrical column is overturned sometimes gives the
-means by which we can determine the direction from which a movement
-came.
-
-_The rotation of bodies._—It has often been observed that almost all
-large earthquakes have caused objects like tombstones, obelisks,
-chimneys, &c., to rotate.
-
-One of the most natural and at the same time most simple explanations
-is to suppose that during the shock there had been a twisting, or
-backward and forward screw-like motion in the ground. Amongst the
-Italians and the Mexicans earthquakes producing an effect like this are
-spoken of as ‘vorticosi.’ In the Calabrian earthquake, not only were
-bodies like obelisks twisted on their bases, but straight rows of trees
-seem to have been left in interrupted zigzags. These latter phenomena
-have been explained upon the assumption of the interference of direct
-waves and reflected waves, the consequence of which being that points
-in close proximity might be caused to move in opposite directions.
-Reflections such as these would be most likely to occur near to the
-junction of strata of different elasticity, and it may be remarked that
-it is often near such places that much twisting has been observed.
-
-Another way in which it is possible for twisting to have taken place
-would be by the interference of the normal and transverse waves which
-probably always exist in an earthquake shock, or by the meeting of
-the parts of the normal wave itself, one having travelled in a direct
-line from the origin, whilst the other, travelling through variable
-material, has had its direction changed.
-
-Mallet, however, has shown that the rotation may have been in many
-cases brought about without the supposition of any actual twisting
-motion of the earth—a simple backward and forward motion being quite
-sufficient. If one block of stone rests upon another, and the two are
-shaken backwards and forwards in a straight line, and if the vertical
-through the centre of gravity of the upper block does not coincide with
-the point where there is the greatest friction between the blocks,
-rotation must take place. If the vertical through the centre of gravity
-falls on one side of the centre of friction, the rotation would be in
-one direction, whilst, if on the other side, the rotation would be in
-the opposite direction.
-
-Although the above explanation is simple, and also in many cases
-probably true, it hardly appears sufficient to account for all the
-phenomena which have been observed.
-
-Thus, for instance, if the stones in the Yokohama cemetery, at the time
-of the earthquake of 1880, had been twisted in consequence of the cause
-suggested by Mallet, we should most certainly have found that some
-stones had turned in one direction whilst others had been twisted in
-another. By a careful examination of the rotated stones, I found that
-every stone—the stones being in parallel lines—had _revolved in the
-same direction_, namely in a direction opposite to that of the hands of
-a watch.
-
-As it would seem highly improbable that the centre of greatest friction
-in all these stones of different sizes and shapes should have been at
-the same side of their centres of gravity, an effect like this could
-only be explained by the conjoint action of two successive shocks, the
-direction of one being transverse to the other.
-
-Although fully recognising the sufficiency of two transverse shocks
-to produce the effects which have been observed in Yokohama, I will
-offer what appears to me to be the true explanation of this phenomenon:
-it was first suggested by my colleague, Mr. Gray, and appears to be
-simpler than any with which I am acquainted.
-
-[Illustration: FIG. 30.]
-
-If any columnar-like object, for example a prism which the basal
-section is represented by A B C D (see fig. 30), receives a shock at
-right angles to B C, there will be a tendency for the inertia of the
-body to cause it to overturn on the edge B C. If the shock were at
-right angles to D C, the tendency would be to overturn on the edge D C.
-If the shock were in the direction of the diagonal C A, the tendency
-would be to overturn on the point C. Let us, however, now suppose the
-impulse to be in some direction like E G, where G is the centre of
-gravity of the body. For simplicity we may imagine the overturning
-effect to be an impulse given through G in an opposite direction—that
-is, in the direction G E. This force will tend to tip or make the body
-bear heavily on C, and at the same time to whirl round C as an axis,
-the direction of turn being in the direction of the hands of a watch.
-If, however, the direction of impulse had been E′ G, then, although the
-turning would still have been round C, the direction would have been
-_opposite_ to that of the hands of a watch.
-
-To put these statements in another form, imagine G E′ to be resolved
-into two components, one of them along G C and the other at right
-angles, G F. Here the component of the direction G C tends to make the
-body tip on C, whilst the other component along G F causes revolution.
-Similarly G E may be resolved into its two components G C and G F′, the
-latter being the one tending to cause revolution.
-
-From this we see that if a body has a rectangular section, so long
-as it is acted upon by a shock which is parallel to its sides or to
-its diagonals, there ought not to be any revolution. If we divide
-our section A B C D up into eight divisions by lines through these
-directions, we shall see that any shock the direction of which passes
-through any of the octants which are shaded will cause a _positive_
-revolution in the body—that is to say, a revolution corresponding in
-its direction to that of the movements of the hands of a watch; whilst
-if its direction passes through any of the remaining octants the
-revolution will be _negative_, or opposite to that of the hands of a
-watch. From the direction in which any given stone has turned, we can
-therefore give two sets of limits between one of which the shock must
-have come.
-
-Further, it will be observed that the tendency of the turning is to
-bring a stone, like the one we are discussing, broadside on to the
-shock; therefore, if a stone with a rectangular cross section has
-turned sufficiently, the direction of a shock will be parallel to one
-of its faces, but if it has not turned sufficiently it will be more
-nearly parallel to its faces in their new position than it was to its
-faces when in their original position.
-
-If a stone receives a shock nearly parallel with its diagonal, on
-account of its instability it may turn either positively or negatively
-according as the friction on its base or some irregularity of surface
-bearing most influence. Similarly, if a stone receives a shock parallel
-to one of its faces, the twisting may be either positive or negative,
-but the probability is that it would only turn slightly; whereas in the
-former case, where the shock was nearly parallel to a diagonal, the
-turning would probably be great.
-
-_Determination of direction from instruments._—When speaking about
-earthquakes it was shown, as the result of many observations, that
-the same earthquake in the space of a few seconds, although it may
-sometimes have only one direction of motion, very often has many
-directions of motion. In certain cases, therefore, our records, if we
-assume the most permanent motions to be normal ones, give definite and
-valuable results. In other cases it is necessary to carefully analyse
-the records, comparing those taken at one station with those taken at
-another.
-
-One remarkable fact which has been pointed out in reference to
-artificial earthquakes produced by exploding charges of gunpowder or
-dynamite, and also with regard to certain earthquakes, is that the
-greatest motion of the ground is _inwards_, towards the point from
-which the disturbance originated. Should this prove the rule, it gives
-a means of determining, not only the direction of earthquake, but the
-side from which it came.
-
-_Determination of earthquake origins by time observations._—The times
-at which an earthquake was felt at a number of stations are among the
-most important observations which can be made for the determination of
-an earthquake origin. The methods of making time observations, and the
-difficulties which have to be overcome, have already been described.
-When determining the direction from which a shock has originated, or
-determining the origin of the shock by means of time observations,
-it has been usual to assume that the velocity of propagation of the
-shock has been uniform from the origin. The errors involved in this
-assumption appear to as follows:—
-
-1. We know from observations on artificial earthquakes that the
-velocity of propagation is greater between stations near to the origin
-of the shock than it is between more remote stations; and also the
-velocity of propagation varies with the initial force which produced
-the disturbance. If our points of observation are sufficiently close
-together as compared with their distance from the origin of the
-disturbance, it is probable that errors of this description are small
-and will not make material differences in the general results.
-
-2. We have reasons for believing that the transit velocity of an
-earthquake is dependent on the nature of the rocks through which it
-is propagated. Errors which arise from causes of this description
-will, however, be practically eliminated if our observation points are
-situated on an area sufficiently large, so that the distribution of the
-causes tending to alter the velocity of a shock balance each other. It
-must be remarked, that causes of this description may also produce an
-alteration in the direction of our shock.
-
-Other errors which may sometimes enter into our results, when
-determining the origin of shocks by means of observations on
-velocities, are the assumptions that the disturbance has travelled
-along the surface from the _epicentrum_ and not in a direct line from
-the _centrum_. Again, it is assumed that the origin is a point, whereas
-it may possibly be a cavity or a fissure. Lastly, if we desire extreme
-accuracy, we must make due allowance for the sphericity of the earth
-and the differences of elevation of the observing stations.
-
-I. _The method of straight lines._—Given a number of pairs of points
-A_{0}, A_{1}, B_{0}, B_{1}, C_{0}, C_{1}, &c., at each of which the
-shock was felt simultaneously, to determine the origin.
-
-Theoretically if we bisect the line which joins A_{0} and A_{1} by a
-line at right angles to A_{0}, A_{1}, and similarly bisect the lines
-B_{0}, B_{1}, C_{0}, C_{1}, all these bisecting lines _a__{0}, _a__{1},
-_b__{0}, _b__{1}, _c__{0}, _c__{1}, &c., ought to intersect in a point,
-which point will be the _epicentrum_ or the point above the origin.
-
-This method will fail, first, if A_{0}, A_{1}, B_{0}, B_{0}, C_{0},
-C_{1} form a continuous straight line, or if they form a series of
-parallel lines.
-
-Hopkins gives a method based on a principle similar to the one which
-is here employed—namely, given that a shock arrives simultaneously
-at _three_ points to determine, the centre. In this case, the
-relative positions of the three points, where the time of arrival was
-simultaneous, must be accurately known, and these three points must
-not lie in a straight line, or the method will fail. For practical
-application the problem must be restricted to the case of three points
-which do not lie nearly in the same straight line.
-
-II. _The method of circles._—Given the times _t__{0}, _t__{1}, _t__{2},
-&c., at which a shock arrived at a number of places A_{0}, A_{1},
-A_{2}, &c., to determine the position from which the shock originated.
-
-Suppose A_{0} to be the place which the shock reached first, and that
-it reached A_{1}, A_{2}, A_{3}, &c., successively afterwards.
-
-  Let     _t__{1} - _t__{0} = _a_
-          _t__{2} - _t__{0} = _b_
-          _t__{3} - _t__{0} = _c_, &c.
-
-With A_{1}, A_{2}, A_{3}, &c. as centres, describe circles with radii
-proportional to the known qualities _a_, _b_, _c_, &c., and also a
-circle which passes through A_{0} and touches these circles. The centre
-of the last circle will be the _epicentrum_. The radii proportional to
-_a_, _b_, _c_, &c. may be represented by the quantities _ax_, _bx_,
-_cx_, &c., where _x_ is the velocity of propagation of the shock.
-
-It will be observed that the times at which the shock arrived at three
-places might alone be sufficient. If, instead of taking the times of
-arrival of a shock, the arrival of a sea wave be taken, the result
-would be a closer approximate to the absolute truth.
-
-It will be observed that this method is not a direct one, but is
-one of trial. If, however, an imaginary case be taken, and three
-given points of observation, A_{0}, A_{1}, A_{2}, be plotted on a
-piece of paper, it will be found that it is not a difficult matter to
-determine two numbers proportional to _a_ and _b_ which will allow
-you to draw two circles so that they may be touched by a third circle
-drawn through A_{0}. This problem has practically been applied in the
-case of the arrival of a sea wave at a number of places on the South
-American coast, at the time of the earthquake of May 9, 1877. This is
-illustrated as follows. The places which were chosen were Huanillos,
-Tocopilla, Cobija, Iquique, Mejillones.
-
-In the following table the first column gives the times at which the
-sea wave arrived at each of these places in Iquique time; in the second
-column the difference between these times and the time at which it
-reached Huanillos is given; in the third column the distances through
-which a sea wave, propagated at the rate of 350 feet per second, could
-travel during the intervals noted in the second column is given.
-
- +------------+----------+----------+------------+
- |            | Arrival  |Time after| Distance at|
- |            |  of sea  |arrival at|350 feet per|
- |            |   wave   | Huanillos|   second   |
- +------------+----------+----------+------------+
- |            |  h. m.   |  minutes |   miles    |
- | Huanillos  |  8  30   |     0    |     0      |
- | Tocopilla  |  8  32   |     2    |     8      |
- | Cobija     |  8  38   |     8    |    32      |
- | Iquique    |  8  40   |    10    |    40      |
- | Mejillones |  8  46   |    16    |    64      |
- +------------+----------+----------+------------+
-
-The distances marked in the third column are used as radii of the
-circles drawn round the places to which they respectively refer.
-
-The centre of the circle drawn to touch the circles of the first
-column, and at the same time to pass through Huanillos, is marked C.
-
-The position from which the shock originated appears therefore to have
-occurred very near to a place lying in Long. 7° 15′ W. and Lat. 21° 22′
-S.
-
-[Illustration: FIG. 31.]
-
-The actual operations which were gone through in making the
-accompanying map were as follows. First, the places with which we had
-to deal were represented on a map in orthographical projection, the
-centre of projection being near to the centre of the map. This was
-done so that the measurements which were made upon the map might be
-more correct than those we should obtain from an ordinary chart where
-this portion of the world was not the centre of projection. Next, a
-number was taken as equal to the velocity with which the sea wave had
-travelled. The first velocity taken was about 400 feet per second—this
-being about the velocity with which, theoretically, it must have
-travelled in an ocean having a depth equal to that indicated upon the
-charts—also it seemed to have travelled at this rate from the various
-times of arrival as recorded at places along the coast. Circles were
-then drawn round Tocopilla, Cobija, Iquique, and Mejillones with radii
-equal to 2, 8, 10, and 15, each multiplied by (60 × 400). It was then
-seen _by trial_ that it was impossible to draw a single circle which
-should touch four circles and also pass through Huanillos. These four
-circles were, in fact, too large. Four new but smaller circles, which
-are shown in the map, were next drawn, their radii being respectively
-equal to the numbers 2, 8, 10, and 16, each multiplied by (60 × 350),
-and it was found that a circle, with a centre C, could be drawn which
-would practically touch the four circles, and at the same time would
-pass through Huanillos.
-
-III. _The method of hyperbolas._—The method which I call that of
-hyperbolas is only another form of the method of circles. It is,
-however, useful in special cases, as, for instance, where we have the
-times of arrival of earthquakes at only two stations. Between Tokio
-and Yokohama, at which places I frequently obtain tolerably accurate
-time records, the method has been applied on several occasions with
-advantage. In the preceding example let us suppose that the only time
-records which we had were for Huanillos and Mejillones, and that the
-wave was felt at the latter place sixteen minutes or 960 seconds
-after it was experienced at the former. Calling these places H and M
-respectively, round M draw a circle equal to the 960 multiplied by
-the velocity with which the wave was propagated. It is then evident
-that the origin of this disturbance must be the centre of a circle
-which passes through H and touches the circle drawn round M. Join H
-M, cutting the circle round M in Y. Bisect Y H in V. It is evident
-that V is one possible origin for the disturbance. Next, from M, in
-the direction of H, draw any line M Z P; join Z H; bisect Z H at right
-angles by the line O P N. Because PH = PZ, it is evident that P is
-a second possible origin. Proceeding in this way a series of points
-lying to the right and left of V on the curve R V T may be found, and
-we may therefore say that the origin lies somewhere in the curve R
-V T. By increasing or decreasing our velocity we vary the position
-of the curve R V T, and, instead of a line on which our origin may
-be, we obtain a band. As the assumed velocity increases, the circle
-round M becomes larger, and has its limit when it passes through H,
-where the two arms of the curve R V T will close together and form a
-prolongation of the line M Y H as the assumed velocity diminishes.
-The circle round M becomes smaller until it coincides with the point
-M. At such a moment the curve R V T opens out to form a straight line
-bisecting M H at right angles. The curve R V T is a hyperbola with a
-vertex V and foci H and M. Inasmuch as PM - PH = a constant quantity.
-If we have the time given at which the shock or wave arrived at a third
-station as at Iquique, it is evident that a second hyperbola R′ V′ T′
-might be drawn with Iquique and Huanillos as foci, and that the mutual
-intersection of these two hyperbolas with a third hyperbola, having for
-its foci Iquique and Mejillones, would give the origin of the wave.
-The obtaining of a mutual intersection would depend on the assumed
-velocity, and the accuracy of the result, like that of the method
-of circles, would depend upon the trials we made. The method here
-enunciated may be carried farther by describing hyperboloids instead
-of hyperbolas, the mutual intersection of which surfaces would, in the
-case of an earth wave, give the actual origin or _centrum_ rather than
-the point above the origin or _epicentrum_.
-
-IV. _The method of co-ordinates._—Given the times at which a shock
-arrived at five or more places, the position of which we have marked
-upon a map, or chart, to determine the position on the map of the
-centre of the shock, its depth, and the velocity of propagation.
-
-Commencing with the place which was last reached by the shock, call
-these places _p_, _p__{1}, _p__{2}, _p__{3}, and _p__{4}, and let the
-times taken to reach these places from the origin be respectively _t_,
-_t__{1}, _t__{2}, _t__{3}, and _t__{4}.
-
-Through _p_ draw rectangular co-ordinates, and with a scale measure the
-co-ordinates of _p__{1}, _p__{2}, _p__{3}, and _p__{4}, and let these
-respectively be _a__{1}, _b__{1}; _a__{2}, _b__{2}; _a__{3}, _b__{3};
-_a__{4}, _b__{4}. Then if _x_, _y_, and _z_ be the co-ordinates of the
-origin of the shock, _d_, _d__{1}, _d__{2}, _d__{3}, and _d__{4}, the
-respective distances of _p_, _p__{1}, _p__{2}, _p__{3}, and _p__{4}
-from this origin, and _v_ the velocity of the shock, we have
-
-  1. _x_^2 + _y_^2 + _z_^2 = _d_^2 = _v_^2 _t_^2
-  2. (_a__{1} - _x_)^2 + (_b__{1} - _y_)^2 + _z_^2 = _v_^2 _t__{1}^2
-  3. (_a__{2} - _x_)^2 + (_b__{2} - _y_)^2 + _z_^2 = _v_^2 _t__{2}^2
-  4. (_a__{3} - _x_)^2 + (_b__{3} - _y_)^2 + _z_^2 = _v_^2 _t__{3}^2
-  5. (_a__{4} - _x_)^2 + (_b__{4} - _y_)^2 + _z_^2 = _v_^2 _t__{4}^2
-
-Because we know the actual times at which the waves arrived at the
-places _p_, _p__{1}, _p__{2}, _p__{3}, _p__{4}, we know the values
-_t_—_t__{1}, _t_—_t__{2}, _t_—_t__{3}, _t_—_t__{4}. Call these
-respectively _m_, _p_, _q_, and _r_. Suppose _t_ known, then
-
-                          _t__{1} = _t_ - _m_
-                          _t__{2} = _t_ - _p_
-                          _t__{3} = _t_ - _q_
-                          _t__{4} = _t_ - _r_.
-
-Subtracting equation No. 1 from each of the equations 2, 3, 4, and 5,
-we obtain,
-
-  _a__{1}^2 + _b__{1}^2 - 2_a__{1} _x_ - 2_b__{1} _y_
-      = _v_^2 (_t__{1}^2 - _t_^2) = _v_^2 (_m_^2 - 2_t_ _m_)
-
-  _a__{2}^2 + _b__{2}^2 - 2_a__{2} _x_ - 2_b__{2} _y_
-      = _v_^2 (_t__{2}^2 - _t_^2) = _v_^2 (_p_^2 - 2_t_ _p_)
-
-  _a__{3}^3 + _b__{3}^2 - 2_a__{3} _x_ - 2_b__{3} _y_
-      = _v_^2 (_t__{3}^2 - _t_^2) = _v_^2 (_q_^2 - 2_t_ _q_)
-
-  _a__{4}^2 + _b__{4}^2 - 2_a__{4} _x_ - 2_b__{4} _y_
-      = _v_^2 (_t__{4}^2 - _t_^2) = _v_^2 (_r_^2 - 2_t_ _r_)
-
-Now let _v_^2 = _u_, and 2_v_^2 _t_ = _w_.
-
-Then
-
-  1. 2_a__{1} _x_ + 2_b__{1} _y_ + _u_ _m_^2 - _n_ _m_
-      = _a__{1}^2 + _b__{1}^2
-
-  2. 2_a__{2} _x_ + 2_b__{2} _y_ + _u_ _p_^2 - _n_ _p_
-      = _a__{2}^2 + _b__{2}^2
-
-  3. 2_a__{3} _x_ + 2_b__{3} _y_ + _u_ _q_^2 - _n_ _q_
-      = _a__{3}^2 + _b__{3}^2
-
-  4. 2_a__{4} _x_ + 2_b__{4} _y_ + _u_ _r_^2 - _n_ _r_
-      = _a__{4}^2 + _b__{4}^2
-
-We have here four simple equations containing the four unknown
-quantities _x_, _y_, _u_, and _w_.
-
-_x_ and _y_ determine the origin of the shock. The absolute velocity
-_v_ equals √ _u_. From _v_ and _w_ we obtain _t_. Substituting
-_x_, _y_, _v_, and _t_ in the first equation we obtain _z_.
-
-We have here assumed that the points of observation have all about the
-same elevation above sea level.
-
-If it is thought necessary to take these elevations into account, a
-sixth equation may be introduced.
-
-If the propagation of the wave is considered as a horizontal one, as
-would be done when calculating the position of the _epicentrum_ or
-point above the origin, by means of the times of arrival of a sea wave,
-the ordinate _z_ of the first five equations would be omitted. Working
-in this way the resulting four equations, viz.
-
-  2_a__{1} _x_ + 2_b__{1} _y_ + _u__m_^2 - _w__m_^2 = _a__{1}^2 + _b__{1}^2
-              &c.                      &c.              &c.
-
-remained unchanged.
-
-Applying this method to the same example as that used as illustration
-for the two previous methods, we obtain for the co-ordinates of
-Mejillones, Iquique, Cobija, Tocopilla, and Huanillos, measured in
-geographical miles, and the times in Iquique time at which the wave
-reached each, as given in the following table; _ox_ and _oy_ being,
-drawn through Mejillones.
-
- +------------+--------------------------------+---------------+
- |            |         Co-ordinates           |Time of arrival|
- +------------+----------------+---------------+---------------+
- |            |      OX        |      OY       |  h.  m.       |
- | Mejillones | _a_     or   0 | _b_     or  0 |  8  46 p. m.  |
- | Iquique    | _a_{1}_ or 150 | _b_{1}_ or 96 |  8  40   „    |
- | Cobija     | _a_{2}_ or  36 | _b_{2}_ or 14 |  8  38   „    |
- | Tocopilla  | _a_{3}_ or  66 | _b_{3}_ or 31 |  8  32   „    |
- | Huanillos  | _a_{4}_ or 102 | _b_{4}_ or 58 |  8  30   „    |
- +------------+----------------+---------------+---------------+
-
-From this data we find the co-ordinates _x_ and _y_ of this origin to
-be 85·8 and 56·7; whilst the velocity of propagation = 45 feet per
-second.
-
-Measuring these ordinates upon the map, we obtain a centre lying very
-near Long. 71° 5′ W. and Lat. 21° 22′ S., a position which is very near
-to that which has already been obtained by other methods.
-
-If instead of Huanillos we substitute the ordinates and time of arrival
-of the sea wave for Pabalon de Pica, another point for the origin will
-be obtained lying farther out at sea. To obtain the best result, the
-method to be taken will evidently be, first to reject those places
-at which it seems likely that some mistake has been made with the
-time observations, and then with the remaining places to form as many
-equations as possible, and from these to obtain a mean value. This
-is a long and tedious process, and as the time observations of this
-particular earthquake are probably one and all more or less inaccurate,
-it is hardly worth while to follow the investigation farther.
-
-In this example, as in the preceding ones, it will be observed that
-it has been sea waves that have been dealt with, rather than earth
-vibrations. It is evident, however, that these latter vibrations may be
-dealt with in a similar manner.
-
-In these determinations it will also have been observed that it
-is assumed that the disturbance has radiated from a centre, and,
-therefore, approached the various stations in different directions.
-If we assume that we have three stations very near to each other as
-compared with their distances from the origin, so that we can assume
-that the wave fronts at the various stations were parallel, the
-determination of the direction in which the wave advanced appears to be
-much simplified. The determination of the direction in which a wave has
-passed across three stations was first given by Professor Haughton.
-
-_Haughton’s method._—Given, the time of an earthquake shock at three
-places, to determine its horizontal velocity and coseismal line.
-
-The solution of this is contained in the formula
-
-                       _a_ (_t_{2}_ - _t_{1}_) sin β
-    tan φ = ——————————————————————————————————————————————————————.
-            _c_ (_t_{3}_ - _t_{2}_) + _a_ (_t_{2}_ - _t_{1}_ cos β
-
-When A, B, and C are three stations at which a shock is observed at
-the times _t_{1}_, _t_{2}_, and _t_{3}_; _a_, _b_, and _c_ are the
-distances between A, B, and C, and φ is the angle made by the coseismal
-lines _x_ A _x_, _y_ B _y_, and the line A B, which are assumed to be
-parallel.
-
-This I applied in the case of the Iquique earthquake, but owing to the
-smallness of the angles between the three stations A, B, and C, the
-result was unsatisfactory. The problem ought to be restricted, first,
-to places which are a long distance away from a centre, and, secondly,
-to places which are not nearly in a straight line. This problem may be
-solved more readily by geometrical methods. Plot the three stations A,
-B, and C on a map, join the two stations between which there was the
-greatest difference in the time observation. Let these, for example,
-be A and C. Divide the line A C at point D, so that A D : D C as the
-interval between the shock felt at A and B is to the interval between
-the shock felt at B and C. The line B D will be parallel to the
-direction in which the wave advanced.
-
-_The difference in time of the arrival of two disturbances._—In the
-various calculations which have been made to determine an origin
-based on the assumption of a known or of a constant velocity, we have
-only dealt with a single wave, which may have been a disturbance in
-the earth or in the water. A factor which has not yet been employed
-in this investigation is the difference in time between the arrival
-of two disturbances; one propagated, for instance, through the
-earth, and the other, for example, through the ocean. The difference
-in the times of the arrival of two waves of this description is a
-quantity which is so often recorded that it is well not to pass it
-by unnoticed. To the waves mentioned we might also add sound waves,
-which so frequently accompany destructive earthquakes, and, in some
-localities, as, for instance, in Kameishi, in North Japan, are also
-commonly associated with earthquakes of but small intensity. It was by
-observing the difference in time between the shaking and the sound in
-different localities that Signor Abella was enabled to come to definite
-conclusions regarding the origin of the disturbances which affected the
-province of Neuva Viscoya in the Philippines, in 1881; the places where
-the interval of time was short, or the places where the two phenomena
-were almost simultaneous, being, in all probability, nearer to the
-origin than when the intervals were comparatively large. I myself
-applied the method with considerable success when seeking for the
-origin of the Iquique earthquake of 1877. The assumptions made in that
-particular instance were, first, that the velocity of the disturbance
-through the earth was known, and, secondly, that the velocity with
-which a sea wave was propagated was also known.
-
-A method similar to the above was first suggested by Hopkins. It
-depended on the differences of velocity with which normal and
-transversal waves are propagated.[85]
-
-_Seebach’s method._—To determine the true velocity of an earthquake,
-the time of the first shock, and the depth of the centre.
-
-[Illustration: FIG. 32.]
-
-Let the straight line M, _m__{1}, _m__{2}, _m__{3} represent the
-surface of the earth shaken by an earthquake. For small earthquakes, to
-consider the surface of the earth as a plane will not lead to serious
-errors.
-
-If an earthquake originates at C, then to reach the surface at M it
-traverses a distance _h_ in the time _t_. To reach the surface at M_{1}
-it traverses a distance _h_ + _x__{1} in a time _t__{2}. If _v_ equals
-the velocity of propagation,
-
-                          _h_             _h_ + _x__{1}
-               then _t_ = ———, _t_{1}_ = —————————————,
-                          _v_                 _v_
-
-                               _h_ + _x__{2}
-                     _t_{2}_ = —————————————, &c.
-                                    _v_
-
-Seebach now says that _if we have given the position of_ M _or
-epicentrum of the shock_, and draw through it rectangular axes like M
-_m_{3}_ and M T_{3}, and lay down on M _m_{3}_ in miles the distances
-from M of the various stations which have been shaken, and in equal
-divisions for minutes lay down on M T_{3} the differences of time at
-which M, _m_{1}_, _m_{2}_, &c. were shaken, then M_{1} T_{1}, M_{2}
-T_{2}, &c. are the co-ordinates of points on an hyperbola. The degree
-of exactness with which this hyperbola is in any given case constructed
-is a check upon the accuracy of the time observations and the position
-of the _epicentrum_. The apex of the hyperbola is the _epicentrum_.
-
-The intersection of the asymptote with the ordinate axis is the time
-point of the first shock, which, because the scale for time and
-for space were taken as equal, gives the absolute position of the
-_centrum_. This intersection is shown by dotted lines. Knowing the
-position of the _centrum_, we can directly read from our diagram how
-far the disturbance has been propagated in a given time.
-
-
-
-
-                              CHAPTER XI.
-
-                  THE DEPTH OF AN EARTHQUAKE CENTRUM.
-
-  The depth of an earthquake centrum—Greatest possible depth of an
-    earthquake—Form of the focal cavity.
-
-
-_Depth of centrum._—The first calculations of the depth at which an
-earthquake originated were those made by Mallet for the Neapolitan
-earthquake of 1857. These were made on the assumption that the earth
-wave radiated in straight lines from the origin, and, therefore, at
-points at different distances from the _epicentrum_ it had different
-angles of emergence. These angles of emergence were chiefly calculated
-from the inclination of fissures produced in certain buildings, which
-were assumed to be at right angles to the direction of the normal
-motion. If we have determined the _epicentrum_ of an earthquake and
-the muzoseismal circle, and make either the assumption that the angle
-of emergence in this circle has been 45° or 54° 44′ 9″ (see page
-54), it is evidently an easy matter by geometrical construction to
-determine the depth of the _centrum_. Höfer followed this method when
-investigating the earthquake of Belluno.
-
-Other methods of calculation which have been employed are based on
-time observations, as, for instance, the method of Seebach, the method
-of co-ordinates, the method of hyperboloids or spheres (see pages
-200–212).
-
-By means of a number of lines parallel to twenty-six angles of
-emergence, drawn in towards the seismic vertical, Mallet found that
-twenty-three of these intersected at a depth of 7⅛ geographical miles.
-The maximum depth was 8⅛ geographical miles, and the minimum depth 2¾
-geographical miles.
-
-The mean depth was taken at a depth of 5¾ geographical miles where,
-within a range of 12,000 feet, eighteen of the wave paths intersected
-the seismic vertical.
-
-The point where these wave paths start thickest is at a depth not
-greater than three geographical miles, and this is considered to be the
-vertical depth of the focal cavity itself.
-
-For the Yokohama earthquake of 1880, from the indications of
-seismometers, and by other means, certain angles of emergence were
-obtained, leading to the conclusion that the depth of origin of that
-earthquake might be between 1½ and 5 miles.
-
-Possibly, perhaps, the earthquake may have originated from a fissure
-the vertical dimensions of which was comprised between these depths.
-
-A source of error in a calculation of this description is that the
-vertical motions may have been a component of transverse motions or
-perhaps due to the slope of surface waves.
-
-The following table of the depths at which certain earthquakes have
-originated has been compiled from the writings of several observers.
-
- +---------------------------------+-----------------------------+
- |                                 |           In feet           |
- +----------------+----------------+---------+---------+---------+
- |                |                | Minimum |   Mean  | Maximum |
- | Rhineland      | 1846 (Schmidt) |         | 127,309 |         |
- | Sillien        | 1858 (Schmidt) |         |  86,173 |         |
- | Middle Germany | 1872 (Seebach) |  47,225 |  58,912 |  70,841 |
- | Herzogenrath   | 1873 (Lasaulx) |  16,553 |  36,516 |  56,477 |
- | Neapolitan     | 1857 (Mallet)  |  16,705 |  34,930 |  49,359 |
- | Yokohama       | 1880 (Milne)   |   7,920 |  17,260 |  26,400 |
- +----------------+----------------+---------+---------+---------+
-
-A table similar to this has been compiled by Lasaulx.[86]
-
-With the exception of the determination for the two last disturbances
-these calculations have been made with the assistance of the method
-of Seebach, which depends, amongst other things, on the assumptions
-of exact time determinations, direct transmission of waves from the
-centrum, and a constant velocity of propagation.
-
-Admitting that our observations of time are practically accurate, it
-appears that the other assumptions may often lead to errors of such
-magnitude that our results may be of but little value.
-
-From what has been said respecting the velocity with which earth
-disturbances are propagated, it seems that these velocities may vary
-between large limits, being greatest nearest to the origin.
-
-If we refer to Seebach’s method, we shall see that a condition of this
-kind would tend to make the differences in time between various places,
-as we recede from the _epicentrum_, greater than that required for the
-construction of the hyperbola. The curve which is obtained would, in
-consequence, have branches steeper than that of the hyperbola, and the
-resultant depth, obtained by the intersection of the asymptotes of this
-curve with the seismic vertical, indicates an origin which may be much
-too great.
-
-Another point worthy of attention, which is common to the method
-of Mallet as well as to that of Seebach, is the question whether
-the shock radiates directly from the origin, or is propagated from
-the origin more or less vertically to the surface, and then spreads
-horizontally. We know that earthquakes, both natural and artificial,
-may be propagated as undulations on the surface of the ground, and
-that the vertical motion of the latter, as testified by the records
-of well-constructed instruments, has no practical connection with the
-depth from which the disturbance originated.
-
-In cases like these, the direction of cracks in buildings, and other
-phenomena usually accredited to a normal radiation, may in reality be
-due to changes in inclination of the surface on which the disturbed
-objects rested. When our points of observation are at a distance from
-the _epicentrum_ of the disturbance which, as compared with the depth
-of the same, is not great, calculations or observations based on the
-assumption of a direct radiation of the disturbance may possibly lead
-to results which are tolerably correct. The calculations of Mallet
-for the Neapolitan earthquake appear to have been made under such
-conditions.
-
-For smaller earthquakes, and for places at a distance from the seismic
-vertical of a destructive earthquake, the results which are deduced
-from the observations on shattered buildings, and all observations
-based upon the assumption of direct radiation, we must accept with
-caution.
-
-Another error which may enter into calculations of this description
-is one which has been discussed by Mallet at some length. This is the
-effect which the form and the position of the focal cavity may have
-upon the transmission of waves.
-
-Should the impulse originate from a point or spherical cavity, then
-we might, in a homogeneous medium perhaps, regard the isoseismals as
-concentric circles, and expect to find that equal effects had been
-produced at equal distances from the _epicentrum_. Should, however,
-this cavity be a fissure, it is evident that even in a homogeneous
-medium the inclination of the plane of such a cavity will have
-considerable effect upon the form of the waves which would radiate from
-its two walls.
-
-For example, let it be assumed that the first impulse of an earthquake
-is due to the sudden formation of a fissure, rent open from its centre,
-and that the waves leave the walls at all points normal to its surface.
-Then, as Mallet points out, it is evident that the disturbance will
-spread out in ellipsoidal waves, the greatest axis of which will be
-perpendicular to the plane of the fissure.
-
-By taking a number of cases of fissures lying in various directions
-and drawing the ellipsoidal waves which would result from an elastic
-pressure, like that of steam suddenly admitted into such cavities, the
-differences in effect which would be simultaneously produced by these
-waves on reaching the surface can be readily understood. The following
-example of an investigation on this subject will serve as an example to
-illustrate the general nature of the many other cases which might be
-taken.
-
-[Illustration: FIG. 33.]
-
-Let a disturbance simultaneously originate from all points of the
-fissure _f_ _f_. This will spread outwards in ellipsoidal shells to
-the surface of the earth _e_ _e_. The major axis of these ellipsoidal
-shells will be the direction of greatest effect. In the direction _c_
-_d_ the waves will plunge into the earth, and places to the right side
-of the fissure will, to use an expression due to Stokes, when speaking
-of analogous phenomena connected with sound, be in _earthquake shadow_.
-The same expression has been employed, somewhat differently, when
-speaking of the effects produced on buildings.
-
-For places, like _s_ and _p_, situated at equal distances from the
-seismic vertical, it is evident that the intensity of the shock will be
-different, and also its time of arrival. It will also be observed that
-the isoseismals will take the form of ovals or distorted ellipses, the
-larger or fuller end of which being to the left of the fissure.
-
-Other cases, like those just given, which are discussed by Mallet in
-his account of the Neapolitan earthquake, are where the fissure forms
-the division between materials of different elasticities. In the hard
-and more elastic material the waves will be more crowded, the velocity
-of a wave particle will be greater, and the transit will be quicker
-than in the less elastic medium.
-
-The result is that the distance of equal effect from the seismic
-vertical will be greatest in the direction of the more compressible
-material.
-
-Unless these considerations are kept carefully before the mind when
-investigating the depth and, we may add, the position and form of the
-centrum of an earthquake, serious errors may arise.
-
-_Greatest depth of an earthquake origin._—A curious but instructive
-calculation which Mallet made was a determination of the greatest
-possible depth at which an earthquake may occur. This calculation is
-based upon the idea that the impulsive effect of an earthquake has an
-intimate relationship with the height of neighbouring volcanoes, the
-column of lava supported on a volcanic cone being a measure of the
-internal pressure tending to rupture the adjacent crust of the earth.
-
-Mitchell, in 1700, virtually propounded this idea, when he suggested that
-the velocity of propagation of an earthquake was related to the height
-of such a column.[87]
-
-Mallet showed that there was probably considerable truth in such a
-supposition by appealing to the results of actual observation. The
-pressure gauge of the Neapolitan district would be Vesuvius, the height
-of which has in round numbers varied between 3,500 to 4,000 feet. One
-of the most destructive earthquakes in this district—namely, the one of
-1857—projected bodies with an initial velocity of about fifteen feet
-per second. The Riobamba earthquake, which projected bodies with an
-initial velocity of eighty feet per second, appears to have been the
-most violent earthquake, so far as its impulsive effort is concerned,
-of which we have any record. It occurred amongst the Andes, where there
-are volcanoes from 16,000 to 21,000 feet in height.
-
-Comparing these two earthquakes together, we see that the Riobamba
-shock had a destructive power 5·33 times that of the Neapolitan shock,
-and we also see that the Riobamba volcanoes were about 5·33 times
-higher than Vesuvius. The accordance in these quantities is certainly
-interesting, and tends to substantiate the idea that volcanoes are
-barometrical-like pressure gauges of a district.
-
-Carrying the argument still further. Mallet says that if the depth of
-origin of earthquakes were the same, then the _area of disturbance_
-would, for like formations and configuration of surface, be a measure
-of the earthquake effort, and also some function of the velocity of the
-wave. From this we may generally infer ‘that earthquakes, like that of
-Lisbon, which have a _very great area_ of sensible disturbance, have
-also a very deep seismal focus, and also the greatest depth of seismal
-focus within our planet is probably not greater than that ascertained
-for this Neapolitan earthquake, multiplied by the ratio that the
-velocity of the Riobamba wave bears to that of its wave, or, what is
-the same thing, by the ratio of the altitudes of the volcanoes of the
-Andes to that of Vesuvius.’
-
-Now, as the depth of the Neapolitan shock may be taken at 34,930
-feet, the greatest probable depth of origin of any earthquake impulse
-occurring in our planet is limited to 5·333 × 4,930 feet, or 30·64
-geographical miles.
-
-Ingenious as this argument is, we can hardly admit it without certain
-qualifications.
-
-First, we are called upon to admit the identity of the originating
-cause of the volcano and the earthquake—as to what may be the
-originating cause of earthquakes we have yet to refer, but certainly in
-the case of particular earthquakes, as, for instance, those which occur
-in countries like Scotland, Scandinavia, and portions of Siberia, the
-direct connection between these phenomena are not at first sight very
-apparent.
-
-Secondly, even if we admit the identity of the origin of these
-phenomena, it is not difficult to imagine that the fluid pressure
-brought to bear upon certain portions of the crust of the earth may
-possibly in many instances be infinitely greater than that indicated
-by the height of the column of liquid lava in the throat of a volcano,
-the true height of which we are unable to obtain. Further, in certain
-instances such a column only appears to be a measure of the pressure
-upon the crust of the earth in the immediate vicinity of the cone.
-
-Thus, in the Sandwich Islands, we have lava standing in the throat
-of the volcano of Mauna Loa 10,000 feet higher than it stands in the
-crater Kilauea, only twenty miles distant. That these columns should be
-measures of the same pressure, originating in a general subterranean
-liquid layer with which they are connected, is a supposition difficult
-to satisfactorily substantiate.
-
-Another measure of the impulsive efforts which subterranean forces
-may exert upon the crust above them is evidently the height to which
-volcanoes eject materials. Cotopaxi is said to have hurled a 200-ton
-block of stone nine miles. Sir W. Hamilton tells us that in 1779
-Vesuvius shot up a column of ashes 10,000 feet in height; and Judd
-tells us that this same mountain in 1872 threw up vapours and rock
-fragments to the enormous height of 20,000 feet. This would indicate an
-initial velocity of 1,131 feet per second.
-
-Notwithstanding Mallet’s calculation that thirty miles is the limiting
-depth for the origin of an earthquake, the origin of the Owen’s
-Valley earthquake of March 1872 was estimated as being at least fifty
-miles.[88]
-
-_Form of the focal cavity._—Among the various problems which are put
-before those who study the physics of the interior of our earth it
-would at first sight appear that there was none more difficult than the
-attempt to determine the form of the cavity, if it be a cavity, from
-which an earthquake originates. Almost all investigators of seismology
-have recognised that the birthplace of an earthquake is not a point,
-and have made suggestions about its general nature. The ordinary
-supposition is that the earthquake originates from a fissure, and if
-the focus of a disturbance could be laid bare to us it would have the
-appearance of a fault such as we so often see exposed on the faces of
-cliffs.
-
-A strong argument, tending to demonstrate that some of the shakings
-which are felt in Japan are due to the production of such fissures,
-is the fact that the vibrations which are recorded are transverse to
-a line joining the point of observation and the district from which,
-by time observations, we know the shock to have originated. The most
-probable explanation of this phenomena appears to be that one mass of
-rock has been sliding across another mass, giving rise to shearing
-strains, and producing waves of distortion.
-
-The first seismologist who attacked the problem of finding out the
-dimensions and position of such a fissure was Mallet, when working on
-the Neapolitan earthquake of 1857. The reasons that the origin should,
-in the first place, have been a fissure, rather than any other form of
-cavity, was that such a supposition seemed to be _a priori_ the most
-probable, and, further, that it afforded a better explanation of the
-various phenomena which were observed, than that obtained from any
-other assumption.
-
-The method on which Mallet worked to determine the form and position of
-the assumed fissure, which method was subsequently more or less closely
-followed by other investigators, was as follows:—
-
-From an observation of the various phenomena produced upon the surface
-of the disturbed area, a map of isoseismals was constructed. These were
-seen, as has been the case with many earthquakes, not to distribute
-themselves in circles round the _epicentrum_, but as distorted oval or
-elliptical figures, the major axes of which roughly coincided with each
-other. Further, the _epicentrum_, did not lie in the centre of these
-ovals, but was near to the narrow end where they converged.
-
-This at once showed, if the reasoning respecting the manner in which
-waves are propagated from an inclined fissure be correct, that the
-fissure was at right angles to the major axis of the curves, dipping
-from their narrow end downwards, in the direction of their larger
-widespread ends.
-
-The next weapon which Mallet employed to attack this problem was the
-sound which was heard at different points round about the focus. These
-sounds appear to have been of the nature of sudden explosive reports
-accompanied by rushing, rolling sounds. The form of the area in which
-these sounds were heard was closely similar to that of the first two
-isoseismals. Except in the central area of great disturbance, no sound
-was heard to accompany the shock.
-
-Those at the northern and southern extremity of the sound area all
-described what they heard as a ‘low, grating, heavy, sighing rush, of
-twenty to sixty seconds’ duration.’ Those in the middle and towards the
-east and west boundaries of this area described a sound of the same
-tone, but shorter and more abrupt, and accompanied with more rumbling.
-
-The nature of the arguments which were followed in discussing the sound
-observations will be found in the chapter relating to these phenomena.
-
-A portion of the argument which it is difficult to follow relates to
-the maximum rate at which it can be supposed possible for a fissure to
-be rent in rocks, which rate depends on the density and elasticity of
-these rocks and other constant factors.
-
-Next it was observed that the paths of the waves drawn on the surface,
-although generally intersecting in a point, did not do so absolutely,
-but along a line passing through the main focus some 7½ miles in
-length. This, coupled with the observations of sounds, led to the
-supposition that the centre of disturbance, considered horizontally,
-originated along a curved line passing through the chief focus and the
-various intersections of the wave paths.
-
-The last phenomena brought forward to assist in the solution of this
-interesting problem were a study of the tremulous movements that
-preceded and followed the shock, and their relation to the sound
-phenomena.
-
-If the earthquake originated by the formation of a fissure, after the
-rending has gone on for a certain time the focal cavity is enlarged
-to a certain extent, and the great shock takes place. This would be
-followed by concluding tremulous waves. A succession of phenomena like
-those accompanied the shock about which Mallet writes.
-
-By observations such as these, coupled with what has been said about
-the maximum and mean depths of the focal cavity, Mallet came to the
-conclusion that the focal cavity was a fissure, the rending open of
-which produced the earthquake. The vertical dimensions of this cavity
-were not more than 5·3 miles, but were probably limited to three miles.
-
-From the intersection of the wave paths upon the surface and the
-observed emergences, this fissure followed horizontally a curve of
-double flexure, about nine geographical miles in length. The area of
-this fissure was twenty-seven geographical miles. The time of rending
-it open in Apennine limestone would be about 7½ seconds, which
-should be the same as the period during which tremors were felt. The
-time actually recorded was six or eight seconds.
-
-Briefly, this is, then, the line of reasoning which was followed by
-Mallet in an investigation the results of which are as interesting
-as they are startling. Since the line of investigation has been
-opened, and the existence of new problems has been indicated, other
-investigators, although not exactly following Mallet’s method in
-all their details, have, when endeavouring to attain the same ends,
-employed similar weapons.
-
-Thus, for example, Seebach, when determining the depth and nature of
-the origin of the earthquake of Middle Germany, reasoned somewhat as
-follows:—
-
-Had the origin been more or less of a spherical cavity, then the region
-of most violent disturbance upon the surface would, according to a
-theorem we have already mentioned, have been upon or near a circle of
-about 8·8 miles in radius round the _epicentrum_. This region, however,
-was found by observation to lie along a curved band about forty miles
-in length, altogether on one side of the _epicentrum_.
-
-To explain this anomaly Seebach followed Mallet, and assumed that the
-origin was not a spherical cavity, but a fissure.
-
-The depth and strike of this fissure was determined by the observation
-that the area of greatest disturbance was along a curved line lying
-radial to the _epicentrum_. Such a condition it was assumed indicated
-that the fissure of origin must be inclined towards this area of
-greatest disturbance. A line was then drawn from this area to the
-_centrum_. A second line at right angles to this one gave the dip of
-the fissure.
-
-Höfer, when working on the earthquake of Belluno, came to the
-conclusion that the disturbance originated from two faults meeting
-each other at an angle of 60°. In this determination he was chiefly
-influenced by the form of a certain homoseist which was of the form of
-an elongated ellipse met on one side by a second ellipse, the principal
-axes of the two ellipses giving the strike of the two faults.
-
-
-
-
-                             CHAPTER XII.
-
-            DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.
-
-  General distribution of earthquakes—Occurrence along lines—Examples
-    of distribution—Italian earthquake of 1873—In Tokio—Extension
-    of earthquake boundaries—Seismic energy in relation to
-    geological time; to historical time—Relative frequency of
-    earthquakes—Synchronism of earthquakes—Secondary earthquakes.
-
-
-_General distribution of earthquakes._—The records of earthquakes
-collected by various seismologists lead us to the conclusion that
-at some time or other every country and every ocean in the world
-has experienced seismic disturbances. In some countries earthquakes
-are felt daily, and from what will be said in the chapter on earth
-pulsations it is not unlikely that every large earthquake might with
-proper instrumental appliances be recorded at any point on the land
-surfaces of our globe. The area over which any given earthquake extends
-is indeterminate. The area over which an earthquake is sensible is
-sometimes very great. The Lisbon shock of 1755 is estimated as having
-been sensible over an area of 3,300 miles long and 2,700 miles wide,
-but in the form of tremors and pulsations it may have shaken the whole
-globe.
-
-The regions in which earthquakes are frequent are indicated in the
-accompanying map, which, to a great extent, is a reproduction of a map
-drawn by Mallet. The regions coloured with the darkest tint are those
-where great earthquakes are the most frequent. The actual number of
-earthquakes which have been felt in the differently coloured areas are
-given, when speaking of the relation of seismic energy to season.
-
-When looking at this chart it must be remembered that if we were
-to make a detailed map of any one of the different countries where
-earthquakes are frequent, we should find in it all the differences that
-we observe in the general chart. For instance, one portion of Japan,
-where perhaps sixty shocks are felt per year, would be coloured with
-a dark tint, whilst other portions of the same country, where there
-is only one slight shaking felt every few years, would be left almost
-uncoloured. The black dots indicating the position of volcanic vents
-are even more general in their signification than the tinted areas.
-Professor Haughton gives for the world a list of 407 volcanoes, 225 of
-which are active. These numbers are the same as those given by A. von
-Humboldt. Of the active volcanoes 172 are on the margin of the Pacific,
-and of the total number eight are in Japan. From my own observations
-in Japan independently of the Kurile Islands, I have enumerated
-fifty-three volcanoes which are either active or have been active
-within a recent period. In a few years’ time this list will probably be
-increased. I mention this fact to show how very imperfect our knowledge
-is respecting the number of volcanic vents existing on our globe. If we
-were in a position to indicate the volcanoes which had been in eruption
-during the last 4,000 years, the probability is that they would number
-several thousands rather than four or five hundred.
-
-An inspection of the map shows that earthquakes chiefly occur in
-volcanic and mountainous regions. The most earthquake-shaken regions of
-the world form the boundaries of the Pacific ocean. It may be remarked
-that these boundaries slope beneath the neighbouring ocean at a much
-steeper angle than the boundaries of countries where earthquakes occur
-but seldom. The coasts of South America, Kamschatka, the Kuriles,
-Japan, and the Sandwich Islands, for example, have slopes beneath the
-Pacific from one in twenty to one in thirty. The coasts of Australia,
-Scandinavia, and the eastern parts of South America, where earthquakes
-are practically unknown, have slopes from one in fifty to one in two
-hundred and fifty. Many earthquakes have taken place in mid-ocean. In
-the Atlantic Ocean M. Perrey has given about 140 instances of such
-occurrences.
-
-The majority of the earthquakes which shake Japan appear to have their
-origin in the neighbouring ocean. If we could draw a map of earthquake
-origins, it is probable that the greater number of the marks indicating
-these origins would be found to be suboceanic and along lines parallel
-to the shores of continents and islands which rise steeply from the
-bed of deep oceans. In countries like Switzerland and India, our marks
-would hold a relationship to the mountains of these countries.[89]
-Looking at the broad features of the globe, we see on its surface
-many vast depressions. Some of these saucer-like hollows form land
-surfaces, as in central Asia. The majority of these, however, are
-occupied by the oceans. Active volcanoes chiefly occur near the rim of
-the hollows which have the steepest slopes. The majority of earthquakes
-probably have their origin on or near the bottom of these slopes. To
-these, however, there are exceptions, as for instance the earthquakes
-in the Alps, in the hills of Scotland, and the shakings which are
-occasionally felt in countries like Egypt. The earthquakes which shake
-the borders of the Pacific have their origins in, and their effects are
-almost exclusively felt on, the sides of the bounding ridge facing this
-ocean. In Japan it is the eastern sides of the islands which suffer,
-the western side being almost as free from these convulsions as England.
-
-Similar remarks may be made about the eastern side of South America,
-especially the southern portion of the continent. At Buenos Ayres, for
-example, there has been no disturbance since Mendoza was destroyed,
-some twenty years ago. In British Guiana slight shocks are occasionally
-felt in the low delta which forms the settled portion of the colony,
-but they are extremely rare.
-
-_Disturbances in lines or zones._—It has often been observed that
-disturbances are propagated along the length of mountains or valleys,
-and it is but seldom that earthquakes cross them transversely. Thus the
-valleys of the Rhone, the Rhine, and the Danube are lines along which
-disturbances travel.
-
-The major axes of the elliptical areas of disturbances which have
-shaken India have a general direction parallel to the valley of the
-Ganges along the flanks of the Himalayas.
-
-The disturbances which have shaken London appear to have been chiefly
-east and west, or along the valley of the Thames. In South America the
-line of disturbance is along the western sides of the Andes. Another
-line is along the northern coast of the continent through Andalusia
-and Caraccas towards the Antilles and Trinidad. The shocks of the
-Pyrenees are chiefly felt along the southern side of these mountains.
-In the middle and on the northern side they are but seldom felt. This
-propagation in lines or zones may in certain cases be apparent rather
-than real. Thus the north and south ranges of mountains in Japan are
-mountains almost simultaneously shaken along their eastern flanks,
-giving the impression that an earthquake had originated simultaneously
-from a fissure parallel to this line, or else, starting at one end,
-had run down their lengths. Time observations have, however, shown
-that such disturbances had their origin at some distance in the ocean,
-and, travelling inwards, had reached all points on the flanks of these
-mountains almost simultaneously. The same explanation will probably
-hold for the so-called linear disturbances of western South America.
-
-All earthquake disturbances have probably a tendency to radiate from
-their source, and are only prevented from doing so by meeting with
-heavy mountainous districts, which by their mass and structure absorb
-the energy communicated to them. Much energy is also lost by emergence
-on the open flanks of a range of mountains. Rather than say that
-high mountains often bound the extension of an earthquake, or that
-earthquakes appear to run along the flanks of such mountains, we might
-say that earthquakes have boundaries parallel to the strike of the
-rocks in a given district, that such a direction is the one in which
-the propagation is the easier.
-
-Rossi is of opinion that volcanic fractures play an important part in
-governing the distribution of seismic disturbances. When a volcano is
-formed, a series of starlike fractures are formed round the central
-crater. Secondary craters may indicate the line of these fissures. The
-mountains about Rome are regarded as typical of this radial structure.
-The more distant the secondary craters are from the centre of the
-system, the smaller will they be, and also the younger. If two fissures
-intersect we get a larger crater at the junction. Earthquakes are
-propagated along the direction of these fissures, whilst the rising
-and falling of these lips throw off transverse waves. Rossi adduces
-observations which appear to meet with explanation on such suppositions.
-
-Suess, who has written upon the earthquakes of lower Austria, shows how
-the majority of the disturbances have had their origin along certain
-lines which form a break in the continuity of the Alps. One line runs
-north-east from Bruck towards Vienna. Near Wiener Neustadt, where the
-greatest number and heaviest shocks have occurred, this line is met by
-a north-north-west line crossing the Danube and following the valley
-of the river Kamp.[90] Hoeffer has drawn similar lines from the head
-of the Adriatic, one set running north-north-east to intersect near
-Litschau, and the other north-north-west to intersect near Frankfort in
-the valley of the Rhine.[91]
-
-_Examples of distribution._—A curious example of the distribution of
-seismic movement is that of the earthquake of March 12, 1873, worked
-out by Professor P. A. Serpieri. This earthquake appears to have been
-simultaneously felt on the Dalmatian coast and in central Italy, in a
-region lying north-east from Rome and south-east from Florence. In both
-of these areas the motion was from south-east to north-west. The shock
-then radiated from the central Italian regions, so that at places on
-the western shore of the Adriatic it was felt after it had been felt on
-the Dalmatian coast.
-
-Many explanations might be offered for this peculiar distribution
-of seismic activity. Possibly the shock originated at a great depth
-beneath the bed of the southern part of the Adriatic, and by following
-existing lines of weakness simultaneously reached the surface of the
-earth in central Italy and Dalmatia.
-
-In Tokio, which is built partly on a flat plain, partly in valleys
-denuded from a low tableland, and partly on the spurs of the tableland
-itself, the distribution of earthquakes is a subject yet requiring
-attention. Sometimes it has happened that persons in one house have
-been sufficiently alarmed to escape into the open air, whilst others,
-not more than a mile distant, have not been aware that the city had
-been shaken.
-
-[Illustration: FIG. 34.
-  Areas almost simultaneously struck from S.E. to N.W. [graphic]
-
-  Subsequent radial disturbance [graphic]]
-
-_Extension of earthquake boundaries._—Natural obstructions which may be
-sufficient to retard small earthquakes may in certain instances not be
-found sufficient to retard the larger disturbances. Thus the shocks of
-Calabria are usually only felt on the western side of the Apennines,
-but instances have occurred when they have crossed this barrier. In
-1801 the earthquake of Cumana crossed a branch of the coast range.
-
-Sometimes earthquake boundaries give way, and countries which they
-sheltered subsequently become exposed to all disturbances. The true
-explanation of this is probably in a shifting of the centre of seismic
-activity. Thus up to December 14, 1797, although Cumana was often
-devastated, the peninsula of Araya was not hurt. On this date Araya
-commenced to suffer, and has continued to suffer ever since.
-
-Fuchs gives an example of the movement of a seismic centre in the case
-of the Calabrian earthquake. The first shock commenced near Oppiedo,
-the second shock commenced four or five miles farther to the north, and
-the third shock had its origin five or six miles still farther, near to
-Girifalco.
-
-
-
-
-                             CHAPTER XIII.
-
-          DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_).
-
-
-_Seismic energy in relation to geological time._—If we admit that
-seismic energy is only a form of volcanic energy, it must also be
-admitted that any cause tending to produce a general decrease in the
-amount of the latter will also produce an alteration in the amount of
-the former.
-
-The nebular hypothesis of Laplace tells us that the solar system is the
-result of the whirling of a heated gaseous mass, which as it cooled
-continually contracted and consequently whirls the faster. With this
-hypothesis before us, we understand why all the planets and their
-satellites have a similarity in the directions of their movements, why
-they revolve nearly in the same plane, in orbits nearly circular, why
-some have a flattened figure and are surrounded by rings or belts, why
-the exterior planets should have a greater velocity of rotation, a
-greater number of satellites, and a less density as compared with the
-interior planets, the similarity of the elements in meteoric stones,
-the sun, the stars, and those found upon our earth, and lastly why
-there should be an increase in temperature as we descend into our
-earth.[92] This increase in temperature as we descend into the earth
-as deduced from many observations appears to be about 1° F. for every
-fifty or sixty feet of descent.
-
-To explain this and other kindred phenomena it is assumed that the
-earth was once very much hotter than it is at present, and to reach its
-present stage it has been gradually cooling. As the laws of cooling are
-perfectly known, to calculate how many years it must have taken a body
-like our earth to cool down to its present temperature is a definite
-problem. Sir William Thomson, starting with the temperature of 7,000°
-F., when all the rocks of the earth must have been molten and a skin or
-crust upon the surface, such as is so quickly produced upon the surface
-of molten lava, finds by calculation that the time taken to reach the
-present temperature must have been about one hundred million years.
-Into this period he and other physicists desire to compress the history
-of all the stratified deposits. Geologists find this period too short.
-Others seeking to reconcile the views of physicists and geologists
-endeavour to show that the various agencies engaged in degrading rocks
-and accumulating sediments in former ages are not to be judged of by
-the agencies we now see around us; in former times they were more
-active. At one period the elastic tides in the earth may have been so
-great that they resulted in the fracturing off from our planet its
-satellite the moon, and subsequently the moon, acting on the waters of
-the earth, may, even as late as 150,000 years ago, have produced every
-three hours tides 150 feet in height.
-
-Whatever may be the value of the figures here quoted, reasonings like
-these bring us to the conclusions that the various agencies which we
-now know to be acting upon our earth were once far more potent than
-they are at present, and if the moon, as a producer of elastic tides,
-has any influence upon the occurrence of earthquakes, it must have had
-a much greater influence in bygone times.
-
-We might speak similarly with regard to the internal heat of the earth.
-
-From the present heat gradient of our globe it is possible to calculate
-how much heat flows from the earth every year.
-
-This is equivalent to a quantity which would raise a layer of water ·67
-centimetres thick, covering the whole of our globe, from a temperature
-of 0° to 100° C.
-
-Similarly, we might calculate the quantity of heat which would be lost
-when the average heat gradient, instead of being 1° F. for fifty feet
-of descent, was 1° F. for twenty-five feet of descent.
-
-We might also calculate how many years ago it was since such a gradient
-existed.
-
-The general result which we should arrive at would be that in past
-ages the loss of heat was more rapid than it is at present. Now the
-contraction of a body as it cools is for low temperatures proportional
-to its loss of heat, and this law is also probably true for contraction
-as it takes place from high temperatures.
-
-Contraction being more rapid, it is probable that phenomena like
-elevations and depressions would be more rapid than they are at
-present, and generally all changes due to plutonic action, as has
-already been pointed out by Sir William Thomson, must have been more
-active.
-
-We have, therefore, every reason to imagine that earthquakes which
-belong to the category of phenomena here referred to were also numerous
-and occurred on a grander scale during the earlier stages of the
-world’s history than they do at present, and seismic and volcanic
-energy, when considered in reference to long periods of time, is
-probably a decreasing energy.
-
-In making this statement we must not overlook the fact that in
-geological time, as testified by the records of our rocks, volcanic
-action, and with it probably seismic action, has been continually
-shifting, first appearing in one area and then in another, and even
-in the same area we have evidence to show that these have periods
-of activity and repose successively succeeding each other. Thus in
-Britain, during the Palæozoic times, we have many evidences of an
-intense volcanic activity. During the Mesozoic or Secondary period
-volcanic energy appears to have subsided, to wake up with renewed
-vigour in the Cainozoic or Tertiary period.
-
-During this latter period it is not at all improbable that Scotland
-was in past times as remarkable for its earthquakes as Japan is at the
-present day.
-
-Later on it will also be shown that earthquakes are concomitant
-phenomena, with those elevatory processes which we have reason to
-believe are slowly going on in certain portions of the earth’s crust.
-If, therefore, we are able by the examination of the rocks which
-constitute the accessible portions of our globe to determine which
-periods were characterised by elevation, we may assume that such
-periods were also periods of seismic activity.
-
-Amongst these periods we have those in which various mountain ranges
-appeared. Thus the Grampians, and the mountains of Scandinavia, were
-probably produced before the deposition of the Old Red sandstone. The
-Urals were upheaved prior to Permian times. The chief upheaval in the
-Alps took place after Eocene times. The Rigi and other sub-Alpine
-mountains were formed after the deposition of the Miocene beds. About
-this same time the Himalayas were upheaved.[93]
-
-The earthquakes which from time to time shake those newer mountains
-apparently indicate that the process of mountain-making is hardly ended.
-
-_Seismic energy in relation to historical time._—The distribution
-of seismic energy with regard to historical time is a subject which
-has been very carefully examined by Mallet, who collected together
-a catalogue of between six and seven thousand earthquakes, embraced
-between the periods B.C. 1606 and A.D. 1850. The earthquake of B.C.
-1606 was on the occasion of the delivery of the law at Mount Sinai.
-Between B.C. 1604 and B.C. 1586 an earthquake probably occurred in
-Arabia, when Korah, Dathan, and Abiram were swallowed up. Another
-biblical record is that of B.C. 1566, when the walls of Jericho were
-overthrown.
-
-The earliest records from China is in B.C. 595; in Japan B.C. 285; in
-India A.D. 894.
-
-By using the number of earthquakes which have been recorded in each
-century as ordinates, Mallet constructed a curve, which apparently
-shows a continual increase in seismic energy, especially during recent
-times. This, Mallet remarks, contradicts all the analogies of the
-physics of the globe, and points out that the rapid increase in the
-number of earthquakes in latter years is chiefly due to the greater
-number of records which have been made, and the increase of the area
-of observation. No doubt many of the records made by the ancients have
-been lost.
-
-If we compare Mallet’s records, as he invites us to do, with the
-great outlines of human progress, we see that the two increase
-simultaneously, and we come to the conclusion that, taken as a whole,
-during the historical period the seismic activity of the world has been
-tolerably constant.
-
-These conclusions, based on the evidence at our command, are not to
-be confuted. If, however, instead of considering the seismic energy of
-the whole world, we consider the seismic energy of particular areas,
-it seems reasonable to expect that in certain instances sometimes a
-decrease and sometimes an increase in this energy might be discovered,
-especially, perhaps, in areas which are highly volcanic.
-
-In France we know that volcanic activity ceased at a period closely
-bordering on historical times, and it is not unlikely that seismic
-activity may have ceased at a corresponding time.
-
-In a country like Japan, it is possible that in one district seismic
-energy may be on the increase, whilst in another upon the decrease.
-
-In a country like England, it is probable that the seismic state is
-constant, and, whatever changes may be now occurring, they are taking
-place at so slow a rate that, even if our records of the historical
-period were complete, we could hardly be expected to find these changes
-sufficiently marked to be observable.
-
-For purposes of reference, and also for examining the present question,
-the table, page 240, has been compiled. The earthquakes given are
-chiefly those which have been recorded in histories as being more or
-less destructive.
-
-In the second column of this table will be seen the number of
-earthquakes which have occurred in Japan during each century, the
-centuries being marked in the first column. In columns 3 to 18
-inclusive are given the number of earthquakes which have occurred
-during different centuries in the various countries and districts
-mentioned at the head of each column. These latter, which are taken
-from the writings of Mallet, are given for the sake of comparison with
-the Japanese earthquakes. If we commence with the seventh century in
-the column for Japan, we see that a great increase in the number of
-earthquakes, as we come towards the present time, is not so observable
-as it is in the other columns.
-
- Key:
-    1 Centuries
-    2 Japan
-    3 Scandinavia and Iceland
-    4 British Isles and Northern Isles
-    5 Spanish Peninsula
-    6 France, Belgium, Holland
-    7 Rhine Basin
-    8 Switzerland and Rhine Basin
-    9 Danube Basin
-   10 Italy, Sicily, Sardinia, and Malta
-   11 Supplemental table for Italy, Sardinia, and Malta
-   12 Turco-Hellenic Territory, Syria, Ægean Isles, and Levant
-   13 United States and Canada
-   14 Mexico and Central America
-   15 Antilles
-   16 Cuba
-   17 Chili and La Plata Basin
-   18 Northern Zone of Asia
-   19 Approximate Intensity in the Kioto District of Japan
- +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+
- |   1  |2 | 3 | 4 |5 | 6 |7 | 8 | 9 | 10|11| 12|13|14| 15|16| 17|18|19|
- +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+
- |I.    | 1| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--|
- |II.   |--| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--|
- |III.  | 1| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--|
- |IV.   |--| --| --|--| --|--| --| --|  6|--| 23|--|--| --|--| --|--|--|
- |V.    | 1| --| --|--|  1|--| --| --|  5|--| 19|--|--| --|--| --|--|--|
- |VI.   | 1| --| --|--|  6|--| --|}  |  3|--| 27|--|--| --|--| --|--|--|
- |VII.  |12| --| --|--| --|--| --|}  |  1|--|  8|--|--| --|--| --|--|15|
- |VIII. |11| --| --|--| --|--| --|}  |  2| 1| 12|--|--| --|--| --|--|17|
- |IX.   |40| --| --|--| 21|--| 19|}  |  6|--|  7|--|--| --|--| --|--|60|
- |X.    |17| --| --|--|  2|--|  2|}19|  3| 3|  5|--|--| --|--| --|--|24|
- |XI.   |20| --|  8| 3| 16|--|  9|}  |  7| 5| 18|--|--| --|--| --|--|28|
- |XII.  |18| --| 11| 4| 12|--|  8|}  | 18|22| 23|--|--| --|--| --|--|20|
- |XIII. |16|}  | 15| 3|  9|--|  3|}  | 15|26| 13|--|--| --|--| --|--|16|
- |XIV.  |19|}  |  4| 8| 21|--| 18|}  | 20|51|  8|--|--| --|--| --|--|25|
- |XV.   |36|}28|  1| 4| 14|--| 12|}  | 18|47| 11|--|--| --|--| --|--|29|
- |XVI.  |17|}  |  8|10| 61|10| 52| 35| 32| 5| 22|--| 6|  1| 4|  5|--|17|
- |XVII. |26|}  | 14|10| 91|29|120| 31|121| 9| 53|10| 7| 16| 4|  9|--|11|
- |XVIII.|31|111| 63|93|237|71|141| 88|438|20|124|88|24| 85| 2| 10|32| 8|
- |XIX.  |27|113|110|85|211|81|173|145|390|88|194|51|30|145|50|170|57| 8|
- +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+
-
-The explanation for this probably lies in the fact that Japan has
-practised civilised arts for a longer period than many of the European
-and other countries mentioned in the table.
-
-In Japan, no doubt, the records of later years have been more perfect
-than they were in early times, but this, although so potent an effacer
-of what was probably the true state of natural phenomena in the case of
-Europe, has not quite obliterated the truth in Japan; for instead of an
-apparent increase of seismic energy since early times it shows a slight
-decrease.
-
-To draw up a table of earthquakes such as the one which has just been
-given, and then, after the inspection of it, draw conclusions as to
-whether there has been an increase or decrease in seismic energy, is,
-however, hardly a just method of reasoning. The earthquakes, taken as
-they are, for the whole of Japan, represent a collection of places some
-of which are 1,000 miles apart. When we consider that many earthquakes
-which occurred at one end of this line were never felt at the other
-end, in order to justly estimate the periodicity of seismic phenomena
-it would seem that we ought either to take some particular seismic area
-or else the whole world.
-
-The particular area which has been taken is that of Kioto in Central
-Japan, and the earthquakes which have been felt there are enumerated in
-the table.
-
-In order to show the variation in seismic activity of this district a
-curve has been plotted, fig. 35, with ordinates equal to the values
-given for the Kioto earthquakes during succeeding centuries. The upper
-points of these ascending and descending lines are joined by a free
-curve. The lower points are similarly joined. The points of bisection
-of ordinates drawn between these two curves are taken as points in a
-curve to show the true secular change in seismic energy.
-
-[Illustration: FIG. 35.—Curve of Seismic Intensity for Kioto.]
-
-By looking at this wavy line it will be seen that the intervals between
-maxima and minima are closer together in early times than they are
-later on.
-
-Thus, between the eighth century and the ninth century, points of
-maximum and minimum seismic efforts occurred at times a century apart,
-whilst later on, from the eleventh to the fifteenth century, they were
-at intervals of 300 years apart.
-
-By inspecting either the wavy line or the resultant curve, it will be
-seen that since the ninth century down to the present time there has
-been a decided decrease in seismic energy. From the ninth century down
-to the fifteenth century this decrease is represented by a regular
-curve. At this point, however, the decrease becomes slightly more
-rapid, and is represented by a second curve. If, instead of calculating
-ordinates for my curve, in which intensity has been considered, simply
-the number of earthquakes are counted, a similar result is obtained.
-From this it appears that the rate at which seismic energy decreased
-during the last 500 years was about the same as that at which it
-decreased during the 500 years previous to this period.
-
-If the lists for the Italian and Turco-Hellenic districts could be
-similarly analysed, and the earthquakes of any particular district
-picked out from the others, it is very probable that a similar decrease
-or alteration in seismic energy might be observed.
-
-Provided that we have at our disposal records of the various
-earthquakes which have occurred in any given district during a
-sufficiently long period of time, one conclusion that we may expect
-to arrive at is that we shall be able to trace some variation in
-the seismic activity of that district. For the Kioto area, it has
-been shown that there is a diminution in seismic activity, In other
-districts, however, there may possibly be an increase.[94]
-
-_Relative frequency of earthquakes._—A question which is of great
-interest to those who dwell in shaken districts is as to how often
-disturbances may be expected to occur.
-
-From a general examination of this question, considering the
-earthquakes of the whole world. Mallet arrived at the following
-conclusions:—
-
-1. While the smallest or minimum paroxysmal intervals may be a year
-or two, the average interval is from five to ten years of comparative
-repose.
-
-2. The shorter intervals are in connection with periods of fewer
-earthquakes—not always with those of least intensity, but usually so.
-
-3. The alternations of paroxysm and of repose appear to follow no
-absolute law deducible from these curves.
-
-4. Two marked periods of extreme paroxysm are observable in each
-century, one greater than the other—that of greatest number and
-intensity occurring about the middle of each century, the other towards
-the end of each.
-
-The form of the curves which Mallet has drawn seem to indicate that
-seismic energy came in sudden bursts, and then subsided, gradually
-gathering strength for another exhibition. This is continually seen
-in the shocks experienced in various seismic areas—a large shock, or
-the maximum of the activity dying out by repeated small shocks on
-succeeding days.
-
-Mr. I. Hattori, writing on the large earthquakes of Japan, remarks that
-on the average there has been one large earthquake every ten years.
-They, however, occur in groups, as shown in the following table.
-
- +------+---------------+----------+
- |No. of|               |          |
- |shocks|    Period     | Interval |
- +------+---------------+----------+
- |  6   | A.D. 827–836  | 10 years |
- |  6   |  „   880–890  | 10   „   |
- |  4   |  „  1040–1043 |  4   „   |
- |  5   |  „  1493–1507 |  5   „   |
- |  4   |  „  1510–1513 |  4   „   |
- |  5   |  „  1645–1650 |  6   „   |
- |  5   |  „  1662–1664 |  3   „   |
- |  4   |  „  1853–1856 |  4   „   |
- +------+---------------+----------+
-
-Dr. E. Naumann, who has also written on the earthquakes in Japan,
-remarks that if periods of seismic activity do not occur every 490
-years, there is a repetition of the cycle after 980 years, but there
-is much variability. A period of 68 years is very marked. On the
-average, large earthquakes have occurred every 5·9 years. Fuchs
-gives some interesting examples of the repetition of earthquakes at
-definite intervals, of which the following are examples. Sometimes
-earthquakes appear to have repeated themselves after 100 years. One
-remarkable example of this is that of Lima, on June 17, 1578, which was
-repeated on the same day in the year 1678. In Copiapo it is believed
-that earthquakes occur every twenty-three years, and examples of such
-repetitions are found in the years 1773, 1796, and 1819. In Canada,
-near to Quebec, earthquakes lasting forty days are said to occur every
-twenty-five years. The plateau of Ardebil is said to be regularly
-shaken by earthquakes every two years.
-
-A. Caldcleugh, writing on the earthquake of Chili, in 1835,[95] remarks
-that the Spaniards first had the idea that a great earthquake occurs
-every century. Afterwards they thought the period was every fifty
-years. As a matter of fact, however, there were large earthquakes in
-1812, at Caracas; in 1818, at Copiapo; in 1822, at Santiago; in 1827,
-at Bogota; in 1828, at Lima; in 1829, at Santiago; and in 1832, at
-Huasco.
-
-The average period of seismic disturbances in any country probably
-depends upon the subterranean volcanic activity of that country.
-When the activity is great the large earthquakes may occur at short
-intervals; but when the activity is small, as in England, shocks of
-moderate intensity may not be felt more than once or twice per century.
-A general idea of the relative frequency of the large earthquakes in
-various parts of the world may be easily obtained by an inspection of
-the table on page 240.
-
-Between the years 1850 and 1857 Kluge found that in the world there had
-been 4,620 earthquakes, which is, upon the average, nearly two per day.
-This estimate of the frequency of earthquakes of sufficient intensity
-to be recorded without the aid of instruments is, however, much below
-the truth. In Japan alone there probably occurs, as a daily average, a
-number at least equal to that which has been just given for the whole
-world. Boussingault considered that, in the Andes, earthquakes were
-occurring every instant of time.[96]
-
-To state definitely how many earthquakes are felt in the world on the
-average every day is, from the data which we have at our command, an
-impossibility. Perhaps there may be ten, perhaps there may be 100. The
-question is one which remains to be decided by statistics which have
-yet to be compiled.
-
-After a large earthquake, smaller shocks usually occur at short
-intervals. At first the succession of disturbances are separated from
-each other by perhaps only a few minutes or hours. Later on, the
-intensity of these shocks usually decreases, and the intervals between
-them become greater and greater, until, finally, after perhaps a few
-months, the seismic activity of the area assumes a quiescent state.
-
-The great earthquake which overtook Concepcion on February 20, 1835,
-was followed by a succession of shocks like those just referred to,
-there being registered, between the date of the destructive shock and
-March 4, 300 smaller disturbances.
-
-During the twenty-four hours succeeding the destruction of Lima
-(October 28, 1746), 200 shocks were counted, and up to the 24th of
-February in the following year 451 shocks were felt.
-
-At St. Thomas, in 1868, 283 shocks were counted in nine and a quarter
-hours.
-
-Similar examples might be taken from the description of almost
-all destructive earthquakes of which we have records. For a large
-earthquake to occur, and not to be accompanied by a train of succeeding
-earthquakes, is exceptional. Sometimes we find that a large number of
-small earthquakes have occurred without a large one being felt. Seismic
-storms of this description have happened, even in England—for instance,
-in the year 1750, which appears to have been a year of earthquakes for
-many portions of the globe.
-
-In this year, which is known as the ‘earthquake year,’ shocks were felt
-in England as follows: On March 14, in Surrey; March 18, in south-west
-of England; April 2, at Chester; June 7, at Norwich; August 23, in
-Lincolnshire; September 30, Northamptonshire.
-
-_Synchronism of earthquakes._—One of the first writers who drew
-attention to the fact that two shocks of earthquakes have been felt
-simultaneously at distant places was David Milne, who published a list
-of these occurrences.[97]
-
-In two instances, February and March 1750, shocks were simultaneously
-felt in England and Italy. In September 1833 shocks appear to have been
-simultaneously felt in England and Peru. These and many other similar
-examples are discussed by Mallet, who thinks with Milne that these
-coincidences are in every probability matters of accident. According to
-Fuchs, Calabria and Sicily appear often to have had earthquakes at the
-same time, as for instance in 1169, 1535, 1638, when the town Euphemia
-sank, and in the years 1770, 1776, 1780, and 1783.
-
-A remarkable example of coincidence occurred on November 16, 1827, when
-a terrible earthquake was felt in Columbia, and at the same time a
-shock occurred on the Ochotsk plains, nearly antipodal to each other.
-
-Kluge also gives a large number of instances of simultaneous
-earthquakes; thus, on January 23, 1855, on the same day that
-Wellington, New Zealand, so severely suffered, there was a heavy
-earthquake in the Siebengeberge, and also in North America. To this
-might be added the fact that the last destructive earthquake in Japan
-occurred within a few days of this time.
-
-Sometimes neighbouring countries where earthquakes are common are
-equally remarkable by their utter want of synchronism. For example,
-Southern Italy and Syria are said never to be shaken simultaneously.
-
-_Secondary earthquakes._—Although it is possible that the simultaneous
-occurrence of earthquakes in distant regions may sometimes be a
-matter of chance, it must also be remarked that the shaking produced
-by one earthquake may be sufficient to cause ground which is in a
-critical state to give way, and thus the first disturbance becomes the
-originator of a second earthquake. Admitting that an earthquake, as
-it radiates from its centre, may act in such a manner, we see that a
-feeble disturbance might be the ultimate cause in the production of a
-destructive earthquake, just as the disturbance of a stone upon the
-face of a scarp might, by its impact upon other stones, cause many tons
-of material to be dislodged.
-
-It is also easy to conceive how the seismic activity of two districts
-may be dependent upon each other. Inasmuch as these secondary shocks
-are direct effects of primary disturbances, they might have been
-treated in a previous chapter.
-
-As examples of consequent or secondary earthquakes Fuchs tells us that
-when small earthquakes take place in Constantinople and Asia Minor,
-earthquakes are felt in Bukharest, Galazy, and Kronstadt.
-
-The great Lisbon earthquake also appears to have given rise to several
-consequent disturbances. One was in Derbyshire, occurring at 11 a.m.
-It was sufficiently violent to cause plaster to fall from the sides
-of a room and a chasm to open on the surface of the ground. Some
-miners working underground were so alarmed that they endeavoured to
-escape to the surface. During twenty minutes there were three distinct
-disturbances.
-
-Another shock was felt at Cork.[98]
-
-Although these disturbances own a consequence of the Lisbon earthquake
-they might properly perhaps be attributed to the pulsations produced by
-the shock at Lisbon, which spread through England and other countries
-without being felt.
-
-The shocks which men felt in New Zealand and New South Wales in 1868
-were probably secondary shocks, due to the disturbance at Arequipa and
-other places on the South American coast.
-
-These so-called secondary earthquakes, although in many instances
-they may be due to earth pulsations produced by earthquake, or to the
-immediate sensible shaking of a large earthquake, may perhaps, in
-certain instances, be attributed to some widespread disturbance beneath
-the crust of the earth. The occurrence of periods where all earthquake
-countries suffer, unusual disturbances indicate the probability of such
-underground phenomena.
-
-
-
-
-                             CHAPTER XIV.
-
-          DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_).
-
-  The occurrence of earthquakes in relation to the position of the
-    heavenly bodies—Earthquakes and the moon—Earthquakes and the sun;
-    and the seasons; the months—Planets and meteors—Hours at which
-    earthquakes are frequent—Earthquakes and sun spots—Earthquakes
-    and the aurora.
-
-
-_The position of the heavenly bodies and the occurrence of
-earthquakes._—Since the earliest times, in searching for the cause of
-various natural phenomena, man has turned his energies towards the
-heavens. One of the earliest observations was the connection that
-exists between the season of the year and the motions of the heavenly
-bodies. Tides were seen to be influenced by the moon. In later times
-it has been discovered that periods of maximum magnetic disturbances
-occur every ten or eleven years with the sun spots, and Herr Kreil, of
-Vienna, tells us our satellite, the moon, has also an influence upon
-the magnet.
-
-From day to day we see the bond connecting our planet with the sun, the
-moon, and other heavenly bodies which are outside us gradually becoming
-closer.
-
-Inasmuch as many phenomena, like the motion of the tides, the rise and
-fall of the barometer, fluctuations in temperature, are all more or
-less directly connected with the relative position of our planet with
-regard to the sun and moon, any coincidence between the phases of
-these bodies and the occurrence of earthquakes more or less involves a
-time relationship with the other phenomena resultant on lunar and solar
-influences.
-
-_Earthquakes and the position of the moon._—Many earthquake
-investigators have attempted to show the connection between earthquakes
-and the phases of the moon.
-
-The first and most successful worker in this branch of seismology was
-Professor Alexis Perrey, of Dijon, who, after many years of arduous
-labour in tabulating and examining catalogues of earthquakes, showed
-that earthquakes were more likely to occur at the following periods
-than at others.
-
-1. They are more frequent at new or full moon (syzygies) than at half
-moon (quadratures).
-
-2. They are more frequent when the moon is nearest the earth (perigee)
-than when she is farthest off (apogee).
-
-3. They are more frequent when the moon is on the meridian than when
-she is on the horizon.
-
-These results were obtained by Perrey after analysing his catalogues
-by three different and independent methods, and they were confirmed
-by the report of a committee appointed by the Academy of Sciences. It
-must, however, be remarked that in several instances anomalies occur,
-and also that the difference between the number of earthquakes at any
-two periods is not a very large one. Thus, for instance, the annual
-catalogues compiled by Perrey from 1844 to 1847, the earthquakes in
-perigee are to those in apogee as 47 : 39. Between the years 1843 and
-1872 Perrey finds that 3,290 shocks occurred at the moon’s perigee, and
-3,015 at the apogee.[99]
-
-Between 1761 and 1800 earthquakes occurred as follows:—
-
-  In Perigee       526
-  Apogee           465
-
-The following table shows the results which enabled Perrey to deduce
-his first law.
-
-Dividing the period of lunation into quarters, with the time of
-syzygies and quadratures as the centres of these quarters, he found
-that the earthquakes were distributed as follows.
-
- +-----------+--------+----------+-------------+---------------+
- |           |        |          |             | Difference in |
- |           | Totals | Syzygies | Quadratures | favour of the |
- |           |        |          |             |   Syzygies    |
- +-----------+--------+----------+-------------+---------------+
- | 1843–1847 |  1,604 |   850·48 |    753·52   |     69·96     |
- | 1848–1852 |  2,049 | 1,053·53 |    995·47   |     58·06     |
- | 1853–1857 |  3,018 | 1,534·13 |  1,483·87   |     50·26     |
- | 1858–1862 |  3,140 | 1,602·99 |  1,537·41   |     65·98     |
- | 1863–1867 |  2,845 | 1,463·42 |  1,381·58   |     81·84     |
- | 1868–1872 |  4,593 | 2,333·48 |  2,259·52   |     73·96     |
- | 1843–1872 | 17,249 | 8,838·03 |  8,410·97   |    427·06     |
- +-----------+--------+----------+-------------+---------------+
-
-The reported earthquakes between 1751 and 1843 are shown to conform
-with the same rule.[100] Julius Schmidt, astronomer at Athens, found
-for the earthquakes of Eastern Europe and adjacent countries for the
-years 1776 to 1873 that there were more earthquakes when the moon was
-in perigee. Other maxima were at new moon, and two days after the first
-quarter. There was a diminution at full moon, and a minimum on the day
-of the last quarter. As one example of results which are antagonistic
-to the general results obtained by Perrey may be quoted the results of
-an examination by Professor W. S. Chaplin of the earthquake recorded at
-the meteorological observatory in Tokio. The list of earthquakes, 143
-in number, extending over a period of three years, was recorded by one
-of Palmieri’s instruments. The results were as follows:—
-
-1. There have been maxima of earthquakes when the moon was two and
-nine hours east and seven hours west. At the upper transit there is a
-minimum.
-
-2. Considering the moon’s position with regard to the sun, at
-conjunction there were 32, at opposition 37, and at quadrature 74. East
-of the meridian the maximum was at least four hours.
-
-3. When the moon was north of the equator these were 68, when south 82.
-
-4. A maximum of earthquakes seven and eleven days after the moon’s
-perigee. The fact that these results were obtained for the earthquakes
-of a special small seismic area renders them more interesting.[101]
-
-_Frequency of earthquakes in relation to the position of the sun._—The
-question as to whether there is a connection between the frequency of
-earthquakes and the relative position of the sun is to a great extent
-identical with the question as to the relative frequency of earthquakes
-in the various seasons. It is a subject which we find referred to
-by writers in the earliest ages. Pliny and Aristotle thought that
-earthquakes occurred chiefly in spring and autumn. In later times it
-has been a subject which has been most carefully considered by Merian,
-von Hoff, Perrey, Mallet, Volger, Kluge, and others who have devoted
-attention to seismology. In a résumé of the earthquakes of Europe, and
-of the adjacent parts of Asia and Africa, from A.D. 306–1843, Mallet
-gives the following results:—
-
- +------------------+------------------------+----------------------+
- |                  | For Nineteenth Century | For the whole period |
- +------------------+------------------------+----------------------+
- | Winter Solstice  |   177 } Solstices      | 253 }  Solstices     |
- | Spring Equinox   |   151 } }  306         | 170 } }  403         |
- | Summer Solstice  |   129 } } Equinoxes    | 150 } } Equinoxes    |
- | Autumnal Equinox |   164   }   315        | 159   }   329        |
- +------------------+------------------------+----------------------|
-
-The above periods were called by Perrey _critical epochs_, because
-as a general result of his researches he found that at such periods
-there was a greater frequency of earthquakes. Fuchs, quoting from
-Kluge’s tables, extending from 1850–1857, tells us that the recorded
-earthquakes occurred as follows:—
-
-  In the Northern Hemisphere—
-      Equinoxes                =1324=
-      Solstices                 1202
-  In the Southern Hemisphere—
-      Equinoxes                 =301=
-      Solstices                  261
-
-Earthquakes are, therefore, more frequent at the equinoxes, and this
-especially at the autumnal equinox. In the northern hemisphere, at
-the solstices, the greater number of shocks occur about the winter
-solstices, whilst in the southern hemisphere, about the summer
-solstices.
-
-Exceptions, however, are found in Central America and the West Indies,
-in the Caucasus, and the Ægean Sea.
-
-The idea that earthquakes had a periodicity dependent upon the position
-of the heavenly bodies is by no means confined to Europe. In a Japanese
-work called ‘Jishin Setsu’ (an opinion about earthquakes) by a priest
-called Tensho, it is stated that the relative positions and movements
-of the twenty-eight constellations with respect to the moon cause
-earthquakes. This Tensho asserts after careful calculation, and Falb
-tells us that all future earthquakes can be predicted.
-
-In the Kuriles and Kamschatka, Sicily, and in parts of South America,
-it is said that the equinoxes are regarded as dangerous seasons.
-
-_Frequency of earthquakes in relation to the seasons and months._—What
-is here said respecting the relative frequency of earthquakes at the
-different seasons and months is little more than an extension and
-critical examination of the results which have been given respecting
-the frequency of earthquakes in regard to the position of the sun.
-
-That there is a difference between the number of earthquakes which are
-felt at one season of the year as compared with those felt at another
-is a fact which, as seismoscopic observations are extended, is becoming
-more and more recognised.
-
-Some of the more important results which were arrived at by Mallet from
-5,879 observations made in the northern hemisphere, and 223 in the
-southern hemisphere, may be expressed as follows:—
-
- +---------------------+------------------------+-------------------+
- |                     |        Maxima          |       Minima      |
- +---------------------+------------------------+-------------------+
- | Northern Hemisphere | January, also a slight | May, June, and    |
- |                     |   rise in August and   |   July            |
- |                     |   October              |                   |
- | Southern Hemisphere | November, also May     | March, extending  |
- |                     |   and June             |   over one month, |
- |                     |                        |   also August     |
- +---------------------+------------------------+-------------------+
-
-Julius Schmidt, of Athens, who so carefully examined the earthquakes of
-eastern Europe, came to the following conclusions:—
-
-For the earthquakes between 1200 and 1873, a maximum on September 26
-and January 17; a minimum on December 3 and June 13.
-
-For the earthquakes between 1873 and 1874, a maximum on March 1 and
-October 1; a minimum on July 7 and December 15.
-
-For all the earthquakes of eastern Europe, a maximum on January 3; a
-minimum on July 8, or there was a maximum at perihelion and aphelion.
-
-When the months are grouped together according to the seasons, spring,
-summer, autumn, and winter, we find that in the northern hemisphere the
-minimum is in summer and the maximum in winter, whilst in the southern
-hemisphere (giving the proper months corresponding to its seasons) we
-find two maxima, one at the commencement of winter, and the other at
-midsummer, whilst the minima are in spring and autumn.
-
-[Illustration: FIG. 36.—Curves of Monthly Seismic Intensity (Mallet).]
-
-In the following table the difference in the number of earthquakes felt
-at different seasons is given more in detail.
-
-In examining this table, we must remember that for countries like Peru,
-Chili, and New Zealand, lying in the southern hemisphere, the records
-given for the months April to September correspond to the winter months
-of those countries. The Roman numerals indicate the centuries between
-which the records date.
-
- +----------+----------------------------------------+-------+---------+
- |          |                                        |October|  April  |
- |          |                                        |  to   |   to    |
- |          |                                        | March |September|
- +----------+----------------------------------------+-------+---------+
- |         {| 1. Scandinavia and Iceland, xii–xix    | =129= |    91   |
- |         {| 2. British and Northern Isles, xi–xix  | =123= |    94   |
- |         {| 3. Belgium, France, and Holland, iv–xix| =395= |   272   |
- |         {| 4. Rhone Basin, xvi–xix                | =115= |    69   |
- |         {| 5. Switzerland and Rhine Basin, ix–xix | =327= |   205   |
- |         {| 6. Danube Basin, v–xix                 | =147= |   128   |
- |         {| 7. Spanish Peninsula, xi–xiv           | =114= |    87   |
- |         {| 8. Italy, Sicily, Sardinia, and Malta, |       |         |
- |Northern {|      iv–xix                            |  650  |   581   |
- | Regions {| 9. Turco-Hellenic Territory, Syria,    |       |         |
- |         {|    Ægean Isles, and Levant, iv–xix     |  214  |  =222=  |
- |         {|10. Northern Zone of Asia, xviii–xix    |  =46= |    36   |
- |         {|11. Japan (Tokio area), 1872–1880       |       |         |
- |         {|    (small earthquakes)                 | =213= |   157   |
- |         {|12. Japan B.C. 295-A.D. 1872 (large     |       |         |
- |         {|    earthquakes)                        |  165  |  =188=  |
- |         {|13. Algeria and Northern Africa         |  =26= |    20   |
- |         {|14. United States and Canada, xvii–xix  |  =86= |    48   |
- |        { |15. Java, Sumatra, and neighbouring     |       |         |
- |        { |      Islands, 1873–4–7–8               | =194= |   182   |
- |Central { |16. Mexico and Central America, xvi–xix |   26  |    26   |
- |Regions { |17. West Indies (Mallet), xvi–xix       |  108  |  =114=  |
- |        { |18. West Indies, xvi–xix                |  296  |  =343=  |
- |        { |19. Cuba, xvi–xix                       |  =28= |    23   |
- |         {|20. Chili, and La Plata Basin, xvi–xix  |   89  |    89   |
- |Southern {|21. Peru, Columbia, Basin of Amazons,   |       |         |
- |Regions  {|      xvi–xix                           |  506  |  =541=  |
- |         {|22. New Zealand, 1869–1879              |  166  |  =176=  |
- +----------+----------------------------------------+-------+---------+
-
-Neglecting those records which show as many earthquakes for the winter
-months as for the summer months, we see at a glance that generally
-the greater number of shocks have happened during the colder seasons.
-In the southern hemisphere, so far as the records go, this is not
-true. In the northern regions, out of fourteen examples there are
-two exceptions. In the central regions there are two cases where the
-greatest number of earthquakes have been recorded in the winter months,
-and two cases where the greatest number have been recorded for the
-summer.
-
-Altogether, out of twenty-two examples, there are only six exceptions
-to the rule. These exceptions altogether occur among records many of
-which are ancient, and are, therefore, more open to error than lists
-which have been compiled in modern times.
-
-Because small earthquakes are seldom noticed by persons out in the
-open air, it might be expected that the number of earthquakes observed
-in warm countries at one portion of the year would be equal to those
-observed in any other season. Such an argument, however, would hardly
-apply to most of the records which are quoted, as they refer to
-destructive disturbances.
-
-If, however, we take the records made in tropical countries from the
-table just given, we see that in such countries there have been almost
-as many observations of earthquakes at one season as at any other.
-
-Another fact which might be adduced against the rule that the
-greater number of earthquakes occur during the winter months would
-be the comparison of a table of earthquakes recorded previous to the
-nineteenth century. By doing this we see that for certain countries the
-winter rule is inverted, and that the greater number of shocks are felt
-during the summer.
-
-Notwithstanding these objections to Perrey’s conclusions, the balance
-of evidence is in favour of his general result, and we may conclude
-that during the colder portions of the year we may expect more shakings
-than during the warmer portions. Comparing the number of earthquakes of
-winter and autumn to those of summer and spring, they are to each other
-in the proportion of 4 : 3.
-
-A fairer way to examine this question, and to determine what is
-probably the present state of seismic activity in our globe, would
-be only to consider the earthquakes which have taken place in
-comparatively recent times, laying especial stress upon those
-observations which have been made with the assistance of automatic
-instruments, or those which have been collected by persons interested
-in these investigations.
-
-For this purpose the following table, showing the distribution of
-earthquakes in different countries during the nineteenth century, has
-been compiled.
-
-The arrangement is mensual. Where the number of earthquakes in any
-month is above the average, the number is printed in large type; where
-below the average, in small type.
-
-
-      EARTHQUAKES OF THE NINETEENTH CENTURY, CHIEFLY FROM PERREY.
-
- Key:
-   Jan January
-   Feb February
-   Mar March
-   Apr April
-   May May
-   Jun June
-   Jul July
-   Aug August
-   Sep September
-   Oct October
-   Nov November
-   Dec December
-   Ave Average per month
- +------------------------------------------------------------------------------+
- |                         |Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec|Ave |
- |-------------------------+---+---+---+---+---+---+---+---+---+---+---+---+----|
- |Scandinavia and Iceland  | 17| 11| 11|  7|  7|  6|  8|  8| 10| 10| 11|  6| 9·3|
- |British Isles and        |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  Northern Isles         |  9|  9| 10|  7|  8|  6|  5| 11| 12|  8| 11| 12| 9  |
- |France, Belgium, Holland | 27| 17| 21| 13| 13|  8| 15| 17| 15| 17| 21| 25|17  |
- |Basin of the Rhone       | 12| 12|  8|  3|  3|  2|  2|  4|  6|  6|  8| 14| 6·6|
- |Basin of the Rhine and   |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  Switzerland            | 15| 17| 13| 12| 11|  6| 12| 11| 10| 17| 24| 25|14  |
- |Basin of the Danube      | 14| 15|  9|  8| 12|  8| 16| 11| 11| 16| 10| 12|11·8|
- |Spanish Peninsula        | 10|  5|  6|  7|  4|  6| 10|  5|  9| 11|  7|  5| 7  |
- |Italian Peninsula,       |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  Sicily, Sardinia,      |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  and Malta              | 44| 44| 48| 43| 40| 34| 41| 46| 27| 45| 26| 39|39  |
- |Turco-Hellenic Territory,|   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  Syria, Ægean Islands,  |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  and Levant             | 22| 20| 10| 10| 16| 15| 14| 22| 14| 17| 12| 14|16  |
- |Northern Zone of Asia    |  4|  6|  6|  4|  4|  3|  5|  7|  6|  3|  4|  5| 4·7|
- |1876–1881, Japan (Tokio  |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  area)                  | 39| 41| 41| 30| 33| 30| 27| 21| 10| 28| 34| 43|31·4|
- |Japan (large earthquakes)| --|  5|  3|  3|  1| --|  5|  4| --| --|  1|  5| 2  |
- |Algeria and North’rn     |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  Africa                 |  5|  2|  6|  7|  3|  2|  2|  5|  1|  4|  8|  1| 3·8|
- |United States and Canada |  4|  4|  3|  3|  3| --|  4|  6|  3|  2|  7|  5| 3·8|
- |Java, Sumatra, &c.,      |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  1873–4–5–7 and 9       | 35| 30| 38| 33| 22| 36| 27| 40| 24| 35| 30| 26|31  |
- |Mexico and Central       |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  America                |  3|  2|  2|  2|  6|  2|  2|  1|  1|  3|  2|  3| 2·5|
- |Antilles                 |  9|  8| 19| 12| 12| 10|  9| 16| 12| 10| 13| 12|11·8|
- |Cuba                     |  4|  3|  2|  3|  3|  4|  5|  2|  6|  5|  6|  4| 4  |
- |Chili and La Plata       | 14| 10| 14|  8| 19| 11| 16| 15| 16|  9| 27|  8|13·9|
- |Peru, Columbia, Basins   |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |  of Amazons, xvi–xix    | 92| 83| 92| 27|106| 79| 94| 93| 97| 77| 72| 90|87  |
- |New Zealand, 1869–79     | 31| 27| 37| 23| 22| 31| 27| 36| 37| 21| 27| 23|28·5|
- |    Jan. 1850, Dec. 1857 |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |Northern Hemisphere      |153|162|143|161|126|124|141|156|154|171|151|168|150 |
- |Southern Hemisphere      | 75| 43| 61| 66| 46| 42| 53| 39| 54| 55| 57| 46|53  |
- |      1821–1830          |   |   |   |   |   |   |   |   |   |   |   |   |    |
- |Northern Hemisphere      | 31| 36| 31| 29| 33| 33| 20| 31| 24| 41| 26| 34|30  |
- |Southern Hemisphere      |  2| --|  1|  1|  3|  1|  3|  2|  3|  2|  1|  1| 1·6|
- +-------------------------+---+---+---+---+---+---+---+---+---+---+---+---+----+
-
-A glance at this table shows that for most countries in the northern
-hemisphere the rule that there are generally more earthquakes during
-the winter months—that is, from October to March—holds good. For
-countries which lie comparatively near to the Equator, and also for
-those countries in the southern hemisphere, the rule is not so clear.
-When examining this table it must be remembered that it does not enable
-us to judge of the relative frequency of earthquakes in different
-countries, inasmuch as the periods over which the records were taken
-are different in different cases.
-
-To the above table might be added the records of P. Merian, who
-examined the earthquakes felt in Basle up to 1831. As a result he
-found that during the winter months eighty shocks had been felt,
-whilst during the summer only forty. Taking the records for the two
-hemispheres from 1850–1857, compiled by Kluge,[102] in the northern
-hemisphere we have in the months between October and March 948 shocks
-against 862 in the remainder of the year. In the same months in the
-southern hemisphere we have for the corresponding periods the numbers
-337 and 300, and thus both hemispheres would appear to follow the same
-rule. If, however, we examine the table we see that the two seasons
-are not so pronounced for the southern hemisphere as they are for
-the northern, and that there may be two or three periods of maximum
-disturbance as has been previously indicated.
-
-_Earthquakes and the planets and meteors._—Just as the moon and the sun
-may exert an attractive influence upon the earth and cause earthquakes
-to predominate at certain seasons rather than at others, several
-investigators of seismic phenomena have thought that the planets might
-act in a similar manner.
-
-M. J. Delauney, from a study of Perrey’s tables of earthquakes from
-1750–1842, found two groups of maxima each with a period of about
-twelve years, one commencing in 1759 and the other in 1756. Two
-other groups with twenty-eight year periods respectively commence in
-1756 and 1773. These groups coincide with the times when Jupiter and
-Saturn reach the mean longitudes of 265° and 135°. From this Delauney
-concludes that earthquakes have a maximum when the planets are in the
-mean longitudes just mentioned.
-
-The increased number of earthquakes, especially in November, are
-attributed to the passage of the earth through swarms of meteors, and
-in like manner supposes the influence of Jupiter and Saturn to be due
-to their passing through meteor streams situated in mean longitudes
-135° and 265°.
-
-As a consequence of this he predicts an increase of earthquakes in the
-years 1886, 1891, 1898, 1900, &c.[103]
-
-Dr. E. Naumann, who critically examined the large earthquakes of Japan,
-showed that there was an approximate coincidence between many of the
-disturbances and the thirty-three year period of meteoric showers.[104]
-
-Humboldt states that a great shower of meteors was seen at Quito before
-the great earthquake of Riobamba (Feb. 4, 1797). The earthquakes of
-1766 and 1799 at Cumana are also said to have been accompanied with
-meteoric showers. Mallet gives a list of large earthquakes which
-occurred at the times when meteors were observed.[105]
-
-_The hours at which earthquakes are most frequent._—From the
-examination of a catalogue of over 2,000 earthquakes which occurred in
-various parts of the world between the years 1850 and 1857, made by
-Kluge, it is found that both for the northern and southern hemispheres
-the observations which were made during the night generally exceed
-those which were made during the day.
-
- +---------------------------+-----------------------+
- |                           | Number of Earthquakes |
- +---------------------------+-----------------------+
- |                           |   Day   |    Night    |
- |In the Northern Hemisphere |   938   |    1592     |
- |In the Southern Hemisphere |   292   |     357     |
- +---------------------------+---------+-------------+
-
-In the northern hemisphere the greatest number were observed between
-10 P.M. and 12 P.M. (360 shocks), and the fewest between 12 and 2 P.M.
-(139 shocks). In the southern hemisphere, the greatest number were
-observed at night between 12 and 1, and the smallest number between 1
-and 2 and 4 and 5 in the afternoon.[106] These distinctions, however,
-are less distinctly marked as we approach the Equator. Schmidt found
-for the earthquakes of the Orient between 1774 and 1873, that shocks
-had been most frequent about half-past two A.M., and less frequent
-about 1 P.M. With regard to these conclusions, which have been reached
-with much labour, we might be inclined to think that they are partially
-to be explained on the supposition that more observations are made
-during the night than during the day—the personal experience of
-residents in an earthquake country being, that many earthquakes which
-occur during the day are passed by unnoticed, whilst those which occur
-during the night are recorded by thousands of observers. Such a view is
-certainly confirmed by the instrumental records obtained in Japan. From
-1872 to 1880 inclusive there were 261 shocks recorded, 132 of which
-occurred between the hours of 6 P.M. and 6 A.M.
-
-_Earthquakes and sun spots._—Of late years considerable attention
-has been drawn to a coincidence between the occurrence of sun spots,
-magnetic disturbances, rainfall, and other natural phenomena.
-
-These periods of sun spots occur about every eleven years, and appear
-to be coincident with the periodical return of the planet Jupiter. In
-Japan, Dr. E. Naumann sought for a coincidence between these periods of
-sun spots and earthquakes, but without any marked results.
-
-Schmidt, who carefully compared his lists of earthquakes with the
-appearance of sun spots, came to the conclusion that there was no
-marked coincidence. The occurrence of earthquakes had sometimes
-synchronised with sun spots, whilst at other times there had been a
-maximum of sun spots and no earthquakes.
-
-M. R. Wolf[107] apparently considers that earthquakes, like volcanic
-eruptions and the appearance of the aurora, are coincident with sun
-spots.
-
-Kluge, however, came to the conclusion that when there are few sun
-spots, earthquakes, like volcanic eruptions and magnetic disturbances,
-have been at a maximum.
-
-M. A. Poey, who examined a catalogue of the earthquakes of Mexico
-and the Antilles, extending from 1634 to 1870, shows by a table that
-earthquakes have come in groups, first at the maxima and then at the
-minima period of sun spots. Out of thirty-eight groups, seventeen
-being at the maximum and seventeen at the minimum, the remaining four
-are exceptions to the rule, being between the maximum and minimum.
-Phenomena which are dependent upon heat occur with the minima of sun
-spots, and those dependent upon cold with the maxima.[108]
-
-_Earthquakes and the aurora._—The possible connection between
-earthquakes and the aurora is a subject which has attracted some
-attention. Boué has especially made a careful examination of this
-subject.[109]
-
-He comes to the conclusion that if we compare the monthly periods of
-earthquake frequency and the aurora there is an agreement between
-the two. Comparing Perrey’s tables of earthquakes from the fourth to
-the nineteenth century, with tables of the aurora, one-third of both
-phenomena have occurred, not only in the same day, but often at the
-same hour. Between 1834 and 1847, 457 earthquakes are given and 351
-notices of the aurora.
-
-Out of these:—
-    48 occur on the same day,
-     5 occur in the same hour,
-    30 approximate to the same time.
-
-The nearer together that these phenomena have occurred the stronger
-have they been.
-
-Professor M. S. di Rossi brings forward many examples where there
-has been a coincidence between the appearance of the aurora and
-earthquakes. On 139 nights out of 211 days the aurora was seen in
-some parts of Italy, and ninety-three times earthquakes were felt. On
-forty-six occasions earthquakes and aurora took place together.[110] In
-considering the probability of a connection existing between these two
-phenomena, we must bear in mind that the aurora is at no great height
-above the surface of our earth, and, further, that it can be partially
-imitated. The fact that in earthquake countries, like Japan, the aurora
-is practically never seen, would indicate that we can neither regard
-this imperfectly understood phenomenon either as an effect or cause
-of earthquakes. That earthquakes and the appearance of the aurora in
-certain countries should not sometimes coincide is an impossibility.
-
-Dr. Stukeley, who, it must be remembered, attempted to correlate
-the phenomena of earthquakes and electricity, when writing of the
-disturbances which shook England in 1849 and 1850, says that the
-weather had been unusually warm, the aurora borealis frequent and of
-unusually bright colours, whilst the whole year was remarkable for its
-fire-balls, lightnings, and corruscations.[111]
-
-The aurora was observed before the commencement of the Maestricht
-earthquakes in 1751[112]; whilst at the time of the shock flashes of
-light like lightning were observed in the sky.
-
-Glimmering lights were seen in the sky before the New England
-earthquakes (Nov. 18, 1755), and again, before the disturbances which
-occurred in the same region in 1727, peculiar flashes of light were
-seen.
-
-Preceding the Sicilian earthquake of 1692 strange lights were seen
-in the sky. Ignis fatui have also been observed with earthquakes. At
-the time of auroral displays Bertelli has observed microseismical
-disturbances, and M. S. di Rossi, who has made similar observations,
-thinks that there is an intimate connection between the aurora and
-earthquakes; the aurora either occurring in a period of earthquakes, or
-else taking the place of earthquakes.
-
-
-
-
-                              CHAPTER XV.
-
- BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE
-                           AND EARTHQUAKES.
-
-
-_Changes in the barometer and earthquakes._—Mallet, who collected
-together a number of examples of earthquakes which have occurred with
-a fall of the barometer, and a number which have happened with a rise,
-concludes that there are as many instances of the one as of the other.
-At the great earthquake of Calabria, in 1783, the barometer was very
-low. The earthquake of the Rhine (February 23, 1828) was preceded by
-a gradual fall of the barometer, which reached its lowest point upon
-that day. After the earthquake the barometer again rose. The earthquake
-of February 22, 1880, in Japan, was accompanied by exactly similar
-phenomena. Caldcleugh, who observed the heavy shocks in Chili (February
-20, 1835), noticed that on February 17 and 18 the barometer fell 5/10
-inches. Similar phenomena were observed before the succeeding smaller
-shocks. After the shocks the barometer again rose. Principal Dawson,
-speaking of the earthquakes of Canada, observes that some of the shocks
-have been accompanied with a low barometer.
-
-P. Merian, who examined the connection between the Swiss earthquakes
-and atmospheric pressure, found that out of twenty-two earthquakes
-observed in Basle between 1755 and 1836, thirteen of these were
-local shocks, of which eight were accompanied with sudden changes of
-pressure. Of the remaining nine, which were only felt slightly in
-Basle, no change in atmospheric pressure was observed. Of thirty-six
-earthquakes which, between 1826 and 1836, were felt in Switzerland,
-thirty were chiefly confined to Switzerland, and ten of these occurred
-with a low or falling barometer.
-
-Humboldt is of opinion that earthquakes only occur with changes in
-barometric pressure in those countries where earthquakes are few; and
-he gives examples where the regular variations of the barometer have
-gone on without interruption at the time of earthquakes.
-
-Frederick Hoffmann, who examined fifty-seven earthquakes which occurred
-at Palermo between 1788 and 1838, came to the following result:—
-
-  The barometer was sinking                 in 20 cases
-      „          „  rising                  in 16   „
-      „          „  at a minimum            in  7   „
-      „          „    „  maximum            in  3   „
-      „          „  undetermined            in 11   „[113]
-
-The observations of M. S. di Rossi apparently show that the earthquakes
-in Italy chiefly occur with a barometrical depression and with sudden
-jumps in atmospheric pressure.
-
-Schmidt, who examined the earthquakes of the Orient, which occurred
-between 1858 and 1873, says that they were rare with a high barometer,
-but numerous when the barometer was low.
-
-From an examination of a table of 396 earthquakes (May 8, 1875-Dec.
-1881) felt in Tokio, furnished to me by Mr. Arai Ikunosuke, the
-director of the meteorological department, I obtained the following
-results:—
-
-  The barometer was rising                  in  169 cases
-      „          „  falling                 in  154   „
-      „          „  steady                  in   73   „
-      „          „  below the monthly mean  in  189   „
-      „          „  above        „          in  192   „
-
-From this it would appear that in Japan at least the movements of the
-barometer do not show any marked connection with the occurrence of
-earthquakes.
-
-When considering this question we must remember the marked effects
-which a lowering of the barometer produces upon certain volcanoes and
-solfataras. The volumes of steam emitted from Stromboli and from some
-of the solfataras in Tuscany hold a marked connection with atmospheric
-pressure as the quantity of fire damp given off from coal seams—these
-being greatest when the barometer is low. At certain changes of
-the weather it is said that the volcano of Vulture, near Melfi,
-emits noises. These phenomena at once place volcanic phenomena and
-barometrical pressure in direct relationship.
-
-_Changes in temperature._—If, with an earthquake, it should happen
-that there is a change in the height of the barometer, we should
-naturally expect that this might be accompanied with the changes in the
-temperature, in the wind, and in other atmospheric phenomena which are
-more or less connected with the height of the barometer.
-
-Many times it has been observed that after an earthquake there has been
-a sudden fall in the temperature. Such was the case with the Yokohama
-earthquakes of 1880.
-
-Cotte endeavours to show that the earthquakes of Lisbon produced a
-change upon the temperature of all Europe. In the year which followed
-this earthquake storms were more common than usual.
-
-Kluge has collected together a large number of examples when there has
-been a fall of temperature at the time of an earthquake.[114]
-
-At Kiachta, in Siberia, at the time of the earthquake of December
-27, 1856, the thermometer fell from 12° to 25° R. We must, however,
-remember that there are many cases known where the thermometer rose.
-
-M. S. di Rossi remarks that we have the highest records of temperature
-in the years richest in earthquakes. Thus, in 1873, at the time of the
-earthquakes in Central and Northern Italy, an abnormal high temperature
-was remarked. Japanese writers have remarked upon the unusual heat
-which has shaken their countries. The temperature of subterranean
-waters have been known to increase before earthquakes.
-
-
-
-
-                             CHAPTER XVI.
-
-              RELATION OF SEISMIC TO VOLCANIC PHENOMENA.
-
-  Want of synchronism between earthquakes and volcanic
-    eruptions—Synchronism between earthquakes and volcanic
-    eruptions—Conclusion.
-
-
-_Connection between earthquakes and volcanic eruptions._—Insomuch as
-it is a recognised fact that regions which are characterised by their
-seismic activity are chiefly those which are also characterised by
-the number of their volcanoes, it is generally assumed that these
-two phenomena have an intimate relation. The residents in a volcanic
-country, when seeking for the origin of an earthquake, invariably turn
-towards the volcanoes which surround them. If a neighbouring volcano
-is in a state of activity, it is often regarded as a safeguard against
-seismic convulsions, in other cases it is looked upon as being the
-cause of such disturbances. In certain instances both of these views
-have apparently been corroborated. When we consider that an earthquake
-and a volcanic eruption may both be the result of some great internal
-convulsion, and that first one and then the other may take place in the
-same neighbourhood, it is natural to expect that when these internal
-forces have expended themselves in the production of one of these
-phenomena, it is not so likely that they should exhibit themselves in
-the other. The inhabitants of Sicily and Naples, we are told, regard
-eruptions of Etna and Vesuvius as safeguards against earthquakes. A
-similar belief is to be found in portions of South America with regard
-to the volcanoes for which that country is so celebrated.
-
-From an examination of the records of the large earthquakes and the
-volcanic eruptions which have taken place in Japan during the last
-2,000 years, Dr. Naumann found that there was often an approximate
-coincidence between the times of the occurrence of these phenomena,
-suggesting the idea that the efforts which had been sufficient to
-establish the volcano had at the same time been sufficient to shake the
-ground.
-
-Of destructive earthquakes which have occurred at the time of volcanic
-eruptions, and of examples when these phenomena have occurred at widely
-separated intervals, the records are extremely numerous.
-
-_Want of synchronism between earthquakes and volcanic eruptions._—Many
-of the great earthquakes of South America do not appear to have been
-connected with volcanic eruptions.
-
-The great earthquakes of the world, like those of Calabria and Lisbon,
-which took place in regions which are not volcanic, have not, Fuchs
-tells us, taken place in conjunction with volcanic outbursts.
-
-In Japan, as in the Sandwich Islands and in many other parts of the
-globe, the small earthquakes which occur almost daily do not appear to
-show any marked connection with volcanic disturbances.
-
-In 1881, during the eruption of Natustake, a volcano lying about a
-hundred miles north of Tokio, there was neither an increase nor a
-decrease in the earthquakes which were felt in Tokio. Similar remarks
-apply to the state of seismic activity of 1876–77, when Oshima, a
-volcanic island about seventy miles to the south of Tokio, was in
-eruption. In the Sandwich Islands Mauna Loa seems to have its eruptions
-independently of the disturbances which shake these islands.[115]
-
-_Synchronism of earthquakes and volcanic eruptions._—Although many
-examples like the above may be quoted, which apparently show an utter
-want of connection between earthquakes and volcanoes, we must not
-overlook that class of earthquakes which almost invariably accompany
-all great volcanic disturbances. In fact the sudden explosions which
-take place at volcanic foci, as, for instance, at the commencement
-of an eruption, are enumerated as one of the causes which produce
-earthquakes. Earthquakes like these usually continue until the pressure
-of the steam and lava have found for themselves an opening. As compared
-with the total number of earthquakes which are recorded, they form but
-an insignificant portion.
-
-The direct connection which exists between these phenomena has, no
-doubt, done very much to spread the popular belief that all earthquakes
-may be connected with volcanic eruptions. As examples where this
-connection has existed we might quote from almost all the volcanic
-countries in the world.
-
-Thus, Fuchs tells us that on October 6, 1737, almost the whole of
-Kamschatka and the Kurile Islands were disturbed by movements which
-were simultaneous with the outbreak of the great volcano Klutschenskja
-of North Kamschatka.
-
-One of the earliest records of a severe earthquake and a volcanic
-eruption occurring simultaneously is found in the accounts of the
-destruction of Herculaneum and Pompeii. The throwing up of Monte Nuovo
-in the neighbourhood of Pozzuoli was accompanied with a dreadful
-earthquake.[116]
-
-In 1868 the earthquake of Arequipa was accompanied by the opening of
-the volcano Misti, on its north side. The distance to the volcano is
-about fourteen miles.
-
-At the time of the eruptions of Kilauea in 1789 the ground shook and
-rocked so that persons could not stand.
-
-The first eruption of the volcano Irasu, in Costa Rica (1783), was
-accompanied by violent earthquakes.[117] The smoke and flames which
-are said to have issued from the side of Mount Fojo at the time of the
-Lisbon earthquake are regarded by some as having been volcanic. Others
-thought that the phenomena, rather than being on the side of Fojo,
-which showed no traces of volcanic action, had taken place in the ocean.
-
-At the time of the great earthquake at Concepcion (1835), whilst the
-waves were coming in, two great submarine eruptions were observed. One,
-behind the Isle of Quiriquina, appeared like a column of smoke. The
-other, in the bay of San Vicente, appeared to form a whirlpool. The
-sea-water became black, and had a sulphurous smell, there being a vast
-eruption of gas in bubbles. Many fish were killed.[118]
-
-With this same earthquake, near to Juan Fernandez, about one mile from
-the shore, the sea appeared to boil, and a high column of smoke was
-thrown into the air. At night flames were seen.
-
-In 1861, when Mendoza was destroyed and 10,000 inhabitants killed, a
-volcano at the foot of which Mendoza is situated burst into eruption.
-
-The earthquake of 1822 at Valdivia was accompanied by eruptions of the
-neighbouring mountains, which only lasted a few minutes.
-
-At the time of the Leghorn shocks (January 16–27, 1742) some fishermen
-observed a part of the sea to rage violently, to raise itself to a
-great height, and then rush landwards.[119]
-
-In 1797, when Riobamba was destroyed, the neighbouring volcanoes were
-not affected, but Mount Pasto, 120 miles distant, suddenly ceased to
-throw out its usual column of water.
-
-On the night of December 10, 1874, a strong shock was felt in New
-England, whilst at 4.45 A.M. on December 11 a shock was felt in the Pic
-du Midi, in the Pyrenees. In the middle of December there were volcanic
-outbursts in Iceland.[120]
-
-It is possible that these occurrences might be the results of some
-widespread disturbance beneath the crust of the earth, or perhaps even
-of widely extended earth pulsations. The probability, however, is that
-these coincidences are accidental. When we remember that in a small
-area like the northern half of Japan alone there are periods when there
-are at least two shocks per day on the average, it is impossible for
-these coincidences not to exist. Less frequently coincidences between
-the larger disturbances must occur. Over and above these accidental
-coincidences, it would appear that in the world’s history periods
-have occurred when earthquakes were unusually frequent, and at such
-times distant countries have suffered simultaneously. This approximate
-coincidence in period, which has been referred to when speaking of the
-distribution of destructive earthquakes in historical time, does not
-imply an exact synchronism in the single shocks.
-
-Small earthquakes, or, more properly speaking, local tremblings, are a
-necessary accompaniment of almost all volcanic eruptions. Tremors of
-this description are seldom, however, felt beyond the crater, or at the
-most upon the flanks of the mountain where the eruption is going on.
-
-They are due to the explosive action of steam bursting through the
-molten lava.
-
-_Volcanic eruption succeeding earthquakes._—Sometimes it has happened
-that an earthquake, or a series of earthquakes, have terminated with
-the formation of volcanic vents.
-
-As an example of a volcanic outburst terminating a seismic disturbance,
-may be mentioned the appearance of a new volcano in the centre of
-Lake Ilopango, as a sequel to the shocks which had disturbed that
-neighbourhood in 1879.[121]
-
-In 1750 there were continuous shakings lasting over three months at
-Manilla. These terminated with an eruption of a small island in the
-middle of a neighbouring lake. Three days after the commencement of
-this eruption, four other small islands rose in the same lake.[122]
-
-Antonio d’Ulloa, when speaking of the Andes, remarks that after a
-volcanic eruption the shocks cease.[123]
-
-_Conclusion._—Looking at this question generally, insomuch as the
-greatest number of volcanic eruptions appear, according to Fuchs,
-to have taken place in summer, whilst the greatest number of its
-earthquakes have apparently taken place in winter, it would seem that
-the two phenomena are without any direct connection, unless it be that
-both are different effects of a common cause.
-
-Regarded in this manner, an earthquake may be looked upon as an
-uncompleted effort to establish a volcano. To use the words of Mallet,
-‘The forces of explosion and impulse are the same in both; they differ
-only in degree of energy, or on the varying sorts and degrees of
-resistance opposed to them.’[124]
-
-Although we have many examples of earthquakes having occurred without
-volcanic eruptions, and, on the other hand, of volcanic eruptions
-without earthquakes, volcanoes may still be regarded as ‘safety-valves
-of the earth’s crust,’ which, by giving relief to internal stresses,
-guard us against the effects of earthquakes.
-
-That many earthquakes are felt at Copiapo is attributed to the fact
-that in the neighbouring mountains there are no volcanic vents.
-
-We must not, however, overrate the protective influence of volcanoes.
-In the Sandwich Islands we see the columns of liquid lava in
-neighbouring mountains standing at different heights, indicating a
-want of subterranean connection between these vents. In consequence
-of this it would seem that enormous pressures might be generated in
-the neighbourhood of one of these mountains without finding relief at
-the other. When we have conditions like these, it would seem that the
-eruption of a volcano may have little or no influence in protecting
-neighbouring districts.
-
-This may possibly be the explanation of the fact that in 1835
-Concepcion was destroyed, notwithstanding there being an unusual
-activity in the volcanic vents of the neighbouring mountains.
-
-
-
-
-                             CHAPTER XVII.
-
-                       THE CAUSE OF EARTHQUAKES.
-
-  Modern views respecting the cause of earthquakes—Earthquakes due
-    to faulting—To explosions of steam—To volcanic evisceration—To
-    chemical degradation—Attractive influence of the heavenly
-    bodies—The effect of oceanic tides—Variation in atmospheric
-    pressure—Fluctuation in temperature—Winds and earthquakes—Rain
-    and earthquakes—Conclusion.
-
-
-As the results of modern inquiries respecting the cause of earthquakes,
-we see many investigators chiefly attributing these phenomena to
-special causes. A few attribute them to several causes. It seems to us
-that they might be attributed to very many causes which often act in
-a complex manner. The primary causes are telluric heats, solar heat,
-and variations in gravitating influences. These may be the principal,
-and sometimes the immediate, cause of an earthquake. The secondary
-causes are those dependent upon the primary causes, such as expansions
-and contractions of the earth’s crust, variations in temperature,
-barometrical pressure, rain, wind, the attractive influences of the
-sun and moon in producing tides in the ocean or the earth’s crust,
-variations in the distribution of stress upon the earth’s surface
-caused by processes of degradation, the alterations in the position of
-isogeothermal surfaces, &c.
-
-The part which may be played by these various causes in the production
-of oscillations, pulsations, and tremors will be referred to.
-
-_Earthquakes consequent on faulting._—In the chapter on Earth
-Oscillations, the causes producing the phenomena of elevation and
-depression are briefly indicated.
-
-By the variations in stress accompanying elevations and depressions,
-cracks are produced. Inasmuch as compression would crush the rocks
-constituting the earth’s crust, we must conclude with Captain Dutton
-that these cracks are formed by tension. By elevation, the upper rigid
-crust of the earth is stretched, and fissures are produced. The sudden
-formation of these fissures or faults gives rise to earthquakes, and
-perhaps also to volcanic vents. That earthquake and volcanic regions
-are situated on areas where there is evidence of rapid elevation is
-strikingly illustrated round the shores of the Pacific.
-
-Lasaulx considered that the earthquake of Herzogenrath was more
-or less intimately connected with the great mountain fissure—the
-_Feldbiss_—which crosses the coal region of the Wurm.[125] The sudden
-elevation or sinking of large areas at the time of an earthquake may be
-a consequence of these dislocations.
-
-It has already been pointed out that the earthquake region of Japan
-is the one where we have evidence of recent and rapid elevation. That
-certain earthquakes of this region may possibly be the result of
-faulting we have the evidence of our senses and of our instruments. The
-sudden blows and jolts which are sometimes felt are indicative of the
-sliding of one mass of rock across another.
-
-Should the ground be simply torn asunder, this tearing would give rise
-to a series of waves of distortion, vibrating in directions parallel
-to the plane of the fissure. Supposing this motion to be propagated
-to a number of surrounding stations, it would be recorded at each of
-these as having the same direction. To those situated on a line forming
-a continuation of the strike of the fissure, the vibrations would
-advance so to speak _end on_, whilst to those stations lying in a line
-perpendicular to the strike of the fissure, the motion would advance
-_broadside on_.
-
-Motions like these latter have been recorded in Tokio, where
-earthquakes which from time observations were known to have come from
-the faulted and rising region to the south have been registered as a
-series of east and west motions, or vibrations transverse to this line
-of propagation.
-
-It must, however, be here mentioned that the registration of only
-transverse motion may possibly be due to the extinction of normal
-motion, although this is not generally regarded as probable.
-
-It would therefore appear that certain earthquakes and faults are
-closely related phenomena, the former being an immediate effect of
-the latter. Faults are due to earth oscillations, and to a variety of
-causes producing disturbances in the equilibrium of the earth’s crust;
-the principal cause of all these phenomena being alterations in the
-distribution of heat, and the attractive force of gravity.
-
-_Earthquakes consequent on the explosion of steam._—Humboldt regarded
-volcanoes and earthquakes as the results of a common cause, which he
-formulated as ‘the reaction of the fiery interior of the earth upon
-its rigid crust.’ Certain investigators, who have endeavoured to
-reduce Humboldt’s explanation to definite limits, have suggested that
-earthquakes may be due to sudden outbursts of steam beneath the crust
-of the earth, and its final escape through cracks and fissures.
-
-Admitting that steam may accumulate by separating out from the cooling
-interior of our globe, its sudden explosion might be brought about by
-its own expansive force, or by the movements in the bubbling mass from
-which it originated.
-
-Others, however, rather than regard the steam as being a primeval
-constituent of the earth’s interior, imagine it arises from the gradual
-percolation of water from the surface of the earth down to volcanic
-foci, into which it is admitted against opposing pressures, by virtue
-of capillary action.
-
-Mallet, in his account of the Neapolitan earthquake, shows that the
-whole of the observed phenomena can be accounted for by the admission
-of steam into a fissure, which by the expansive force exerted on its
-walls was rent open. Just as at the Geysers we hear the thud and
-feel the trembling produced by the sudden evolution and condensation
-of steam, so may steam by its sudden evolution and condensation in
-the ground beneath us give rise to a series of shocks of varying
-intensity, accompanied by intermediate vibratory motions—that is to
-say, a motion which, as judged of by our feelings, is not unlike many
-earthquakes. Often it may happen that the result of the explosion may
-be the production of a fault, or at least a fissure; and thus in the
-resulting movements we may have a variety of vibrations, some being
-those of compression and distortion, produced by the blow of the
-explosion, and others being those of distortion alone, produced by the
-shearing action which may have taken place by the opening of the fault.
-Sometimes one set of these vibrations may be prominent, and sometimes
-the other. Thus, when we say that an earthquake has shown evidence by
-the nature of its vibrations that it was produced by a fault, this
-by no means precludes the possibility that an explosion of steam may
-also have been connected with the production of the disturbance.
-Mallet threw out the suggestion that the opening of fissures beneath
-the ocean might admit water to volcanic foci. During the time that the
-water was in the spheroidal state, the preliminary tremors, so common
-to many earthquakes, would be produced. These would be followed by the
-explosion, or series of explosions, constituting the shock or shocks of
-the earthquakes.
-
-The chief reasons for believing that the earthquakes of North-Eastern
-Japan are partly due to explosive efforts are:—
-
-1. That the greater number of disturbances, perhaps ninety per cent.,
-originate beneath the sea, where we may imagine that the ground,
-under the superincumbent hydrostatic pressure, is continuously being
-saturated with moisture.
-
-2. Many of the diagrams show that the prominent vibrations, of which
-there are usually from one to three, in a given disturbance have the
-same character as those produced by an explosive like dynamite, the
-greatest and probably the most rapid motions being inwards towards the
-origin.
-
-It may here be remarked that a very large proportion of the destructive
-earthquakes of the world have originated beneath the sea, as has often
-been testified by the succeeding sea waves. Also, it must be observed,
-that earthquake countries, like volcanic countries, are chiefly those
-which have a coast line sloping at a steep angle beneath the sea—that
-is to say, earthquakes are frequent along coasts bordered by deep water.
-
-The earthquakes which occur at volcanic foci constitute another class
-of disturbances which may be accredited to the explosive efforts of
-steam.
-
-_Earthquakes due to volcanic evisceration._—By the ejection of ashes
-and lava from volcanic vents, there is an extensive evisceration of
-the neighbouring ground. When we look at a volcano like Fujiyama,
-13,000 feet in height, and at least fifty miles in circumference, and
-remember that the mass of cinders and slag of which it is composed
-came from beneath the area on which it rests, the point to be wondered
-at is, that earthquakes, consequent on the collapse of subterranean
-hollows, are not more frequent than they are. At the time of a single
-eruption of a volcano, the quantity of lava ejected amounts to many
-thousand millions of cubic feet. In 1783 the quantity of lava ejected
-from Skaptas Joknee, in Iceland, was estimated as surpassing ‘in
-magnitude the bulk of Mont Blanc.’[126] Admitting that hollow spaces
-are the results of these eruptions, and that in consequence of this
-evisceration the ground is rendered unstable, the instability being
-increased by the additional load placed above the eviscerated area, it
-would seem that from time to time earthquakes are inevitable.
-
-Facts, however, teach us that volcanoes act as safety valves, and
-that, as a rule, at or shortly after an eruption, earthquakes cease.
-The relationship of earthquakes to volcanic eruptions would therefore
-indicate, notwithstanding the arguments put forward to show that an
-area loaded by a volcano has in consequence of the evisceration and the
-load a quaquaversal dip, that evisceration does not take place beneath
-volcanoes as is usually supposed, and we may conclude that it is but
-few earthquakes which have an origin due to these causes.
-
-_Earthquakes and evisceration by chemical degradation._—A powerful
-agent, which tends to the formation of subterranean hollows, is
-chemical degradation. The effects of this have been often measured by
-quantitative analysis of the solid materials which are daily carried
-away by many of our springs. In limestone districts this is very great.
-Prof. Ramsay estimates that the mineral matter discharged annually by
-the hot springs of Bath is equivalent in bulk to a column 140 feet in
-height and 9 feet in diameter. At San Filippo, in Tuscany, the solid
-matter discharged from the springs has formed a hill a mile and a
-quarter long, a third of a mile broad, and 250 feet in thickness.[127]
-Many other examples of subterranean chemical degradation will be found
-in text-books of geology.
-
-By this chemical action large cavernous hollows are produced. Beneath
-a volcano it is probable that liquid material immediately takes the
-place of that which is ejected, and that hollows are not formed as in
-the case of chemical degradation. If a cavern becomes too large, it
-eventually collapses.
-
-Of the falling in of large excavations we have examples in large mines.
-As a consequence, not only is a trembling produced, but also a noise,
-which is so like that produced by certain earthquakes that the South
-American miners have but one word, ‘bramido,’ to express both.[128]
-
-Boussingault, who was an advocate for the theory that many earthquakes
-are produced by the sinking of the ground, calls attention to the fact
-that we have evidences of the subsidence of great mountains, like
-the Andes, the districts around which are so well known for their
-earthquakes. Capac Urcu is one of these mountains which legends tell us
-has decreased in height.
-
-The variation in the height of mountains is a subject which deserves
-attention. That mountains may possibly be hollow, we have the
-remarkable results attained by Captain Herschel, who found that the
-attractive force of gravity in the neighbourhood of the Himalayas was
-not so great as it ought to have been had these mountains been solid.
-The Rev. O. Fisher gives another explanation of this phenomenon.
-Palmieri considers that the terrible earthquake which devastated
-Casamicciola (1881) was due to the hot springs having gradually eaten
-out cavernous spaces beneath the town. The extremely local character of
-this shock was certainly favourable to such a view.
-
-The earthquake which, in 1840, caused Mount Cernans, in the Jura, to
-fall, is also attributed to the solvent action of waters in undermining
-its foundations. This undermining action was in great measure probably
-due to a large spring, which, twenty-five years previously, had
-disappeared, and which subsequently may possibly have been slowly
-disintegrating the foundations of the mountain. Earthquakes of this
-order would be principally confined to districts where there are rocks
-which are more or less soluble, as, for instance, rock salt, gypsum,
-and limestone.
-
-_Earthquakes and the attractive influences of the heavenly bodies._—The
-most important attractions exercised upon our planet are those due to
-the sun and moon. To these influences we owe the tides in our ocean,
-and possibly elastic tides in the earth’s crust. Some theorists would
-also insist upon liquid tides in the fluid interior of our earth. The
-nature of the earth’s interior is, however, a question on which there
-is a diversity of opinion.
-
-One doctrine, which, until recent years, received much support, was
-that the interior of the earth was a reservoir of molten matter
-contained within a thin crust. Hopkins showed that the least
-possible thickness of such a crust must be from 800 or 1,000 miles,
-otherwise the motions of precession and nutation would be subject to
-interference.
-
-M. Delauney objected to the views of Hopkins, on the supposition that
-the fluid interior of the earth had a certain viscosity.
-
-Sir William Thomson arrives at the conclusion that the earth on the
-whole must be more rigid than a continuous solid globe of glass. Mr.
-George H. Darwin’s investigations on the bodily tides of viscous or
-semi-elastic spheroids tend to strengthen the arguments of Sir William
-Thomson.
-
-Some philosophers hold the view that the central portion of the earth,
-although intensely hot, is solid by pressure, whilst the outer crust is
-solid by cooling. Between the two there is a shell of liquid or viscous
-molten matter.
-
-Another argument is, that although the interior of the globe may be
-solid, it is only retained in that condition by an immense pressure, on
-the relief of which it is liquefied—it is potentially liquid.
-
-As these views, and the arguments for and against them, are to be
-found in all modern text-books of geology, we will at once proceed
-to consider the effect of solar and lunar attractive influences in
-producing earthquakes upon a globe which is either solid, partially
-solid, or which has an interior wholly liquid.
-
-_Effect of the attractive influences of the sun and moon._—In 1854
-M. F. Zantedeschi put forward the view—that it is probable there is
-a continual tendency of the earth to protuberance in the direction
-of the radii vectores of the two luminaries which attract it. In
-consequence of these protuberances, pendulums ought at one time to
-swing more slowly than at others. Zantedeschi remarks that the periods
-of earthquakes appear to confirm such a view, insomuch as they occur
-more often at the syzygies, or epoch of the spring tides, than at neap
-tides—an observation found in the works of Georges Baglivi (1703) and
-Joseph Toaldo (1770).[129]
-
-Prof. Perrey, of Dijon, who did so much for seismology, held the view
-that the preponderance in the number of earthquakes felt at particular
-seasons was possibly due to the attractive influence of the sun and
-moon producing a tide in the fluid interior of the earth, which, acting
-on the solid crust, produced fractures.
-
-Rudolf Falb, whose writings have of late years attracted considerable
-attention, brings forward views which may be regarded as amplifications
-of those suggested by Perrey.
-
-According to Falb, the inner portion of the earth must be regarded as
-fluid. In the crust above this fluid reservoir are cracks and channels,
-into which, by the attraction of the moon and sun, the fluid is drawn.
-On entering these cracks cooling takes place, together with explosions
-of gas and subterranean volcanic disturbances. The attractions
-producing the internal tides required by Falb are chiefly dependent
-upon the following factors:—
-
-1. The nearness and distance of the sun from the earth (January 1 and
-July 1).
-
-2. The position of the moon with regard to the earth, which in every
-twenty-seven days is once near and once distant.
-
-3. The phases of the moon—whether full or new moon (syzygies), or
-whether first or last quarter (quadratures).
-
-4. The equinoxes, the position of the sun in the equator, and the
-relative position of the earth.
-
-5. The position of the moon relative to the equator.
-
-6. The concurrence of the ‘centrifugal force’ of the earth with the
-last quarter of the moon.
-
-7. The entrance of the moon on the ecliptic—the so-called nodes.
-
-Assuming that earthquakes are wholly consequent on these attractions,
-it at once becomes possible to predict their occurrence. This Falb
-does, and when his predictions have been fulfilled he has certainly
-gained notoriety.
-
-He commenced by the predictions of great storms. In 1873 he predicted
-the destructive earthquake of Belluno, which earned for himself a
-eulogistic poem, which he has republished in his ‘Gedanken und Studien
-über Vulkanismus.’ After this, in 1874, he predicted the eruption of
-Etna. He also explained why, in B.C. 4000, there should have been a
-great flood, and for A.D. 6400 he predicts a repetition of such an
-occurrence.
-
-When we approach the question of the extent to which the attraction
-of the sun and moon may influence the production of earthquakes, a
-question which we have to answer is, whether it is likely that the
-attractive power of the moon is so great that it could draw up the
-crust the earth beyond its elastic limits. We know what it can do with
-water. It can lift up a hemispherical shell 8,000 miles in diameter
-about two or three feet higher at its crown than it lifts the earth.
-Even supposing the solid crust to be lifted 100 times the apparent
-rise of the tide, is it likely that a hemispherical arch 8,000 miles
-in diameter when it is raised 200 feet at its crown could by any
-possibility suffer fracture? If an arch is 12,000 miles in length, all
-that we here ask is, whether the materials which compose the arch are
-sufficiently elastic to allow themselves to be so far stretched that
-the crown may be raised 200 feet. The result which we should arrive
-at is apparently so obvious that actual calculation seems hardly
-necessary. If we regard the earth as being solid, the question resolves
-itself into the inquiry as to whether a column of rock, which is equal
-in length to the diameter of the earth, or about 8,000 miles, can be
-elongated 200 feet without a fracture. This is equivalent to asking
-whether a piece of rock one yard in length can be stretched one seventy
-thousandth of a foot. Considering that this is a quantity which is
-scarcely appreciable under the most powerful of our microscopes, we
-must also regard this as a question which it is hardly necessary to
-enter into calculations about before giving it an answer. To vary
-the method of treating such a question, may we not ask what is the
-utmost limit to which it would be possible to raise up or stretch the
-crust of the earth without danger of a fracture? Thus, for instance,
-to what extent might a column of rock be elongated without danger
-of its being broken? From what we know of the tenacity of materials
-like brick and their moduli of elasticity, it would seem possible to
-stretch a bar of rock 8,000 miles in length for approximately half
-a mile before expecting it to break. As to whether there is a wave,
-the height of which is equal to half this quantity, running round our
-earth as successive portions of its surface pass beneath the attracting
-influences of the sun and moon, is a phenomenon which, if it exists,
-would probably long ago have met with a practical demonstration.
-
-The deformation which a solid globe or spherical shell would
-experience under the attractive influences of the sun and moon has
-been investigated by Lamé, Thomson, Darwin, and other physicists and
-mathematicians.
-
-A conclusion that we are led to as one result of these valuable
-investigations is, that if the interior of the earth be fluid, and
-covered with a thin shell, then enormous elastic tides must be
-produced. A consequent phenomenon, dependent on the existence of these
-tides, would be a marked regularity in the occurrence of earthquakes.
-As this marked regularity does not exist, we must conclude that
-earthquakes are not due to the attractive influences of the sun and
-moon acting upon the thin crust of the earth covering a fluid interior.
-The periodicity of earthquakes corroborates the conclusions of Sir
-William Thomson, who remarks that if the earth were not extremely rigid
-the enormous elastic tides which must result would be sufficient to
-lift the waters of the ocean up and down so that the oceanic tide would
-be obliterated.
-
-Assuming that the earth has the rigidity assigned to it by mathematical
-and physical investigators, we nevertheless have travelling round our
-earth, following the attractions of the moon and sun, a tidal stress.
-This stress, imposed upon an area in a critical state, may cause it to
-give way, and thus be the origin of an earthquake. Earthquakes ought
-therefore to be more numerous when these stresses are the greatest.
-
-The periods of maximum stress or greatest pull exerted by the moon and
-sun will occur when these bodies are nearest to our planet—that is, in
-perigee and perihelion, and again when they are acting in conjunction
-or at the syzygies. That earthquakes are _slightly_ more numerous
-at these particular periods than at others is a strong reason for
-believing that the attractions of the moon and sun enter into the list
-of causes producing these phenomena.
-
-Had there been a strongly marked distinction in the number of
-earthquakes occurring at these particular seasons as compared with
-others, we might have attributed earthquakes to the existence of
-elastic tides of a sensible magnitude. As the facts stand, it appears
-that the maximum pulls exerted by the moon and sun are only sufficient
-to cause a slight preponderance in the number of earthquakes felt at
-particular seasons, and therefore that these pulls only result in
-earthquakes when the distorting effort has been exerted on an area
-which, by volcanic evisceration, the pressure of included gases, and
-other causes, is on the verge of yielding.
-
-_Earthquakes and the tides._—If we assume that earthquakes are in many
-cases due to the overloading of an area and its consequent fracture,
-such loading may occur by the rising of the tide. A belief that the
-earthquakes of Japan were attributable to the tides may be found in the
-diary of Richard Cocks under the date November 7, 1618, who remarks:—
-
-‘And, as we retorned, about ten aclock, hapned a greate earthquake,
-which caused many people to run out of their howses. And about the lyke
-hower the night following hapned an other, this countrey being much
-subject to them. And that which is comunely markd, they allwais hapen
-at a hie water (or full sea); so it is thought it chauseth per reason
-is much wind blowen into hollow caves under ground at a loe water, and
-the sea flowing in after, and stoping the passage out, causeth these
-earthquakes, to fynd passage or vent for the wind shut up.’[130]
-
-Although we may not acquiesce in Cock’s views respecting the imprisoned
-wind, it would seem that a comparison of the occurrence of earthquakes
-and the state of the tide would be a legitimate research. Inasmuch
-as the stresses which are brought to bear upon an area by the rising
-of the tide are so very much greater than those due to barometrical
-changes, it is not unlikely that a marked connection would be found.
-But it must be remembered that because researches, so far as they have
-gone, tend to show that earth movements are more frequent when an area
-is relieved of a load, it is not unlikely that the greatest number of
-earthquakes may be found to occur at low water. Prof. W. S. Chaplin
-attempted to make this investigation in Japan, but not being able to
-obtain the necessary information respecting the tides, was compelled to
-relinquish this interesting work.
-
-Every foot of rise in a tide is equivalent to a load being placed on
-the area over which the tide takes place of sixty-two pounds to the
-square foot. This load is not evenly distributed, but stops abruptly at
-a coast line. Lastly, it may be observed that many coast lines are not
-simultaneously subjected to stresses consequent upon this load. Japan,
-for instance, may be regarded as an arch placed horizontally. The
-area near the crown of this arch is loaded by the tidal wave crossing
-the Pacific before the areas near the abutment, and farther there is
-a horizontal pressure at the crown which, if Japan were like a raft,
-would tend, as the tide advanced, to straighten its bow-like form, but
-as the wave passed its abutments to increase its curvature.
-
-Prof. G. Darwin has calculated the amount of rise and fall of a shore
-line due to tidal loads (see p. 336, ‘Earth Pulsations’). The result
-of these calculations apparently indicates that these loads may have a
-considerable influence upon the stability of an area in a more or less
-critical condition.
-
-Mr. J. Carruthers suggests that tidal action may hold a general but
-indirect relationship to volcanic and seismic action by the retardation
-it causes on the earth’s rotation. By this retardation the polar axis
-tends to lengthen, and tensile stresses are induced, resulting in
-fracture. The fluid interior of the earth, being no longer restrained,
-would move polewards, and, leaving equatorial portions unsupported,
-this would gradually collapse. The primary fractures would be north and
-south, while the secondary fractures would be east and west.[131]
-
-That the rise of the tide is accompanied by a greater percolation
-of water to volcanic foci, which, in consequence, assume a greater
-state of activity, is a theory which was advanced many years ago. To
-determine how far tides may directly be connected with earthquakes, the
-necessary records have yet to be examined.
-
-_Variations in atmospheric pressure._—When we consider the immense
-load which, by a sudden rise of the barometer, is placed upon the area
-over which this rise takes place, it is not difficult to imagine that
-this rise may occasionally be the final cause which makes the crust of
-the earth to give way. A barometric rise of an inch is equivalent to a
-load of about seventy-two pounds being put upon every square foot of
-area over which this rise takes place. On the other hand, a fall in the
-barometric column indicates that a load has been removed, and whatever
-elastic effort may be exerted by subterranean forces in endeavouring to
-escape, being met by less resistance, they may burst these bonds, and
-an earthquake will result. For reasons such as these the final cause
-of earthquakes has often been attributed to variations in atmospheric
-pressure. In Japan there are practically as many earthquakes with a
-high barometer as with a low one.
-
-The extent to which barometric fluctuations have acted as final causes
-in the production of earthquakes may be judged of by a comparison of
-the times of barometric variation and the times at which earthquakes
-have occurred.
-
-Three important laws of barometric variation are the following:—
-
-1. In the world generally the average barometric pressure is highest in
-winter. (Exceptions occur near Iceland and in the North Pacific.)
-
-2. The summer and winter monthly mean barometer differs least near the
-equator and over the great oceans. They differ most over the great
-continents and generally with increasing latitude.
-
-3. The greatest number of barometrical fluctuations usually take place
-in winter.
-
-Inasmuch as there are generally more earthquakes in winter than in
-summer, the first of these laws would indicate that this might be due
-to the greater load which acts upon the crust of the earth at that
-season. The second law would indicate that the distinction between
-the winter and summer earthquakes ought to be most marked in high
-latitudes, which, if we refer to the table on p. 257, we observe to be
-borne out by the results of observation. The countries where there are
-as many earthquakes in winter as in summer are chiefly those in low
-latitudes. The number of these countries from which we have records
-are, however, few.
-
-Facts opposed to the idea that earthquakes may be caused by an increase
-of barometric pressure are the results of observations like those of
-Schmidt and Rossi, which show that earthquakes chiefly occur with a low
-barometer.
-
-Assuming that these latter observations will be found by future
-investigators to be generally true, we must conclude that the relief
-of atmospheric pressure has an influence upon the occurrence of
-earthquakes. Such a conclusion would partially accord with the third
-barometrical law, or the fact that there are more occasions on which we
-get a low barometer during the winter months.
-
-Other writers who have examined this question are Volger, Kluge,
-Andrès, and Poly. The latter investigator sought a connection between
-earthquakes and revolving storms, in the centres of which there is
-usually an abnormal decrease of atmospheric pressure. If an area over
-which such a sudden change in pressure took place was in a critical
-state, it is not difficult to see that storms such as Poly refers to
-might sometimes be accompanied by earthquakes.
-
-_Fluctuations in temperature._—Inasmuch as fluctuations in temperature
-are governed by the sun, it may at once be said that there is a
-connection between earthquakes and readings of the thermometer.
-Certainly earthquakes occur mostly during the cold months or in
-winter. Similarly, as changes in temperature are so closely connected
-with barometric fluctuations, and these are said to have a direct
-influence upon the yielding of the earth’s crust, seismic phenomena are
-indirectly linked to fluctuations in temperature. A rise in temperature
-is usually accompanied by a fall in the barometer, and this in turn may
-be a condition favourable for the occurrence of an earthquake.
-
-If we regard solar heat as an agent causing expansions or contractions
-in the earth’s crust, then fluctuations in temperature become an
-immediate cause of earthquakes. The probability, however, is that
-solar heat has little or no connection with the final cause producing
-earthquakes, although at the same time coincidences between the
-occurrence of earthquakes and unusual fluctuations in temperature may
-from time to time be observed.
-
-_Winds and earthquakes._—Although it may be admitted that high winds
-exert enormous pressures upon mountain ranges, and might occasionally
-give rise to stresses causing rocky masses in unstable equilibrium to
-give way, the coincidences which have been established between the
-occurrence of storms and earthquakes can usually only be regarded as
-occurrences which have synchronised by chance.
-
-Storms are usually accompanied with a barometric depression, and the
-relation of diminutions in atmospheric pressure to earthquakes has been
-discussed.
-
-_Rain and earthquakes._—It has already been shown that earthquakes
-have occasionally been found to coincide with rain and rainy seasons.
-Whether the saturation of the ground with moisture or the percolation
-of the same to volcanic foci may be a direct effect producing
-earthquakes it is difficult to say. The probability, however, is
-that, rain being dependent on phenomena like changes in temperature,
-barometric fluctuations, and winds, we must regard it and the
-earthquakes which happen to coincide with these precipitations of
-moisture as congruent effects of more general causes.
-
-_Conclusion._—Although it would be an easy matter to discuss the
-relationship of earthquakes and other phenomena, we must conclude
-that the primary cause of earthquakes is endogenous to our earth, and
-that exogenous phenomena, like the attraction of the sun and moon and
-barometric fluctuations, play but a small part in the actual production
-of these phenomena, their greatest effect being to cause a slight
-preponderance in the number of earthquakes at particular seasons. They
-may, therefore, sometimes be regarded as final causes. The majority of
-earthquakes are due to explosive efforts at volcanic foci. The greater
-number of these explosions take place beneath the sea, and are probably
-due to the admission of water through fissures to the heated rocks
-beneath. A smaller number of earthquakes originate at actual volcanoes.
-Some earthquakes are produced by the sudden fracture of rocky strata or
-the production of faults. This may be attributable to stresses brought
-about by elevatory pressure. Lastly, we have earthquakes due to the
-collapse of underground excavations.
-
-
-
-
-                            CHAPTER XVIII.
-
-                      PREDICTION OF EARTHQUAKES.
-
-  General nature of predictions—Prediction by the observation of
-    unusual phenomena (alteration in the appearance and taste of
-    springs; underground noises; preliminary tremors; earthquake
-    prophets—warnings furnished by animals, &c.)—Earthquake warning.
-
-
-_General nature of predictions._—Ever since seismology has been
-studied, one of the chief aims of its students has been to discover
-some means which would enable them to foretell the coming of an
-earthquake, and the attempts which have been made by workers in various
-countries to correlate these occurrences with other well-marked
-phenomena may be regarded as attempts in this direction.
-
-Ability to herald the approach of these calamities would unquestionably
-be an inestimable boon to all who dwell in earthquake-shaken countries,
-and the attempts which have been made both here and in other places are
-extremely praiseworthy. In almost all countries where earthquakes are
-of common occurrence these movements of the earth have been more or
-less connected with certain phenomena which, in the popular mind, are
-supposed to be associated with an approach of an earthquake.
-
-If predictions were given in general terms, and they only referred
-to time, inasmuch as on the average there are in the world several
-shakings per day, we should always find that predictions were
-verified. We might even go further and predict that on certain days
-earthquakes would occur in certain countries, and still find that in
-many instances our supposed power of foresight had not deceived us.
-Thus, for instance, in Japan, where on the average there are probably
-one or two shakings every day, if prognostications were never correct
-there would be a violation of the laws of chance.
-
-What is required from those who undertake to forewarn us of an
-earthquake is an indication not only of the time at which the
-disturbance will happen, but also an indication of the area in which
-it is to occur. Those who dwell in an area where there are certain
-well-defined periods during which seismic activity is at a maximum—if
-ten or fourteen days should have passed without a shock—might, in many
-instances, find that a prophecy that there would be an earthquake
-within the next few days would prove itself correct. Also, if a severe
-shock had taken place, a prophecy that there would be a second or
-third smaller disturbance within a short period would also meet with
-verification.
-
-Certain persons with whom I am intimate appear to have persuaded
-themselves that they can foretell the coming of an earthquake by the
-sultry state of the atmosphere or a certain oppressiveness they feel,
-and an instinctive feeling arises that an earthquake is at hand.
-
-It is said that a few minutes before many of the shocks which shook
-New England between 1827 and 1847 people could foretell the coming
-disturbance by an alteration in their stomach.[132] No doubt many who
-dwell in earthquake countries, and have been alarmed by earthquakes,
-are at times subject to nervous expectancy.
-
-The author has had such sensations himself, due, perhaps, to a
-knowledge that it was the earthquake season, that there had been no
-disturbance for some weeks, and a consequent increasing state of
-nervous presentments. In consequence of this, not only has he carefully
-prepared his instruments for the coming shock, but he has written and
-telegraphed to friends to do the same.
-
-Sometimes these guesses have proved correct. One remarkable instance
-was a few hours prior to the severe shock of February 22, 1880, when
-he communicated with his friends in Yokohama and asked them to see
-that their instruments were in good order. Oftener, however, his
-prognostications have been incorrect. The point in connection with
-this subject which he wishes to be remarked is, that the instances
-where earthquakes occurred shortly after the receipt of his letters
-are carefully remembered, and often mentioned, but the instances in
-which earthquakes did not occur appear to be entirely forgotten. He is
-led to mention these facts because they appear to be an experimental
-proof of what has taken place in bygone times, and what still takes
-place, especially amongst savages—namely, that the record of that which
-is remarkable survives, whilst that which is of every-day occurrence
-quickly dies. Had the records of all prognostications been preserved,
-the probability is that we should find that they had, in the majority
-of cases, been incorrect, whilst it would have been but in very few
-instances they had been fulfilled.
-
-_Prediction by the observation of natural phenomena._—The above remarks
-may perhaps help us to understand the prognostications of the ancient
-philosophers about which Professor Antonio Favaro, of Padua, has
-written.[133] Cicero in the ‘De Divinatione,’ speaking on this subject,
-says that ‘God has not predicted so much as the divine intelligence of
-man.’—‘Non Deus prævidet tantum, sed et divini in genii viri.’ Favaro
-regards these predictions, however, as the result of observations of
-nature which show it is possible that indications of coming earthquakes
-had been announced by variations in the gas given out from subterranean
-sources, the change in colour, taste, level, temperature of the water
-in springs, &c.
-
-In 1843 a bishop of Ischia forewarned his people of a conning
-earthquake, and thus was instrumental in the saving of many lives.
-Naturally, in an age of superstition, the bishop would be regarded as
-a prophet, but Favaro considers that the prognostication was probably
-due to a knowledge of premonitory signs as exhibited in changes in the
-characters of mineral waters.
-
-The shock of 1851, at Melfi, was in this way predicted by the Capuchin
-fathers, who observed that a lake near their door became frothy and
-turbulent.
-
-Underground noises have led persons to the belief that an earthquake
-was at hand. It was in this way that Viduari, a prisoner at Lima,
-predicted the destruction of that city.
-
-Before the earthquake of 1868, so severely felt at Iquique, the
-inhabitants were terrified by loud subterranean noises.
-
-That underground noises have preceded earthquakes by considerable
-intervals appears to be a fact, but, at the same time, it must
-be remembered that similar noises have often occurred without an
-earthquake having taken place.
-
-Farmers predicted the earthquake of St. Remo, in 1831, by underground
-noises.
-
-On the day before the earthquake which, in 1873, shook Mount Baldo,
-the inhabitants of Puos, a village north of Lake Santa Croce, heard
-underground noises.
-
-Before the earthquakes which, in 1783, shook Calabria and Sicily, fish
-are said to have appeared in great numbers on the coast of Sicily, and
-the whirlpool of Charybdis assumed an unusual excited state.
-
-It is said that Pherecydes predicted the earthquakes of Lacedemon and
-Helm out, by the taste of the water in the very deep well at the castle
-of Lovain.[134]
-
-The writer of an article on the Lisbon earthquake says that ‘after the
-24th I felt apprehensive, as I observed the same prognostics as on the
-afternoon of October 31, that is, the weather was severe, the wind
-northerly, a fog came from the sea, the water in a fountain ran of a
-yellow clay colour, and’ he adds, ‘from midnight to the morning of the
-25th I felt five shocks.’[135]
-
-At the present time Rudolf Falb, following a theory based upon the
-attractive influences of the sun and moon, tells us the time at which
-we are to expect earthquakes.
-
-That occasionally there are signs attendant on earthquakes, although
-we cannot give them a physical explanation, we cannot doubt. Also we
-know that in certain areas earthquakes are more likely to occur at
-one season than at another. Should earthquakes be foretold with the
-assistance of knowledge of this description, the predictions at once
-become the result of the application of certain natural laws, and are
-not to be regarded as predictions in the popularly accepted sense of
-that term, any more than the arrival of a friend is predicted by the
-previous receipt of a telegram announcing his coming.
-
-Rather than accredit the ancients and those of more modern times who,
-in consequence of their feelings, have recorded the coming of an
-earthquake, with a knowledge of premonitory signs, we might in many
-instances regard the records of those prognostications as the survival
-of accidental guesses, and, as such, examples of the survival of the
-useless.
-
-The effect of accidental occurrences of this description upon an
-uneducated mind, in engendering superstition, is a subject which has
-often been dwelt upon, and the difficulty of eradicating the same—as
-may be judged of by the following accident which came under the
-observation of Mr. T. B. Lloyd and the author, in 1873, when travelling
-in Newfoundland—will be easily appreciated.
-
-At the time to which I refer, my companion was bringing a canoe down
-the rapids below the Grand Pond in a country which is practically
-uninhabited, and where an Indian trapper would perhaps be the only
-person met with, and this not more than once a year. Whilst shooting
-the rapids one of the Indians, Reuben Soulian, shot at a deer passing
-up one bank of the river. That the deer had been hit was testified by a
-trail of blood which bespattered the rocks. Subsequently several more
-shots were fired, and it was believed by all that the deer was killed.
-Soulian quickly followed the animal to the spot where it was supposed
-to have fallen. Some time after he returned, having failed to find any
-trace of the animal. He was greatly agitated, but eventually became
-melancholy, saying that the sudden disappearance of the animal was a
-sure sign that some of his relations had suddenly died. About two hours
-afterwards Mr. Lloyd’s party met with a party of Indians coming up
-the river, the first they had seen for four weeks, who told them that
-Soulian’s sister had just died on the coast.
-
-In the northern part of South America certain shocks are anticipated by
-preliminary vibrations which cause a little bell attached to a T-shaped
-frame (cruz sonante) to ring. There are, however, persons (trembloron)
-who are supposed to be endowed with seismic foresight, whose verdicts
-are much relied upon.
-
-In Caraccas it is said that nearly every street in the river suburb has
-an earthquake Cassandra or two. Some of these go so far not only as
-to predict the coming seisms, but also the vicissitudes of particular
-streets.[136] Earthquake prophets are, however, by no means confined to
-the new world, and many examples of them may be found in the histories
-of countries where earthquakes have been felt.
-
-The story of the crazy lifeguardsman who prophesied an earthquake to
-take place in London on April 4, 1691, is an example. The Rev. Sig.
-Pasquel E. Perdini, writing on the earthquakes at Leghorn in 1742,
-says that ‘a Milanese astrologer predicted this earthquake for January
-27, by which “misfortune” the faith and credit given to the astrologer
-gained him more reverence and honour than the prophets and holy
-gospel.’ Before the time at which he predicted a second shock, people
-removed away from Leghorn.
-
-_Warnings furnished by animals._—A study of the warnings furnished by
-animals is also interesting. Several of the natives in Caraccas possess
-oracular quadrupeds, such as dogs, cats, and jerboas, which anticipate
-coming dangers by their restlessness.
-
-Before the catastrophe of 1812, at Caraccas, a Spanish stallion broke
-out from its stable and escaped to the highlands, which was regarded
-as the result of the prescience of a coming calamity. Before the
-disturbances of 1822 and 1835, which shook Chili, immense flocks of sea
-birds flew inland, as if they had been alarmed by the commencement of
-some suboceanic disturbance. Before this last shock it is also related
-that all the dogs escaped from the city of Talcahuano.
-
-_Earthquake warning._—What has here been said respecting the prediction
-of earthquakes is necessarily imperfect—many of the signs which are
-popularly supposed to enable persons to foretell the coming of an
-earthquake having already been mentioned in previous chapters. That
-we shall yet be able to prepare ourselves against the coming of
-earthquakes, by the applications of laws governing these disturbances,
-is not an unreasonable hope.
-
-With an electric circuit which is closed by a movement of the ground,
-we are already in a position to warn the dwellers in surrounding
-districts that a movement is approaching.
-
-An earthquake which travelled at the rate of four seconds to the
-mile might, if it were allowed to close a circuit which fired a gun
-at a station fifteen miles distant, give the inhabitants at that
-place a minute’s warning to leave their houses. The inhabitants of
-Australia and the western shores of the Pacific might, by telegraphic
-communication, receive eighteen to twenty-five hours’ warning of the
-coming of destructive sea waves resulting from earthquakes in South
-America.
-
-Although warnings like these might have their value, that which is
-chiefly required is to warn the dwellers at and near an earthquake
-centre of coming disturbances.
-
-What the results of the observations on earth tremors will lead to is
-problematical.
-
-Should microseismic observation enable us to say when and where the
-minute movements of the soil will reach a head, a valuable contribution
-to the insurance of human safety in earthquake regions will have been
-attained.
-
-As to whether the movements of tromometers are destined to become
-barometric-like warnings of increased activity beneath the earth
-crust, or whether they are only due to vibrations of the earth
-crust produced by variations in atmospheric pressure, has yet to be
-investigated.
-
-Other phenomena which may probably forewarn us of the coming of an
-earthquake are phenomena resultant on the stresses brought to bear upon
-the rocky crust previous to its fracture, or phenomena due to changes
-in the position and condition of heated materials beneath the earth’s
-surface. Amongst these may be mentioned electrical disturbances, which
-appear to be so closely related to seismic phenomena.
-
-At the time of earthquakes telegraph lines have been disturbed, but
-as to what may happen before an earthquake we have as yet but little
-information. The subject of earthquake warning is of importance to many
-countries, and is deserving of attention.
-
-As our knowledge of earth movements, and their attendant phenomena,
-increases, there is but little doubt that laws will gradually be
-formulated, and in the future, as telluric disturbances increase, a
-large black ball gradually ascending a staff may warn the inhabitants
-on the land of a coming earthquake, with as much certainty as the ball
-upon a pole at many seaports warns the mariner of coming storms.
-
-
-
-
-                             CHAPTER XIX.
-
-                            EARTH TREMORS.
-
-  Artificially produced tremors—Observations of Kater, Denman,
-    Airy, Palmer, Paul—Natural tremors—Observations of Zöllner,
-    M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With
-    seismoscopes, microphones, pendulums—Work in Italy—Bertelli,
-    Count Malvasia, M. S. di Rossi—Instruments employed in
-    Italy—Tromometers, microseismographs, microphones—Results
-    obtained in Italy—in Japan—Cause of microseismic motion.
-
-
-During the past few years considerable attention has been drawn towards
-the study of small vibratory motions of the ground, which to the
-unaided senses are usually passed by without recognition. These motions
-are called _earth tremors_. Their discovery appears to have been due
-to accident, and not to the results of inductive reasoning. No sooner
-had philosophers contrived astronomical and other instruments for the
-purpose of making refined measurements and observations than they at
-once discovered that they had an enemy to contend against in the form
-of microscopic earthquakes.
-
-_Artificially produced tremors._—Artificial disturbances of this
-description exist in all our towns, and near a railway line they are
-perceptible with every passing train. Those who have used microscopes
-of high power will readily appreciate how small a disturbance of
-the ground is visible in the apparent movement of the object under
-examination.
-
-Captain Kater found that he could not perform his pendulum experiments
-in London on account of the vibrations produced by the rolling of
-carriages. Captain Denman, who made some observations on artificially
-produced tremors, found that a goods train produced an effect 1,100
-feet distant in marshy ground over sandstone. Vertically, however,
-above a tunnel through the sandstone, the effects only extended 100
-feet.
-
-A remarkable example of the trouble which artificially produced earth
-vibrations have occasioned those who make astronomical observations
-occurred some twenty years ago at the Greenwich Observatory. When
-determining the collimation error of the transit circle by means of the
-reflexion of a star in a tray of mercury, it was found that on certain
-nights the surface of the mercury was in such a state of trembling
-that the observers were unable to complete their observations until
-long after midnight. After obtaining a series of dates on which these
-disturbances occurred, it was found that they coincided with public and
-bank holidays, on which days crowds of the poorer classes of London
-flocked to Greenwich Park, and there amused themselves with running and
-rolling down the hill on which the observatory is situated. On these
-occasions it was found that the disturbances in the mercury were such
-that observations could not be made until two or three hours after the
-crowds had been turned out of the neighbouring park.[137]
-
-To obviate this difficulty Sir George Airy suspended his dish of
-mercury in a system of india-rubber bands, and in this way succeeded in
-eating the intruders up.
-
-Lieutenant-Colonel H. S. Palmer, R.E., when engaged with the transit of
-Venus expedition in New Zealand, in 1874, was troubled with vibrations
-produced from a neighbouring railway. To escape the enemy he intrenched
-his instruments by placing them in pits. With pits 3½ feet deep he
-found himself sufficiently protected. The distance from the line was
-about 400 yards, and the soil through which the disturbances were
-propagated was a coarse pebbly gravel.[138]
-
-Before the United States Naval Observatory was established at
-Washington, Professor H. M. Paul was deputed to make a tremor survey
-to discover stable ground. The results of these experiments were
-exceedingly interesting. By watching the reflected image of a star in
-a dish of mercury a passing train would be noticed at the distance of
-a mile. Its approach could be detected by the trembling of the image
-before its coming could be heard. At one point of observation the
-disturbance appeared to be cut off by a ravine. The strata was gravel
-and clay.[139]
-
-These few examples of artificially produced tremors, to which many
-more might be added, have been given because they teach us something
-respecting their nature. Hitherto earth tremors have only been regarded
-as intruders, which it was necessary to escape from or destroy. From
-what has been said they appear to be a superficial disturbance which
-is propagated to an enormous distance. This distance appears to
-depend upon the propagating medium, upon the intensity of the initial
-disturbance, and upon its duration. In the observation of these
-artificial disturbances, which are accessible to every one, and which
-hitherto have been so neglected, we have undoubtedly a fruitful source
-of study.
-
-_Natural tremors._—Next let us turn to those microscopical
-disturbances of our soil which are due to natural causes. Thus far they
-seem to have been recorded wherever instruments suitable for their
-detection have been erected, and it is not improbable that they are
-common to the surface of the whole globe.
-
-Some of the more definite observations which have been made upon earth
-tremors were those made in connection with experiments on the deviation
-of the vertical due to the attractive influence of the moon and sun.
-
-Professor Zöllner, who invented the horizontal pendulum which he used
-in the attempt to measure the change in level due to lunar and solar
-attraction, found his instruments so sensitive that the readings were
-always changing.
-
-The most interesting observations which were made upon small
-disturbances of the soil were those of M. d’Abbadie, who carried on his
-experiments at Abbadia, in Subernoa, near Hendaye, 400 mètres distant
-from the Atlantic, and 62 mètres above sea level. The soil was a loamy
-rock. Here M. d’Abbadie constructed a concrete cone 8 mètres in height,
-which was pierced down the centre by a vertical hole or well, which was
-continued two mètres below the cone into the solid rock. At the bottom
-of this hole or well a pool of mercury was formed which reflected
-the image of cross wires placed at the top of the hole. These cross
-wires and their reflection were observed by means of a microscope.
-The observations consisted in noting the displacement and azimuth of
-the reflected image relatively to the real image of the wires. After
-allowing this structure five years to settle, M. d’Abbadie commenced
-his observations. To find the surface of the mercury tranquil was
-a rare occurrence. Sometimes the mercury appeared to be in violent
-motion, although both the air and neighbouring sea were perfectly calm.
-At times the reflected image would disappear as if the mercury had
-been disturbed by a microscopic earthquake.
-
-The relative positions of the images were in part governed by the state
-of the tide. Altogether the movements were so strange that M. d’Abbadie
-did not venture any speculations as to their cause, but he remarks
-that the cause of the changes he observed were sometimes neither
-astronomical nor thermometrical. These observations, the principal
-object of which was to determine changes in level rather than earth
-vibrations, were carried on between the years 1868 and 1872.[140]
-
-_Observations at Cambridge._—Another instructive set of observations
-were those which were made in the years 1880–1882, by George and Horace
-Darwin, in the Cavendish Laboratory, at Cambridge. The main object
-in these experiments was to determine the disturbing influence of
-gravity produced by lunar attraction. The result which was obtained,
-however, showed that the soil at Cambridge was in such an incessant
-state of vibration that whatever pull the moon may have exerted upon
-the instrument which was employed was masked by the magnitude of the
-effects produced by the earth tremors, and the experiments had, in
-consequence, to be abandoned.
-
-The principle of this instrument was similar to one devised by Sir
-William Thomson, and put up by him in his laboratory at Glasgow.
-As erected by the brothers Darwin, at Cambridge, it was briefly as
-follows: A pendulum, which was a massive cylinder of pure copper,
-was hung by a copper wire, about four feet long, inside a hollow
-cylindrical tube rising from a stone support. A small mirror was then
-hung by two silk fibres, one of which was fastened to the bob and the
-other to the stone basement. A ray of light sent from a lamp on to
-the mirror was reflected to a scale seven feet distant, and by this
-magnification any motion of the bob relatively to the stone support
-was magnified 50,000 times. In several ways the apparatus was insulated
-from all accidental disturbances. The spot of light was observed from
-another room by means of a telescope. This instrument was so delicate
-that even at the distance of sixteen feet the shifting of your weight
-from one foot to the other caused the spot of light to run along the
-scale. So sensitive was the instrument that, notwithstanding its being
-cut off from the surrounding soil by a trench filled with water and
-the whole instrument being immersed in water to damp out the small
-vibrations, it would seem that the ground was in a constant state of
-tremor; in fact, so persistent and irregular were these movements that
-it seemed impossible to separate them from the perturbations due to the
-attraction of the moon.[141]
-
-As a result of observations like these, the world had gradually
-forced upon it the fact that the ground on which we live is probably
-everywhere in what is practically an incessant state of vibration.
-
-This led those who were interested in the study of earth movements to
-establish special apparatus for the purpose of recording these motions
-with the hope of eventually discovering the laws by which they were
-governed.
-
-_Experiments in Japan._—The simpler forms of apparatus which have been
-used in Japan may be described as delicate forms of seismoscopes,
-which, in addition to recording earth tremors, also record the
-occurrence of small earthquakes.
-
-A simple contrivance which may be used for the purpose of recording
-small earthquakes can be made with a small compass needle.
-
-If a light, small sensitive compass needle be placed on a table, it
-will be found that a small piece of iron like a nail may be pushed so
-near to it that the needle assumes a position of extremely unstable
-equilibrium. If the table now receives the slightest tap or shake this
-condition is overcome, and the needle flies to the iron and there
-remains. By making the support of the needle and the iron the poles of
-an electric circuit it is possible to register the time at which motion
-took place with considerable accuracy.
-
-With crude apparatus like this a large number of small earth
-disturbances have been recorded.
-
-Another form of apparatus, employed in Japan, has been a delicately
-constructed _circuit closer_. The motions of this instrument were
-recorded by causing an electro-magnet to deflect a pencil which was
-tracing a circle on a revolving dial. The revolving dial was a disc of
-wood covered with paper fixed to the hour-hand axle of a common clock.
-
-A third form of apparatus used in Japan consisted of a small piece of
-sheet lead about the size of a threepenny piece suspended by a short
-loop from a rigid support. Projecting from the lead a fine wire, about
-two inches in length, passed freely through a hole in a metallic plate.
-By the slightest motion of the support the small pendulum of lead was
-set into a state of tremor and caused its pointer to come in contact
-with one or other side of the hole in the metal plate and thus to close
-an electric circuit.
-
-A more refined kind of apparatus which has been employed in Japan was
-similar to that used by the Darwins at Cambridge. This was so arranged
-that any deflection of the mirror was permanent until the instrument
-was reset, and in this way the maximum disturbance which had taken
-place between each observation was recorded.
-
-In addition to these and other contrivances, experiments were made with
-microphones.
-
-The microphones used were small doubly pointed pencils of carbon
-about three centimetres long, saturated with mercury, and supported
-vertically in pivot holes bored in other pieces of carbon, which were
-the terminals of an electric circuit. These microphones were screwed
-down on the top of stakes driven deeply into the ground. They were
-covered with a glass shade thickly greased at its base. The stakes were
-in the ground at the bottom of a small pit—about two feet square and
-two feet deep—which was lined with a box. The box was covered with a
-lid, and earth to the depth of nine inches or one foot. One of these
-pits was in the middle of a lawn in the front of my house, and the
-other was at the foot of a hill at the back of the house. The wires
-from the microphone passed through the side of the box into a bamboo
-tube and thence up to my dining-room and bed-room. In one of the
-circuits there were three Daniell’s cells, a telephone, and a small
-galvanometer. I used the galvanometer because I found that when there
-was sufficient motion in the microphone to produce a sound in the
-telephone a motion in the needle of a galvanometer was produced. If in
-any case motion took place in the magnetic needle during my absence, it
-was held deflected by a small piece of iron with which it was brought
-into contact by the movement.
-
-The sensitiveness of the arrangement may be judged of from the fact
-that if a person walked on the grass within six feet of the microphone,
-each step caused a creak in the telephone, and the needle of the
-galvanometer was caused to swing and come in contact with the iron.
-Dogs running on the grass had no effect. A small stone one or two
-inches in diameter thrown from the house so that it fell near to the
-microphone pit caused a sharp creak in the telephone and a movement in
-the needle.
-
-The nature of the records I received from this contrivance may be
-judged of from the following extract from my papers.
-
-                  h. m.
-  1879. Nov. 12th 7  0 P.M. contact of needle
-                  7  2  „   difficult to set the needle
-                  7  3  „   needle swings and telephone creaks
-                  7  4  „         „           „           „
-                  7  5  „         „           „           „
-                  7  6  „         „           „           „
-                  7 10  „   3 more swings
-                  7 11  „   again    „
-
-Here I went out, took away the covering, and examined the microphone.
-Nothing wrong was to be observed. All that I saw was one small ant. I
-do not think that this could have caused the disturbance, because it
-could not get near the instrument.
-
-On the succeeding nights I experienced similar disturbances, and it
-seemed as if they might possibly have been the prelude to several small
-shocks which occurred about this time (November 15, 16, and 17). On
-November 17, at 8 A.M., the needle was found in contact, and again at
-5 P.M., and at 6 P.M. the shock of a small earthquake was felt _which
-caused a rattling sound in the telephone for about one minute after the
-motion had appeared to cease_. The needle swung considerably, but did
-not come in contact.
-
-The great objection to these observations is that it is possible
-that the movements and sounds which I have recorded might, with the
-exception of one case when the shaking was actually felt, possibly have
-been produced by causes other than that of the movement of the ground.
-To determine this I subsequently put up two distinct sets of apparatus
-to determine whether the motions of each were synchronous. So far as
-I went this appeared only to be sometimes the case:—but this is a
-question difficult to determine, unless a recorder of time be added to
-the apparatus.
-
-The greatest objection to observations of this sort is that the
-sensibility of the instrument is not constant. After a current has been
-running for several days it is no longer sensible to slight shocks, it
-appears as if its resistance had been increased. To overcome this it is
-necessary to resharpen the carbon points and bore out the pivot holes
-every three or four days. Farther, the battery varies. This might to
-some extent be overcome by using a battery with large plates. These
-two causes tend to reduce the sensitiveness of the galvanometer-like
-recorder—the deflection of the needle gradually becoming less and less,
-and therefore day by day needing a greater swing to bring it into
-contact with the iron. For reasons such as these this instrument, to be
-used successfully, appears to require considerable attention.
-
-Another form of microphone employed by the author consisted of an
-aluminium wire standing vertically on a metallic plate, its upper end
-passing loosely through a hole in an aluminium wire standard.
-
-The upper end of the vertical wire was loaded with lead. This
-contrivance possesses all the sensitiveness of an ordinary microphone,
-whilst, if it receives a sudden impulse, there is a sudden break in the
-current, and the vertical wire is thrown from one side to the other of
-the hole in the standard.
-
-After many months of tiresome observation with instruments of this
-description, and after eliminating all motions which might have been
-produced by accidental causes, the general result obtained showed that
-in Tokio there were movements of the soil to be detected every day,
-and sometimes many times per day, which to ordinary persons were passed
-by unnoticed.
-
-_Work in Italy._—The most satisfactory observations which have been
-made upon microseismic disturbances are those which have been made
-during the last ten years in Italy. The father of systematical
-microseismical research appears to have been Father Timoteo Bertelli,
-of Florence.
-
-In 1870 Father Bertelli suspended a pendulum in a cellar, and observed
-it with a microscope. As the result of his observations it was
-announced that he had perceived the earthquakes which shook Romagna,
-although to the ordinary observer in Florence these shakings had not
-been perceptible.
-
-In 1873 Bertelli, by means of microscopes fixed in several azimuths,
-made 5,500 observations on free pendulums. He also made observations on
-reflections from the surface of mercury.[142]
-
-One result of these observations was to show that microseismic motions
-increased with a fall of the barometer. Similar observations were made
-at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near
-Rome. On January 14, 15, and February 25 these three observers at their
-respective stations simultaneously observed great disturbances.
-
-Similar investigations were made at Nice by M. le Baron Prost.
-
-Although doubt was cast upon Bertelli’s observations they appear to
-have been the origin of a series of microseismical observations, a
-distinguished leader in which is Professor Rossi, who, in 1874, found
-that large earthquakes were almost always preceded or accompanied with
-microseismical storms. In 1878 Professor Rossi worked upon these small
-disturbances with the assistance of the microphone and telephone, and
-his first results were published by Professor Palmieri.
-
-Many of Professor Rossi’s observations were made in the grotto of Roca
-de Papa, 700 mètres high and eighteen mètres under the soil. Here over
-6,000 observations were made by means of microscopes, on pendulums of
-different lengths, suspended in tubes cut in the solid rock.
-
-_Instruments employed in Italy._—It is impossible to describe in detail
-the various forms of apparatus which have been used by the Italian
-investigators. A description of one or two of the more important
-instruments may not, however, be out of place, inasmuch as they will
-assist the reader to understand the manner in which the various results
-respecting the laws governing microseismic movements have been arrived
-at.
-
-The most important of these instruments is the _Normal Tromometer_ of
-Bertelli and Rossi.
-
-This consists of a pendulum 1½ metres long, carrying, by means of
-a very fine wire, a weight of 100 grammes. To the base of the bob
-a vertical stile is attached, and the whole is enclosed in a tube
-terminated, at its base, by a glass prism of such a form that when
-looked through horizontally the motion of the stile can be seen in
-all azimuths. In front of this prism a microscope is placed. Inside
-the microscope there is a micromatic scale, so arranged that it can
-be turned to coincide with the apparent direction of oscillation of
-the point of the stile. In this way not only can the amplitude of the
-motion of the stile be measured, but also its azimuth. The extent of
-vertical motion is measured by the up and down motion of the stile due
-to the elasticity of the supporting wire. This instrument is shown in
-the accompanying drawing.
-
-[Illustration: FIG. 37.—Normal Tromometer.
-
-B, bob of pendulum; P, prism; M, microscope; S, rim of scale.]
-
-Another apparatus is the _Microseismograph_ of Professor Rossi. Here we
-have an arrangement which gives automatic records of slight motions.
-It consists of four pendulums, each about three feet long, suspended
-so that they form the corners of a square platform. In the centre
-of this platform a fifth, but rather longer, pendulum is suspended.
-The four pendulums are each connected just above their bobs to the
-central pendulum with loose silk threads. Fixed to the centre of each
-of these threads, and held vertically by a light spring, is a needle,
-so adjusted that each thread is depressed to form an obtuse angle of
-about 155°. These needles form the terminals of an electric circuit,
-the other termination of which is a small cup of mercury placed just
-below the lower end of the needle. By a horizontal swing of one of the
-pendulums this arrangement causes the needle to move vertically, but
-with a slightly multiplied amplitude. By this motion the needle comes
-in contact with the mercury, and an electro-magnet with a lever and
-pencil is caused to make a mark on a band of paper moved by clockwork.
-The five pendulums being of different lengths allows the apparatus to
-respond ‘to seismic waves of different velocities.’[143]
-
-Lastly, we have Professor Rossi’s _Microphone_. This consists of a
-metallic swing arranged like the beam of a balance. By means of a
-movable weight at one end of the beam this is so adjusted that it
-falls down until it comes in contact with a metallic stop. This can be
-so adjusted that a slight tap will cause the beam to slightly jump from
-the stop. The beam and the stop form two poles of an electric circuit,
-in which there is a telephone. The slightest motion in a _vertical_
-direction causes a fluctuation in the current passing between the stop
-and the beam, and a consequent noise is heard in the telephone.
-
-With instruments analogous to these, observations have been made by
-various observers in all portions of Italy, extending over a period
-of ten years. Every precaution appears to have been taken to avoid
-accidental disturbances, and the experiments have been repeated in a
-variety of forms.
-
-_Results obtained in Italy._—The results which from time to time have
-been announced are of the greatest interest to those who study the
-physics of the earth’s crust, and they appear to be leading to the
-establishment of laws of scientific value.
-
-It would seem that the soil of Italy is in incessant movement, there
-being periods of excessive activity usually lasting about ten days.
-Such periods are called seismic storms. These storms are separated by
-periods of relative calm. These storms have their greater regularity
-in winter, and sharp maximums are to be observed in spring and autumn.
-In the midst of such a period or at its end there is usually an
-earthquake. Usually these storms are closely related to barometric
-depressions. To distinguish these movements from those which occur
-under high pressure, the latter are called baro seismic movements, and
-the former _vulcano seismic_ movements. The relation of these storms
-to barometric fluctuation has been observed to have been very marked
-during the time of a volcanic eruption.
-
-At the commencement of a storm the motions are usually small, and one
-storm, lasting two or three days, may be joined to another storm.
-In such a case the action may be a local one. It has been observed
-that a barometrical depression tended to bring a storm to a maximum,
-whilst an increase of pressure would cause it to disappear. Sometimes
-these actions are purely local, but at other times they may affect a
-considerable tract of land.
-
-If a number of pendulums of different length are observed at the same
-place, there is a general similarity in their movements, but it is also
-evident that the free period of the pendulum more or less disturbs
-the character of the record. The greatest amplitude of motion in a
-set of pendulums is not reached simultaneously by all the pendulums,
-and at every disturbance the movement of one will predominate. From
-this Rossi argues that the character of the microseismical motions is
-not constant. Bertelli observed that the direction of oscillation of
-the pendulums is different at different places, but each place will
-have its particular direction dependent upon the direction of valleys
-and chains of mountains in the neighbourhood. Rossi shows that the
-directions of movement are perpendicular to the direction of lines of
-faults, the lips of these fractures rising and falling, and producing
-two sets of waves, one set parallel to the line of fracture, and the
-other perpendicular to such a direction. These movements, according
-to Bertelli, have no connection with the wind, rain, change of
-temperature, and atmospheric electricity.
-
-[Illustration: FIG. 38.]
-
-The disturbances, as recorded at different towns, are not always
-strictly synchronous, but succeed each other at short intervals. If,
-however, we take monthly curves of the disturbances as recorded at
-different towns in Italy, we see that these are similar in character.
-The maximum of disturbance occurs about the winter solstice, and the
-minimum about the summer solstice, and in this respect they exhibit
-a perfect accordance with the curves drawn by Mallet to show the
-periodicity of earthquakes. The accompanying curves taken from one
-of Bertelli’s original memoirs not only show this general result but
-also show the close accord there is between the results obtained at
-different towns during successive months.
-
-At Florence, before a period of earthquakes there is an increase in the
-amplitude and frequency of vertical movements. These vertical movements
-do not appear to coincide with the barometrical disturbances, but they
-appear to be connected with the seismic disturbances.
-
-They are usually accompanied with noises in the telephone, but as the
-microphone is so constructed as to be more sensitive to vertical motion
-than to horizontal motion, this is to be expected. This vertical motion
-would appear to be a local action, inasmuch as the accompanying motions
-of an earthquake which originates at a distance are horizontal.
-
-Storms of microseismical motions appear to travel from point to point.
-
-Sometimes a local earthquake is not noticed in the tromometer,
-whilst one which occurred at a distance, although it may be small,
-is distinctly observed. To explain this, Bertelli suggests the
-existence of nodes. Similar conclusions were arrived at by Rossi when
-experimenting on different portions of the sides of Vesuvius. Galli
-noticed an augmentation in microseismic activity when the sun and moon
-are near the meridian. Grablovitz found from Bertelli’s observations
-a maximum two or three days before the syzygies, and a minimum
-three days after these periods. He also found that the principal
-large disturbances occurred in the middle of periods separating
-the quadrature from the syzygies, the apogee from perigee, and the
-lunistigi period from the nodes, whilst the smallest disturbances
-happened in the middle of periods opposed to these.
-
-P. C. Melzi says that the curves of microseismical motions,
-earthquakes, lunar and solar motions, show a concordance with each
-other.
-
-With the microphone Rossi hears sounds which he describes as roarings,
-explosions, occurring isolated or in volleys, metallic and bell-like
-sounds, ticking, &c., which, he says, revealed natural telluric
-phenomena. Sometimes these have been intolerably loud. At Vesuvius the
-vertical shocks corresponded with a sound like volleys of musketry,
-whilst the undulatory shocks gave the roaring. Some of these sounds
-could be imitated artificially by rubbing together the conducting wires
-in the same manner in which the rocks must rub against each other in
-an earthquake. Other sounds were imitated by placing the microphone
-on a vessel of boiling water, or by putting it on a marble slab and
-scratching and tapping the under side of it.
-
-These, then, are some of the more important results which have been
-arrived at by the study of microseismic motions. One point which seems
-worthy of attention is that they appear to be more law-abiding than
-their more violent relations, the earthquakes, and as phenomena in
-which natural laws are to be traced they are certainly deserving our
-attention. As to whether they will ever become the means of forewarning
-ourselves against earthquakes is yet problematical. Their systematic
-study, however, will enable us to trace the progress of a microseismic
-storm from point to point, and it is not impossible that we may yet
-be enabled to foretell where the storm may reach its climax as an
-earthquake. These, I believe, are the views of Professor di Rossi, who
-is at the present time engaged in the establishment of a system of
-microseismic observations throughout Italy.
-
-Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer
-was in a state of agitation, and similar disturbances were observed at
-Livorno, Florence, and Bologna.
-
-Since February 1883 I have observed a tromometer in Japan, and such
-results as have been obtained accord with results obtained in Italy.
-The increase in microseismical activity with a fall of the barometer
-is very marked. Other peculiarities in the behaviour of the instrument
-will be referred to under ‘Earth Pulsations.’
-
-_Cause of microseismic movements._—As to the cause of tromometric
-movements, we have a field for speculation. Possibly they may be
-due to slight vibratory motions produced in the soil by the bending
-and crackling of rocks produced by their rise upon the relief of
-atmospheric pressure. If this were so we should expect similar
-movements to be produced at the time of an increase of pressure. Rossi
-suggests that they may be the result of an increased escape of vapour
-from the molten materials beneath the crust of the earth, consequent
-upon a relief of pressure. The similarity of some of the sounds which
-are heard with the microphone to those produced by boiling water are
-suggestive of this, and Rossi quotes instances when underground noises
-like those which we should expect to hear from a boiling fluid have
-been heard before earthquakes without the aid of microphones. One
-instance was that of Viduari, a prisoner in Lima, who, two days before
-the shock of 1824, repeatedly predicted the same in consequence of the
-noises he heard.
-
-A possible cause of disturbances of this order may be small but
-sudden fluctuations in barometric pressure, which are visible during
-a storm. During a small typhoon on September 15, 1881, when in the
-Kurile Islands, I observed that the needle of an aneroid worked back
-and forth with a period of from one to three seconds. This continued
-for several hours. With every gust of wind the needle suddenly rose
-and then immediately fell. At times it trembled. These movements were
-observed in the open air. The extent of these sudden variations
-was approximately from ·03 to ·05 inches. Beckoning an increase of
-barometrical pressure of one hundredth of an inch as equivalent to a
-load of twenty million pounds on the square mile, during this storm
-there must have been the equivalent of loads of from 60 to 100 million
-pounds to the square mile continually placed on and removed from a
-considerable tract of the earth’s surface. If the period of application
-of these stresses approximately coincide with the natural vibrational
-period of the area affected, it would surely seem, especially when
-we reflect upon the effect of an ordinary carriage, that tremors of
-considerable magnitude ought to be produced.
-
-An inspection of the following few observations taken from my note-book
-for the same typhoon will suggest that even the large and slower
-variations are capable of producing tremulous motions.
-
-                        Time           Barometer
-                        h.  m.          reading
-                        12  5 P.M.       29·02
-                        12 10  „         29·05
-                        12 12  „         29·07
-                        12 13  „         29·05
-                        12 25  „         29·10
-                        12 50  „         29·00
-                         1 10  „         29·00
-                         1 20  „         29·07
-
-
-
-
-                              CHAPTER XX.
-
-                           EARTH PULSATIONS.
-
-  Definition of an earth pulsation—Indications of
-    pendulums—Indications of levels—Other phenomena indicating
-    the existence of earth pulsations—Disturbances in lakes and
-    oceans—Phenomena resultant on earth pulsations—Cause of earth
-    pulsations.
-
-
-The object of the present chapter is to show that from time to time it
-is very probable that slow but large wave-like undulations travel over
-or disturb the surface of the globe.
-
-These movements, which have escaped our attention on account of their
-slowness in period, for want of another term I call earth pulsations.
-
-The existence of movements such as these may be indicated to us by
-changes in the level of bodies of water like seas and lakes, by the
-movements of delicate levels, by the displacement of the bob of a
-pendulum relatively to some point on the earth above which it hangs,
-and by other phenomena which will be enumerated.
-
-_Indication of pendulums._—Pendulums which have been suspended for the
-purposes of seismometrical observations have, both by observers in
-Italy and Japan, been seen to have moved a short distance out from, and
-then back to, their normal position.
-
-This motion has simply taken place on one side of their central
-position, and is not due to a swing. The character of these records
-is such that we might imagine the soil on which the support of the
-pendulum had rested to have been slowly tilted, and slowly lowered.
-They are the most marked on those pendulums provided with an index
-writing a record of its motions on a smoked glass plate, which index is
-so arranged that it gives a multiplied representation of the relative
-motion between it and the earth. As motions of this sort might be
-possibly due to the action of moisture in the soil tilting the support
-of the pendulum, and to a variety of other accidental causes, we cannot
-insist on them as being certain indications that there are slow tips in
-the soil, but for the present allow them to remain as possible proof of
-such phenomena.
-
-Evidence of displacement of the vertical, which are more definite than
-the above, are those made by Bertelli, Rossi, Count Malvasia, and other
-Italian observers, who, whilst recording earth tremors, have spent so
-much time in watching the vibrations of stiles of delicate pendulums
-by means of microscopes. As a result of these observations we are
-told that the point about which the stile of a pendulum oscillates
-is variable. These displacements take place in various azimuths, and
-they appear to be connected with changes of the barometer. I have made
-similar observations in Japan.
-
-From this, and from the fact that it is found that a number of
-different pendulums differently situated on the same area give similar
-evidence of these movements, it would hardly seem that these phenomena
-could be attributed to causes like changes in temperature and moisture.
-M. S. di Rossi lays stress on this point, especially in connection with
-his microseismograph, where there are a number of pendulums of unequal
-length which give indications of a like character. The direction in
-which these tips of the soil take place—which phenomena are noticeable
-in seismic as well as microseismic motions—Rossi states are related to
-the direction of certain lines of faulting.
-
-_Indications of levels._—Bubbles of delicate levels can be easily seen
-to change their position with meteorological variations; but Rossi also
-tells us that they change their position, sometimes not to return for
-a long time, during a microseismic storm. Here again we have another
-phenomenon pointing to the fact that microseismic disturbances are the
-companions of slow alterations in level.
-
-One of the most patient observers of levels has been M. Plantamour,
-who commenced his observations in 1878, at Sécheron, on the Lake of
-Geneva. He used two levels, one placed north and south, and the other
-east and west. During the summer of 1878 the east end rose, but at the
-end of September a depression set in. The diurnal movements had their
-maximum and minimum at 6 and 7.45 A.M. and P.M. The total amplitude was
-4·89″. The variations of the east and west level appeared to be due to
-the temperature, but the movements of the north and south level were
-dependent upon an unknown cause.
-
-Between October 1, 1879, and September 30, 1880, the east end fell
-rapidly, from the middle of November up to December 26, amounting to
-88·71″. It then rose 6·55″ to January 5, and then fell again. On
-January 28 it reached 89·95″, after which it rose.
-
-Between October 4, 1879, and January 28, 1880, the movement was 95·8″,
-against 28·08″ of the previous year.
-
-These movements were not due alone to temperature. The north and south
-level, which was not influenced by the cold of the winter, moved
-4·56″. In the previous year 4·89″.[144]
-
-From February 17 to June 5, 1883, the author observed in Tokio the
-bubbles of two delicate levels, one placed north and south, and the
-other east and west. They were placed under glass cases on the head of
-a stone column. The column, which is inside a brick building, rests
-on a concrete foundation, and is about ten years old. It is in no way
-connected with the building. The temperature of the room has a daily
-variation of about 1° Fahr.
-
-In both these levels diurnal changes are very marked. Occasionally they
-are enormously great. Thus, on March 25, the readings of the south end
-of the north south level were as follows:—
-
-                            Time           Readings.
-                            h. m.
-                      25th. 4 00 P.M.       104·5
-                            4  5  „         103
-                            4 10  „         102
-                            4 25  „         101
-                            4 30  „         100
-                            4 40  „          98
-                            4 42  „          99·5
-                            4 45  „         100
-                            4 50  „         101
-                            4 55  „         101
-                            5 00  „         100
-                      26th. 7 00 A.M.       105
-
-Usually this level moves through about three divisions per day.
-
-From March 25 to May 4 it travelled from 98 to 127. Since then, to June
-5, it has descended to 116. During this period the east west level
-has been _comparatively_ quiet. One division of the north south level
-equals about 2″ of arc.
-
-Many of these changes may be due to changes in temperature, variations
-in moisture, and other local actions. Some of them, however, are hardly
-explicable on such assumptions. The fact that the general direction in
-change of the vertical, as indicated by a tromometer standing on the
-same column with the levels, showed that the change which was taking
-place was rather in the column than in the instruments.
-
-The fact also that at the time of a barometrical depression a
-_pulse-like surge_ can be seen in the levels, having a period averaging
-about three seconds and sometimes amounting to about one second of
-arc, is a phenomenon hardly to be attributed to sudden fluctuations in
-moisture or temperature, but indicates real changes in level.[145]
-
-In addition to variation in the bubbles of levels which come on more or
-less gradually, we have many recorded instances of _sudden_ alterations
-taking place in these instruments.
-
-Examples of what may have been a slow oscillating motion of the earth’s
-crust are referred to by Mr. George Darwin in a Report to the British
-Association in 1882.
-
-One of them was made by M. Magnus Nyrén, at Pulkova, who, when engaged
-in levelling the axis of a telescope, observed spontaneous oscillation
-in the bulb of the level.
-
-This was on May 10 (April 28), 1877. The complete period was about
-20 seconds, the amplitude being 1·5″ and 2″. One hour and fourteen
-minutes before this he observes that there had been a severe earthquake
-at Iquique, the distance to which in a straight line was 10,600
-kilomètres, and on an arc of a great circle, 12,500 kilomètres.
-On September 20 (8), in 1867, Mr. Wagner had observed at Pulkova
-oscillations of 3″, seven minutes before which there had been an
-earthquake at Malta. On April 4 (March 23), 1868, an agitation of the
-level had been observed by Mr. Gromadzki, five minutes before which
-there had been an earthquake in Turkestan. Similar observations had
-been made twice before. These, however, had not been connected with any
-earthquakes—at least, Mr. Darwin remarks—with certainty.
-
-_Phenomena analogous to the pendulum and level observations._—As
-examples of phenomena which are analogous to those made on pendulums
-and levels, the following may be noticed. On March 20, 1881, at 9 P.M.
-a watchmaker in Buenos Ayres observed that all his clocks oscillating
-north and south suddenly began to increase their amplitude, until some
-of them became twice as great as before. Similar observations were
-made in all the other shops. No motion of the earth was detected.
-Subsequently it was learnt that this corresponded with an earthquake in
-Santiago and Mendoza.[146]
-
-Another remarkable example illustrating the like phenomena is furnished
-by the observations which were made on December 21, 1860, by means of a
-barometer in San Francisco, which oscillated, with periods of rest, for
-half an hour. No shock was felt, nor is it likely that it was a local
-accident, as it could not be produced artificially. On the following
-day, however, a violent earthquake was experienced at Santiago.[147]
-
-At the time or shortly after the great Lisbon earthquake, curious
-phenomena were observed in distant countries, which only appear to be
-explicable on the assumption of the existence of earth pulsations.
-
-Thus at Amsterdam and other towns, chandeliers in churches were
-observed to swing. At Haarlem water was thrown over the sides of tubs,
-and it is expressly mentioned that no motion was perceived in the
-ground.
-
-At the Hague a tallow chandler was surprised at the clashing noise
-made by his candles, and this the more so because no motion was felt
-underfoot.
-
-_Unusual disturbances in bodies of water._—At the time of large
-earthquakes it would appear that earth pulsations are produced, which
-exhibit themselves in countries where the actual shaking of the
-earthquake is not felt, by disturbances in bodies of waters like lakes
-and seas.
-
-Some remarkable examples of these disturbances are to be found in the
-records of the great Lisbon earthquake. This earthquake, as a violent
-movement of the ground, was chiefly felt in Spain, Portugal, northern
-Italy, the south of France and Germany, northern Africa, Madeira, and
-other Atlantic islands. In other countries further distant, as, for
-instance, Great Britain, Holland, Scandinavia, and North America,
-although the records are numerous, the only phenomena which were
-particularly observed was the slow oscillations of the waters in lakes,
-ponds, canals, &c. In some instances the observers especially remark
-that there was no motion in the soil.
-
-Pebley Dam, in Derbyshire, which is a large body of water covering
-some thirty acres, commenced to oscillate from the south. A canal near
-Godalming flowed eight feet over the walk on the north side.
-
-Coniston Water, in Cumberland, which is about five miles long,
-oscillated for about five minutes, rising a yard up its shores. Near
-Durham a pond, forty yards long and ten broad, rose and fell about one
-foot for six or seven minutes. There were four or five ebbs and flows
-per minute.
-
-Loch Lomond rose and fell through about two and a half feet every
-five minutes, and all the other lochs in Scotland seem to have been
-similarly agitated.
-
-At Shirbrun Castle, in Oxfordshire, where the water in some moats and
-ponds was very carefully observed, it was noticed that the floods
-began gently, the velocity then increased, till at last with great
-impetuosity they reached their full height. Here the water remained for
-a little while, until the ebb commenced, at first gently, but finally
-with great rapidity. At two extremities of a moat about 100 yards long,
-it was found that the sinkings and risings were almost simultaneous.
-The motions in a pond a short distance from the moat were also
-observed, and it was found that the risings and sinkings of the two did
-not agree.
-
-During these motions there were several maxima.
-
-These few examples of the motions of waters, without any record of the
-motions of the ground, at the time of the Lisbon earthquake, must be
-taken as examples of a very large number of similar observations of
-which we have detailed accounts.
-
-Like agitations, it must also be remembered, were perceived in North
-America and in Scandinavia, and if the lakes of other distant countries
-had been provided with sufficiently delicate apparatus, it is not
-unlikely that similar disturbances would have been recorded.
-
-Besides these movements in the waters of seas and lakes, at or about
-the time of great earthquakes, we have records of like movements, which
-take place as independent phenomena.
-
-Thus we read that on October 22, 1755, the waters of Lake Ontario rose
-and fell five and a half feet several times in the course of half an
-hour.[148] On March 31, 1761, Loch Ness rose suddenly for the period
-of three-quarters of an hour.[149]
-
-As another example of the disturbance of water at the time of a great
-earthquake in districts where the earthquake was not felt, may be
-mentioned the swelling of the waters of the Marañon, in 1746, on the
-night when Callao was overwhelmed.
-
-Sudden variations in the level of the water have been many times
-observed in the North American lakes. The changes in level which
-sometimes take place in the Genfer and Boden lakes are supposed to have
-some relation to the condition of the atmosphere. A rising and falling
-of especial note took place on April 18, 1855.
-
-In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’
-
-From the observations and calculations of Prof. Forel it would seem
-that the period of the ‘seiches’ depends upon the dimensions of the
-lakes; the calculated periods dependent on the depths of the lakes
-being approximately equal to the observed periods.[150]
-
-W. T. Bingham, writing on the volcanoes of the Hawaiian Islands,
-remarks that it is not unusual for the sea to be agitated by great and
-unusual tides, and that such sea waves have not been attended with
-volcanic eruptions or seismic disturbances. Thus in May 1819 the tide
-rose and fell thirteen times. On November 7, 1837, there was an ebb and
-flow of eight feet every twenty-eight minutes. Again, on May 17, 1841,
-like phenomena, unaccompanied by any other unusual occurrences, were
-recorded.[151]
-
-Phenomena which may possibly hold a relationship to earth pulsations
-are the periodical swellings of the ocean on the coast of Peru. Dr. C.
-F. Winslow, who made a long period observation upon the coast of Peru,
-found ‘the highest tides to prevail at Callao and Paita in December
-and January,’ and ‘also a series of enormous waves or sea-swells to
-be thrown from time to time upon the coast, varying from twenty-four
-to twenty-seven hours in continuance, accompanied by unusual height
-of the tide during the same period.’ During June and July the ocean
-was unusually tranquil. These phenomena do not appear to be connected
-with great atmospheric storms, nor do they hold any relation to the
-prevailing wind. They increase with and accompany the swelling of the
-tides, and occur generally, but not always, about full moon.
-
-Sometimes they break suddenly upon the coast. ‘_They are annual and
-constant in their periodicity._’
-
-The periodical swellings are most noticeable between Tumbez 3° S.L. and
-the Chincha Islands 14° S.L.
-
-These oceanic phenomena synchronise with the periodic intensity of
-earthquake phenomena in that part of the globe, and these with tidal
-movements.[152]
-
-_Other phenomena possibly attributable to earth pulsations._—If we
-assume that earth pulsations have an existence, these many phenomena
-which are otherwise difficult to understand meet with an explanation.
-The curious effects which were produced in the springs at Toplitz at
-the time of the Lisbon earthquake may have been due to a pulse-like
-wave. The flow of the principal spring was greatly increased. Before
-the increase it became turbid and at one time stopped. Subsequently it
-became clear and flowed as usual, but the water was hotter and more
-strongly mineralised. Sudden changes in the flow of underground waters
-which from time to time are observed may be attributed to like causes.
-Secondary earthquakes such as occurred after the Lisbon earthquake,
-as for instance in Derbyshire, may have been produced by pulsations
-disturbing the equilibrium of ground in a critical state.
-
-The falling in of subterranean excavations is also possibly connected
-with these phenomena.
-
-_Possible causes of earth pulsations._—Mr. George Darwin, in a report
-to the British Association (1882), has shown that movements of
-considerable magnitude may occur in the earth’s crust in consequence of
-fluctuations in barometrical pressure. (A rise of the barometer over an
-area is equivalent to loading that area with a weight, in consequence
-of which it is depressed. When the barometer falls, the load is removed
-from the area, which, in virtue of its elasticity, rises to its
-original position. This fall and rise of the ground completes a single
-pulsation.)
-
-On the assumption that the earth has a rigidity like steel, Mr. Darwin
-calculates that if the barometer rises an inch over an area like
-Australia, the load is sufficient to sink that continent two or three
-inches.
-
-The tides which twice a day load our shores cause the land to rise and
-fall in a similar manner. On the shores of the Atlantic, Mr. Darwin
-has calculated that this rise and fall of the land may be as much as 5
-inches. By these risings and fallings of the land the inclination of
-the surface is so altered that the stile of a plummet suspended from a
-rigid support ought not always to hang over the same spot. There would
-be a deflection of the vertical.
-
-In short, calculations respecting the effects of loads of various
-descriptions, which we know are by natural operations continually
-being placed upon and removed from the surface of various areas of the
-earth’s surface, indicate that slow pulsatory movements of the earth’s
-surface must be taking place, causing variations in inclination of one
-portion of the earth’s crust relatively to another.
-
-Although it is possible that phenomena like the surging of levels may
-be attributable to causes like these, we can hardly attribute the other
-phenomena to such agencies.
-
-Rather than seek an explanation from agencies exogenous to our earth,
-we might perhaps with advantage appeal to the endogenous phenomena of
-our planet. When the barometer falls, which we have shown corresponds
-to an upward motion of the earth’s crust, we know, from the results of
-experiments, that microseismic motions are particularly noticeable.
-
-As a pictorial illustration of what this really means, we may imagine
-ourselves to be residing on the loosely fitting lid of a large
-cauldron, the relief of the external pressure over which increases the
-activity of its internal ebullition—the jars attendant on which are
-gradually propagated from their endogenous source to the exterior of
-our planet. This travelling outwards would take place much in the same
-way that the vibrations consequent to the rattle and jar of a large
-factory slowly spread themselves farther and farther from the point
-where they were produced.
-
-Admitting an action of this description to take place, it would then
-follow that this extra liberation of gaseous material beneath the
-earth’s crust would result in an increased upward pressure from within,
-and a tendency on the part of the earth’s crust to elevation. If we
-accept this as an explanation of the increased activity of a tremor
-indicator, then such an instrument may be regarded as a barometer,
-measuring by its motions the variations in the internal pressure of our
-planet.
-
-The relief of external pressure and the increase of internal pressure,
-it will be observed, both tend in the same direction—namely, to an
-elevation of the earth’s crust.
-
-This explanation of the increased activity of earth tremors, which
-has also been suggested by M. S. di Rossi, is here only advanced as a
-speculation, more probable perhaps than many others.
-
-We know how a mass of sulphur which has been fused in the presence of
-water in a closed boiler gives up in the form of steam the occluded
-moisture upon the relief of pressure. In a similar manner we see steam
-escaping from volcanic vents and cooling streams of lava. We also
-know how gas escapes from the pores and cavities in a seam of coal
-on the fall of the barometrical column. We also know that certain
-wells increase the height of their column under like conditions. The
-latter of these phenomena, resulting in an increase in the rate of
-drainage of an area by its tendency to render such an area of less
-weight, facilitates its rise. If we follow the views of Mr. Mallet in
-considering that the pressures exerted on the crust of our earth may in
-volcanic regions be roughly estimated by the height of a column of lava
-in the volcanoes of such districts, we see that in the neighbourhood of
-a volcano like Cotopaxi the upward pressures must be enormously great.
-Further, the phenomena of earthquakes and volcanoes indicate that these
-pressures are variable. Before a volcano bursts forth we should expect
-that there would be in its vicinity an upward bulging of the crust, and
-after its formation a fall. Further, it is not difficult to conjecture
-other possible means by which such pressures may obtain relief.
-
-Should these pressures then find relief without rupturing the
-surface, it is not difficult to imagine them as the originators of
-vast pulsations which may be recorded on the surface of the earth as
-wave-like motions of slow period.
-
-As an explanation of the strange movements observed on seas and lakes,
-Kluge brings forward the following strange and remarkable theory. The
-oxygen of the air is magnetic, whilst water is diamagnetic and the
-earth magnetic: we have, therefore, in our seas and lakes a diamagnetic
-body lying between and being, consequently, repelled from two magnetic
-bodies. By variation in temperature, the balance of repulsions
-exerted by the air and the earth is destroyed. Thus, by an elevation
-of temperature the air expands and flows away from the heated area,
-where, in consequence, there is less oxygen. The result of this is,
-that the repulsion of the air upon the waters is less than that of the
-earth upon the waters, and the waters are in consequence raised up. By
-a falling of temperature the waters may be depressed, and by either
-of these actions waves may be produced without the intervention of
-earthquakes or earth pulsations.
-
-The more definite kinds of information which we have to bring forward,
-tending to prove the existence of earth pulsations, too slow in period
-to be experienced by ordinary observers, are those which appear to be
-resultant phenomena of great earthquakes.
-
-The phenomena that we are certain of in connection with earth
-vibrations, whether these vibrations are produced artificially by
-explosions of dynamite in bore-holes, or whether they are produced
-naturally by earthquakes, are, first, that a disturbance as it dies out
-at a given point often shows in the diagrams obtained by seismographs a
-decrease in period; and, secondly, a similar decrease in the period of
-the disturbance takes place as the disturbance spreads.
-
-As examples of these actions I will quote the following.
-
-The diagram of the disturbance of March 1, 1882, taken at Yokohama,
-shows that the vibrations at the commencement of the disturbance had a
-period of about three per second, near the middle of the disturbance
-the period is about 1·1, whilst near the end the period has decreased
-to ·46. That is to say, the backward and forward motion of the ground
-at the commencement of the earthquake was six times as great as it was
-near the end, when to make one complete oscillation it took between two
-and three seconds. Probably the period became still less, but was not
-recorded owing to the insensibility of the instruments to such slow
-motions.[153]
-
-We have not yet the means of comparing together diagrams of two or more
-earthquakes, one having been taken near to the origin, and the other
-at a distance. The only comparisons which I have been enabled to make
-have been those of diagrams taken of the same earthquake, one in Tokio
-and the other in Yokohama. As this base is only sixteen miles, and the
-earthquake may have originated at a distance of several hundreds of
-miles, comparisons like these can be of but little value.
-
-Other diagrams illustrating the same point are those obtained at three
-stations in a straight line, but at different distances from the
-origin of a disturbance produced by exploding a charge of dynamite in
-a bore-hole. A simple inspection of these diagrams shows that at the
-near station the disturbance consisted of backward and forward motions,
-which, as compared with the same disturbance as recorded at a more
-distant station, were very rapid. Further, by examining the diagram
-of the motions, say, at the near station, it is clearly evident that
-the period of the backward and forward motion rapidly decreased as the
-motion died out.
-
-These illustrations are given as examples out of a large series of
-other records, all showing like results.
-
-An observation which confirms the records obtained from seismographs
-respecting the increase in period of an earthquake as it dies out
-I have had opportunities of twice making with my levels. After all
-perceptible motion of the ground subsequent upon a moderately severe
-shock had died away, I have distinctly seen the bubble in one of these
-levels slowly pulsating with an irregular period of from one to five
-seconds.
-
-Although we must draw a distinction between earth waves and water
-waves, we yet see that in these points they present a striking
-likeness. Let us take, for example, any of the large earthquake waves
-which have originated off the coast of South America, and then radiated
-outwards, until they spread across the Pacific, to be recorded in Japan
-and other countries perhaps twenty-five hours afterwards, at a distance
-of nearly 9,000 miles from their origin. Near this origin they appeared
-as walls of water which were seen rapidly advancing towards the coast.
-These have been from twenty to two hundred feet in height, and they
-succeeded each other at rapid intervals, until finally they died out as
-a series of gentle waves. By the time these walls of water traversed
-the Pacific, to, let us say, Japan, they broadened out to a swell so
-flat that it could not be detected on the smoothest water excepting
-along shore lines where the water rose and fell like the tide. Instead
-of a wall of water sixty feet in height, we had long flat undulations
-perhaps eight feet in height, but with a distance from crest to crest
-of from one to two hundred miles.
-
-If we turn to the effects of large earthquakes as exhibited on the
-land, I think that we shall find records of phenomena which are only
-to be explained on the assumption of an action having taken place
-analogous to that which takes place so often in the ocean, or an action
-similar to that exhibited by small earthquakes, and artificially
-produced disturbances, if greatly exaggerated.
-
-The only explanation for the phenomena accompanying the Lisbon
-earthquake appears to be that the short quick vibrations which had
-ruined so many cities in Portugal had, by the time that they had
-radiated to distant countries, gradually become changed into long flat
-waves having a period of perhaps several minutes. In countries like
-England these pulse-like movements were too gentle to be perceived,
-except in the effects produced by tipping up the beds of lakes and
-ponds.
-
-The phenomenon was not unlike that of a swell produced by a distant
-storm. It would seem possible that in some cases pulsations producing
-phenomena like the ‘seiches’ of Switzerland might have their origin
-beneath the ocean, or deep down beneath the earth’s crust. Perhaps,
-instead of commencing with the ‘snap and jar’ of an earthquake, they
-may commence as a heaving or sinking of a considerable area, which
-may be regarded as an uncompleted effort in the establishment of an
-earthquake or a volcano.
-
-From what has now been said it would seem that earth pulsations
-are phenomena with a real existence, and that some of these are
-attributable to earthquakes. On the other hand, certain earthquakes
-are attributable to earth pulsations. Some of the phenomena which
-have been brought forward have only a possible connection with these
-movements, and they yet require investigation. Elastic tides in the
-earth’s crust have for long been realities in the minds of physicists.
-These, however, are due to lunar and solar influences, and are regular
-in their action. The tidal-like movements called pulsations are of
-greater magnitude, and their goings and comings are irregular.
-
-
-
-
-                             CHAPTER XXI.
-
-                          EARTH OSCILLATIONS.
-
-  Evidences of oscillation—Examples of oscillation—Temple of Jupiter
-    Serapis—Observations of Darwin—Causes of oscillation.
-
-
-_Evidences of oscillation._—By earth oscillations are meant those slow
-and quiet changes in the relative level of the sea and land which
-geologists speak of as elevations or subsidences. These movements are
-especially characteristic of volcanic and earthquake-shaken countries.
-
-As evidences of elevations we appeal to phenomena like raised beaches,
-sea-worn caves, raised coral reefs, and the remains of other dead
-organisms like barnacles, and the borings of lithodomous shells in and
-on the rocks of many coasts high above the level of the highest tides.
-As a proof that subsidence has taken place, there is the evidence
-afforded by submerged forests, the prolongation of certain valleys
-beneath the bed of the ocean, the formation of coral islands, the
-peculiar distribution of the plants and animals which we find in many
-countries, and the submergence of works of human construction. Inasmuch
-as these phenomena are discussed so fully in many treatises on physical
-geology, the references to them here will be made as brief as possible.
-Elevations and depressions which have taken place at the time of large
-earthquakes in a paroxysmal manner have already been mentioned. The
-movements referred to in this chapter, although generally taking place
-with extreme slowness, in certain instances, by an increase in their
-rapidity, have approached in character to earth pulsations. In most
-instances it would appear that the upward movement of the ground, which
-may be likened to a process of tumefaction, goes on so gently that it
-only becomes appreciable after the lapse of many generations.
-
-_Examples of movements._—Lyell estimated that the average rate of rise
-in Scandinavia has been about two and a half feet per century. At the
-North Cape the rise may have been as much as five or six feet per
-century. Observations made at the temple of Jupiter Serapis, between
-October 1822 and July 1838, showed that the ground was sinking at the
-rate of about one inch in four years. Since the Roman period, when this
-temple was built, the ground has sunk twenty feet below the waves. Now
-the floor of the temple is on the level of the sea. Lyell remarks that
-if we reflect on the dates of the principal oscillations at this place
-there appears to be connection between the movements of upheaval and a
-local development of volcanic heat, whilst periods of depression are
-concurrent with periods of volcanic quiescence.[154]
-
-As examples of movements even more rapid than those at the Temple of
-Jupiter Serapis we refer to an account of the earthquakes in Vallais
-(November 1755), when the ground about a mountain at a small distance
-from Brigue sank about a thumb’s-breadth every twenty-four hours. This
-took place between December 9 and February 26.[155]
-
-Another remarkable example of earth movement is given in the account
-of the earthquake at Scarborough, on December 29, 1737, when the head
-of the spa water well was forced up in the air about ten yards high. At
-this time the sands on the shore are said to have risen so slowly that
-people came out to watch them.[156]
-
-Two other examples of rapid earth movement are taken from Professor
-Rossi’s ‘Meteorologia Endogena.’ Professor D. Seghetti, writing to
-Professor Rossi, says that a few lustres ago (one lustre = twenty
-years) Mount S. Giovanni hid the towns Jenne and Subiaco from each
-other. From Subiaco the church at Jenne is now visible, which a few
-years ago was invisible. The people at Jenne also can see more than
-formerly. The supposition is that the side of Mount S. Giovanni is
-lowered. This fact corresponds to a fact stated by Professor Carina,
-who says that forty or fifty years ago from Granaiola you could not see
-either the church of S. Maria Assunta di Citrone or the church of S.
-Pietro di Corsena. Now you can see both.[157]
-
-For a remarkable example illustrating the connection between seismic
-activity and elevation we are indebted to the patient labours of
-Darwin, who carefully investigated the evidences of elevation which
-are visible upon the western coasts of South America. These evidences,
-consisting of marks of erosion, caves, ancient beaches, sand dunes,
-terraces of gravel, &c., were traced between latitudes 45° 35′ to 12°
-5′, a distance north and south of 2,075 geographical miles, and there
-is but little doubt that they extend much farther. As deduced from
-observations upon upraised shells alone, a summary of Mr. Darwin’s
-observations are contained in the following table:—
-
-                                                    Feet
-  At Chiloe the recent elevation has been            350
-  „  Concepcion            „         „      625 to 1,000
-  „  Valparaiso            „         „             1,300
-  „  Coquimbo              „         „               252
-  „  Lima                  „         „                85
-
-Shells, similar to those clinging to uplifted rocks, which are
-evidences of these elevations, still exist in the neighbouring seas,
-and in the same proportionate numbers as they are found in the upraised
-beds. In addition to this, Mr. Darwin shows us that at Lima, during the
-Indo-human period, the elevation has been at least eighty-five feet.
-At Valparaiso, during the last 220 years, the rise was about nineteen
-feet, and in the seventeen years subsequent to 1817 the rise has been
-ten or eleven feet, a portion only of which can be attributed to
-earthquakes. In 1834 the rise there was apparently still in progress.
-
-At Chiloe there has been a gradual elevation of about four feet in four
-years. These, together with numerous other examples, testify to the
-gradual but, as compared with other parts of the globe, exceedingly
-rapid rise of the ground upon the western shores of South America.[158]
-The most important point to be noticed is that this district of rapid
-elevation is one of the most earthquake-shaken regions of the world.
-And further, judging from Darwin’s remarks, in those portions of it
-where the movements have been the most extensive, and at the same time
-probably the most rapid, the seismic disturbances appear to have been
-the most noticeable.
-
-Similar remarks may be applied to Japan, it being in those districts
-where evidences of recent elevation are abundant that earthquakes
-are numerous. Thus, in the bay of Yedo, where we have borings of
-lithodomi in the tufaceous cliffs ten feet above high-water mark,
-which, inasmuch as the rock in which they are found is soft and easily
-weathered, indicate an exceedingly rapid elevation, earthquakes are of
-common occurrence.
-
-From the evidences of elevation which we have upon the South American
-coast, Japan, and in other countries, it appears that these movements
-are intermittent, there being periods of rest, when sea cliffs are
-denuded, and perhaps even periods of subsidence. There is also evidence
-to show that, although these movements have been gradual from time to
-time, they have been aided by starts occasioned by earthquakes.
-
-As to whether earthquakes are more numerous during periods of
-elevation, or of subsidence, or during the intermediate periods of
-rest, we have no evidence.
-
-Sudden displacements which occasionally accompany earthquakes might, it
-was said, sometimes be regarded as the _cause_ of an earthquake, and
-sometimes as the _effect_.
-
-The slow elevations here referred to may be looked upon as being one of
-the more important factors in the production of earthquakes. By various
-causes the rocky coast is bent until, having reached the limit of its
-elasticity, it snaps, and, in flying back like a broken spring, causes
-the jars and tremors of an earthquake.
-
-If this is the case, then the number of earthquakes felt in a district
-which is being elevated may possibly be a function of the rate of
-elevation.
-
-
-
-
-                               APPENDIX.
-
-      LIST OF THE PRINCIPAL BOOKS, PAPERS, PERIODICALS, WHICH ARE
-                  REFERRED TO IN THE PRECEDING PAGES.
-
-                   •       •       •       •       •
-
-  _For a more complete bibliography of earthquakes refer to Mallet’s
-      catalogue of works given in his report to the British Association
-      in 1858._
-
-                   •       •       •       •       •
-
-  A True and Particular Relation of the Dreadful Earthquake which
-      happened at Lima, &c. (1746). 1768.
-  Abbot, Gen. H. L. On the Velocity of Transmission of Earth Waves.
-      _Am. Jour. Sci._ XV., March 1878.
-  — Shock of the Explosion at Hallet’s Point, Nov. 14, 1876.
-      _Battalion Press._
-  Alexander, Prof. T. See _Trans. Seis. Soc. of Japan_.
-  American Journal of Science.
-  Annali del reale osservatorio meteorologico Vesuviano.
-  Annual Register, The.
-  Anonymous, A Chronological and Historical Account of the most
-      Memorable Earthquakes in the World, &c. 1750.
-  — A Vindication of the Bishop of London’s Letter occasioned by the
-      Late Earthquake. 1750.
-  — Phenomena of the Great Earthquake of Nov. 1, 1755.
-  — Serious Thoughts occasioned by the Late Earthquake at Lisbon.
-      1755.
-  Asiatic Society of Japan, Transactions of.
-  Ayrton, Prof. W. E. _See_ Perry, J.
-
-  Bárceno, M. Estudio del Terremoto (May 17, 1879) Mexico. 1879.
-  Beke, Dr. C. T. Mount Sinai a Volcano.
-  Bissett, Rev. J. A Sermon (on account of the Earthquake at Lisbon,
-      Nov. 1, 1755). 1757.
-  Bittner, A. Beiträge zur Kenntniss des Erdbebens von Belluno vom
-      29. Juni 1873.
-  — Sitzungsb. der K. Akad. d. Wissensch., lxix. II. Abth., 1874.
-  Bollettino del Vulcanismo Italiano.
-  Boué, Dr. A. Ueber das Erdbeben welches Mittel-Albanien im
-      October d. J. so schrecklich getroffen hat. _Die K. Akad. d.
-      Wissenschaften_, Nov. 1851.
-  — Parallele der Erdbeben, des Nordlichtes und des Erdmagnetismus.
-  — Ueber die Nothwendigkeit die Erdbeben und vulkanischen
-      Erscheinungen genauer als bis jetzt beobachten zu lassen. _Die
-      K. Akad. d. Wissenschaften_, 1851 and 1857.
-  Bouguer, M. Of the Volcanoes and Earthquakes in Peru.
-  British Association, Reports of.
-  Brunton, R. H. Constructive Art in Japan. _Trans. Asiatic Soc. of
-      Japan_, II. and III., Pt. 2.
-  Bryce, J. Report to British Association, 1841.
-  Buffour, M. The Natural History of Earthquakes and Volcanoes.
-
-  C. H. A Physical Discussion of Earthquakes, &c. 1693.
-  Canterbury, Thomas, Lord Archbishop of, The Theory and History of
-      Earthquakes.
-  Casariego, E. A. See _Trans. Seis. Soc. of Japan_.
-  Cawley, G. Some Remarks on Construction in Brick and Wood, &c.
-      _Trans. Asiatic Soc. of Japan_, VI. Plate ii.
-  Chaplin, Prof. W. S. An Examination of the Earthquakes recorded
-      at the Meteorological Observatory, Tokio. _Trans. Asiatic Soc.
-      of Japan_, VI. Part ii.
-  Comptes Rendus.
-  Credner, H. Das Dippoldiswalder Erdbeben vom Oktober 1877.
-  — Zeitschr. f. d. Naturwiss. f. Sachsen u. Thüringen.
-  — Das Vogtländisch-erzgebirgische Erdbeben, 23. Nov. 1875.
-  — Zeitschr. f. d. gesammt. Naturwissenschaften, xlviii., Oktober.
-
-  Dan, T. See _Trans. Seis. Soc. of Japan_.
-  Darwin, Charles. Researches on Geology and Natural History.
-  — Geological observations.
-  Darwin, G. H. Reports on Lunar Disturbance of Gravity to British
-      Association, 1881. 1882.
-  Diffenbach, F. Plutonismus und Vulkanismus in der Periode von
-      1868–1872, und ihre Beziehungen zu den Erdbeben im Rheingebiet.
-  Doelter, C. von. Ueber die Eruptivgebilde von Fleims, nebst einigen
-      Bemerkungen über den Bau älterer Vulcane.
-  — lxxiv. Band d. Sitzungsb. d. K. Akad. d. Wissensch., I. Abth.,
-      Dec. Heft, Jahrg. 1876.
-  Doolittle, Rev. T. Earthquakes Explained and Practically Improved,
-      &c. 1693.
-  Doyle, P. See _Trans. Seis. Soc. of Japan_.
-
-  Emerson, Prof., B.A. Review of Von Seebachs’ Earthquake of March 6,
-      1872. _Am. Jour. Sci._, Series III.
-  Ewing, Prof. J. A. Earthquake Measurement. A memoir published by
-      the Tokio University. 1883.
-  — See _Trans. Seis. Soc. of Japan_.
-
-  Falb, R. Gedanken und Studien über den Vulkanismus, &c. 1875.
-  — Grundzüge zu einer Theorie der Erdbeben und Vulkanausbrüche.
-  — Das Erdbeben von Belluno. ‘Sirius,’ Bd. VI., Heft ii.
-  Flamstead, J. A Letter concerning Earthquakes. 1693.
-  Forel, F. A. Les Tremblements de Terre (Suisse). _Arch. des
-      Sciences Physiques et Naturelles_, VI. p. 461.
-  — Tremblement de Terre du 30 Décembre 1879.
-  Fuchs, Karl. Vulkane und Erdbeben.
-  — _Die Vulkanischen Erscheinungen der Erde._
-
-  Garcia, J. C. See _Trans. Seis. Soc. of Japan_.
-  Geinitz, Dr. E. Das Erdbeben von Iquique am 9. Mai 1877, &c. _Die
-      K. Leop.-Carol.-Deutschen Akademie der Naturforscher_, Band
-      xl., Nr. 9.
-  Gentleman’s Magazine, The.
-  Geographical Society, Proceedings of.
-  Geological Society, Proceedings of.
-  Girard, Dr. H. Ueber Erdbeben und Vulkane. 1845.
-  Gray, T. See _Trans. Seis. Soc. of Japan_.
-  — On Instruments for Measuring and Recording Earthquake Motions.
-      Phil. Mag. Sept. 1881.
-  — On Recent Earthquake Investigation. _The Chrysanthemum_, 1881.
-      Guiscardi, Prof. G. Notizie del Vesuvio. 1857.
-  — Il terremoto di Casamicciola del 4 Marzo. 1881.
-
-  Hales, S., D.D., F.R.S. Some Considerations on the Causes of
-      Earthquakes. 1750.
-  Hamilton, Sir W. Observations on Mount Vesuvius, Mount Etna, &c.
-      1774.
-  Hattori, I. Destructive Earthquakes in Japan. _Trans. Asiatic Soc.
-      of Japan_, V. Plate i.
-  Heim, Prof. A. Les Tremblements de Terre et leur Etude
-      Scientifique. 1880.
-  — Prof. A. Die Schweizerischen Erdbeben in 1881–1882.
-  Hoeffer, Prof. H. Die Erdbeben Kärntens und deren Stosslinien.
-      _Die Kais. Akademie d. Wissenschaften_, Band xlii.
-  Höfer, Prof. H. Das Erdbeben von Belluno, am 29. Juni 1873.
-      _Sitzungsb. der K. Akad. d. Wissensch._, I. Abth., Band lxxiv.
-  Hoff, K. E. A. von. Geschichte der durch Ueberlieferung
-      nachgewiesenen natürlichen Veränderungen der Erdoberfläche.
-      1822.
-  Hooke, R., M.D., F.R.S. Discourses concerning Earthquakes.
-  Hopkins, William. Report to the British Association on the
-      Geological Theories of Elevation and Earthquakes. 1847.
-  Horton, Rev. Mr. An Account of the Earthquake which happened at
-      Leghorn in Italy (Jan. 1742). 1750.
-  Humboldt, Alexander von. Cosmos.
-  — Travels.
-
-  Jeitteles, L. A. Bericht über das Erdbeben am 15. Januar 1858.
-  — Sitzungsberichte der mathem.-naturw. Classe d. K. Akad. d.
-      Wissensch., xxxv. S. 511.
-  Judd, J. W., Prof. Volcanoes, What they Are and What they Teach.
-
-  Knipping, E. Verzeichniss von Erdbeben wahrgenommen in Tokio,
-      &c. _Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde
-      Ostasiens_, Heft 14.
-  — See _Trans. Seis. Soc. of Japan_.
-
-  Lasaulx, A. von. Das Erdbeben von Herzogenrath am 22. October 1873.
-  Lemery, M. A Physico-Chemical Explanation of Subterranean Fires,
-      Earthquakes, &c.
-  Lescasse, M. J. Etude sur les Constructions Japonaises, &c.
-      _Mémoires de la Société des Ingénieurs Civils_.
-  Lister, M., M.D., F.R.S. Of the Nature of Earthquakes.
-  Little, Rev. J. Conjectures on the Physical Causes of Earthquakes
-      and Volcanoes. 1820.
-
-  Mallet, R. The Neapolitan Earthquake, Vol. II. _Reports to the
-      British Association_, 1850, 1851, 1852, 1854, 1858, 1861.
-  — Secondary Effects of the Earthquake of Cachar. _Proc. Geolog.
-      Soc._, 1872.
-  — Dynamics of Earthquakes. _Trans. Royal Irish Acad._ 1846.
-  Milne, David. Reports to British Association, 1841, 1843, 1844.
-  Milne, John. See _Trans. Seis. Soc. of Japan_.
-  — On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) _Trans.
-      Royal Soc._ 1882.
-  — On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) _Proc.
-      Royal Soc._ No. 217, 1881.
-  — Earthquake Observations and Experiments in Japan (with T. Gray,
-      B.Sc., F.R.S.E.) _Phil. Mag._, Nov. 1881.
-  — On the Elasticity and Strength Constants of certain Rocks (with
-      T. Gray, B.Sc., F.R.S.E.) _Jour. Geolog. Soc._, 1882.
-  — A Visit to the Volcano of Oshima. _Geolog. Mag._, Dec. 2, Vol.
-      IV., pp. 193–197, 255.
-  — On the Form of Volcanoes. _Geolog. Mag._, Dec. 2, Vol. V., and
-      Dec. 2, Vol. VI.
-  — Note upon the Cooling of the Earth, &c. _Geolog. Mag._, Dec. 2.,
-      Vol. VII., p. 99.
-  — Investigation of the Earthquake Phenomena of Japan. _Rep. Brit.
-      Assoc._, 1881 and 1882.
-  — A Large Crater. _Popular Science Review._
-  — The Volcanoes of Japan (a series of Articles). _Japan Gazette._
-  — Earthquake Literature of Japan (a series of Articles). _Japan
-      Gazette._
-  — The Earthquake of Dec. 23, 1880. _The Chrysanthemum_, 1881.
-  — Earthquake Motion. _The Chrysanthemum_, 1882.
-  — Seismology in Japan. _Nature_, Oct. 1882.
-  — Earth Movements. _The Times_, Oct. 12, 1882.
-  Mitchell, Rev. J. Conjectures Concerning the Cause and Observations
-      upon the Phenomena of Earthquakes. 1760.
-  Mohr, Dr. F. Geschichte der Erde. 1875.
-
-  Naturkundig Tijdschrift voor Nederlandsch Indie. 1875–1880.
-  Naumann, Dr. E. Ueber Erdbeben und Vulkanausbrüche in Japan.
-      _Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde
-      Ostasiens._ Heft 15.
-  Noggerath, Dr. J. Die Erdbeben vom 29. Juli 1846 im Rheingebiet,
-      &c.
-  — Die Erdbeben im Vispthale (1855).
-  — Die Erdbeben im Rheingebiet in den Jahren 1868, 1869, 1870.
-  — Jahrgänge d. Verbandlungen d. Natur. Vereins für 1870. _Rheinland
-      u. Westphalen_, xxvii.
-
-  Oldham, Dr. Secondary Effects of the Earthquake of Cachas. _Proc.
-      Geolog. Soc._ 1872.
-  — Thermal Springs of India. _Memoirs of Geolog. Survey of India_,
-      XIX. Plate 2.
-  — A Catalogue of Indian Earthquakes. _Memoirs of Geolog. Survey of
-      India_, XIX. Plate 3.
-  — The Cachas Earthquake. _Memoirs of Geolog. Survey of India_, XIX.
-      Plate 1.
-
-  Palmer, Col. H. S. See _Trans. Seis. Soc. of Japan_.
-  Palmieri, Prof. L., e Scacchi, A. Della Regione Volcanica del Monte
-      Vulture, e del Tremuoto ivi avvenuto nel dì 14 Agosto 1851, 1852.
-  — Annali del reale Osservatorio Meteorologico Vesuviano.
-  — Il Vesuvio, il Terremoto d’ Isernia e l’eruzione sottomarina di
-      Santorino. _R. Accad. d. Sci. Fis. e Mat. di Napoli_, iv. 1866.
-  — Sul recente Terremoto di Corleone. _R. Accad. d. Sci. Fis. e
-      Mat._, v. 1876.
-  — Il Terremoto di Scio del dì 4 Aprile. _R. Accad. d. Sci. Fis. e
-      Mat. di Napoli_, v. 1881.
-  — Sul Terremoto di Casamicciola del 4 Marzo 1881. _R. Accad. d.
-      Sci. Fis. e. Mat. di Napoli_. 1881.
-  Paul, Prof. H. M. See _Trans. Seis. Soc. of Japan_.
-  Perrey, Prof. A. Earthquake Catalogue and Memoirs. (For list see
-      Mallet’s Report to British Association. 1858.)
-  — See _Trans. Seis. Soc. of Japan_.
-  Perry, J., and W. E. Ayrton. On a Neglected Principle that may be
-      Employed in Earthquake Measurement.
-  — See _Trans. Seis. Soc. of Japan_.
-  Philosophical Magazine.
-  Pickering, Rev. R. An Address to those who have either retired
-      or intend to leave Town under the Imaginary Apprehension of
-      the Approaching Shock of another Earthquake. 1750.
-
-  Ray, J., F.R.S. A Summary of the Causes of the Alterations which
-      have happened to the Face of the Earth.
-  Rockstroh, E. Informe de la Comision Científica del Instituto
-      Nacional de Guatemala, nombrada por el Sr. Ministro de
-      Instruccion Pública para el Estudio de los Fenómenos
-      Volcánicos en el Lago de Ilopango. 1880.
-  Rockwood, Prof. C. G. Notes on Earthquakes. Annually in the _Am.
-      Jour. Sci._
-  — Japanese Seismology. _Am. Jour. Sci._, XXII. Dec. 1881.
-  Romaine, W. A Discourse occasioned by the Late Earthquake. 1755.
-  Rossi, Prof. M. S. di. Intorno all’ odierna fase dei Terremoti in
-      Italia, e segnatamente sul Terremoto in Casamicciola del 4
-      Marzo 1881. _Società Geografica Italiana._ 1881.
-  — La Meteorologia Endogena, 2 vols.
-  Royal Society, Transactions of.
-
-  Scacchi, A. _See_ Palmieri.
-  Schmidt, Dr. J. F. Untersuchungen über das Erdbeben am 15. Januar
-      1858.
-  — Studien über Erdbeben. 1879.
-  — Die Eruption des Vesuv (1855). 1856.
-  Scrope, G. P. Volcanoes.
-  Seebach. Das mittle Deutsche Erdbeben (1872). _Mitt. der K.K.
-      geograph. Gesellsch._, II. Jahrg., 2. Heft, 1873.
-  Serpieri, Prof. A. C. S. Nuove Osservazioni sul Terremoto avvenuto
-      in Italia il 12 Marzo 1873. _Istituto Lombardo._ 1873.
-  — Il Terremoto di Rimini della notte 17–18 Marzo 1875.
-  — Documenti nuove e Riflessioni sul Terremoto della notte 17–18
-      Marzo 1875. _Meteorologia Italiana_, iv. 1875.
-  — Determinazione delle fasi e delle leggi del grande Terremoto
-      avvenuto in Italia nella notte 17–18 Marzo 1875. _Istituto
-      Lombardo._ 1875.
-  — Dell’ influenza del Lume Solare sui Terremoti. _Istituto
-      Lombardo._ 1882.
-  Sherlock, T., D.D. (Lord Bishop of London). A Letter on the
-      occasion of the late Earthquakes. 1750.
-  Shower, Rev. J., D.D. Practical Reflections on the Earthquakes that
-      have happened in Europe and America, &c. 1750.
-  Stübel, A. (see Reiss, W.)
-  Stukeley, Rev. W., M.D., F.R.S. The Philosophy of Earthquakes,
-      Natural and Religious, &c. Plates 1, 2, and 3. 1756.
-  Sturmius, J. C. A Methodical Account of Earthquakes.
-  Suess, E. Die Erdbeben Niederösterreiches. _Die Kais. Akademie der
-      Wissenschaften_, Bd. xxxiii.
-  — Die Erdbeben des südlichen Italiens. _Die Kais. Akademie der
-      Wissenschaften_, Bd. xxxiv.
-
-  Volger, Dr. G. H. Untersuchungen über das Phänomen der Erdbeben.
-      1857.
-
-  Wagener, Dr. G. Bemerkungen über Erdbebenmesser und Vorschläge zu
-      einem neuen Instrumente dieser Art. _Mitt. d. Deutsch.
-      Gesellsch. für Natur- und Völkerkunde Ostasiens_, Heft 15.
-  — See _Trans. Seis. Soc. of Japan_.
-  Winchilsea, The Earl of. A True and Exact Relation of the late
-      Prodigious Earthquake and Eruption of Mount Etna. 1669.
-  Woodward, J., M.D., F.R.S. Earthquake caused by some Accidental
-      Obstruction of a Continual Subterranean Heat.
-
-
-                    SEISMOLOGICAL SOCIETY OF JAPAN.
-
-  The following are a list of the papers published by this Society:—
-
-
-                                VOL. I.
-
-  Milne, J. Seismic Science in Japan. 35 pages.
-  Ewing, J. A. New Form of Pendulum Seismograph. 6 pages, 3 plates.
-  Gray, T. Seismometer and Torsion Pendulum Seismograph. 8 pages, 2
-      plates.
-  Mendenhall, T. C. Acceleration of Gravity at Tokio (abstract). 2
-      pages.
-  Wagener, G., and E. Knipping. New Seismometer and Observations with
-      same. 18 pages, 1 plate.
-  Milne, J. Earthquake in Japan of Feb. 22, 1880. 116 pages, 5
-      plates, 8 woodcuts.
-
-
-                               VOL. II.
-
-  Milne, J. Recent Earthquakes of Yeddo, Effects on Buildings, &c. 38
-      pages, 2 plates, and many tables.
-  Mendenhall, T. C. Gravity on Summit of Fujiyama (abstract). 2 pages.
-  Paul, H. M. Earth Vibrations from Railroad Trains (abstract). 4
-      pages.
-  Ewing, J. A. Astatic Horizontal Lever Seismograph (abstract). 5
-      pages, 1 plate.
-  Milne, J. Peruvian Earthquake of May, 9, 1877. 47 pages, 2 plates,
-      tables. Constitution, Rules, Officers and Members of the
-      Society, Dec., 1881.
-
-
-                               VOL. III.
-
-  Gray, T. Steady Points for Earthquake Measurements. 11 pages, 3
-      plates.
-  Milne, J. Experiments in Observational Seismology. 53 pages, 1
-      plate, tables.
-  — The Great Earthquakes of Japan. 38 pages, 1 plate, many tables.
-  Perry, J. Theory of a Rocking Column. 4 pages.
-  Knipping, E. Earthquake of July 25, 1880, with Dr. Wagener’s
-      Seismometer. 4 pages.
-  Ewing, J. A. Earthquake Observation at three or more Stations, &c.
-      4 pages.
-  — Records of three recent Earthquakes. 6 pages, 3 plates.
-  — Earthquake of March 8, 1881. 8 pages, 1 plate.
-  Milne, J. Horizontal and Vertical Motion in Earthquake of March 8,
-      1881. 8 pages, 3 plates.
-  Gray, T. Seismograph for Registering Vertical Motion. 3 pages, 1
-      plate.
-  Ewing, J. A. Seismometer for Vertical Motion. 3 pages, 1 plate.
-  Gray, T. Seismograph for Large Motions. 2 pages.
-  — Compensating a Pendulum to make it Astatic. 3 pages.
-  Palmer, H. S. Note on Earth Vibrations. 3 pages.
-  Kuwabara, M. The Hot Springs of Atami. 2 pages.
-
-
-                               VOL. IV.
-
-  Milne, J. Distribution of Seismic Activity in Japan. 30 pages, 1
-      plate.
-  Wada, T. Notes on Fujiyama. 7 pages.
-  Casariego, E. Abella y. Earthquakes of Nueva Vizcaya in 1881. 23
-      pages, 2 maps.
-  Milne, J. Utilisation of Earth’s Internal Heat. 12 pages.
-  Ewing, J. A. Earthquake of March 11, 1882. 5 pages.
-  Doyle, P. Note on an Indian Earthquake. 6 pages.
-  Milne, J. Systematic Observation of Earthquakes. 31 pages, 5 plates.
-
-
-                                VOL. V.
-
-  Naumann, Dr. E. Notes on Secular Changes of Magnetic Declination in
-      Japan, p. 1–18.
-  Casariego, Don E. Abella y. Monografía Geológica del Volcan de
-      Albay ó El Máyon. p. 19–43.
-  Garcia, Don J. Centeno y. Abstract of a Memoir on the Earthquakes
-      on the Island of Luzon in 1880. p. 43–89.
-  Ewing, Prof. J. A. Seismological Notes.
-  — A Duplex Pendulum Seismometer.
-  — The Suspension of a Horizontal Pendulum.
-  — A Speed Governor for Seismograph Clocks, p. 89–95.
-  Dan, T., S.B. Notes on the Earthquake at Atami, in the Province of
-      Idzu, on September 29, 1882. p. 95–105.
-
-
-                               VOL. VI.
-
-  Alexander, Prof. T. The Development of the Record given by a
-      Bracket Machine.
-  Milne, J. Earth Pulsations.
-  Ewing, J. A. Note on a Duplex Pendulum with a Single Bob.
-  Gergens, F. Note on an Iron Casting, Supposed to have been
-      Disturbed whilst Cooling by an Earthquake.
-  West, C. D. On a Parallel Motion Seismograph.
-  Ewing, J. A. Certain Methods of Astatic Suspension.
-  Alexander, T. Ball and Cup Seismometer.
-  Knipping, Messrs. Paul and. Report on a System for Earthquake
-      Observation.
-  Catalogues of Earthquakes.
-
-
-
-
-                                INDEX.
-
-
-  Abbadie, M. d’, on earth tremors, 309
-  Abbot, General H. L., on the transmission of vibrations, 62
-  Abella, M., on the earthquake in the Philippines in 1881, 77
-  Activity, on seismic, 6
-  Aristotle, on the classification of earthquakes, 41
-  Artificial earthquakes, experiments on, 57
-  — — intensity of, 61
-  Aurora, on the occurrence of, with earthquakes, 264
-  Ayrton and Perry, on the effect of soft foundations, 130
-  — — on the period of vibration of buildings, 115
-  — — on the principle of, 31
-
-  Barometer, effect of changes of, on earthquakes, 266
-  Bertelli, on aurora and earthquakes, 265
-  — on earth tremors, 316, 320
-  — on the normal tromometer of, 317
-  Bittner, A., on the buildings of Belluno, 100
-  Bridges, on earthquake, 140
-  Brunton, R. H., on buildings in earthquake countries, 123
-  Buckle, on the history of civilisation, 1
-  Builders, interest of the study of earthquakes to, 3
-  Buildings, on cracks in, 98, 108
-  — the effect of earthquakes on, 96
-  — on the irregular destruction of, 96
-  — — effect on the end house in a row, 112
-  — — church of St. Augustin at Manilla, 113
-  — — relation of destruction of, to earthquake motion, 103
-  — — protection of, 143
-  — — pitch of the roof of, 110
-  — — position of openings in the walls of, 111
-  — — swing of, 115
-  — — period of vibration of, 115
-  — — principle of relative periods in, 116
-  — — types of, for earthquake countries, 121
-  — — effect of underlying rocks on, 130
-  — general conclusions regarding, 144
-
-  Cacciatore, definition of the, 18
-  Caldcleugh, A., on earthquake frequency, 245
-  — on barometric height and earthquakes, 266
-  Carruthers, J., on earthquakes and tides, 291
-  Centrum, definition of, 9
-  — on the depth of, 213
-  — on the maximum depth of, 218
-  Centrum, determination of position of, _see_ origins
-  Chaplin, W. S., on the bracket seismometer of, 27
-  — on earthquakes and the position of the moon, 252
-  Coast line, on the movement of, 160
-  Cocks, R., on earthquakes and tides, 290
-  Coseismic lines, definition of, 10
-  Curves, on microseismic, 321
-
-  Darwin, Charles, on the movement of coast lines, 160
-  — George H., on earth tides, 285
-  — on tidal loads, 291
-  — on earth pulsation, 330
-  — on the effect of fluctuations of barometric pressure, 336
-  — experiments at Cambridge, 310
-  Delauney, M. J., on the influence of the planets on earthquakes, 261
-  Diagonic, definition of, 11
-  Diastrophic, definition of, 11
-  Direction of motion, from instrumental records, 198
-  Distribution, on earthquake, 226
-  — examples of, 231
-  Disturbance, on the propagation of, 50
-  Douglas, J., on South American houses, 126
-
-  Earth particle, on the velocity and acceleration of, 79
-  Earthquake motion, nature of, as deduced from the feelings, 67
-  — direction of, derived from instrumental records, 69
-  — duration of, 71
-  — period of vibration in, 74
-  — examples of extent of, 75–77
-  — absolute intensity of force in, 83
-  — radiation of, 85
-  — velocity of propagation of, 87
-  Earthquake at Lisbon, velocity of propagation of, 88
-  Earthquakes, general examples of effects of, 142
-  — geological changes produced by, 161
-  — hunting, 187
-  — distribution of, 226
-  — maps, 189
-  — secondary, 248
-  — table of, for nineteenth century, 259
-  — on the course of, 277–281
-  — and tides, 290
-  — prediction of, 297–304
-  Elastic waves, nature of, 44
-  Emergence, angle of, 9
-  Energy, dissipation of, in earthquakes, 52
-  — seismic, in relation to geological time, 234
-  — — table of, 240
-  Epicentrum, definition of, 9
-  Euthutropic, definition of, 11
-  Ewing, J. A., pendulum seismograph of, 25
-  — astatic pendulum of, 26
-  — bracket seismograph of, 26
-
-  Falb, R., on the influence of the sun and moon on earthquakes, 286
-  Fissures, on the material discharged from, 148
-  — on the explanation of, 151
-  Focal cavity, definition of, 9
-  — on the form of, 221
-  Forbes, D., on an earthquake in Mendoza, 151
-  Frequency of earthquakes, 243
-  Frere, Sir H. Bartle, on geological changes produced by earthquakes, 161
-  Fuchs, on sea waves, 176
-  — on the movement of the seismic centre, 233
-  — on earthquakes and volcanic outbursts, 271
-  — on hot springs, 157
-  Fumaroles, the effect of earthquakes on, 156
-
-  Geinitz, Dr., on sea waves, 182
-  Geologists, on the interest of seismology to, 2
-  Gray, T., astatic pendulum of, 26
-  — bracket seismometer of, 27
-  — conical pendulum of, 29
-  — dead heat pendulum of, 22
-  — on the rotation of bodies, 196
-  — rolling spheres and cylinders of, 29
-  — torsion pendulum seismometer of, 25
-  — vertical motion seismometers of, 32, 33
-  — and Milne, seismograph of, 38
-
-  Hattori, I., on the large earthquakes of Japan, 244
-  Haughton, Prof., list of active volcanoes of, 227
-  — method of finding earthquake origins of, 209
-  Hills, on the want of support on the face of, 136
-  Höfer, on an earthquake at Belluno, 225
-  Hoffmann, F., on the barometer and earthquakes, 267
-  Hooke, on earthquake motion, 42
-  Hopkins, on the thickness of the earth’s crust, 284
-  Humboldt, on meteors and earthquakes, 261
-  — on the barometer and earthquakes, 267
-  — on volcanoes and earthquakes, 279
-
-  Imagination, effect of earthquakes on the, 2
-  Instruments, direction of motion derived from, 198
-  Intensity, on earthquake, 51, 71
-  — seismic curve of, for Kioto, 242
-  Isoseismic circles, definition of, 10
-  — areas, definition of, 10
-
-  Kluge, on sea waves, 175
-  — on earthquake frequency, 246
-  — on simultaneous earthquakes, 248
-  — on earthquakes and sun spots, 263
-  — on earth pulsations, 339
-  Kreil, pendulum seismometer of, 25
-
-  Lakes, on disturbances in, 154
-  Land, effect of earthquakes on, 146–162
-  — on the reason of movements of, 162
-  — on cracks and fissures formed in, 146
-  Level, on the use of for earth pulsations, 328
-  Literature, on seismic, 6
-  — on Japanese earthquake, 7
-
-  Mallet, R., on area of disturbance as a test of seismic energy, 78
-  — on clock stopping, 36
-  — list of works on earthquakes of, 5
-  — curve of seismic energy of, 238
-  — definition of earthquake of, 43
-  — on earthquake frequency, 243
-  — on the influence of the heavenly bodies on earthquakes, 253
-  — on maximum depth of origin, 218
-  — on pendulum seismometers, 20
-  — projection seismometer of, 17
-  — on the Neapolitan earthquake, 69, 77, 83, 97, 103, 132, 142, 218, 280
-  — on sea waves, 170
-  — on the swing of mountains, 135
-  — on propagation from a fissure, 217
-  Mallet on the temperature of focal cavity, 84
-  Malvasia, M. le Conte, on earth tremors, 316
-  Martin, D. S., on the New England earthquake of 1874, 142
-  Meizoseismic area, definition of, 10
-  Melzi, on curves of microseismic motion, 322
-  Meteors, on earthquakes and, 260
-  Microseismic movements, on cause of, 324
-  Milne, D., on the Lisbon earthquake, 87
-  — on earthquake synchronism, 247
-  Mitchell, on earthquake motion, 42
-  Moon, effect of, on earthquakes, 251, 285
-  Mountains, on the swing of, 135
-
-  Naumann, E., on meteors and earthquakes, 261
-  — on sun spots and earthquakes, 263
-
-  Ocean, on disturbances in, 163–186
-  Origin, definition of, 9
-  — on the determination of, 187
-  — position of, deduced from direction of motion, 192
-  — — from destruction of buildings, 194
-  — — from rotation of bodies, 195
-  — — from time of occurrence, 199
-  — — examples of methods of calculating, 200–212
-  Oscillations, on earth, 344
-  Overturning moment, on the area of greatest, 53
-
-  Palmer, Col. H. S., on earth tremors, 307
-  Palmieri, on clock stopping, 36, 62
-  Paul, H. M., on earth tremors, 308
-  Perrey, A., on the influence of the moon on earthquakes, 251
-  Perrey on the periodicity of earthquakes, 8
-  Perry, J., on position of openings in walls, 111
-  Physicists, on the interest of earthquakes to, 2
-  Planets, influence of, on earthquakes, 260
-  Plantamour, M., on earth pulsations, 328
-  Pleistoseists, definition of, 10
-  Poly, M. A., on earthquakes and sun spots, 263
-  — on earthquakes and revolving storms, 294
-  Prost, M. le Baron, on earth tremors, 316
-  Pulsation, on earth, 4, 326–343
-
-  Records, on receivers of, 33
-  Rivers, on disturbances in, 154
-  Rockwood, Prof., on American earthquakes, 6
-  Ronaldson, T., on San Francisco houses, 129
-  Rossi, M. S. di, on an eruption of gas in the Tiber, 153
-  — aurora and earthquakes, 264
-  — earth tremors, 317, 320
-  — earth oscillations, 346
-  — earth pulsations, 327
-  — microseismograph of, 318
-  — microphonic observations of, 319, 323
-  — normal tromometer of, 317
-
-  Schmidt, on the influence of barometric pressure on earthquakes, 267
-  Sea waves, on nature of, 165
-  — on cause of, 171
-  — seldom produced by earthquakes which originate inland, 175
-  — on velocity of propagation of, 177
-  — examples of, 179
-  Seasons, frequency of earthquakes at different, 254
-  Seebach, on the determination of origins, 211
-  — on the focal cavity, 224
-  Seismic vertical, definition of, 9
-  Seismic and volcanic phenomena, relation of, 270
-  — — conclusions regarding, 275, 295
-  Seismology, definition of, 9
-  Seismometers, on various forms of, 17–40
-  Seismoscopes, on various forms of, 13–20
-  Serpieri, P. A., on distribution of seismic movement, 231
-  Shadows, on earthquake, 137
-  Spring, on frequency of earthquakes during, 156
-  Stukeley, on earthquake motion, 42
-  — earthquakes and aurora, 265
-  Succussatore, definition of, 10
-  Sun, on the effect of, on earthquakes, 253, 285
-
-  Temperature, effect of changes of, on earthquakes, 268, 294
-  Terremoto, definition of, 10
-  Thomson, Sir W., on the rigidity of the earth, 285
-  Time, on recording apparatus for, 35
-  Travagini, F., on earthquake motion, 42
-  Trembelores, definition of, 10
-  Tremors, on earth, 3, 306–325
-
-  Understanding, effects of earthquakes on, 2
-
-  Verbeck, on the ball and plate seismometer of, 31
-  Vibration, on the nature of earthquake, 12
-  Vorticose motion, on, 70
-  — definition of, 10
-
-  Wagener, on the pendulum seismometer of, 25
-  — vertical motion seismometer of, 33
-  — list of earthquakes, 76
-  Wave paths, definition of, 9
-  Waves, on the nature of earthquake, 55
-  — on the interference of, 138
-  Wells, on the effect of earthquakes on, 156
-  Wenthrop, on the New Zealand earthquake of 1855, 79
-  West, on the parallel motion seismometer of, 28
-  Winslow, on pulsations of the ocean, 334
-  Wolf, R., on earthquakes and sun spots, 263
-  Woodward, on earthquake motion, 42
-
-  Young, Dr. T., on earthquake motion, 43
-
-  Zantedeschi, M. F., on the influence of the sun and moon on earthquakes, 285
-  Zöllner, on the bracket seismometer of, 27
-  — on earth tremors, 309
-
-
-
-
-  +--------------------------------------------------------------------+
-  |                                                                    |
-  |                             FOOTNOTES:                             |
-  |                                                                    |
-  | [1] _Mémoires de l’Académie Imp. de Dijon_, vols. xiv. and xv.,    |
-  |     2nd Series, 1855–56.                                           |
-  |                                                                    |
-  | [2] _Trans. Seis. Soc. of Japan_, vol. iii. p. 65.                 |
-  |                                                                    |
-  | [3] _Gentleman’s Magazine_, 1753.                                  |
-  |                                                                    |
-  | [4] 1 Kings xix. 11, 12.                                           |
-  |                                                                    |
-  | [5] ‘Notes on the Great Earthquake of Japan.’ J. Milne, _Trans.    |
-  |     Seis. Soc. of Japan_, vol. iii.                                |
-  |                                                                    |
-  | [6] See Mallet’s List of Works on Earthquakes, _Report of the      |
-  |     British Association_, 1858, p. 107.                            |
-  |                                                                    |
-  | [7] _Quarterly Review_, vol. lxiii. p. 61.                         |
-  |                                                                    |
-  | [8] _De Mundo_, c. iv.                                             |
-  |                                                                    |
-  | [9] See _Phil. Trans. R. S._, Part III. 1882.                      |
-  |                                                                    |
-  | [10] _Report of the British Association_, 1851.                    |
-  |                                                                    |
-  | [11] ‘On the Velocity of Transmission of Earth Waves,’ by General  |
-  |     H. L, Abbot, _American, Journal of Science and Arts_, vol. xv. |
-  |     March 1878; ‘Shock of the Explosion at Hallet’s Point,’ by     |
-  |     Bvt. Brig.-Gen. Henry L. Abbot, read before the Essayons Club  |
-  |     of the Corps of Engineers, Nov. 1876.                          |
-  |                                                                    |
-  | [12] _West. Rev._, July 1849.                                      |
-  |                                                                    |
-  | [13] _Phil. Trans._, L., 1755.                                     |
-  |                                                                    |
-  | [14] The solution is taken from Mallet’s _Account of the           |
-  |     Neapolitan Earthquake_, vol. i. p. 155.                        |
-  |                                                                    |
-  | [15] _Neapolitan Earthquake_, ii. p. 300.                          |
-  |                                                                    |
-  | [16] See _Edinburgh Phil. Trans._, vol. xxxi.                      |
-  |                                                                    |
-  | [17] See _Report of British Association_, 1858, p. 10.             |
-  |                                                                    |
-  | [18] _Meteorologia Endogena_, i. p. 306.                           |
-  |                                                                    |
-  | [19] See remarks on the Earthquake ‘Push,’ p. 162.                 |
-  |                                                                    |
-  | [20] See _Researches in Geology and Natural History_, p. 374.      |
-  |                                                                    |
-  | [21] ‘The City of Earthquakes,’ H. D. Warner, _Atlantic Monthly_,  |
-  |     March, 1883.                                                   |
-  |                                                                    |
-  | [22] Mallet, _Dynamics of Earthquakes_.                            |
-  |                                                                    |
-  | [23] Stud Mill at Haywards.                                        |
-  |                                                                    |
-  | [24] See ‘Constructive Art in Japan,’ by R. H. Brunton, C.E.,      |
-  |     F.R.G.S., F.G.S., _Transactions of Asiatic Society of Japan_,  |
-  |     December 22, 1873, and January 13, 1875.                       |
-  |                                                                    |
-  | [25] _Journal of the American Geographical Society_, vol. x.       |
-  |                                                                    |
-  | [26] _Phil. Trans._, li. 1760.                                     |
-  |                                                                    |
-  | [27] _Ibid._, xviii.                                               |
-  |                                                                    |
-  | [28] ‘The City of Earthquakes,’ H. D. Warner, _Atlantic Monthly_,  |
-  |     March 1883.                                                    |
-  |                                                                    |
-  | [29] T. Ronaldson, _A Treatise on Earthquake Dangers &c._          |
-  |                                                                    |
-  | [30] _Principles of Geology_, Lyell, vol. ii. p. 106.              |
-  |                                                                    |
-  | [31] _The Neapolitan Earthquake of 1857_, R. Mallet, vol. ii. p.   |
-  |     359.                                                           |
-  |                                                                    |
-  | [32] _Am. J. Sci._ x. 191.                                         |
-  |                                                                    |
-  | [33] _Reports of British Association_, 1858, p. 106.               |
-  |                                                                    |
-  | [34] See chapter ‘Causes of Earthquakes’ for details of this myth. |
-  |                                                                    |
-  | [35] _Am. Jour. Sci._ vol. x. p. 191.                              |
-  |                                                                    |
-  | [36] _The Earth_, p. 599.                                          |
-  |                                                                    |
-  | [37] Lyell, _Principles of Geology_, vol. ii. chap. xxix.          |
-  |                                                                    |
-  | [38] _Gent. Mag._ vol. xx. p. 212.                                 |
-  |                                                                    |
-  | [39] _Trans. Seis. Soc._ vol. v. p. 67–68.                         |
-  |                                                                    |
-  | [40] _Am. Jour. Sci._ vol. iv.                                     |
-  |                                                                    |
-  | [41] _Phil. Trans._ vol. xviii.                                    |
-  |                                                                    |
-  | [42] Oldham and Mallet, ‘Cachar Earthquake,’ _Proc. Geolog. Soc._  |
-  |     1872.                                                          |
-  |                                                                    |
-  | [43] _Phil. Trans._ vols. li. and xviii.; _Gent. Mag._ vol. xx.    |
-  |     212.                                                           |
-  |                                                                    |
-  | [44] _Trans. Royal Geog. Soc._ vol. vi.                            |
-  |                                                                    |
-  | [45] _Phil. Trans._ vols. xxxvi. and xxxix.                        |
-  |                                                                    |
-  | [46] _Am. Jour. of Sci._ 1865, vol. xl. p. 365.                    |
-  |                                                                    |
-  | [47] _Proc. Geolog. Soc. Ap._ 1875, p. 270.                        |
-  |                                                                    |
-  | [48] _Gent. Mag._ vol. xxi. p. 569.                                |
-  |                                                                    |
-  | [49] _Jahrb. f. Min._ 1840, p. 173.                                |
-  |                                                                    |
-  | [50] Oldham and Mallet, ‘Cachar Earthquake,’ _Trans. Geolog. Soc.  |
-  |     Ap._ 1872.                                                     |
-  |                                                                    |
-  | [51] O. Volger, _Unters üb. d. Phän. d. Erdb._ vol. iii. p. 414.   |
-  |                                                                    |
-  | [52] _Meteorologia Endogena_, vol. i. p. 166.                      |
-  |                                                                    |
-  | [53] _Gent. Mag._ vol. xxvi. p. 91.                                |
-  |                                                                    |
-  | [54] _Compte Rendu_, 1873, p. 66.                                  |
-  |                                                                    |
-  | [55] _An Historical Account of Earthquakes_, p. 46.                |
-  |                                                                    |
-  | [56] _Phil. Trans._ vol. xlix. p. 436.                             |
-  |                                                                    |
-  | [57] _Am. Jour. Sci._ vol. xlv. p. 129.                            |
-  |                                                                    |
-  | [58] _Phil. Trans._ vol. xlix. p. 547.                             |
-  |                                                                    |
-  | [59] _Ibid._ vols. xlii. and xxxix.                                |
-  |                                                                    |
-  | [60] _Phil. Trans._ vol. xlix, part i.                             |
-  |                                                                    |
-  | [61] _Compte Rendu_, 1873, part ii. p. 66.                         |
-  |                                                                    |
-  | [62] _Die Vulcan. Ers. d. Erde_, C. W. C. Fuchs.                   |
-  |                                                                    |
-  | [63] _Comte Rendu_, 1875, p. 693.                                  |
-  |                                                                    |
-  | [64] _Gent. Mag._ vol. xix. p. 190.                                |
-  |                                                                    |
-  | [65] _Phil. Trans._ vol. xlix. p. 115.                             |
-  |                                                                    |
-  | [66] _Gent. Mag._ vol. xxi. 1751.                                  |
-  |                                                                    |
-  | [67] _Jour. Royal Geo. Soc._ vol. vi. p. 319.                      |
-  |                                                                    |
-  | [68] Darwin, _Geolog. Observations_, p. 232.                       |
-  |                                                                    |
-  | [69] _Ibid._ p. 245.                                               |
-  |                                                                    |
-  | [70] Lyell, _Principles of Geology_, vol. ii. pp. 107–8.           |
-  |                                                                    |
-  | [71] _Gent. Mag._ 1733, vol. iii. p. 217.                          |
-  |                                                                    |
-  | [72] ‘Earthquakes of Cutch,’ _Jour. Royal Geo. Soc._ vol. xl.      |
-  |                                                                    |
-  | [73] M. Daussy, ‘Sur l’existence probable d’un volcan sousmarin    |
-  |     situé ar environ 0° 20′  de lat. S., et 22° 0′  de long, ouest,’ |
-  |     _Comptes Rendus_, vol. vi. p. 512.                             |
-  |                                                                    |
-  | [74] _Am. Jour. Sci._ vol. xlv. p. 133.                            |
-  |                                                                    |
-  | [75] _Am. Jour. Sci._ vol. xiv. p. 209.                            |
-  |                                                                    |
-  | [76] D. C. F. Winslow, ‘Tides at Tahiti,’ _Am. Jour. Sci._ 1865,   |
-  |     p. 45; also Mallet’s _Catalogue of Earthquakes_.               |
-  |                                                                    |
-  | [77] _Am. Jour. Sci._ vol. i. p. 469.                              |
-  |                                                                    |
-  | [78] Darwin, _Researches in Geology, &c._, p. 378.                 |
-  |                                                                    |
-  | [79] Kluge, _Jahrb. f. Min._ 1861, p. 977.                         |
-  |                                                                    |
-  | [80] Darwin, _Voyage of a Naturalist_, p. 309.                     |
-  |                                                                    |
-  | [81] Prof. A. D. Bache, _United States Coast Survey Report_,       |
-  |     1855, p. 342.                                                  |
-  |                                                                    |
-  | [82] _United States Coast Surrey Report_, or _Am. Jour. Sci._ vi.  |
-  |     p. 77.                                                         |
-  |                                                                    |
-  | [83] _Petermann’s Mittheilungen_, 1877, Heft xii. S. 454,          |
-  |     and _Nova Acta der Ksl. Leop. Carol. Deutschen Acad. d         |
-  |     Naturforscher_, Band xl. No. 9.                                |
-  |                                                                    |
-  | [84] J. Milne: ‘Peruvian Earthquake of May 9, 1877.’ See _Trans.   |
-  |     Seis. Soc. of Japan_, vol. ii.                                 |
-  |                                                                    |
-  | [85] _Report of British Association_, 1847, p. 84.                 |
-  |                                                                    |
-  | [86] _Das Erdbeben von Herzogenrath, &c._, p. 134.                 |
-  |                                                                    |
-  | [87] _Phil. Trans._ vol. li.                                       |
-  |                                                                    |
-  | [88] See _Am. Jour. Sci._ 1872.                                    |
-  |                                                                    |
-  | [89] David Milne says that ‘out of 110 shocks recorded in          |
-  |     England, thirty-one originated in Wales, thirty-one along the  |
-  |     south coast of England, fourteen on the borders of Yorkshire   |
-  |     and Derbyshire, and five or six in Cumberland.’                |
-  |                                                                    |
-  | [90] E. Suess, _Die Erdbeben Niederösterreiches_.                  |
-  |                                                                    |
-  | [91] H. Hoeffer, _Die Erdbeben Kärntens_.                         |
-  |                                                                    |
-  | [92] _Six Lectures on Physical Geography_, by Rev. S. Haughton,    |
-  |     F.R.S., chap. i.                                               |
-  |                                                                    |
-  | [93] Ramsay, ‘Geological History of Mountain Chains,’ _Mining      |
-  |     Journal_.                                                      |
-  |                                                                    |
-  | [94] A notable example of a rapid diminution in the number of      |
-  |     earthquakes felt at a place is that of Comrie in Scotland. In  |
-  |     1839–40, no less than sixty shocks were felt in eleven months. |
-  |     In 1842–43, about thirty shocks were felt, and in the          |
-  |     following year thirty-seven. Since this time the number of     |
-  |     shocks has decreased until they are almost of as rare          |
-  |     occurrence at Comrie as in other portions of the British       |
-  |     Isles.                                                         |
-  |                                                                    |
-  | [95] _Phil. Trans._ vol. i. 1836.                                  |
-  |                                                                    |
-  | [96] _Am. Jour. of Sci._ vol. xxxvii. p. 1.                        |
-  |                                                                    |
-  | [97] Milne, ‘British Earthquakes,’ _Edin. Phil. Jour._ vol. xxxi. |
-  |                                                                    |
-  | [98] _Phil. Trans._ vol. xlix. pt. i.                              |
-  |                                                                    |
-  | [99] _Compte Rendus_, 1875, p. 690.                                |
-  |                                                                    |
-  | [100] _Am. Jour. Sci._ vol. xi. p. 233.                            |
-  |                                                                    |
-  | [101] _Transactions of the Asiatic Society of Japan_, vol. vi.     |
-  |     pt. i. p. 353.                                                 |
-  |                                                                    |
-  | [102] Kluge, _Ueber die Ursachen_, &c., p. 74.                     |
-  |                                                                    |
-  | [103] _Am. Jour. Sci._ vol. xix. p. 162.                           |
-  |                                                                    |
-  | [104] _Mitt. d. Deutsch._ Ges., Aug. 1878.                         |
-  |                                                                    |
-  | [105] _Report to British Association_, 1850, p. 74.                |
-  |                                                                    |
-  | [106] Fuchs, _Die Vulkanischen Erscheinungen der Erde_, p. 424.    |
-  |                                                                    |
-  | [107] _Bern. Naturf. Gesellschaft_, 1852.                          |
-  |                                                                    |
-  | [108] _Comptes Rendus_, 1874, Jan. to June, p. 51.                 |
-  |                                                                    |
-  | [109] Boué, _Parallele der Erdbeben, Nordlichter und               |
-  |     Erdmagnetismus, in Sitz. der K. A. d. Wissensch_. 1856, vol.   |
-  |      iv. p. 395.                                                   |
-  |                                                                    |
-  | [110] _Meteorologia Endogena_, vol. i. p. 107, &c.                 |
-  |                                                                    |
-  | [111] _Phil. Trans._ vol. lxviii. p. 221.                          |
-  |                                                                    |
-  | [112] _Gent. Mag._ vol. xxvii. p. 508.                             |
-  |                                                                    |
-  | [113] _Die Vulkanischen Erscheinungen der Erde_, p. 419.           |
-  |                                                                    |
-  | [114] Petermann’s _Geogr. Mitth._ 1858, sec. 246.                  |
-  |                                                                    |
-  | [115] _Notes on volcanoes of the Hawaiian Islands_, W. T.          |
-  |     Brigham, Mem. Boston Soc. of Nat. Hist., 1868.                 |
-  |                                                                    |
-  | [116] _Gent. Mag._ vol. xxiii., 1753.                              |
-  |                                                                    |
-  | [117] _Jour. Royal Geog. Soc._ vol. vi.                            |
-  |                                                                    |
-  | [118] _Ibid._ vol. vi.                                             |
-  |                                                                    |
-  | [119] _Phil. Trans._ vol. xlii.                                    |
-  |                                                                    |
-  | [120] _Am. Jour. Sci._ vol. x. p. 191.                             |
-  |                                                                    |
-  | [121] ‘Earthquakes of San Salvador, December 21–30, 1879.’ _Am.    |
-  |     Jour. Sci._ vol. xix. p. 415.                                  |
-  |                                                                    |
-  | [122] _Gent. Mag._ 1757, p. 323.                                   |
-  |                                                                    |
-  | [123] _Phil. Trans._ vol. li., 1760.                               |
-  |                                                                    |
-  | [124] Mallet, _Report to Brit. Ass._, 1858, p. 67.                 |
-  |                                                                    |
-  | [125] Von Lasaulx, _Earthquakes of Herzogenrath_.                  |
-  |                                                                    |
-  | [126] Lyell, _Principles_, vol. ii. p. 51.                         |
-  |                                                                    |
-  | [127] Lyell, _Principles_, vol. i. p. 402.                         |
-  |                                                                    |
-  | [128] Fuchs, p. 464.                                               |
-  |                                                                    |
-  | [129] _Comptes Rendus_, August 1854.                               |
-  |                                                                    |
-  | [130] _Nature_, April 26, 1883.                                    |
-  |                                                                    |
-  | [131] _Phil. Soc._, Wellington, New Zealand, 1875.                 |
-  |                                                                    |
-  | [132] _Phil. Trans._, vol. xlii.                                   |
-  |                                                                    |
-  | [133] M. S. di Rossi, _Earthquakes of Casamicciola_.               |
-  |                                                                    |
-  | [134] _Phil. Trans._, vol. xviii. 1683–5.                          |
-  |                                                                    |
-  | [135] _Ibid._ vol. xlix.                                           |
-  |                                                                    |
-  | [136] H. D. Warner, ‘The City of Earthquakes,’ _Atlantic           |
-  |     Monthly_, March 1833.                                          |
-  |                                                                    |
-  | [137] Palmer, _Trans. Seis. Soc. of Japan_, vol. iii. p. 148.      |
-  |                                                                    |
-  | [138] Palmer, _Trans. Seis. Soc. of Japan_, vol. iii. p. 148.      |
-  |                                                                    |
-  | [139] Paul, _Trans. Seis. Soc. of Japan_, vol. ii. p. 41.          |
-  |                                                                    |
-  | [140] G. H. and H. Darwin, _Reports of British Association_, 1881. |
-  |                                                                    |
-  | [141] _Reports of British Association_, 1881.                      |
-  |                                                                    |
-  | [142] _Comptes Rendus_, 1875, January to June, p. 685.             |
-  |                                                                    |
-  | [143] _Tel. Jour._, November 15, 1881.                            |
-  |                                                                    |
-  | [144] _Minutes and proceedings of the Institute of Civil           |
-  |     Engineers_, vol. lx. p. 412, and vol. lxiv. p. 343.            |
-  |                                                                    |
-  | [145] _See_ ‘Earth Tremors,’ p. 309, experiments of M. d’Abbadie,  |
-  |     &c.                                                            |
-  |                                                                    |
-  | [146] _Meteorologia Endogena._                                     |
-  |                                                                    |
-  | [147] _Ibid._                                                      |
-  |                                                                    |
-  | [148] _Phil. Trans._ vol. xlix. p. 544.                            |
-  |                                                                    |
-  | [149] _Annual Register_, vol. iv. 1761, p. 92.                     |
-  |                                                                    |
-  | [150] _Phil. Mag._, May 1876, p. 447.                              |
-  |                                                                    |
-  | [151] _Boston Soc. Nat. Hist._, 1868.                              |
-  |                                                                    |
-  | [152] ‘Notes on Tides at Tahiti,’ &c., _Am. Jour. Sci._ 1866,      |
-  |     vol. xlii. p. 45.                                              |
-  |                                                                    |
-  | [153] _Trans. Seis. Soc. of Japan_, vol. iv. Milne, _Systematic    |
-  |     Observation of Earthquakes_.                                   |
-  |                                                                    |
-  | [154] _Principles of Geology_, vol. ii. 177.                       |
-  |                                                                    |
-  | [155] _Gent. Mag._, vol. xxvii. p. 448.                            |
-  |                                                                    |
-  | [156] _Phil. Trans._, vol. xli. p. 805.                            |
-  |                                                                    |
-  | [157] _Meteorologia Endogena_, vol. i. pp. 186, 187.               |
-  |                                                                    |
-  | [158] Darwin, _Geological Observations_, p. 275 _et seq._          |
-  |                                                                    |
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-
-Project Gutenberg's Earthquakes and Other Earth Movements, by John Milne
-
-This eBook is for the use of anyone anywhere in the United States and most
-other parts of the world at no cost and with almost no restrictions
-whatsoever.  You may copy it, give it away or re-use it under the terms of
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-www.gutenberg.org.  If you are not located in the United States, you'll have
-to check the laws of the country where you are located before using this ebook.
-
-Title: Earthquakes and Other Earth Movements
-
-Author: John Milne
-
-Release Date: July 29, 2019 [EBook #60007]
-
-Language: English
-
-Character set encoding: UTF-8
-
-*** START OF THIS PROJECT GUTENBERG EBOOK EARTHQUAKES, OTHER EARTH MOVEMENTS ***
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-Produced by deaurider, Robert Tonsing, and the Online
-Distributed Proofreading Team at http://www.pgdp.net (This
-file was produced from images generously made available
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-</pre>
-
-
-  <div class="figcenter">
-    <img id="coverpage" src="images/cover.jpg" width="529" height="800" alt="" />
-  </div>
-
-  <hr class="page" />
-
-  <div class="center xlarge">
-    <b>THE INTERNATIONAL SCIENTIFIC SERIES<br />
-    VOLUME LV</b>
-  </div>
-
-  <hr class="page" />
-
-  <div class="chapter">
-    <div class="center mb1"><b>THE INTERNATIONAL SCIENTIFIC SERIES</b></div>
-    <hr class="r15" />
-    <h1><span class="gespertt">EARTHQUAKES</span><br />
-      <span class="small">AND</span><br />
-      <span class="xlarge gespertt">OTHER EARTH MOVEMENTS</span>
-    </h1>
-  </div>
-
-  <div class="center mt10">
-    <span class="small">BY</span><br />
-    <span class=" large gespertt">JOHN MILNE</span><br />
-    <span class="xsmall">PROFESSOR OF MINING AND GEOLOGY IN THE IMPERIAL COLLEGE OF ENGINEERING,<br />
-    TOKIO, JAPAN</span>
-  </div>
-
-  <div class="center small mt10"><i>WITH THIRTY-EIGHT FIGURES</i></div>
-
-  <div class="center mt10">NEW YORK<br />
-    <span class="large gespertt">D. APPLETON AND COMPANY</span><br />
-    <span class="smcap small">1, 3, and 5 BOND STREET</span><br />
-    1886
-  </div>
-
-  <hr class="page" />
-  <div class="chapter" id="PREFACE">
-    <h2 class="gespertt xxlarge">PREFACE</h2>
-  </div>
-
-  <div class="figcenter">
-    <img src="images/divider.png" width="80" height="9" alt="" />
-  </div>
-
-  <p class="noindent"><span class="smcap">In</span> the following pages it has been my object to give a systematic
-    account of various Earth Movements.</p>
-
-  <p>These comprise <em>Earthquakes</em>, or the sudden violent movements of the
-    ground; <em>Earth Tremors</em>, or minute movements which escape our attention
-    by the smallness of their amplitude; <em>Earth Pulsations</em>, or movements
-    which are overlooked on account of the length of their period; and
-    lastly, <em>Earth Oscillations</em>, or movements of long period and large
-    amplitude which attract so much attention from their geological
-    importance.</p>
-
-  <p>It is difficult to separate these Earth Movements from each other,
-    because they are phenomena which only differ in degree, and which are
-    intimately associated in their occurrence and in their origin.</p>
-
-  <hr class="tb" />
-
-  <p>Because Earthquakes are phenomena which have attracted a universal
-    attention since the earliest times, and about them so many observations
-    have been made, they are treated of at considerable length.</p>
-
-  <p>As very much of what might be said about the other Earth Movements is
-    common to what is said about Earthquakes,
-    <span class="pagenum">vi</span>it has been possible to make
-    the description of these phenomena comparatively short.</p>
-
-  <p>The scheme which has been adopted will be understood from the following
-    table:—</p>
-
-  <table class="ssmall" summary="Book scheme">
-    <tr>
-      <td colspan="3" class="tdc"><div>I. <span class="smcap">Earthquakes</span>.</div></td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdl">1. Introduction.</td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdl">2. Seismometry.</td>
-    </tr>
-    <tr>
-      <td class="tdl">3. Earthquake Motion.</td>
-      <td><div class="x300">{</div></td>
-      <td class="tdl0">
-        <div>(<i>a</i>) Theoretically.</div>
-        <div>(<i>b</i>) As deduced from experiments.</div>
-        <div>(<i>c</i>) As deduced from actual Earthquakes.</div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">4. Earthquake Effects.</td>
-      <td><div class="x200">{</div></td>
-      <td class="tdl0">
-        <div>(<i>a</i>) On land.</div>
-        <div>(<i>b</i>) In the ocean.</div>
-      </td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdl">5. Determination of Earthquake origins.</td>
-    </tr>
-    <tr>
-      <td class="tdl">6. Distribution of Earthquakes.</td>
-      <td><div class="x300">{</div></td>
-      <td class="tdl0">
-        <div>(<i>a</i>) In space.</div>
-        <div>(<i>b</i>) In time (geological time, historical time, annual, seasonal, diurnal, &amp;c.)</div>
-      </td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdl">7. Cause of Earthquakes.</td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdl">8. Earthquake prediction and warning.</td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdc"><div>II. <span class="smcap">Earth Tremors</span>.</div></td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdc"><div>&nbsp; III. <span class="smcap">Earth Pulsations</span>.</div></td>
-    </tr>
-    <tr>
-      <td colspan="3" class="tdc"><div>&nbsp; &nbsp; &nbsp;IV. <span class="smcap">Earth Oscillations</span>.</div></td>
-    </tr>
-  </table>
-
-  <p class="noindent">In some instances the grouping of phenomena according to the above
-    scheme may be found inaccurate, as, for example, in the chapters
-    referring to the effects and causes of Earthquakes.</p>
-
-  <p>This arises from the fact that the relationship between Earthquakes
-    and other Earth phenomena are not well understood. Thus the sudden
-    elevation of a coast line and an accompanying earthquake may be
-    related, either as effect and cause, or <i xml:lang="la">vice versâ</i>, or they may both
-    be the effect of a third phenomenon.</p>
-
-  <p><span class="pagenum">vii</span></p>
-
-  <p>Much of what is said respecting Earthquake motion will show how little
-    accurate knowledge we have about these disturbances. Had I been writing
-    in England, and, therefore, been in a position to make references to
-    libraries and persons who are authorities on subjects connected with
-    Seismology, the following pages might have been made more complete,
-    and inaccuracies avoided. A large proportion of the material embodied
-    in the following pages is founded on experiments and observations
-    made during an eight years’ residence in Japan, where I have had the
-    opportunity of recording an earthquake every week.</p>
-
-  <p>The writer to whom I am chiefly indebted is Mr. Robert Mallet. Not
-    being in a position to refer to original memoirs, I have drawn many
-    illustrations from the works of Professor Karl Fuchs and M. S. di
-    Rossi. These, and other writers to whom reference has been made, are
-    given in an appendix.</p>
-
-  <p>For seeing these pages through the press, my thanks are due to
-    Mr. Thomas Gray, who, when residing in Japan, did so much for the
-    advancement of observational Seismology.</p>
-
-  <p>For advice and assistance in devising experiments, I tender my thanks
-    to my colleagues, Professor T. Alexander, Mr. T. Fujioka, and to my
-    late colleague. Professor John Perry.</p>
-
-  <p>For assistance in the actual observation of Earthquakes, I have to
-    thank my friends in various parts of Japan, especially Mr. J. Bissett
-    and Mr. T. Talbot, of Yokohama. For assistance in obtaining information
-    from<span class="pagenum">viii</span>
-    Italian sources I have to thank Dr. F. Du Bois, from German
-    sources Professor C. Netto, and from Japanese sources Mr. B. H.
-    Chamberlain. For help in carrying out experiments, I am indebted to
-    the liberality of the British Association, the Geological Society of
-    London, the Meteorological and Telegraph departments of Japan, and to
-    the officers of my own institution, the Imperial College of Engineering.</p>
-
-  <p>And, lastly, I offer my sincere thanks to those gentlemen who have
-    taken part in the establishment and working of the Seismological
-    Society of Japan, and to my publishers, whose liberality has enabled me
-    to place the labours of residents in the Far East before the European
-    public.</p>
-
-  <p class="rightinset"><span class="smcap">John Milne.</span></p>
-
-  <p class="small"><span class="smcap">Tokio, Japan</span>; <i>June</i> 30, 1883</p>
-
-  <hr class="page" />
-  <div class="chapter">
-    <span class="pagenum">ix</span>
-    <h2 class="gespertt xxlarge">CONTENTS.</h2>
-  </div>
-
-  <div class="figcenter">
-    <img src="images/divider.png" width="80" height="9" alt="" />
-  </div>
-
-  <table class="mt0" summary="Contents">
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_I">CHAPTER I.</a><br />
-          <span class="subhead">INTRODUCTION.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td></td>
-      <td class="tdpage"><div>PAGE</div></td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Relationship of man to nature—The aspect of a country is dependent
-        on geological phenomena—Earthquakes an important
-        geological phenomenon—Relationship of seismology to the
-        sciences and arts—Earth movements other than earthquakes—Seismological
-        literature—(Writings of Perrey, Mallet, Eastern
-        writings, the Philosophical Transactions of the Royal Society,
-        the ‘Gentleman’s Magazine,’ the Bible, Herodotus, Pliny, Hopkins,
-        Von Hoff, Humboldt, Schmidt, Seebach, Lasaulx, Fuchs,
-        Palmieri, Bertelli, Seismological Society of Japan)—Seismological
-        terminology
-      </td>
-      <td class="tdrb"><div>1</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_II">CHAPTER II.</a><br />
-          <span class="subhead">SEISMOMETRY.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Nature of earthquake vibrations—Many instruments called seismometers
-        only seismoscopes—Eastern seismoscopes, columns, projection
-        seismometers—Vessels filled with liquid—Palmieri’s
-        mercury tubes—The ship seismoscope—The cacciatore—Pendulum
-        instruments of Kreil, Wagner, Ewing, and Gray—Bracket
-        seismographs—West’s parallel motion instrument—Gray’s conical
-        pendulums, rolling spheres, and cylinders—Verbeck’s ball
-        and plate seismograph—The principle of Perry and Ayrton—Vertical
-        motion instruments—Record receiver—Time-recording
-        apparatus—The Gray and Milne seismograph
-      </td>
-      <td class="tdrb"><div>12</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <span class="pagenum">x</span>
-          <a href="#CHAPTER_III">CHAPTER III.</a><br />
-          <span class="subhead">EARTHQUAKE MOTION DISCUSSED THEORETICALLY.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Ideas of the ancients (the views of Travagini, Hooke, Woodward,
-        Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves
-        and vibrations—Possible causes of disturbance in the earth’s
-        crust—The time of vibration of an earth particle—Velocity and
-        acceleration of a particle—Propagation of a disturbance as determined
-        by experiments upon the elastic moduli of rocks—The
-        intensity of an earthquake—Area of greatest overturning moment—Earthquake
-        waves—Reflexion, refraction, and interference
-        of waves—Radiation of a disturbance
-      </td>
-      <td class="tdrb"><div>41</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_IV">CHAPTER IV.</a><br />
-          <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Experiments with falling weights—Experiments with explosives—Results
-        obtained from experiments—Relative motion of two
-        adjacent points—The effect of hills and excavations upon the
-        propagation of vibrations—The intensity of artificial disturbances—Velocity
-        with which earth vibrations are propagated—Experiments
-        of Mallet—Experiments of Abbot—Experiments
-        in Japan—Mallet’s results—Abbot’s results—Results obtained
-        in Japan
-      </td>
-      <td class="tdrb"><div>57</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_V">CHAPTER V.</a><br />
-          <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION
-            ON EARTHQUAKES.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Result of feelings—The direction of motion—Instruments as indicators
-        of direction—Duration of an earthquake—Period of
-        vibration—The amplitude of earth movements—Side of greatest
-        motion—Intensity of earthquakes—Velocity and acceleration of
-        an earth particle—Absolute intensity of an earthquake—Radiation
-        of an earthquake—Velocity of propagation
-      </td>
-      <td class="tdrb"><div>67</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <span class="pagenum">xi</span>
-          <a href="#CHAPTER_VI">CHAPTER VI.</a><br />
-          <span class="subhead">EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        The destruction of buildings is not irregular—Cracks in buildings—Buildings
-        in Tokio—Relation of destruction to earthquake
-        motion—Measurement of relative motion of parts of a building
-        shaken by an earthquake—Prevention of cracks—Direction of
-        cracks—The pitch of roofs—Relative position of openings in a
-        wall—The last house in a row—The swing of buildings—Principle
-        of relative vibrational periods
-      </td>
-      <td class="tdrb"><div>96</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_VII">CHAPTER VII.</a><br />
-          <span class="subhead">EFFECTS PRODUCED UPON BUILDINGS (<i>continued</i>).</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Types of buildings used in earthquake countries—In Japan, in Italy,
-        in South America, in Caraccas—Typical houses for earthquake
-        countries—Destruction due to the nature of underlying rocks—The
-        swing of mountains—Want of support on the face of hills—Earthquake
-        shadows—Destruction due to the interference of
-        waves—Earthquake bridges—Examples of earthquake effects—Protection
-        of buildings—General conclusions
-      </td>
-      <td class="tdrb"><div>122</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_VIII">CHAPTER VIII.</a><br />
-          <span class="subhead">EFFECTS OF EARTHQUAKES ON LAND.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        1. Cracks and fissures—Materials discharged from fissures—Explanation
-        of fissure phenomena. 2. Disturbances in lakes, rivers,
-        springs, wells, fumaroles, &amp;c.—Explanation of these latter phenomena.
-        3. Permanent displacement of ground—On coast lines—Level
-        tracts—Among mountains—Explanation of these movements
-      </td>
-      <td class="tdrb"><div>146</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_IX">CHAPTER IX.</a><br />
-          <span class="subhead">DISTURBANCES IN THE OCEAN.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Sea vibrations—Cause of vibratory blows—Sea waves: preceding
-        earthquakes; succeeding earthquakes—Magnitude of waves
-        <span class="pagenum">xii</span>—Waves
-        as recorded in countries distant from the origin—Records
-        on tide gauges—Waves without earthquakes—Cause of waves—Phenomena
-        difficult of explanation—Velocity of propagation—Depth
-        of the ocean—Examples of calculations—Comparison of
-        velocities of earthquake waves with velocities which ought to
-        exist from the known depth of the ocean
-      </td>
-      <td class="tdrb"><div>163</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_X">CHAPTER X.</a><br />
-          <span class="subhead">DETERMINATION OF EARTHQUAKE ORIGINS.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Approximate determination of an origin—Earthquake-hunting in
-        Japan—Determinations by direction of motion—Direction indicated
-        by destruction of buildings—Direction determined by
-        rotation—Cause of rotation—The use of time observations—Errors
-        in such observations—Origin determined by the method
-        of straight lines—The method of circles, the method of hyperbolas,
-        the method of co-ordinates—Haughton’s method—Difference
-        in time between sound, earth, and water waves—Method of Seebach
-      </td>
-      <td class="tdrb"><div>187</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_XI">CHAPTER XI.</a><br />
-          <span class="subhead">THE DEPTH OF AN EARTHQUAKE CENTRUM.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        The depth of an earthquake centrum—Greatest possible depth of
-        an earthquake—Form of the focal cavity
-      </td>
-      <td class="tdrb"><div>213</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_XII">CHAPTER XII.</a><br />
-          <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        General distribution of earthquakes—Occurrence along lines—Examples
-        of distribution—Italian earthquake of 1873—In Tokio—Extension
-        of earthquake boundaries—Seismic energy in relation
-        to geological time; to historical time—Relative frequency of
-        earthquakes—Synchronism of earthquakes—Secondary earthquakes
-      </td>
-      <td class="tdrb"><div>226</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <span class="pagenum">xiii</span>
-          <a href="#CHAPTER_XIII">CHAPTER XIII.</a>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td><div class="center small">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</div></td>
-      <td class="tdrb"><div>234</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_XIV">CHAPTER XIV.</a><br />
-          <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        The occurrence of earthquakes in relation to the position of the
-        heavenly bodies—Earthquakes and the moon—Earthquakes and
-        the sun; and the seasons; the months—Planets and meteors—Hours
-        at which earthquakes are frequent—Earthquakes and
-        sun spots—Earthquakes and the aurora
-      </td>
-      <td class="tdrb"><div>250</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_XV">CHAPTER XV.</a><br />
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl small">
-        BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE AND EARTHQUAKES
-      </td>
-      <td class="tdrb"><div>266</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_XVI">CHAPTER XVI.</a><br />
-          <span class="subhead">RELATION OF SEISMIC TO VOLCANIC PHENOMENA.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Want of synchronism between earthquakes and volcanic eruptions—Synchronism
-        between earthquakes and volcanic eruptions—Conclusion
-      </td>
-      <td class="tdrb"><div>270</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-        <div class="tdh">
-          <a href="#CHAPTER_XVII">CHAPTER XVII.</a><br />
-          <span class="subhead">THE CAUSE OF EARTHQUAKES.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Modern views respecting the cause of earthquakes—Earthquakes
-        due to faulting—To explosions of steam—To volcanic evisceration—To
-        chemical degradation—Attractive influence of the
-        heavenly bodies—The effect of oceanic tides—Variation in atmospheric
-        pressure—Fluctuation in temperature—Winds and
-        earthquakes—Rain and earthquakes—Conclusion
-      </td>
-      <td class="tdrb"><div>277</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-       <div class="tdh">
-         <span class="pagenum">xiv</span>
-          <a href="#CHAPTER_XVIII">CHAPTER XVIII.</a><br />
-          <span class="subhead">PREDICTION OF EARTHQUAKES.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        General nature of predictions—Prediction by the observation of unusual
-        phenomena (Alteration in the appearance and taste of
-        springs; underground noises; preliminary tremors; Earthquake
-        prophets—warnings furnished by animals, &amp;c.)—Earthquake warning
-      </td>
-      <td class="tdrb"><div>297</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-       <div class="tdh">
-          <a href="#CHAPTER_XIX">CHAPTER XIX.</a><br />
-          <span class="subhead">EARTH TREMORS.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Artificially produced tremors—Observations of Kater, Denman,
-        Airy, Palmer, Paul—Natural tremors—Observations of Zöllner,
-        M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With
-        seismoscopes, microphones, pendulums—Work in Italy—Bertelli,
-        Count Malvasia, M. S. di Rossi—Instruments employed
-        in Italy—Tromometers, microseismographs, microphones—Results
-        obtained in Italy—In Japan—Cause of microseismic motion
-      </td>
-      <td class="tdrb"><div>306</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-       <div class="tdh">
-          <a href="#CHAPTER_XX">CHAPTER XX.</a><br />
-          <span class="subhead">EARTH PULSATIONS.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Definition of an earth pulsation—Indications of pendulums—Indications
-        of levels—Other phenomena indicating the existence of
-        earth pulsations—Disturbances in lakes and oceans—Phenomena
-        resultant on earth pulsations—Cause of earth pulsations
-      </td>
-      <td class="tdrb"><div>326</div></td>
-    </tr>
-    <tr>
-      <td colspan="2">
-       <div class="tdh">
-          <a href="#CHAPTER_XXI">CHAPTER XXI.</a><br />
-          <span class="subhead">EARTH OSCILLATIONS.</span>
-        </div>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl">
-        Evidences of oscillation—Examples of oscillation—Temple of Jupiter
-        Serapis—Observations of Darwin—Causes of oscillation
-      </td>
-      <td class="tdrb"><div>344</div></td>
-    </tr>
-    <tr>
-      <td><div class="tdl mt3"><a href="#APPENDIX">APPENDIX</a></div></td>
-      <td class="tdrb"><div>349</div></td>
-    </tr>
-    <tr>
-      <td><div class="tdl mt3"><a href="#INDEX">INDEX</a></div></td>
-      <td class="tdrb"><div>359</div></td>
-    </tr>
-  </table>
-
-  <div class="chapter" id="CHAPTER_I">
-    <span class="pagenum" id="Page_1">1</span>
-    <div class="xxlarge center"><b>EARTHQUAKES</b>.</div>
-    <div class="figcenter">
-      <img src="images/divider.png" width="80" height="9" alt="" />
-    </div>
-    <h2>CHAPTER I.<br />
-      <span class="subhead">INTRODUCTION.</span>
-    </h2>
-  </div>
-
-  <div class="summary">Relationship of man to nature—The aspect of a country is dependent
-    on geological phenomena—Earthquakes an important geological
-    phenomenon—Relationship of seismology to the sciences and
-    arts—Earth movements other than earthquakes—Seismological
-    literature—(Writings of Perrey, Mallet, Eastern writings, the
-    Philosophical Transactions of the Royal Society, the ‘Gentleman’s
-    Magazine,’ the Bible, Herodotus, Pliny, Hopkins, Von Hoff,
-    Humboldt, Schmidt, Seebach, Lasaulx, Fuchs, Palmieri, Bertelli,
-    Seismological Society of Japan)—Seismological terminology.</div>
-
-  <p class="noindent"><span class="smcap">In</span>
-    bygone superstitious times lightning and thunder were regarded
-    as supernatural visitations. But as these phenomena became better
-    understood, and men learned how to avoid their destructive power, the
-    superstition was gradually dispelled. Thus it is with Earthquakes:
-    the more clearly they are understood, the more confident in the
-    universality of law will man become, and the more will his mental
-    condition be advanced.</p>
-
-  <p>In his ‘History of Civilisation in England,’ Buckle has laid
-    considerable stress upon the manner in which earthquakes, volcanoes,
-    and other of the more terrible forms in which the workings of
-    nature reveal themselves<span class="pagenum" id="Page_2">2</span>
-    to us, have exerted an influence upon the
-    imagination and understanding; and just as a sudden fright may affect
-    the nerves of a child for the remainder of its life, we have in the
-    annals of seismology records which indicate that earthquakes have not
-    been without a serious influence upon the mental condition of whole
-    communities.</p>
-
-  <p>To a geologist there are perhaps no phenomena in nature more
-    interesting than earthquakes, the study of which is called Seismology.
-    Coming, as shocks often will, from a region of volcanoes, the study
-    of these disturbances may enable us to understand something about
-    the nature and working of a volcano. As an earthquake wave travels
-    along from strata to strata, if we study its reflections and changing
-    velocity in transit, we may often be led to the discovery of certain
-    rocky structures buried deep beneath our view, about which, without the
-    help of such waves, it would be hopeless ever to attain any knowledge.</p>
-
-  <p>By studying the propagation of earthquake waves the physicist is
-    enabled to confirm his speculations respecting the transmission of
-    disturbances in elastic media. For the physicist earthquakes are
-    gigantic experiments which tell him the elastic moduli of rocks as
-    they exist in nature, and when properly interpreted may lead him to
-    the proper comprehension of many ill-understood phenomena. It is not
-    impossible that seismological investigation may teach us something
-    about the earth’s magnetism, and the connection between earthquakes and
-    the ‘earth currents’ which appear in our telegraph wires. These and
-    numerous other kindred problems fall within the domain of the physicist.</p>
-
-  <p>It is of interest to the meteorologist to know the connections which
-    probably exist between earthquakes and the fluctuations of the
-    barometer, the changes of the<span class="pagenum" id="Page_3">3</span>
-    thermometer, the quantity of rainfall,
-    and like phenomena to which he devotes his attention.</p>
-
-  <p>Next we may turn to the more practical aims of seismology and ask
-    ourselves what are the effects of earthquakes upon buildings, and
-    how, in earthquake-shaken countries, the buildings are to be made to
-    withstand them. Here we are face to face with problems which demand
-    the attention of engineers and builders. To attain what we desire,
-    observation, common sense, and subtle reasoning must be brought to bear
-    upon this subject.</p>
-
-  <p>In the investigation of the principle on which earthquake instruments
-    make their records, in the analysis of the results they give, in
-    problems connected with astronomy, with physics, and with construction,
-    seismology offers to the mathematician new fields for investigation.</p>
-
-  <p>A study of the effects which earthquakes produce on the lower animals
-    will not fail to interest the student of natural history.</p>
-
-  <p>A study like seismology, which leads us to a more complete knowledge
-    of earth-heat and its workings, is to be regarded as one of the
-    corner-stones of geology. The science of seismology invites the
-    co-operation of workers and thinkers in almost every department of
-    natural science.</p>
-
-  <p>We have already referred to the influence exerted by earthquakes over
-    the human mind. How to predict earthquakes, and how to escape from
-    their dangers, are problems which, if they can be solved, are of
-    extreme interest to the world at large.</p>
-
-  <p>In addition to the sudden and violent movements which we call
-    earthquakes, the seismologist has to investigate the smaller motions
-    which we call earth tremors. From observations which have been made
-    of late years,<span class="pagenum" id="Page_4">4</span>
-    it would appear that the ground on which we dwell is
-    incessantly in a state of tremulous motion.</p>
-
-  <p>A further subject of investigation which is before the seismologist is
-    the experimental verification of the existence of what may be called
-    ‘earth-pulsations.’ These are motions which mathematical physicists
-    affirmed the existence of, but which, in consequence of the slowness of
-    their period, have hitherto escaped observation.</p>
-
-  <p>The oscillations, or slow changes in the relative positions of land and
-    sea, might also be included; but this has already been taken up as a
-    separate branch of geology.</p>
-
-  <p>These four classes of movements are no doubt interdependent, and
-    seismology in the widest sense might conveniently be employed to
-    include them all. In succeeding chapters we will endeavour to indicate
-    how far the first three of these branches have been prosecuted, and
-    to point out that which remains to be accomplished. It is difficult,
-    however, to form a just estimate of the amount of seismological work
-    which has been done, in consequence of the scattered and uncertain
-    nature of many of the records. Seismology, as a science, originated
-    late, chiefly owing to the facts that centres of civilisation are
-    seldom in the most disturbed regions, and that earthquake-shaken
-    countries are widely separated from each other.</p>
-
-  <p>As every portion of the habitable globe appears to have been shaken
-    more or less by earthquakes, and as these phenomena are so terrible in
-    their nature, we can readily understand why seismological literature is
-    extensive. In the annals of almost every country which has a written
-    history, references are made to seismic disturbances.</p>
-
-  <p>An idea of the attention which earthquakes have received may be
-    gathered from the fact that Professor Alexis Perrey, of Dijon, who
-    has published some sixty memoirs<span class="pagenum" id="Page_5">5</span>
-    on this subject, gave, in 1856, a
-    catalogue of 1,837 works devoted to seismology.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a>
-    In 1858 Mr. Robert
-    Mallet published in the Reports of the British Association a list of
-    several hundred works relating to earthquakes. Sixty-five of these
-    works are to be found in the British Museum. So far as literature is
-    concerned, earthquakes have received as much attention in the East as
-    in the West. In China there are many works treating on earthquakes, and
-    the attention which these phenomena received may be judged of from the
-    fact that in <span class="smcap">a.d.</span> 136 the Government appointed a commission
-    to inquire into the subject. Even the isolated empire of Japan can
-    boast of at least sixty-five works on earthquakes, seven of which
-    are earthquake calendars, and twenty-three earthquake monographs.<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a>
-    Besides those treating especially of earthquakes, there are innumerable
-    references to such disturbances in various histories, in the
-    transactions of learned societies, and in periodicals. To attempt to
-    give a complete catalogue of even the books which have been written
-    would be to enter on a work of compilation which would occupy many
-    years, and could never be satisfactorily finished.</p>
-
-  <p>In the ‘Philosophical Transactions of the Royal Society,’ which were
-    issued in the eighteenth century, there are about one hundred and
-    eighty separate communications on earthquakes; and in the ‘Gentleman’s
-    Magazine’ for 1755 there are no less than fifty notes and articles on
-    the same subject. The great interest shown in earthquakes about the
-    years 1750–60 in England, was chiefly due to the terrible calamity
-    which overtook Lisbon in 1755, and to the fact that about this time
-    several shocks were experienced in various parts of the British
-    Islands. In 1750, which may <span class="pagenum" id="Page_6">6</span>
-    be described as the earthquake year of
-    Britain, ‘a shock was felt in Surrey on March 14; on the 18th of the
-    same month the whole of the south-west of England was disturbed. On
-    April 2, Chester was shaken; on June 7, Norwich was disturbed; on
-    August 23, the inhabitants of Lancashire were alarmed; and on September
-    30 ludicrous and alarming scenes occurred in consequence of a shock
-    having been felt during the hours of Divine service, causing the
-    congregations to hurry into the open air.’<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a>
-    As might be expected,
-    these occurrences gave rise to many articles and notes directing
-    attention to the subject of earthquakes.</p>
-
-  <p>Seismic literature has not, however, at all times been a measure of
-    seismic activity: thus, in Japan, the earthquake records for the
-    twelfth and sixteenth centuries scarcely mention any shocks. At first
-    sight it might be imagined that this was owing to an absence of
-    earthquakes; but it is sufficiently accounted for by the fact that
-    at that time the country was torn with civil war, and matters more
-    urgent than the recording of natural phenomena engaged the attention of
-    the inhabitants. Professor Rockwood, who has given so much attention
-    to seismic disturbances in America, tells us that during the recent
-    contest between Chili and Peru a similar intermission is observable. We
-    see, therefore, that an absence of records does not necessarily imply
-    an absence of the phenomena to be recorded.</p>
-
-  <p>Perhaps the earliest existing records of earthquakes are those which
-    occur in the Bible. The first of these, which we are told occurred in
-    Palestine, was in the reign of Ahab (<span class="smcap">b.c.</span> 918–897).<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a>
-    One of the most terrible earthquakes mentioned in the Bible is that which took
-    place in the days of Uzziah, king of Judah (<span class="smcap">b.c.</span> 811–759),
-    <span class="pagenum" id="Page_7">7</span>
-    which shook the ground and rent the Temple. The awful character of
-    this, and the deep impression produced on men’s minds, may be learned
-    from the fact that the time of its occurrence was subsequently used as
-    an epoch from which to reckon dates.</p>
-
-  <p>The writings of Herodotus, Pliny, Livy, &amp;c., &amp;c., show the interest
-    which earthquakes attracted in early ages. These writers chiefly
-    devoted themselves to references and descriptions of disastrous shocks,
-    and to theories respecting the cause of earthquakes.</p>
-
-  <p>The greater portion of the Japanese notices of earthquakes is simply
-    a series of anecdotes of events which took place at the time of
-    these disasters. We also find references to superstitious beliefs,
-    curious occurrences, and the apparent connection between earthquake
-    disturbances and other natural phenomena. In these respects the
-    literature of the East closely resembles that of the West. The
-    earthquake calendars of the East, however, form a class of books which
-    can hardly be said to find their parallel in Europe;<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a> while, on the
-    other hand, the latter possesses types of books and pamphlets which do
-    not appear to have a parallel elsewhere. These are the more or less
-    theological works—‘Moral Reflections on Earthquakes,’ ‘Sermons’ which
-    have been preached on earthquakes, ‘Prayers’ which have been appointed
-    to be read.<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a></p>
-
-  <p>Speaking generally, it may be said that the writings of the ancients,
-    and those of the Middle Ages, down to the commencement of the
-    nineteenth century, tended to the propagation of superstition and to
-    theories based on speculations with few and imperfect facts for their
-    foundation.</p>
-
-  <p><span class="pagenum" id="Page_8">8</span></p>
-
-  <p>Among the efforts which have been made in modern times to raise
-    seismology to a higher level, is that of Professor Perrey, of Dijon,
-    who commenced in 1840 a series of extensive catalogues embracing
-    the earthquakes of the world. These catalogues enabled Perrey, and
-    subsequently Mallet in his reports to the British Association, to
-    discuss the periodicity of earthquakes, with reference to the seasons
-    and to other phenomena, in a more general manner than it had been
-    possible for previous workers to accomplish. The facts thus accumulated
-    also enabled Mallet to discuss earthquakes in general, and the various
-    phenomena which they present were sifted and classified for inspection.
-    Another great impetus which observational seismology received was Mr.
-    Mallet’s report upon the Neapolitan earthquake of 1857, in which new
-    methods of seismic investigation were put forth. These have formed
-    the working tools of many subsequent observers, and by them, as well
-    as by his experiments on artificially produced disturbances, Mallet
-    finally drew the study of earthquakes from the realms of speculation by
-    showing that they, like other natural phenomena, were capable of being
-    understood and investigated.</p>
-
-  <p>In addition to Perrey and Mallet, the nineteenth century has produced
-    many writers who have taken a considerable share in the advancement of
-    seismology. There are the catalogues of Von Hoff, the observations of
-    Humboldt, the theoretical investigations of Hopkins, the monographs of
-    Schmidt, Seebach, Lasaulx, and others; the books of Fuchs, Credner,
-    Vogt, Volger; the records and observations of Palmieri, Bertelli,
-    Rossi, and other Italian observers. To these, which are only a few
-    out of a long list of names, may be added the publications of the
-    <span class="pagenum" id="Page_9">9</span>
-    Commission appointed for the observation of earthquakes by the Natural
-    History Society of Switzerland, and the volumes which have been
-    published by the Seismological Society of Japan.</p>
-
-  <p>Before concluding this chapter it will be well to define a few of the
-    more ordinary terms which are used in describing earthquake phenomena.
-    It may be observed that the English word <em>earthquake</em>, the German
-    <i xml:lang="de">erdbeben</i>, the French <i xml:lang="fr">tremblement de terre</i>, the Spanish <i xml:lang="es">terremoto</i>,
-    the Japanese <i xml:lang="ja">jishin</i> &amp;c., all mean, when literally translated,
-    <em>earth-shaking</em>, and are popularly understood to mean a sudden and more
-    or less violent disturbance.</p>
-
-  <p>Seismology (σειμός an earthquake, λόγος a discourse)
-    in its simplest sense means the study of earthquakes. To be consistent
-    with a Greek basis for seismological terminology, some writers have
-    thrown aside the familiar expression ‘earthquake,’ and substituted the
-    awkward word ‘seism.’</p>
-
-  <p>The source from which an earthquake originates is called the ‘origin,’
-    ‘focal cavity,’ or ‘centrum.’</p>
-
-  <p>The point or area on the surface of the ground above the origin is
-    called the ‘epicentrum.’ The line joining the centrum and epicentrum is
-    called the ‘seismic vertical.’</p>
-
-  <p>The radial lines along which an earthquake may be propagated from the
-    centrum are called ‘wave paths.’</p>
-
-  <p>The angle which a wave path, where it reaches the surface of the earth,
-    makes with that surface is called the ‘angle of emergence’ of the wave.
-    This angle is usually denoted by the letter <i>e</i>.</p>
-
-  <p>As the result of a simple explosion at a point in a homogeneous medium,
-    we ought, theoretically, to obtain at points on the surface of the
-    medium equidistant<span class="pagenum" id="Page_10">10</span>
-    from the epicentrum, equal mechanical effects.
-    These points will lie on circles called ‘isoseismic’ or ‘coseismic’
-    circles. The area included between two such circles is an ‘isoseismic
-    area.’ In nature, however, isoseismic lines are seldom circles.
-    Elliptical or irregular curves are the common forms.</p>
-
-  <p>The isoseismic area in which the greatest disturbance has taken place
-    is called the ‘meizoseismic area.’ Seebach calls the lines enclosing
-    this area ‘pleistoseists.’</p>
-
-  <p>These last-mentioned lines are wholly due to Mallet and Seebach.</p>
-
-  <p>Many words are used to distinguish different kinds of earthquakes from
-    each other. All of these appear to be very indefinite and to depend
-    upon the observer’s feelings, which, in turn, depend upon his nervous
-    temperament and his situation.</p>
-
-  <p>In South America small earthquakes, consisting of a series of rapidly
-    recurring vibratory movements not sufficiently powerful to create
-    damage, are spoken of as <i xml:lang="es">trembelores</i>.</p>
-
-  <p>The <i xml:lang="es">terremotos</i> of South America are earthquakes of a destructive
-    nature, in which distinct shocks are perceptible. It may be observed
-    that shocks which at one place would be described as <i xml:lang="es">terremoto</i>
-    would at another and more distant place probably be described as
-    <i xml:lang="es">trembelores</i>.</p>
-
-  <p>The <i xml:lang="es">succussatore</i> are the shocks where there is considerable vertical
-    motion. The terrible shock of Riobamba (February 4, 1797), which is
-    said to have thrown corpses from their graves to a height of 100 feet,
-    was an earthquake of this order.</p>
-
-  <p>The <i xml:lang="es">vorticosi</i> are shocks which have a twisting or rotatory motion.</p>
-
-  <p>Another method of describing earthquakes would be
-    <span class="pagenum" id="Page_11">11</span> to refer to
-    instrumental records. When the vibrations of the ground have only
-    been along the line joining the observer and the epicentrum, the
-    disturbance might be called ‘euthutropic.’ A disturbance in which the
-    prominent movements are <em>transverse</em> to the above direction might be
-    called ‘diagonic.’ If motions in both of these directions occur in the
-    records, the shock might be said to be ‘diastrophic.’ If there be much
-    vertical movement, the shock might be said to be ‘anaseismic.’ Some
-    disturbances could only be described by using two or three of these
-    terms.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_II">
-    <span class="pagenum" id="Page_12">12</span>
-    <h2>CHAPTER II.<br />
-      <span class="subhead">SEISMOMETRY.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Nature of earthquake vibrations—Many instruments called
-    seismometers only seismoscopes—Eastern seismoscopes, columns,
-    projection seismometers—Vessels filled with liquid—Palmieri’s
-    mercury tubes—The ship seismoscope—The cacciatore—Pendulum
-    instruments of Kreil, Wagner, Ewing, and Gray—Bracket
-    seismographs—West’s parallel motion instrument—Gray’s conical
-    pendulums, rolling spheres, and cylinders—Verbeck’s ball and
-    plate seismograph—The principle of Perry and Ayrton—Vertical
-    motion instruments—Record receivers—Time-recording apparatus—The
-    Gray and Milne seismograph.</div>
-
-  <p class="noindent"><span class="smcap">Before</span>
-    we discuss the nature of earthquake motion, the determination
-    of which has been the aim of modern seismological investigation, the
-    reader will naturally look for an account of the various instruments
-    which have been employed for recording such disturbances. A description
-    of the earthquake machines which have been used even in Japan would
-    form a bulky volume. All that we can do, therefore, is to describe
-    briefly the more prominent features of a few of the more important of
-    these instruments. In order that the relative merits of these may be
-    better understood, we may state generally that modern research has
-    shown a typical earthquake to consist of a series of small tremors
-    succeeded by a shock, or series of shocks, separated by more or less
-    irregular vibrations of the ground. The vibrations are often both
-    irregular in period<span class="pagenum" id="Page_13">13</span>
-    and in amplitude, and they have a duration of from
-    a few seconds to several minutes. We will illustrate the records of
-    actual earthquakes in a future chapter, but in the meantime the idea
-    that an earthquake consists of a single shock must be dismissed from
-    the imagination.</p>
-
-  <p>To construct an instrument which at the time of an earthquake shall
-    move and leave a record of its motion, there is but little difficulty.
-    Contrivances of this order are called <em>seismoscopes</em>. If, however,
-    we wish to know the period, extent, and direction of each of the
-    vibrations which constitutes an earthquake, we have considerable
-    difficulty. Instruments which will in this way measure or write down
-    the earth’s motions are called <em>seismometers</em> or <em>seismographs</em>.</p>
-
-  <p>Many of the elaborate instruments supplemented with electro-magnetic
-    and clockwork arrangements are, when we examine them, nothing more than
-    elaborate seismoscopes which have been erroneously termed seismographs.</p>
-
-  <p>The only approximations to true seismographs which have yet been
-    invented are without doubt those which during the past few years have
-    been used in Japan. It would be a somewhat arbitrary proceeding,
-    however, to classify the different instruments as seismoscopes,
-    seismometers, and seismographs, as the character of the record given
-    by certain instruments is sometimes only seismoscopic, whilst at other
-    times it is seismometric, depending on the nature of the disturbance.
-    Many instruments, for instance, would record with considerable accuracy
-    a single sudden movement, but would give no reliable information
-    regarding a continued shaking.</p>
-
-  <p><em>Eastern Seismoscopes.</em>—The earliest seismoscope of which we find any
-    historical record is one which owes its origin to a Chinese called
-    Chôko. It was invented in the year <span class="smcap">a.d.</span> 136. A description is
-    given in the Chinese<span class="pagenum" id="Page_14">14</span>
-    history called ‘Gokanjo,’ and the translation of
-    this description runs as follows:—</p>
-
-  <div class="figcenter">
-    <img src="images/i_p014.jpg" width="400" height="473" alt="" />
-    <div class="caption"><span class="smcap">Fig. 1.</span></div>
-  </div>
-
-  <p>‘In the first year of Yōka, <span class="smcap">a.d.</span> 136, a Chinese called Chôko
-    invented the seismometer shown in the accompanying drawing. This
-    instrument consists of a spherically formed copper vessel, the diameter
-    of which is eight feet. It is covered at its top, and in form resembles
-    a wine-bottle. Its outer part is ornamented by the figures of different
-    kinds of birds and animals, and old peculiar-looking letters. In the
-    inner part of this instrument a column is so suspended that it can
-    move in eight directions. Also, in the inside of the bottle, there is
-    an arrangement by which some record of an earthquake is made according
-    to the movement of the pillar. On the outside of the bottle there
-    are eight dragon heads, each<span class="pagenum" id="Page_15">15</span>
-    of which holds a ball in its mouth.
-    Underneath these heads there are eight frogs so placed that they appear
-    to watch the dragon’s face, so that they are ready to receive the ball
-    if it should be dropped. All the arrangements which cause the pillar to
-    knock the ball out of the dragon’s mouth are well hidden in the bottle.’</p>
-
-  <p>‘When an earthquake occurs, and the bottle is shaken, the dragon
-    instantly drops the ball, and the frog which receives it vibrates
-    vigorously; any one watching this instrument can easily observe
-    earthquakes.’</p>
-
-  <p>With this arrangement, although one dragon may drop a ball, it is
-    not necessary for the other seven dragons to drop their balls unless
-    the movement has been in all directions; thus we can easily tell the
-    direction of an earthquake.</p>
-
-  <p>‘Once upon a time a dragon dropped its ball without any earthquake
-    being observed, and the people therefore thought the instrument of
-    no use, but after two or three days a notice came saying that an
-    earthquake had taken place at Rōsei. Hearing of this, those who doubted
-    the use of this instrument began to believe in it again. After this
-    ingenious instrument had been invented by Chōko, the Chinese Government
-    wisely appointed a secretary to make observations on earthquakes.’</p>
-
-  <p>Not only is this instrument of interest on account of its antiquity,
-    but it is also of interest on account of the close resemblance it bears
-    to many of the instruments of modern times.</p>
-
-  <p>Another earthquake instrument also of Eastern origin is the magnetic
-    seismoscope of Japan.</p>
-
-  <p>On the night of the destructive earthquake of 1855, which devastated
-    a great portion of Tokio, the owner of a spectacle shop in Asakusa
-    observed that a magnet dropped<span class="pagenum" id="Page_16">16</span>
-    some old iron nails and keys which had
-    been attached to it. From this occurrence the owner thought that the
-    magnet had, in consequence of its age, lost its powers. About two hours
-    afterwards, however, the great earthquake took place, after which the
-    magnet was observed to have regained its powers. This occurrence led
-    to the construction of the seismoscope, which is illustrated in a book
-    called the ‘Ansei-Kembun-Roku,’ or a description of the earthquake of
-    1855, and examples of the instrument are still to be seen in Tokio.
-    These instruments consist of a piece of magnetic iron ore, which holds
-    up a piece of iron like a nail. This nail is connected, by means of a
-    string, with a train of clockwork communicating with an alarm. If the
-    nail falls a catch is released and the clockwork set in motion, and
-    warning given by the ringing of a bell. It does not appear that this
-    instrument has ever acted with success.</p>
-
-  <p><em>Columns.</em>—One of the commonest forms of seismoscope, and one which
-    has been very widely used, consists of a round column of wood, metal,
-    or other suitable material, placed, with its axis vertical, on a level
-    plane, and surrounded by some soft material such as loose sand to
-    prevent it rolling should it be overturned. The fall of such a column
-    indicates that a shaking or shock has taken place. Attempts have been
-    made by using a number of columns of different sizes to make these
-    indications seismometric, but they seldom give reliable information
-    either as to intensity or direction of shock. The indications as to
-    intensity are vitiated by the fact that a long-continued gentle shaking
-    may overturn a column which would stand a very considerable sudden
-    shock, while the directions in which a number of columns fall seldom
-    agree owing to the rotational motion imparted to them by the shaking.
-    Besides, the direction of motion<span class="pagenum" id="Page_17">17</span>
-    of the earthquake seldom remains in
-    the same azimuth throughout the whole disturbance.</p>
-
-  <p>An extremely delicate, and at the same time simple form of seismoscope
-    may be made by propping up strips of glass, pins, or other easily
-    overturned bodies against suitably placed supports. In this way bodies
-    may be arranged, which, although they can only fall in one direction,
-    nevertheless fall with far less motion than is necessary to overturn
-    any column which will stand without lateral support.</p>
-
-  <p><em>Projection Seismometers.</em>—Closely related to the seismoscopes and
-    seismometers which depend on the overturning of bodies. Mallet has
-    described two sets of apparatus whose indications depend on the
-    distance to which a body is projected. In one of these, which consisted
-    of two similar parts arranged at right angles, two metal balls rest one
-    on each side of a stop at the lower part of two inclined
-    <img class="iglyph-a" src="images/i_p017.png" alt="" width="48" height="25" />
-    like troughs. In this position each of the balls completes an electric
-    circuit. By a shock the balls are projected or rolled up the troughs,
-    and the height to which they rise is recorded by a corresponding
-    interval in the break of the circuits. The vertical component of the
-    motion is measured by the compression of a spring which carries the
-    table on which this arrangement rests. In the second apparatus two
-    balls are successively projected, one by the forward swing, and the
-    other by the backward swing of the shock. Attached to them are loose
-    wires forming terminals of the circuits. They are caught in a bed
-    of wet sand in a metal trough forming the other end of the circuit.
-    The throw of the balls as measured in the sand, and the difference
-    of time between their successive projections as indicated by special
-    contrivances connected with the closing of the circuits, enables
-    the observer to calculate the direction of the wave of shock,
-    <span class="pagenum" id="Page_18">18</span> its
-    velocity, and other elements connected with the disturbance. It will be
-    observed that the design of this apparatus assumes the earthquake to
-    consist of a distinct isolated shock.</p>
-
-  <p>Oldham, at the end of his account of the Cachar earthquake of 1869,
-    recommends the use of an instrument based on similar principles. In his
-    instrument four balls like bullets are placed in notches cut in the
-    corners of the upper end of a square stake driven into the ground.</p>
-
-  <p><em>Vessels filled with liquid.</em>—Another form of simple seismoscope is
-    made by partially filling a vessel with liquid. The height to which the
-    liquid is washed up the side of the vessel is taken as an indication of
-    the intensity of the shock, and the line joining the points on which
-    maximum motion is indicated, is taken as the direction of the shock. If
-    earthquakes all lasted for the same length of time, and consisted of
-    vibrations of the same period, such instruments might be of service.
-    These instruments have, however, been in use from an early date. In
-    1742 we find that bowls of water were used to measure the earthquakes
-    which in that year alarmed the inhabitants of Leghorn. About the same
-    time the Rev. S. Chandler, writing about the shock at Lisbon, tells us
-    that earthquakes may be measured by means of a spherical bowl about
-    three or four feet in diameter, the inside of which, after being dusted
-    over with Barber’s puff, is filled very gently with water. Mallet,
-    Babbage, and De la Bêche have recommended the same sort of contrivance,
-    but, notwithstanding, it has justly been criticised as ‘ridiculous and
-    utterly impracticable.’<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a></p>
-
-  <p><span class="pagenum" id="Page_19">19</span></p>
-
-  <p>An important portion of Palmieri’s well-known instrument consists
-    of horizontal tubes turned up at the ends and partially filled with
-    mercury. To magnify the motion of the mercury, small floats of iron
-    rest on its surface. These are attached by means of threads to a pulley
-    provided with indices which move in front of a scale of degrees. We
-    thus read off the intensity of an earthquake as so many degrees, which
-    means so many millimetres of washing up and down of mercury in a tube.
-    The direction of movement is determined by the azimuth of the tube
-    which gives the maximum indication, several tubes being placed in
-    different azimuths.</p>
-
-  <p>This form of instrument appears to have been suggested by Mallet, who
-    gives an account of the same in 1846. Inasmuch as the rise and fall
-    of the mercury in such tubes depend on its depth and on the period
-    of the earthquake together with its duration, we see that although
-    the results obtained from a given instrument may give us means of
-    making approximate comparisons as to the relative intensity of various
-    earthquakes, it is very far from yielding any absolute measurement.</p>
-
-  <p>Another method which has been employed to magnify and register the
-    motions of liquid in a vessel has been to float upon its surface a
-    raft or ship from which a tall mast projected. By a slight motion of
-    the raft, the top of the mast vibrated through a considerable range.
-    This motion of the mast as to direction and extent was then recorded by
-    suitable contrivances attached to the top of the mast.</p>
-
-  <p>A very simple form of liquid seismoscope consists of a circular trough
-    of wood with notches cut round its side. This is filled with mercury
-    to the level of the notches. At the time of an earthquake the maximum
-    quantity of mercury runs over the notches in the direction of greatest
-    motion. This instrument, which has long been used in Italy, is known as
-    a Cacciatore, being named after its inventor. It is a prominent feature
-    in the collection<span class="pagenum" id="Page_20">20</span>
-    of apparatus forming the well-known seismograph of Palmieri.</p>
-
-  <p><em>Pendulum instruments.</em>—Mallet speaks of pendulum seismoscopes and
-    seismographs as ‘the oldest probably of seismometers long set up in
-    Italy and southern Europe.’ In 1841 we find these being used to record
-    the earthquake disturbances at Comrie in Scotland.</p>
-
-  <p>These instruments may be divided into two classes: first, those which
-    at the time of the shock are intended to swing, and thus record the
-    direction of movement; and second, those which are supposed to remain
-    at rest and thus provide ‘steady points.’</p>
-
-  <p>To obtain an absolutely ‘steady point’ at the time of an earthquake,
-    has been one of the chief aims of all recent seismological
-    investigations.</p>
-
-  <p>With a style or pointer projecting down from the steady point to
-    a surface which is being moved backward and forward by the earth,
-    such a surface has written upon it by its own motions a record of
-    the ground to which it is attached. Conversely, a point projecting
-    upwards from the moving earth might be caused to write a record on the
-    body providing the steady point, which in the class of instruments
-    now to be referred to is supposed to be the bob of a pendulum. It is
-    not difficult to get a pendulum which will swing at the time of a
-    moderately strong earthquake, but it is somewhat difficult to obtain
-    one which will not swing at such a time. During the past few years,
-    pendulums varying between forty feet in length and carrying bobs of
-    eighty pounds in weight, and one-eighth of an inch in length, and
-    carrying a gun-shot, have been experimented with under a great variety
-    of circumstances. Sometimes the supports of these pendulums have been
-    as rigid as it is possible to make a structure from brick and mortar,
-    and at other times they have<span class="pagenum" id="Page_21">21</span>
-    intentionally been made loose and
-    flexible. The indices which wrote the motions of these pendulums have
-    been as various as the pendulums themselves. A small needle sliding
-    vertically through two small holes, and resting its lower end on a
-    surface of smoked glass, has on account of its small amount of friction
-    been perhaps one of the favourite forms of recording pointers.</p>
-
-  <p>The free pendulums which have been employed, and which were intended
-    to swing, have been used for two purposes: first, to determine the
-    direction of motion from the direction of swing, and second, to see if
-    an approximation to the period of the earth’s motion could be obtained
-    by discovering the pendulum amongst a series of different lengths which
-    was set in most violent motion, this probably being the one which had
-    its natural period of swing the most nearly approximating to the period
-    of the earthquake oscillations.</p>
-
-  <p>Inasmuch as all pendulums when swinging have a tendency to change the
-    plane of their oscillation, and also as we now know that the direction
-    of motion during an earthquake is not always constant, the results
-    usually obtained with these instruments respecting the direction of
-    the earth’s motion have been unsatisfactory. The results which were
-    obtained by series of pendulums of different lengths were, for various
-    reasons, also unsatisfactory.</p>
-
-  <p>Of pendulums intended to provide a steady point, from which the
-    relative motion of a point on the earth’s surface could be recorded,
-    there has been a great variety. One of the oldest forms consisted of
-    a pendulum with a style projecting downwards from the bob so as to
-    touch a bed of sand. Sometimes a concave surface was placed beneath the
-    pendulum, on which the record was traced by means of a pencil. Probably
-    the best form was that in which a<span class="pagenum" id="Page_22">22</span>
-    needle, capable of sliding freely up
-    and down, marked the relative horizontal motion of the earth and the
-    pendulum bob on a smoked glass plate.</p>
-
-  <p>It generally happens that at the time of a moderately severe earthquake
-    the whole of these forms of apparatus are set in motion, due partly to
-    the motion of the point of support of the pendulum, and partly to the
-    friction of the writing point on the plate.</p>
-
-  <p>Among these pendulums may be mentioned those of Cavallieri, Faura,
-    Palmieri, Rossi, and numerous others. It is possible that the
-    originators of some of these pendulums may have intended that
-    they should record by swinging. If this is so, then so far as the
-    determination of the actual nature of earthquake motion is concerned,
-    they belong to a lower grade of apparatus than that in which they are
-    here included.</p>
-
-  <p>A great improvement in pendulum apparatus is due to Mr. Thomas Gray
-    of Glasgow, who suggested applying so much frictional resistance to
-    the free swing of a pendulum that for small displacements it became
-    ‘dead beat.’ By carrying out this suggestion, pendulum instruments
-    were raised to the position of seismographs. The manner of applying
-    the friction will be understood from the following description of a
-    pendulum instrument which is also provided with an index which gives a
-    magnification of the motion of the earth.</p>
-
-  <p><span class="smcap">b b b b</span> is a box 113 cm. high and 30 cm. by 18 cm. square.
-    Inside this box a lead ring <span class="smcap">r</span>, 17 cm. in diameter and 3 cm.
-    thick, is suspended as a pendulum from the screw <span class="smcap">s</span>. This screw
-    passes through a small brass plate <span class="smcap">p p</span>, which can be moved
-    horizontally over a hole in the top of the box. These motions in the
-    point of suspension allow the pendulum to be adjusted.</p>
-
-  <div class="figright">
-    <img src="images/i_p023.jpg" width="300" height="539" alt="" />
-    <div class="caption"><span class="smcap">Fig. 2.</span></div>
-  </div>
-
-  <p>Projecting over the top of the pendulum there is a
-    <span class="pagenum" id="Page_23">23</span> wooden arm
-    <span class="smcap">w</span> carrying two sliding pointers <span class="smcap">h h</span>, resting
-    on a glass plate placed on the top of the pendulum. These pointers are
-    for the purpose of giving the frictional resistance before referred to.
-    If this friction plate is smoked, the friction pointers will write upon
-    it records of <em>large</em> earthquakes independently of the records given
-    by the proper index, which only gives satisfactory records in the case
-    of shocks of ordinary intensity. Crossing the inside of the pendulum
-    <span class="smcap">r</span> there is a brass bar perforated with a small conical hole at
-    <span class="smcap">m</span>. A stiff wire passes through <span class="smcap">m</span> and forms the upper
-    portion of the index <span class="smcap">i</span>, the lower portion of which is a thin
-    piece of bamboo. Fixed upon the wire there is a small brass ball which
-    rests on the upper side of a second brass plate also perforated with a
-    conical hole, which plate is fixed on the bar <span class="smcap">o o</span> crossing the
-    box.</p>
-
-  <p>If at the time of an earthquake the upper part of the index <span class="smcap">i</span>
-    remains steady at <span class="smcap">m</span>, then by the motion at <span class="smcap">o</span>, the
-    lower end of the index which carries a sliding needle at <span class="smcap">g</span>,
-    will magnify the motion of the earth in the ratios <span class="smcap">m o</span> :
-    <span class="smcap">o g</span>. In this instrument <span class="smcap">o g</span> is about 17 cm.</p>
-
-  <p>The needle <span class="smcap">g</span> works upon a piece of smoked glass. In
-    <span class="pagenum" id="Page_24">24</span> order to
-    bring the glass into contact with the needle without disturbance, the
-    glass is carried on a strip of wood <span class="smcap">k</span>, hinged at the back of
-    the box, and propped up in front by a loose block of wood <span class="smcap">y</span>.
-    When <span class="smcap">y</span> is removed the glass drops down with <span class="smcap">k</span> out of
-    contact with the needle. The box is carried on bars of wood
-    <span class="smcap">c c</span>, which are fixed to the ground by the stakes <span class="smcap">a</span>
-    <span class="smcap">a</span>.</p>
-
-  <p>The great advantage of a pendulum seismograph working on a stationary
-    plate is, that the record shows at once whether the direction of motion
-    has been constant, or whether it has been variable. The maximum extent
-    of motion in various directions is also easily obtained.</p>
-
-  <p>The disadvantage of the instrument is, that at the time of a large
-    earthquake, owing perhaps to a slight swing in the pendulum, the
-    records may be unduly magnified.</p>
-
-  <p>On such occasions, however, fairly good records may be obtained from
-    the friction pointers, provided that the plates on which they work have
-    been previously smoked. It might perhaps be well to use two of these
-    instruments, one having a comparatively high frictional resistance, and
-    hence ‘dead beat’ for large displacements.</p>
-
-  <p>Many attempts have been made to use a pendulum seismograph in
-    conjunction with a record-receiving surface, which at the time of the
-    earthquake should be kept in motion by clockwork. In this way it was
-    hoped to separate the various vibrations of the earthquake, and thus
-    avoid the greater or less confusion which occurs when the index of the
-    pendulum writes its backward and forward motion on a stationary plate.
-    Hitherto all attempts in this direction, in which a single multiplying
-    index was used, have been unsuccessful because of the moving plate
-    dragging the index in the direction of its motion for a
-    <span class="pagenum" id="Page_25">25</span> short
-    distance, and then allowing it to fall back towards its normal position.</p>
-
-  <p>In connection with this subject we may mention the pendulum
-    seismographs of Kreil, Wagener, Ewing, and Gray.</p>
-
-  <p>In the bob of Kreil’s pendulum there was clockwork, which caused a disc
-    on the axis of the pendulum to continuously rotate. On this continually
-    revolving surface a style fixed to the earth traced an unbroken circle.
-    At the time of an earthquake, by the motion of the style, the circle
-    was to be broken and lines drawn. The number and length of these lines
-    were to indicate the length and intensity of the disturbance.</p>
-
-  <p>Gray’s pendulum consisted of a flat heavy disc carrying on its upper
-    surface a smoked glass plate. This, which formed the bob of the
-    pendulum, was supported by a pianoforte steel wire. When set ready to
-    receive an earthquake, the wire was twisted and the bob held by a catch
-    so arranged that at the time of the earthquake the catch was released,
-    and the bob of the pendulum allowed to turn slowly by the untwisting of
-    the supporting wire. Resting on the surface of this rotating disc were
-    two multiplying indices arranged to write the earth’s motions as two
-    components.</p>
-
-  <p>In the instruments of Wagener and Ewing, the clockwork and moving
-    surface do not form part of the pendulum, but rest independently on
-    a support rigidly attached to the earth. In Wagener’s instrument one
-    index only is used, while in Ewing’s two are used for writing the
-    record of the motion.</p>
-
-  <p>A difficulty which is apparent in all pendulum machines is that when
-    the bob of such a pendulum is deflected it tends to fall back to its
-    normal position. To make a pendulum perfect it therefore requires
-    some compensating<span class="pagenum" id="Page_26">26</span>
-    arrangement, so that the pendulum, for small
-    displacements, shall be in neutral equilibrium, and the errors due to
-    swinging shall be avoided.</p>
-
-  <p>Several methods have been suggested for making the bob of an ordinary
-    pendulum astatic for small displacements. One method proposed by Gray
-    consists in fixing in the bob of a pendulum a circular trough of
-    liquid, the curvature of this trough having a proper form. Another
-    method which was suggested, was to attach a vertical spiral spring
-    to a point in the axis of the pendulum a little below the point of
-    suspension, and to a fixed point above it, so that when the pendulum is
-    deflected it would introduce a couple.</p>
-
-  <p>Professor Ewing has suggested an arrangement so that the bob of the
-    pendulum shall be partly suspended by a stretched spiral spring, and at
-    the same time shall be partly held up from below by a vertically placed
-    strut, the weight carried by the strut being to the weight carried by
-    the spring in the ratio of their respective lengths. As to how these
-    arrangements will act when carried into practice yet remains to be seen.</p>
-
-  <p>Another important class of instruments are <em>inverted pendulums</em>. These
-    are vertical springs made of metal or wood loaded at their upper end
-    with a heavy mass of metal. An arrangement of this sort, provided at
-    its upper end with a pencil to write on a concave surface, was employed
-    in 1841 to register the earthquakes at Comrie in Scotland. In Japan
-    they were largely employed in series, each member of a series having
-    a different period of vibration. The object of these arrangements
-    was to determine which of the pendulums, with a given earthquake,
-    recorded the greatest motion, it being assumed that the one which was
-    thrown into the most violent oscillation would be the one most nearly
-    approximating<span class="pagenum" id="Page_27">27</span>
-    with the period of the earthquake. The result of these
-    experiments showed that it was usually those with a slow period of
-    vibration which were the most disturbed.</p>
-
-  <p><em>Bracket Seismographs.</em>—A group of instruments of recent origin which
-    have done good work, are the bracket seismographs. These instruments
-    appear to have been independently invented by several investigators:
-    the germ from which they originated probably being the well-known
-    horizontal pendulum of Professor Zöllner. In Japan they were first
-    employed by Professor W. S. Chaplin. Subsequently they were used by
-    Professor Ewing and Mr. Gray. They consist essentially of a heavy
-    weight supported at the extremity of a horizontal bracket which is free
-    to turn on a vertical axis at its other end. When the frame carrying
-    this axis is moved in any direction excepting parallel to the length
-    of the gate-like bracket, the weight causes the bracket to turn round
-    a line known as the instantaneous axis of the bracket corresponding to
-    this motion of the fixed axis. Any point in this line may therefore be
-    taken as a steady point for motions at right angles to the length of
-    the supporting bracket. Two of these instruments placed at right angles
-    to each other have to be employed in conjunction, and the motion of
-    the ground is written down as two rectangular components. In Professor
-    Ewing’s form of the instrument, light prolongations of the brackets
-    form indices which give magnified representations of the motion, and
-    the weights are pivoted round a vertical axis through their centre.</p>
-
-  <p>In the accompanying sketch <span class="smcap">b</span> is a heavy weight pivoted at the
-    end of a small bracket <span class="smcap">c a k</span>, which bracket is free to turn
-    on a knife-edge, <span class="smcap">k</span>, above, and a pivot <span class="smcap">a</span>, below,
-    in the stand <span class="smcap">s</span>. At the time of an earthquake <span class="smcap">b</span>
-    remains steady, and the index <span class="smcap">p</span>, forming a continuation
-    <span class="pagenum" id="Page_28">28</span> of
-    the bracket, magnifies the motion of the stand, in the ratio of
-    <span class="smcap">a c</span> : <span class="smcap">c n</span>.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p028a.jpg" width="500" height="225" alt="" />
-    <div class="caption"><span class="smcap">Fig. 3.</span></div>
-  </div>
-
-  <p>In an instrument called a double-bracket seismograph, invented by Mr.
-    Gray, we have two brackets hinged to each other, and one of them to a
-    fixed frame. The planes of the two brackets are placed at right angles,
-    so as to give to a heavy mass supported at the end of the outer bracket
-    two degrees of horizontal freedom.</p>
-
-  <p>In all bracket machines, especially those which carry a pivoted weight,
-    it is doubtful whether the weight provides a truly steady point
-    relatively to the plate on which the record is written for motion
-    parallel to the direction of the arm.</p>
-
-  <div class="figleft">
-    <img src="images/i_p028b.jpg" width="266" height="175" alt="" />
-    <div class="caption"><span class="smcap">Fig. 4.</span></div>
-  </div>
-
-  <p><em>Parallel motion Instrument.</em>—A machine which writes its record as
-    two components, and which promises great stability,
-    is one suggested
-    by Professor C. D. West. Like the bracket machines it consists of two
-    similar parts placed at right angles to each other, and is as follows:
-    A bar of iron <span class="smcap">a</span> is suspended from both sides on pivots at
-    <span class="smcap">c c</span> by a system of light arms hinging with each other at
-    <span class="pagenum" id="Page_29">29</span> the
-    black dots, between the upper and lower parts of the rigid frame
-    <span class="smcap">b c</span>. The arms are of such a length that for small displacements
-    parallel to the length of the bar, <span class="smcap">c c</span> practically move
-    in a straight line, and the bar is in neutral equilibrium. A light
-    prolongation of the bar <i>d</i> works the upper end of the light index <i>e</i>,
-    passing as a universal joint through the rigid support <span class="smcap">f</span>. A
-    second index <i>e′</i> from the bar at right angles also passes through
-    <span class="smcap">f</span>. The multiplying ends of these indices are coupled together
-    to write a resultant motion on a smoked glass plate <span class="smcap">s</span>.</p>
-
-  <p><em>Conical Pendulums.</em>—Another group of instruments which have also
-    yielded valuable records are the conical pendulum seismographs. The
-    idea of using the bob of a conical pendulum to give a steady point in
-    an earthquake machine was first suggested and carried into practice by
-    Mr. Gray. The seismograph as employed consists of a pair of conical
-    pendulums hung in planes at right angles to each other. The bob of each
-    of these pendulums is fixed a short distance from the end of a light
-    lever, which forms the writing index, the short end resting as a strut
-    against the side of a post fixed in the earth. The weight is carried by
-    a thin wire or thread, the upper end of which is attached to a point
-    vertically above the fixed end of the lever.</p>
-
-  <p><em>Rolling Spheres and Cylinders.</em>—After the conical pendulum
-    seismographs, which claim several important advantages over the bracket
-    machines, we come to a group of instruments known as rolling sphere
-    seismographs. Here, again, we have a class of instruments for the
-    various forms of which we are indebted to the ingenuity of Mr. Gray.</p>
-
-  <p>The general arrangement and principle of one of these instruments will
-    be readily understood from the<span class="pagenum" id="Page_30">30</span>
-    accompanying figure. <span class="smcap">s</span> is a
-    segment of a large sphere with a centre near <span class="smcap">c</span>. Slightly below
-    this centre a heavy weight <span class="smcap">b</span>, which may be a lead ring, is
-    pivoted. At the time of an earthquake <span class="smcap">c</span> is steady, and the
-    earth’s motions are magnified by the pointer <span class="smcap">c a n</span> in the
-    proportion of <span class="smcap">c a</span> : <span class="smcap">a n</span>. The working of this pointer
-    or index is similar to that of the pointer in the pendulum.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p030.jpg" width="400" height="356" alt="" />
-    <div class="caption"><span class="smcap">Fig. 5.</span></div>
-  </div>
-
-  <p>Closely connected with the rolling sphere seismographs, are Gray’s
-    rolling cylinder seismographs.</p>
-
-  <p>These are two cylinders resting on a surface plate with their axes at
-    right angles to each other. Near to the highest point in each of these
-    cylinders, this point remaining nearly steady when the surface plate
-    is moved backwards and forwards, there is attached the end of a light
-    index. These indices are again pivoted a short distance from their ends
-    on axes connected with the surface plate. In order that the two indices
-    may be brought parallel, one is cranked at the second pivot.</p>
-
-  <p><span class="pagenum" id="Page_31">31</span></p>
-
-  <p><em>Ball and Plate Seismograph.</em>—Another form of seismograph, which is
-    closely related to the two forms of apparatus just described, is
-    Verbeck’s ball and plate seismograph. This consists of a surface plate
-    resting on three hard spheres, which in turn rests upon a second
-    surface plate. When the lower plate is moved, the upper one tends to
-    remain at rest, and thus may be used as a steady mass to move an index.</p>
-
-  <p><em>The Principle of Perry and Ayrton.</em>—An instrument which is of interest
-    from the scientific principle it involves is a seismograph suggested
-    by Professors Perry and Ayrton, who propose to support a heavy ball on
-    three springs, which shall be sufficiently stiff to have an exceedingly
-    quick period of vibration. By means of pencils attached to the ball by
-    levers, the motions of the ball are to be recorded on a moving band of
-    paper. The result would be a record compounded of the small vibrations
-    of the springs superimposed on the larger, slower, wave-like motions
-    of the earthquake, and, knowing the former of these, the latter might
-    be separated by analysis. Although our present knowledge of earthquake
-    motion indicates that the analysis of such a record would often present
-    us with insuperable difficulties, this instrument is worthy of notice
-    on account of the novelty of the principle it involves, which, the
-    authors truly remark, has in seismometry been a ‘neglected’ one.</p>
-
-  <p><em>Instruments to record Vertical Motion.</em>—The instruments which have
-    been devised to record vertical motion are almost as numerous as those
-    which have been devised to record horizontal motion. The earliest
-    form of instrument employed for this purpose was a spiral spring
-    stretched by weight, which, on account of its inertia, was supposed
-    at the time of a shock to remain steady. No satisfactory results have
-    ever been obtained<span class="pagenum" id="Page_32">32</span>
-    from such instruments, chiefly on account of the
-    inconvenience in making a spring sufficiently long to allow of enough
-    elongation to give a long period of vibration. Similar remarks may
-    be applied to the horizontally placed elastic rods, one end of which
-    is fixed to a wall, whilst the opposite end is loaded with a weight.
-    Such contrivances, furnished with pencil on the weight to write a
-    record upon a vertical surface, were used in 1842 at Comrie, and we
-    see the same principle applied in a portion of Palmieri’s apparatus.
-    Contrivances like these neither give us the true amplitude of the
-    vertical motion, insomuch as they are readily set in a state of
-    oscillation; nor do they indicate the duration of a disturbance, for,
-    being once set in motion, they continue that motion in virtue of their
-    inertia long after the actual earthquake has ceased. They can only be
-    regarded as seismoscopes.</p>
-
-  <div class="figleft">
-    <img src="images/i_p032.jpg" width="280" height="496" alt="" />
-    <div class="caption"><span class="smcap">Fig. 6.</span></div>
-  </div>
-
-  <p>The most satisfactory instrument which has yet been devised for
-    recording vertical motion is Gray’s horizontal lever spring seismograph.</p>
-
-  <p>This instrument will be better understood from the accompanying sketch.
-    A vertical spring <span class="smcap">s</span> is fixed at its upper end by means of a
-    nut <i>n</i>, which rests on the top of the frame <span class="smcap">f</span>, and serves to
-    raise or lower the spring through a short distance as a last adjustment
-    for<span class="pagenum" id="Page_33">33</span>
-    the position of the cross-arm <span class="smcap">a</span>. The arm <span class="smcap">a</span> rests
-    at one end on two sharp points, <i>p</i>, one resting in a conical hole
-    and the other in a <span class="smcap">v</span>-slot; it is supported at <span class="smcap">b</span> by
-    the spring <span class="smcap">s</span>, and is weighted at <span class="smcap">c</span> with a lead ring
-    <span class="smcap">r</span>. Over a pin at the point <span class="smcap">c</span> a stirrup of thread is
-    placed which supports a small trough, <i>t</i>. The trough <i>t</i> is pivoted at
-    <i>a</i>, has attached to it the index <i>i</i> (which is hinged by means of a
-    strip of tough paper at <i>h</i>, and rests through a fine pin on the glass
-    plate <i>g</i>), and is partly filled with mercury.</p>
-
-  <p>Another method of obtaining a steady point for vertical motion is that
-    of Dr. Wagener, who employs a buoy partly immersed in a vessel of
-    water. This was considerably improved upon by Mr. Gray, who suggested
-    the use of a buoy, which, with the exception of a long thin style, was
-    completely sunk.</p>
-
-  <p>Among the other forms of apparatus used to record vertical motion may
-    be mentioned vessels provided with india-rubber or other flexible
-    bottoms, and partially filled with water or some other liquid. As the
-    vessel is moved up and down, the bottom tends to remain behind and
-    provides a more or less steady point. Pivoted to this is a light index,
-    which is again pivoted to a rigid frame in connection with the earth.
-    Instruments of this description have yielded good records.</p>
-
-  <p><em>Record Receivers.</em>—A large number of earthquake machines having been
-    referred to, it now remains to consider the apparatus on which they
-    write their motions. The earlier forms of seismographs, as has already
-    been indicated, recorded their movements in a bed of sand; others
-    wrote their records by means of pencils on sheets of paper. Where we
-    have seismographs which magnify the motion of the earth, it will be
-    observed that methods like the above would involve great frictional
-    resistances,<span class="pagenum" id="Page_34">34</span>
-    tending to cause motion in the assumed steady points of
-    the seismographs. One of the most perfect instruments would be obtained
-    by registering photographically the motions of the recording index by
-    the reflection of a ray of light. Such an instrument would, however,
-    be difficult to construct and difficult to manipulate. One of the best
-    practical forms of registering apparatus is one in which the record is
-    written on a surface of smoked glass. This can afterwards be covered
-    with a coat of photographer’s varnish, and subsequently photographed by
-    the ‘blue process’ so well known to engineers.</p>
-
-  <p>To obtain a record of all the vibrations of an earthquake it is
-    necessary that the surface on which the seismograph writes should at
-    the time of an earthquake be in motion. Of record-receiving machines
-    there are three types. First, there are those which move continuously.
-    The common form of these is a circular glass plate like an old form of
-    chronograph, driven continuously by clockwork. On this the pointers of
-    the seismograph rest and trace over and over again the same circles.
-    At the time of an earthquake they move back and forth across the
-    circles, which are theoretically fine lines, and leave a record of
-    the earthquake. Instead of a circular plate, a drum covered with
-    smoked paper may be used, which, after the earthquake, possesses the
-    advantage, after unrolling, of presenting the record in a straight
-    line, instead of a record written round the periphery of a circle, as
-    is the case with the circular glass plates. Such records are easily
-    preserved, but they are more difficult to photograph.</p>
-
-  <p>The second form of apparatus is one which is set in motion at the
-    time of a shock. This may be a contrivance like one of those just
-    described, or a straight smoked glass plate on a carriage. By means
-    of an electrical<span class="pagenum" id="Page_35">35</span>
-    or a mechanical contrivance called a ‘starter,’ of
-    which many forms have been contrived, the earthquake is caused to
-    release a detent and thus set in motion the mechanism which moves the
-    record receiver.</p>
-
-  <p>The great advantage of continuously-moving machines is that the
-    beginning and end of the shock can usually be got with certainty, while
-    all the uncertainty as to the action of the ‘starter’ is avoided.
-    Self-starting machines have, of course, the advantage of simplicity and
-    cheapness, while there is no danger of the record getting obliterated
-    by the subsequent motion of the plate under the index.</p>
-
-  <p><em>Time-recording Apparatus.</em>—Of equal importance with the instruments
-    which record the motion of the ground, are those instruments which
-    record the time at which such motion took place. The great value of
-    time records, when determining the origin from which an earthquake
-    originates, will be shown farther on. The most important result which
-    is required in connection with time observations, is to determine
-    the interval of time taken by a disturbance in travelling from one
-    point to another. On account of the great velocity with which these
-    disturbances sometimes travel, it is necessary that these observations
-    should be made with considerable accuracy. The old methods of adapting
-    an apparatus to a clock which, when shaken, shall cause the clock to
-    stop, are of little value unless the stations at which the observations
-    are made are at considerable distances apart. This will be appreciated
-    when we remember that the disturbance may possibly travel at the rate
-    of a mile per second, that its duration at any station may often extend
-    over a minute, and that one set of apparatus at one station may stop,
-    perhaps, at the commencement of the disturbance, and the other near
-    the end. A satisfactory time-taking<span class="pagenum" id="Page_36">36</span>
-    apparatus will therefore require,
-    not only the means for stopping a clock, but also a contrivance which,
-    at the same instant that the clock is stopped, shall make a mark on a
-    record which is being drawn by a seismograph. In this way we find out
-    at which portion of the shock the time was taken.</p>
-
-  <div class="figleft">
-    <img src="images/i_p036.jpg" width="270" height="357" alt="" />
-    <div class="caption"><span class="smcap">Fig. 7.</span></div>
-  </div>
-
-  <p>Palmieri stops a clock in his seismograph by closing an electric
-    circuit. Mallet proposes to stop a clock by the falling of a column
-    which is attached by a string to the pendulum of the clock. So long as
-    the column is standing the string is loose and the pendulum is free
-    to move; but when the column falls, the string is tightened and the
-    pendulum is arrested. The difficulty which arises is to obtain a column
-    that will fall with a slight disturbance. The best form of contrivance
-    for causing a column to fall, and one which may also be used in drawing
-    out a catch to relieve the machinery of a record receiver, is shown in
-    the accompanying sketch.</p>
-
-  <p><span class="smcap">s</span> is the segment of a sphere about 4·5 cm. radius, with a
-    centre slightly above <span class="smcap">c</span>. <span class="smcap">l</span> is a disc of lead about 7
-    cm. in diameter resting upon the segment. Above this there is a light
-    pointer, <span class="smcap">p</span>, about 30 cm. long. On the top of the pointer a
-    small cylinder of iron, <span class="smcap">w</span>, is balanced, and connected by a
-    string with the catch to be relieved. When the table on which <span class="smcap">w p s</span>
-    rests is shaken, rotation takes place near to <span class="smcap">c</span>, the
-    motion of the base <span class="smcap">s</span> is magnified at the upper end of the
-    pointer, and the weight overturned. This catch may be used to relieve
-    a<span class="pagenum" id="Page_37">37</span>
-    toothed bar axled at one end, and held up above a pin projecting
-    from the face of the pendulum bob. When this falls it catches the
-    projecting pin and holds the pendulum.</p>
-
-  <p>Another way of relieving the toothed bar is to hold up the opposite
-    end to that at which it is axled by resting it on the extremity of a
-    horizontal wire fixed to the bob of a conical pendulum—for example, one
-    of the indices of a conical pendulum seismograph. The whole of this
-    apparatus, which may be constructed at the cost of a few pence, can be
-    made small enough to go inside an ordinary clock case.</p>
-
-  <p>The difficulty which arises with all these clock-stopping arrangements
-    is that it is difficult for observers situated at distant stations
-    to re-start their clocks so that their difference in time shall
-    be accurately known. Even if each observer is provided with a
-    well-regulated chronometer, with which he can make comparisons, the
-    rating of these instruments is for all ordinary persons an extremely
-    troublesome operation.</p>
-
-  <p>In order to avoid this difficulty the author has of late years used a
-    method of obtaining the time without stopping the clock. To do this a
-    clock with a central seconds hand is taken, and the hour and minute
-    hands are prolonged and bent out slightly at their extremities at right
-    angles to the face, the hour hand being slightly the longest. Each
-    hand is then tipped with a piece of soft material like cork, which is
-    smeared with a glycerine ink. A light flat ring, with divisions in it
-    corresponding to those on the face of the clock, is so arranged that
-    at the time of a shock it can be quickly advanced to touch the inked
-    pads on the hands of the clock and then withdrawn. This is accomplished
-    by suitable machinery, which is relieved either by an electro-magnet
-    or some other<span class="pagenum" id="Page_38">38</span>
-    contrivance which will withdraw a catch. In this way an
-    impression in the form of three dots is received on the disc, and the
-    time known without either stopping or sensibly retarding the clock.</p>
-
-  <p>For ordinary observers, if a time-taker is not used in conjunction
-    with a record receiver, as good results as those obtained by ordinary
-    clock-stopping apparatus are obtainable by glancing at an ordinary
-    watch. Subsequently the watch by which the observation was made should
-    be compared with some good time-keeper, and the local time at which the
-    shock took place is then approximately known.</p>
-
-  <p>From what has now been said it will be seen that for a complete
-    seismograph we require three distinct sets of apparatus—an apparatus to
-    record horizontal motion, an apparatus to record vertical motion, and
-    an apparatus to record time. The horizontal and vertical motions must
-    be written on the same receiver, and if possible side by side, whilst
-    the instant at which the time record is made a mark must be made on
-    the edge of the diagram which is being drawn by the seismograph. Such
-    a seismograph has been constructed and is now erected in Japan. It is
-    illustrated in the accompanying diagram.</p>
-
-  <p><em>The Gray and Milne Seismograph.</em>—In this apparatus two mutually
-    rectangular components of the horizontal motion of the earth are
-    recorded on a sheet of smoked paper wound round a drum, <span class="smcap">d</span>,
-    kept continuously in motion by clockwork, <span class="smcap">w</span>, by means of two
-    conical pendulum seismographs, <span class="smcap">c</span>. The vertical motion is
-    recorded on the same sheet of paper by means of a compensated-spring
-    seismograph, <span class="smcap">s l m b</span>.</p>
-
-  <p>The time of occurrence of an earthquake is determined by causing the
-    circuit of two electro-magnets to be closed by the shaking. One of
-    these magnets relieves a<span class="pagenum" id="Page_39">39</span>
-    mechanism, forming part of a time-keeper, which causes the dial of the
-    timepiece to come suddenly forwards on the hands and then move back
-    to its original position. The hands are provided with ink-pads, which
-    mark their positions on the dial, thus indicating the hour, minute, and
-    second when the circuit was closed. The second
-    <span class="pagenum" id="Page_40">40</span> electro-magnet causes a
-    pointer to make a mark on the paper receiving the record of the motion.
-    This mark indicates the part of the earthquake at which the circuit was
-    closed.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p039.jpg" width="625" height="699" alt="" />
-    <div class="caption"><span class="smcap">Fig. 8.</span></div>
-  </div>
-
-  <p>The duration of the earthquake is estimated from the length of the
-    record on the smoked paper and the rate of motion of the drum. The
-    nature and period of the different movements are obtained from the
-    curves drawn on the paper.</p>
-
-  <p>Mr. Gray has since greatly modified this apparatus, notably by the
-    introduction of a band of paper sufficiently long to take a record for
-    twenty-four hours without repetition. The record is written in ink by
-    means of fine siphons. In this way the instrument, which is extremely
-    sensitive to change of level, can be made to show not only earthquakes,
-    but the pulsations of long period which have recently occupied so much
-    attention.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_III">
-    <span class="pagenum" id="Page_41">41</span>
-    <h2>CHAPTER III.<br />
-      <span class="subhead">EARTHQUAKE MOTION DISCUSSED THEORETICALLY.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Ideas of the ancients (the views of Travagini, Hooke, Woodward,
-    Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves
-    and vibrations—Possible causes of disturbance in the Earth’s
-    crust—The time of vibration of an earth particle—Velocity and
-    acceleration of a particle—Propagation of a disturbance as
-    determined by experiments upon the elastic moduli of rocks—The
-    intensity of an earthquake—Area of greatest overturning
-    moment—Earthquake waves—Reflexion, refraction, and interference
-    of waves—Radiation of a disturbance.</div>
-
-  <p><em>Ideas of Early Writers.</em>—One of the first accounts of the varieties of
-    motion which may be experienced at the time of an earthquake is to be
-    found in the classification of earthquakes given by
-    Aristotle.<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a>
-    It is as follows:—</p>
-
-  <p>1. Epiclintæ, or earthquakes which move the ground obliquely.</p>
-
-  <p>2. Brastæ, with an upward vertical motion like boiling water.</p>
-
-  <p>3. Chasmatiæ, which cause the ground to sink and form hollows.</p>
-
-  <p>4. Rhectæ, which raise the ground and make fissures.</p>
-
-  <p>5. Ostæ, which overthrow with one thrust.</p>
-
-  <p>6. Palmatiæ, which shake from side to side with a sort of tremor.</p>
-
-  <p><span class="pagenum" id="Page_42">42</span></p>
-
-  <p>From the sixth group in this classification we see that this early
-    writer did not regard earthquakes as necessarily isolated events, but
-    that some of them consisted of a succession of backward and forward
-    vibratory motions. He also distinguishes between the total duration of
-    an earthquake and the length of, and intervals between, a series of
-    shocks. Aristotle had, in fact, some idea of what modern writers upon
-    ordinary earthquakes would term ‘modality.’</p>
-
-  <p>The earliest writer who had the idea that an earthquake was a
-    pulse-like motion propagated through solid ground appears to have been
-    Francisci Travagini, who, in 1679, wrote upon an earthquake which
-    in 1667 had overthrown Ragusa. The method in which the pulses were
-    propagated he illustrated by experiments.</p>
-
-  <p>Hooke, who, in 1690, delivered discourses on earthquakes before the
-    Royal Society, divides these phenomena according to the geological
-    effects they have produced; thus there is a <em>genus</em> producing
-    elevations, a <em>genus</em> producing sinkings, a <em>genus</em> producing
-    conversions and transportations, and a <em>genus</em> which produces what, in
-    modern language, we should term metamorphic action.</p>
-
-  <p>Woodward, in his ‘Natural History,’ written in 1695, speaks of
-    earthquakes as being agitations and concussions produced by water in
-    the interior of the earth coming in contact with internal fires.</p>
-
-  <p>Stukeley observed that an earthquake was ‘a tremor of the earth,’ to
-    be explained as a vibration in a solid. The Rev. John Mitchell, writing
-    in 1760, says that the motion of the earth in earthquakes is partly
-    tremulous and partly propagated by waves.</p>
-
-  <p>From these few examples, to which might be added many more, it will
-    be seen that an earthquake disturbance has usually been regarded as
-    a concussion, vibration, trembling, or undulatory movement. Further,
-    it can be<span class="pagenum" id="Page_43">43</span>
-    seen in narratives of earthquakes that it had been often
-    observed that these tremblings and shakings continued over a certain
-    period of time. Although it had been noticed that large areas were
-    almost simultaneously affected by these disturbances, no definite idea
-    appears to have existed as to how earthquake motion was propagated.
-    Usually it was assumed that the disturbance spread through subterranean
-    channels.</p>
-
-  <p>The first true conception of earthquake motion and the manner of its
-    propagation is due to Dr. Thomas Young, who suggested that earthquake
-    motion was vibratory, and it might be ‘propagated through the earth
-    nearly in the same manner as a noise is conveyed through the air.’ The
-    same idea was moulded into a more definite form by Gay Lussac.</p>
-
-  <p>The first accurate definition of an earthquake is due to Mr. Robert
-    Mallet, who, after collecting and examining many facts connected with
-    earthquake phenomena, and reasoning on these, with the help of known
-    laws connected with the production and propagation of waves of various
-    descriptions, formulated his views as follows:—</p>
-
-  <p>An earthquake is ‘<em>the transit of a wave or waves of elastic
-      compression in any direction from vertically upwards to horizontally,
-      in any azimuth, through the crust and surface of the earth, from any
-      centre of impulse or from more than one, and which may be attended
-      with sound and tidal waves, dependent upon the impulse and upon
-      circumstances of position as to sea and land</em>.’</p>
-
-  <p>In brief, so far as motion in the earth is concerned. Mallet defined
-    an earthquake as being a motion due to the transit of waves of elastic
-    compression. In many cases it is possible that this is strictly true,
-    but in succeeding pages it will be shown that earthquake motion may
-    also be due to the transit of waves of elastic distortion.</p>
-
-  <p><span class="pagenum" id="Page_44">44</span></p>
-
-  <p>To obtain a true idea of earthquake motion is a matter of cardinal
-    importance, as it forms the key-stone of many investigations.</p>
-
-  <p>If we know the nature of the motion produced by an earthquake, we are
-    aided in tracking it to its origin, and in reasoning as to how it was
-    produced. If our knowledge of the nature of the motion of an earthquake
-    is incorrect, it will be impossible for us intelligently to construct
-    buildings to withstand the effects of these disturbances. We have thus
-    to consider, in this portion of seismology, a point of great scientific
-    importance, and shall deal with it at some length.</p>
-
-  <p><em>Nature of Elastic Waves and Vibrations.</em>—When it is stated that an
-    earthquake consists of elastic waves of compression and distortion, the
-    student of physics has a clear idea of what is meant and a knowledge
-    of the mechanical laws which govern such disturbances. The ordinary
-    reader, however, and the majority of the inhabitants of earthquake
-    countries, who of all people have the greatest interest in this matter,
-    may not have so clear a conception, and it will, therefore, not be out
-    of place to give some general explanation on this point.</p>
-
-  <p>The ordinary idea of a wave is that it is a disturbance similar to
-    that which we often see in water. Waves like these must not, however,
-    be confounded with elastic waves. A disturbance produced in water,
-    say, for instance, by dropping a stone into a pond, is propagated
-    outwards by the action of gravity. First, a ridge of water is raised
-    up by the stone passing beneath the surface. As this ridge falls
-    towards its normal position in virtue of its weight, it raises a second
-    ridge. This second ridge raises a third ridge, and so on. The water
-    moves vertically up and down, whilst the wave itself is propagated
-    horizontally.</p>
-
-  <p>To understand what is meant by elastic waves, it is
-    <span class="pagenum" id="Page_45">45</span> first necessary
-    to understand what is meant by the term elastic. In popular language
-    the term elastic is confined to substances like india-rubber, and but
-    seldom to rock-like materials, through which earthquake waves are
-    propagated. India-rubber is called elastic because after we remove a
-    compressive force it has a tendency to spring back to its original
-    shape. The elastic force of the india-rubber is in this case the force
-    which causes it to resist a change of form. Now, a piece of rock may,
-    up to a certain point, like the india-rubber, be compressed, and when
-    the compressing force is removed it will also tend to resume its
-    original form. However, as the rock offers more resistance to the
-    compressing force than the india-rubber offers, we say that it is the
-    more elastic. It may be here observed that a substance like granite
-    offers great resistance, not only to compression or a change of volume,
-    but also to a change of form or shape; whereas a substance like air,
-    which is also elastic, only offers resistance to compression, but not
-    to a change of shape.</p>
-
-  <p>With these ideas before us we will now proceed to consider how, after
-    a body has been suddenly compressed or distorted, this disturbance
-    is propagated through the mass. For the elastic body let us take a
-    long spiral spring hung from the ceiling of a room and kept slightly
-    stretched by a weight. If we give this weight an upward tap from below,
-    say with a hammer, we shall observe a pulse-like wave which runs up the
-    spring until it reaches the ceiling of the room. Here it will, so to
-    speak, rebound, like a billiard ball from the end of a table, and run
-    towards the weight from which it started. Whilst this is going on we
-    may also observe that the weight is moving up and down.</p>
-
-  <p>Here, then, we have two distinct things to observe—one being the
-    transmission of motion up to the ceiling,
-    <span class="pagenum" id="Page_46">46</span> which we may liken to the
-    transmission of an earthquake wave between two distant localities on
-    the earth’s surface, and the other being the up and down motion of our
-    weight, which we may compare to the backward and forward swinging which
-    we experience at the time of an earthquake.</p>
-
-  <p>These two motions—namely, the pulse-like wave produced by the
-    transmission of motion, and the backward and forward oscillation of the
-    weight or of any point on the spring—must be carefully distinguished
-    from each other.</p>
-
-  <p>First, we will consider the backward and forward motion of the weight.
-    The distance through which the weight moves depends upon the force
-    of the blow. The number of up and down oscillations it makes, say
-    in a second, depends upon the stiffness of the spring. The weight,
-    supposing it to be always the same, will move more quickly at the end
-    of a stiff spring than at the end of a flaccid one; that is to say, its
-    velocity is quicker. As in any given spring the number of up and down
-    oscillations are always the same in a given interval of time, if these
-    oscillations are of great extent, the weight must move more quickly
-    with large than with small oscillations.</p>
-
-  <p>At the time of an earthquake the manner in which we are moved backward
-    and forward is very similar to the manner in which the weight is moved.
-    If we stand on a hard rock-like granite, we are to a great extent
-    placed as if we were attached to a stiff quickly-vibrating spring.
-    If, however, we are on a soft rock, it is more like being on a loose
-    flaccid spring.</p>
-
-  <p>All that has thus far been considered has been a backward and forward
-    kind of motion, where there is a rectilinear <em>compression</em> and
-    <em>extension</em> amongst the particles on which we stand.</p>
-
-  <p>We might, however, imagine our rock, which for the
-    <span class="pagenum" id="Page_47">47</span> moment we will
-    consider to be a square column, to be twisted, and thus have its
-    <em>shape</em> altered. When the twisting force is taken off it seems evident
-    that the column would endeavour to untwist itself or regain its
-    original form. Now the force which a body offers against a change of
-    <em>volume</em> may be very different from that which it offers against a
-    change of <em>form</em>.</p>
-
-  <p>In disturbances which take place in the rocky crust of our earth,
-    it would seem possible that we may have vibrations set up which are
-    either compressions and extensions or twistings and distortions. These
-    may take place separately or simultaneously, or we may have resultant
-    motions due to their combination.</p>
-
-  <p>The following are examples of possible causes which might give rise to
-    these different orders of disturbance:—</p>
-
-  <p>1. Imagine a large area stretched by elevation until it reaches the
-    limit of its elasticity and cracks. After cracking, in consequence
-    of its elasticity, it will fly back over the whole area like a
-    broken spring, and each point in the area will oscillate round its
-    new position of equilibrium. In this case there will be no waves
-    of distortion excepting near the end of the crack, where waves are
-    transmitted in a direction parallel to the fissure.</p>
-
-  <p>2. The ground is broken and slips either up, down, or sideways, as
-    we see to have taken place in the production of faults. Here we get
-    distortion in the direction of the movement, and waves are produced
-    by the elastic force of the rock, causing it to spring back from its
-    distorted form. In a case like this the production of a fissure running
-    north and south might give rise to north and south vibrations, which
-    would be propagated end on towards the north and south, but broadside
-    on towards the east and west. With disturbances of this kind, on
-    account of the want of homogeneousness in the materials
-    <span class="pagenum" id="Page_48">48</span> in which
-    they are produced, we should expect to find waves of compression and
-    extension.</p>
-
-  <p>3. A truly spherical cavity is suddenly formed by the explosion of
-    steam in the midst of an elastic medium. In this case all the waves
-    will be those of compression, each particle moving backward and forward
-    along a radius.</p>
-
-  <p>Should the cavity, instead of being truly spherical, be irregular, it
-    is evident that, in addition to the normal vibration of compression,
-    transverse waves of distortion will be more or less pronounced,
-    depending upon the nature of the cavity.</p>
-
-  <p>The combination of these two sets of vibrations may cause a point in
-    the earth to move in a circle, an ellipse, the form of a figure eight,
-    and in other curves similar to these, which are produced by apparatus
-    designed to show the combination of harmonic motion. From these
-    examples it will be seen that we have therefore to consider two kinds
-    of vibrations—one produced by compression or the alteration of volume,
-    and the other produced by an alteration in shape.</p>
-
-  <p>Now the resistance which a body offers, either to a change in its
-    volume or in its shape, is called its elasticity, and the law which
-    governs the backward and forward motion of a particle under the
-    influence of this elasticity may be expressed as follows:</p>
-
-  <p>If <span class="smcap">t</span> be the time of vibration, or the time taken by a particle
-    to make one complete backward and forward swing, <span class="smcap">d</span> the density
-    of the material of which this particle forms a part, and <span class="smcap">e</span> the
-    proper modulus of elasticity of the material, then,</p>
-
-  <div class="center">
-    <span class="smcap">t</span>&nbsp;=&nbsp;2π&nbsp;<span class="x200a">√</span><span class="sqrt2"><span class="frac"><sup><span class="smcap">d</span></sup><span>/</span><sub><span class="smcap">e</span></sub></span></span>
-  </div>
-
-  <p>From this formula,
-    <span class="smcap">t</span>&nbsp;=&nbsp;2π&nbsp;<span class="x200a">√</span><span class="sqrt2"><span class="frac"><sup><span class="smcap">d</span></sup><span>/</span><sub><span class="smcap">e</span></sub></span></span>,
-    we see that the<span class="pagenum" id="Page_49">49</span>
-    time of vibration of the earth during an earthquake,
-    or the rate at which we are shaken backwards and forwards, varies
-    directly as the square root of the density of the material on which we
-    stand, and inversely as the square root of a number proportional to its
-    elasticity.</p>
-
-  <p><em>Velocity and Acceleration of an Earth Particle.</em>—Another important
-    point, which the practical seismologist has often brought to
-    his notice, is the question of the velocity with which an earth
-    particle moves. According to the formula,
-    <span class="smcap">t</span>&nbsp;=&nbsp;2π&nbsp;<span class="x200a">√</span><span class="sqrt2"><span class="frac"><sup><span class="smcap">d</span></sup><span>/</span><sub><span class="smcap">e</span></sub></span></span>,
-    we should expect that a particle would
-    make each semi-vibration in an equal time, and from a knowledge of the
-    density and elastic moduli of a body this time might be calculated.
-    Although the time of a semi-oscillation may be constant, we must bear
-    in mind that, like the bob of a pendulum during each of its swings, the
-    particle starts from rest, increases in velocity until it reaches the
-    middle portion of its half swing, from which it gradually decreases in
-    speed until it reaches zero, when it again commences a similar motion
-    in the opposite direction.</p>
-
-  <p>These pendulum-like vibrations are sometimes spoken of as simple
-    harmonic motions. If we know the distance through which an earthquake
-    moves in making a single swing, and the time taken in making this
-    swing, on the assumption that the motion is simple harmonic we can
-    easily calculate the maximum velocity with which the particle moves.</p>
-
-  <p>Thus, if an earth particle takes one second to complete a
-    semi-oscillation, half of which, or the amplitude of the motion, equals
-    <i>a</i>, the maximum velocity equals <i>π</i> × <i>a</i>.</p>
-
-  <p>Again, assuming the earth vibrations to be simple harmonic, the maximum
-    acceleration or rate of change in<span class="pagenum" id="Page_50">50</span>
-    velocity will come about at the ends
-    of each semi-oscillation; and if <span class="smcap">v</span> be the maximum velocity of
-    the particle, and <i>a</i> the amplitude or half semi-oscillation, then the
-    maximum acceleration equals
-    <span class="frac"><sup><span class="smcap">v</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>.</p>
-
-  <p>Later on it will be shown, as the result of experiment, that certain of
-    the more important earth oscillations in an earthquake are not simple
-    harmonic motion. Nevertheless the above remarks will be of assistance
-    in showing how the velocity and other elements connected with the
-    motion of an earth particle, which are required by the practical
-    seismologist, may be calculated, irrespective of assumptions as to the
-    nature of the motion.</p>
-
-  <p><em>Propagation of a Disturbance.</em>—We may next consider the manner in
-    which a disturbance, in which there are both vibrations of compression
-    and of distortion, is propagated. The first or normal set of vibrations
-    are propagated in a manner similar to that in which sound vibrations
-    are propagated. From a centre of disturbance these movements approach
-    an observer at a distant station, so to speak, end on. The other
-    vibrations have a direction of motion similar to that which we believe
-    to exist in a ray of light. These would approach the observer broadside
-    on.</p>
-
-  <p>If the disturbance passed through a formation like a series of
-    perfectly laminated slates, each of these two sets of vibration might
-    be subdivided, and we should then obtain what Mallet has termed
-    ordinary and extraordinary normal and transverse vibrations.</p>
-
-  <p>In consequence of the difference in the elastic forces on which the
-    propagation of these two kinds of vibration depends, the normal
-    vibrations are transmitted faster than the transversal ones—that is
-    to say, if an earthquake originated from a blow, the first thing that
-    would be<span class="pagenum" id="Page_51">51</span>
-    felt at a point distant from the origin of the shock would be
-    a backward and forward motion in the direction to and from the origin,
-    and then, a short interval afterwards, a motion transversal, or at
-    right angles to this, would be experienced.</p>
-
-  <p>From the mathematical theory of vibratory motions it is possible to
-    calculate the velocity with which a disturbance is propagated. As the
-    result of these investigations it has been shown that normal vibrations
-    travel more quickly than transverse vibrations.</p>
-
-  <p>Deductions from experiments on small specimens are, however,
-    invalidated by the fact that the specimens used for experiments are,
-    of course, nearly homogeneous, whilst the earthquake passes through
-    a mass which is heterogeneous and more or less fissured. Mallet, by
-    experiments ‘on the compressibility of solid cubes of these rocks,
-    obtained the mean modulus of elasticity,’ with the result that ‘nearly
-    seven-eighths of the full velocity of wave-transit due to the material,
-    if solid and continuous, is lost by reason of the heterogeneity and
-    discontinuity of the rocky masses as they are found piled together in
-    nature.’ The full velocities of wave-transit, as calculated by Mallet
-    from a theorem given by Poisson, were—</p>
-
-  <blockquote>
-    <p class="noindent">
-      For slate and quartz transverse to lamination, 9,691 feet per second.<br />
-      &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
-      &nbsp;„&nbsp; &nbsp; &nbsp;in line of lamination,&nbsp; &nbsp; &nbsp; &nbsp;5,415
-      &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „
-    </p>
-  </blockquote>
-
-  <p>This more rapid transmission in a direction transverse to the
-    lamination, Mr. Mallet observes, may be more than counterbalanced by
-    the discontinuity of the mass transverse to the same direction.</p>
-
-  <p><em>The Intensity of an Earthquake.</em>—The intensity of an earthquake
-    is best estimated by the intensity of the forces which are brought
-    to bear on bodies placed on the earth’s surface. These forces are
-    evidently proportional<span class="pagenum" id="Page_52">52</span>
-    to the rate of change of velocity in the body,
-    and, as the destructive effect will be proportional to the maximum
-    forces, we may consistently indicate the intensity of an earthquake by
-    giving the maximum acceleration to which bodies were subject during
-    the disturbance. On the assumption that the motion of a point on
-    the earth’s surface is simple harmonic, the maximum acceleration is
-    directly as the maximum velocity and inversely as the amplitude of
-    motion, or as
-    <span class="frac"><sup><i>v</i><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>
-    where <i>v</i> indicates velocity and <i>a</i> amplitude.</p>
-
-  <p>The next question of importance is to determine the manner in which
-    earthquake energy becomes dissipated—that is, to compare together
-    the intensity of an earthquake as recorded at two or more points at
-    different distances from the origin. First let us imagine the origin
-    of our earthquake to be surrounded by concentric shells, each of
-    which is the breadth of the vibration of a particle. Going outwards
-    from the centre, each successive shell will contain a greater number
-    of particles, this number increasing directly as the square of the
-    distance from the origin. Let the blow have its origin at the centre,
-    and give a vibratory movement to the particles in one of the shells
-    near the centre.</p>
-
-  <p>This shell may be supposed to possess a certain amount of energy,
-    which will be measured by its mass and the square of the velocity of
-    its particles. In transferring this energy to the neighbouring shell
-    which surrounds it, because it has to set in motion a greater number
-    of particles than it contains itself, the energy in any one particle
-    of the second layer will be less than the energy in any one particle
-    in the first layer; the total energy in the second shell, however,
-    will be equal to the total energy in the first shell. Neglecting the
-    energy lost during the transfer, if<span class="pagenum" id="Page_53">53</span>
-    the energy in a particle of the
-    first shell at any particular phase of the motion be <span class="smcap">k</span><sub>1</sub>,
-    and the energy in a particle of the second shell <span class="smcap">k</span><sub>2</sub>, these
-    quantities are to each other inversely as the masses of the shells—that
-    is, inversely as the squares of the mean radii of the shells.</p>
-
-  <p>In symbols,
-    <span class="icenter"><span class="frac"><sup><span class="smcap">k</span><sub>2</sub></sup><span>/</span><sub><span class="smcap">k</span><sub>1</sub></sub></span>
-    &nbsp;=&nbsp; <span class="frac"><sup><i>r</i><sub>1</sub><sup>2</sup></sup><span>/</span><sub><i>r</i><sub>2</sub><sup>2</sup></sub></span></span>
-    <span class="right">(1)</span>
-  </p>
-
-  <p>Assuming that energy is dissipated,</p>
-
-  <div class="center">
-    <span class="frac"><sup><span class="smcap">k</span><sub>2</sub></sup><span>/</span><sub><span class="smcap">k</span><sub>1</sub></sub></span>
-    &nbsp;&gt;&nbsp; <span class="frac"><sup><i>r</i><sub>1</sub><sup>2</sup></sup><span>/</span><sub><i>r</i><sub>2</sub><sup>2</sup></sub></span>
-    &nbsp;=&nbsp; <i>f</i>&nbsp; <span class="frac"><sup><i>r</i><sub>1</sub><sup>2</sup></sup><span>/</span><sub><i>r</i><sub>2</sub><sup>2</sup></sub></span>
-    <span class="right">(2)</span>
-  </div>
-
-  <p class="noindent">where <i>f</i> &lt; 1 is the rate of dissipation of energy which is assumed to
-    be constant.</p>
-
-  <p><em>Area of greatest Overturning Moment.</em>—Although the rate of dissipation
-    of the impulsive effects of an earthquake may follow a law like that
-    just enumerated, it must be remembered that if the depth of the origin
-    is comparable with the radius of the area which is shaken, the maximum
-    impulsive effect as exhibited by the actual destruction on the surface
-    may not be immediately above the origin where buildings have simply
-    been lifted vertically up and down, but at some distance from this
-    point, where the impulsive effort has been more oblique.</p>
-
-  <p>At the <em>epicentrum</em> we have the maximum of the true intensity as
-    measured by the acceleration of a particle, or the height to which
-    a body might be projected, but it will be at some distance from
-    this where we shall have the maximum intensity as exhibited by an
-    overturning effort.</p>
-
-  <p>This will be rendered clear by the following diagram.</p>
-
-  <p>In the accompanying diagram let <span class="smcap">o</span> be the origin of a shock,
-    and <span class="smcap">o c</span> the seismic vertical equal to <i>r</i>. Let the direct or
-    normal shock emerge at <span class="smcap">c</span>, <span class="smcap">c</span><sub>1</sub>, <span class="smcap">c</span><sub>2</sub>, and
-    <span class="pagenum" id="Page_54">54</span>
-    at the angles <i>θ</i><sub>1</sub>, <i>θ</i><sub>2</sub>, &amp;c.</p>
-
-  <p>Assuming that the displacement of an earth particle at <span class="smcap">c</span>
-    equals <span class="smcap">c b</span>, and at <span class="smcap">c</span><sub>1</sub> equals <i>c</i><sub>1</sub> <i>b</i><sub>1</sub>,
-    and at <span class="smcap">c</span><sub>2</sub> equals <i>c</i><sub>2</sub> <i>b</i><sub>2</sub>, &amp;c., and let these
-    displacements <span class="smcap">c b</span>, <i>c</i><sub>1</sub> <i>b</i><sub>1</sub>, <i>c</i><sub>2</sub> <i>b</i><sub>2</sub>, &amp;c., for
-    the sake of argument, vary inversely as <i>r</i>, <i>r</i><sub>1</sub>, <i>r</i><sub>2</sub>, &amp;c.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p054.jpg" width="460" height="164" alt="" />
-    <div class="caption"><span class="smcap">Fig. 9.</span></div>
-  </div>
-
-  <p>The question is to determine where the horizontal component <span class="smcap">c a</span>
-    of these normal motions is a maximum.</p>
-
-  <p>First observe that the triangle <span class="smcap">o c</span> <i>c</i> is similar to <i>a</i>,
-    <i>b</i>, <i>c</i>.</p>
-
-  <p>Also <i>r</i> = <span class="frac"><sup><i>h</i></sup><span>/</span><sub>sin <i>θ</i></sub></span>,
-    and therefore the normal component <i>c</i><sub>1</sub>
-    <i>b</i><sub>1</sub> at <span class="smcap">c</span><sub>1</sub> is equal to
-    <span class="smcap">c</span> <span class="frac"><sup>sin <i>θ</i></sup><span>/</span><sub><i>h</i></sub></span>.</p>
-
-  <p>Also <i>c</i><sub>1</sub> <i>a</i><sub>1</sub> = <i>c</i><sub>1</sub> <i>b</i><sub>1</sub>, cos <i>θ</i>.</p>
-
-  <div class="center">
-    ∴ <span class="smcap">c</span><sub>1</sub> <i>a</i> = <span class="smcap">c</span>
-    <span class="frac"><sup>sin <i>θ</i> cos <i>θ</i></sup><span>/</span><sub><i>h</i></sub></span>
-    = <span class="frac"><sup><i>c</i></sup><span>/</span><sub><i>h</i></sub></span>
-    ∙ <span class="frac"><sup>sin 2<i>θ</i></sup><span>/</span><sub>2</sub></span>,
-  </div>
-
-  <p class="noindent">and sin 2<i>θ</i> is greatest when 2<i>θ</i> = 90° or <i>θ</i> = 45°.</p>
-
-  <p>That is to say, the horizontal component reaches a maximum where the
-    angle of emergence equals 45°.</p>
-
-  <p>This question has been discussed on the assumption that the amplitude
-    of an earth particle varies inversely as its distance from the origin
-    of the shock. Should we, however, assume that this amplitude varies
-    inversely as the square of the distance from the origin, we are led to
-    the result that the area of greatest disturbance is nearer to the point
-    where the angle of emergence is 55° 44′ 9″.
-    <span class="pagenum" id="Page_55">55</span> Both of these methods are
-    referred to by Mallet, but the first is considered as probably the more
-    correct.</p>
-
-  <p><em>Earthquake Waves.</em>—Hitherto we have chiefly considered earthquake
-    vibrations; now we will say a few words about earthquake waves. If
-    we strike a long iron rod at one end, we can imagine that, as in the
-    long spring, a pulse-like motion is transmitted. If the rod be struck
-    quickly, the pulses will rapidly succeed each other, and if struck
-    slowly the pulses will be at longer intervals. Each individual pulse,
-    however, will travel along the rod at the same rate, and hence the
-    distance between any two will remain constant; but that distance will
-    depend on the interval between the blows producing these pulses being
-    equal to the distance travelled by one pulse before the next blow is
-    struck.</p>
-
-  <p>From this we see that an irregular disturbance will produce an
-    irregular succession of motions; some will be like long undulations in
-    a wide deep ocean, whilst others will be like ripples in a shallow bay.
-    Again, consider the bar to be struck one blow only, and then left to
-    itself. The bar will propagate a series of pulses along its length, due
-    to the out and in vibration of its end. These will succeed each other
-    at regular intervals, and will be mixed up with the pulses we have
-    previously considered.</p>
-
-  <p>From this we see that in an earthquake, if it be produced by one blow,
-    the motion will be isochronous in its character; but if it be due to
-    a succession of blows at regular intervals, the motion will be the
-    resultant of a series of isochronous motions, and will be periodical.
-    If the impulses are irregular, you have a motion which is the resultant
-    of a number of isochronous motions due to each impulse, but these
-    compounded together in a different manner at each instant during the
-    earthquake, and giving as a result a motion which is in no sense
-    isochronous.<span class="pagenum" id="Page_56">56</span>
-    This approaches more nearly to the actual motions we feel
-    as earthquakes.</p>
-
-  <p>If we can imagine the ground shaken by an earthquake, made of a
-    transparent material which transmitted less light when compressed,
-    and we could look down upon a long extent of this at the time of an
-    earthquake, we should see a series of dark bands indicating strips of
-    country which were compressed. The distances between these bands might
-    be irregular. Keeping our attention on one particular band, this would
-    be seen to travel forward in a direction from the source. If we kept
-    our eye on one particular point, it would appear to open and shut,
-    becoming light and dark alternately.</p>
-
-  <p>As to the existence of these elastic waves in actual earthquakes we
-    have no direct experimental evidence. The only kind of wave with which
-    we are familiar is a true surface undulation, which, although having
-    the appearance of a water-wave, may nevertheless represent a district
-    of compression.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_IV">
-    <span class="pagenum" id="Page_57">57</span>
-    <h2>CHAPTER IV.<br />
-      <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Experiments with falling weights—Experiments with
-    explosives—Results obtained from experiments—Relative motion
-    of two adjacent points—The effect of hills and excavations
-    upon the propagation of vibrations—The intensity of artificial
-    disturbances—Velocity with which earth vibrations are
-    propagated—Experiments of Mallet—Experiments of Abbot—Experiments
-    in Japan—Mallet’s results—Abbot’s results—Results obtained in
-    Japan.</div>
-
-  <p><em>Experiments with Falling Weights.</em>—A series of experiments, as the
-    nature of the disturbance produced in the surface of the earth when
-    a heavy weight is allowed to fall on it, was begun in November 1880
-    by Mr. T. Gray and the author. These experiments were carried out at
-    the Akabane Engineering Works in Tokio. The weight used was a ball
-    of iron weighing about a ton, which in the different experiments was
-    allowed to fall from heights varying between ten and thirty-five feet.
-    The position of the place where the ball was allowed to fall was such
-    that in one direction the vibrations were transmitted up the side of
-    a steep hill, in another direction across a pond with perpendicular
-    sides, and in another direction across a level plain the material of
-    which consisted for the most part of hardened mud extending to a very
-    considerable depth. The vibrations produced by the fall of the ball
-    were transmitted through this hard mud with considerable intensity to a
-    distance of between 300 and 400 feet.</p>
-
-  <p><span class="pagenum" id="Page_58">58</span></p>
-
-  <p>The object of the experiment was to find the nature of the vibrations
-    produced in the crust of the earth by such a blow, the velocity of
-    transmission through this comparatively soft material, the effect of
-    hills and excavations in cutting off such disturbances, and the law
-    according to which the amplitude of the vibrations diminishes with the
-    distance from the source.</p>
-
-  <p>A considerable variety of apparatus was used during these experiments,
-    but the most reliable results were obtained from the records of a
-    rolling sphere seismograph, which wrote the vibrations on a stationary
-    plate, and from the records of two bracket seismographs, similar to
-    Professor Ewing’s horizontal lever seismographs, which gave a record
-    of the vibrations as two rectangular components on a moving plate of
-    smoked glass.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p058.jpg" width="514" height="283" alt="" />
-    <div class="caption"><span class="smcap">Fig. 10.</span></div>
-  </div>
-
-  <p>The general result as to the nature of the disturbance was that two
-    distinct sets of vibrations were set up by the blow. In one set the
-    direction of motion was along a line joining the point of observation
-    with the point from which the disturbance emanated; in the other set
-    the<span class="pagenum" id="Page_59">59</span>
-    direction of motion was at right angles to that line. The nature
-    of the resultant motion will be gathered from fig. 10, which is taken
-    from the records drawn by the rolling sphere seismograph at a distance
-    of 50 feet, 100 feet, and 200 feet respectively from the point where
-    the ball struck the ground. The direct or normal vibrations reached
-    the instrument first, and were followed at an interval depending on
-    the distance of the instrument from the origin by the transverse
-    vibrations. From the records of these two sets of vibrations as
-    separated by the bracket seismographs, combined with the known rate of
-    motion of the glass plate, the velocity of transmission was found to
-    be, for normal vibrations 446–438 feet per second, and for transverse
-    vibrations 357–353 feet per second.</p>
-
-  <p>The effect of the hill in cutting off the disturbance seemed to be
-    slight, but the direction of the vibrations which ascended the side
-    was mostly transverse. The pond, on the other hand, seemed completely
-    to cut off the disturbance, which, however, gradually crept round the
-    side, so that only a comparatively small triangular area was in shadow.</p>
-
-  <p>The amplitude of the vibrations diminished directly as the distance
-    increased for some distance from the origin, but at greater distance
-    the rate of diminution seemed to be slower. The transverse vibration
-    seemed to die out less quickly than the normal vibrations.<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a></p>
-
-  <p>These experiments were afterwards very considerably extended by the
-    author. In these later experiments charges of from one to two pounds of
-    dynamite were placed in bore-holes of various depths and exploded by
-    means of electricity. The results obtained confirmed the conclusions
-    already arrived at from the former experiments. The experiments on
-    velocity, however, seemed to indicate <span class="pagenum" id="Page_60">60</span>
-    that the higher the initial
-    impulse the greater was the velocity. The velocity of propagation of
-    the transverse vibrations seemed to approach more and more to that of
-    the directed vibrations as the distance from the origin of disturbance
-    increased. Fig. 11 shows the nature of the record obtained from the
-    explosion of two pounds of dynamite at the bottom of a bore-hole eight
-    feet deep. These records show the interval of time which elapsed
-    between the arrival of the normal and the transverse vibrations at
-    points distant 100, 250, and 400 feet from the bore-hole. In the case
-    of the 100-feet station it will be observed that the motion towards the
-    origin is greater than that from the origin. It is also to be noticed
-    that the period of vibration becomes greater as the distance from the
-    origin increases.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p060.jpg" width="563" height="458" alt="" />
-    <div class="caption"><span class="smcap">Fig. 11.</span>—Records obtained at three stations
-    of the motion of the ground produced by the explosion of 2 lbs. of
-    dynamite.</div>
-  </div>
-
-  <p><span class="pagenum" id="Page_61">61</span></p>
-
-  <p><em>The Intensity of Artificial Disturbances.</em>—The data which we have at
-    our disposal for determining the intensity of an earth particle which
-    has been caused to vibrate by the explosion of a charge of dynamite are
-    a series of records similar to that given on <a href="#Page_60">p. 60</a>. These disturbances
-    are practically surface movements, and may be compared with the
-    movements of an earthquake which spreads over an area the radius of
-    which is great as compared with its depth.</p>
-
-  <p>To find the mean acceleration of an earth particle, which quantity
-    has been taken to represent intensity, during any simple backward or
-    forward motion of the earth, it will be first necessary to determine
-    the amplitude of this motion and its maximum velocity, the mean
-    acceleration being equal to
-    <span class="frac"><sup><span class="smcap">v</span><sup>2</sup></sup><span>/</span><sub>2<span class="smcap">a</span></sub></span>.</p>
-
-  <div class="figright">
-    <img src="images/i_p061.jpg" width="349" height="265" alt="" />
-    <div class="caption"><span class="smcap">Fig. 12.</span></div>
-  </div>
-
-  <p>The second and third movements in a shock invariably exhibited the
-    greatest intensity, and to a distance of 400 feet from the origin,
-    where about three pounds of dynamite had been exploded in a bore-hole
-    about six feet deep, these intensities decreased directly as the
-    distance from the origin. The less intense movements also decreased
-    directly as the distance from the origin to a certain point, but after
-    that they decreased more slowly. A mean result of the more prominent
-    vibrations in four sets of experiments is shown in the curve, fig. 12,
-    where the horizontal measurements
-    <span class="pagenum" id="Page_62">62</span> represent distance from the origin
-    in feet, and the vertical measurements mean acceleration in thousands
-    of millimetres per second.</p>
-
-  <p>This curve approximates to an equi-angular hyperbola. The area between
-    the curve and its asymptotes is proportional to the whole energy of
-    the shock. The area of the diagram is proportional to the energy given
-    up to the ground by the explosion of three pounds of dynamite. If we
-    call the unit shock the effect produced by the explosion of one pound
-    of dynamite, the above artificial earthquake had an intensity equal to
-    three.</p>
-
-  <p>The only other investigations which have been made in this interesting
-    branch of observational seismology are those by Mr. Robert Mallet,<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a>
-    and those by General Henry L. Abbot.<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a></p>
-
-  <p><em>Mallet’s Results.</em>—The velocity with which earth vibrations were
-    transmitted as deduced by Mr. Mallet were as follows:—</p>
-
-  <table summary="Mallets results">
-    <tr>
-      <th>&nbsp;</th>
-      <th>Feet per second</th>
-    </tr>
-    <tr>
-      <td class="tdhang">In sand</td>
-      <td class="tdr"><div>824·915</div></td>
-    </tr>
-    <tr>
-      <td class="tdhang">In contorted stratified rock, quartz, and slate at Holyhead</td>
-      <td class="tdr"><div>1,088·669</div></td>
-    </tr>
-    <tr>
-      <td class="tdhang">In discontinuous and much shattered granite</td>
-      <td class="tdr"><div>1,306·425</div></td>
-    </tr>
-    <tr>
-      <td class="tdhang">In more solid granite</td>
-      <td class="tdr"><div>1,664·574</div></td>
-    </tr>
-  </table>
-
-  <p>A striking result which was obtained by Mallet in his experiments at
-    Holyhead was that the transit velocity increases with an increase in
-    the intensity of the initial shock. Thus with a charge of 12,000 pounds
-    of powder the transit rate was 1,373 feet per second, whilst with 2,100
-    pounds the transit rate was 1,099 feet per second. In these experiments
-    tremors were observed as preceding and following the main shock.</p>
-
-  <p><span class="pagenum" id="Page_63">63</span></p>
-
-  <p><em>Abbot’s Results.</em>—The important results obtained by General Abbot are
-    contained in the following table:—</p>
-
-  <table id="AbbotsResults" summary="Abbots results">
-    <tr>
-      <th>No. of Observation</th>
-      <th>Date</th>
-      <th>Cause of Shock</th>
-      <th>Distance to<br />Station in miles</th>
-      <th>Type of Seismometer</th>
-      <th>Velocity in<br />feet per second</th>
-    </tr>
-    <tr>
-      <td class="tdr"><div>1</div></td>
-      <td class="tdc">Aug. 18, 1876</td>
-      <td class="tdc">200 lbs. of dynamite</td>
-      <td class="tdr"><div>5 ±&nbsp; &nbsp;&nbsp;</div></td>
-      <td class="tdc">B</td>
-      <td class="tdc">5,280</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>2</div></td>
-      <td class="tdc">Sep. 24, 1876</td>
-      <td class="tdc">Hallet’s Point Explosion</td>
-      <td class="tdr"><div>5·134</div></td>
-      <td class="tdc">A</td>
-      <td class="tdc">3,873</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdc">„</td>
-      <td class="tdc">„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; </td>
-      <td class="tdr"><div>8·330</div></td>
-      <td class="tdc">B</td>
-      <td class="tdc">8,300</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>4</div></td>
-      <td class="tdc">„</td>
-      <td class="tdc">„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; </td>
-      <td class="tdr"><div>9·333</div></td>
-      <td class="tdc">A</td>
-      <td class="tdc">4,521</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>5</div></td>
-      <td class="tdc">„</td>
-      <td class="tdc">„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; </td>
-      <td class="tdr"><div>12·769</div></td>
-      <td class="tdc">B</td>
-      <td class="tdc">5,309</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>6</div></td>
-      <td class="tdc">Oct 10, 1876</td>
-      <td class="tdc">&nbsp; &nbsp; &nbsp; 70 lbs. dynamite</td>
-      <td class="tdr"><div>1·360</div></td>
-      <td class="tdc">A</td>
-      <td class="tdc">1,240</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>7</div></td>
-      <td class="tdc">Sept. 6, 1877</td>
-      <td class="tdc">400&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; </td>
-      <td class="tdr"><div>1·169</div></td>
-      <td class="tdc">A</td>
-      <td class="tdc">3,428</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdc">„</td>
-      <td class="tdc">&nbsp; „&nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp;</td>
-      <td class="tdr"><div>1·169</div></td>
-      <td class="tdc">B</td>
-      <td class="tdc">8,814</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdc">Sept. 12, 1877</td>
-      <td class="tdc">200&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; </td>
-      <td class="tdr"><div>1·340</div></td>
-      <td class="tdc">A</td>
-      <td class="tdc">6,730</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdc">„</td>
-      <td class="tdc">&nbsp; „&nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp;</td>
-      <td class="tdr"><div>1·340</div></td>
-      <td class="tdc">B</td>
-      <td class="tdc">8,730</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>11</div></td>
-      <td class="tdc">„</td>
-      <td class="tdc">&nbsp; 70&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; </td>
-      <td class="tdr"><div>1·340</div></td>
-      <td class="tdc">A</td>
-      <td class="tdc">5,559</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>12</div></td>
-      <td class="tdc">„</td>
-      <td class="tdc">&nbsp; &nbsp;„ &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp;</td>
-      <td class="tdr"><div>1·340</div></td>
-      <td class="tdc">B</td>
-      <td class="tdc">8,415</td>
-    </tr>
-  </table>
-
-  <p>A seismometer of type A means that the telescope used in observing the
-    tremor produced on the surface of a vessel of mercury by the passage of
-    the shock had a magnification of 6, whilst a telescope of the type B
-    had a magnification of 12.</p>
-
-  <p>The mean velocity given by six observations with type A is 4,225 feet
-    per second, while that given by the same number with type B is 7,475
-    feet per second.</p>
-
-  <div class="figright">
-    <img src="images/i_p063.jpg" width="283" height="151" alt="" />
-    <div class="caption"><span class="smcap">Fig. 13.</span></div>
-  </div>
-
-  <p>If we assume that the first tremor observed in the mercury is to
-    determine the true rate of transmission, General Abbot tells us that we
-    must reject all observations made with type A, inasmuch as they do not
-    reveal the velocity of the leading tremor. However, he also tells
-    <span class="pagenum" id="Page_64">64</span> us
-    that a still higher power above 12 might have detected still earlier
-    tremors.</p>
-
-  <p>When gunpowder was the explosive, the observers noted that the
-    disturbance observed in the mercury took a much longer time to reach a
-    maximum than it did when dynamite was employed.</p>
-
-  <p>It was also observed that explosions fired beneath deep water gave
-    a higher velocity than similar explosions which took place beneath
-    shallow water. In the latter case much of the energy was probably
-    expended in throwing a jet of water into the air.</p>
-
-  <p>Another point which was observed appears to have been that the rate
-    varied with the initial shock. Thus:—</p>
-
-  <table summary="Shockwave velocity">
-    <tr>
-      <th>&nbsp;</th>
-      <th>Feet per second</th>
-    </tr>
-    <tr>
-      <td class="left">400 lbs. of dynamite gave</td>
-      <td class="tdc"><div>8,814</div></td>
-    </tr>
-    <tr>
-      <td class="left">200&nbsp; &nbsp;„ &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;</td>
-      <td class="tdc"><div>8,730</div></td>
-    </tr>
-    <tr>
-      <td class="left">&nbsp; 70&nbsp; &nbsp;„&nbsp; powder (deep) gave</td>
-      <td class="tdc"><div>8,415</div></td>
-    </tr>
-  </table>
-
-  <p>Also it is probable that the rate of a wave diminished with its
-    advance. For,</p>
-
-  <table summary="Shockwave velocity per distance">
-    <tr>
-      <th>&nbsp;</th>
-      <th>Feet per second</th>
-    </tr>
-    <tr>
-      <td class="left">&nbsp; 200&nbsp; lbs. of dynamite gave for 1&nbsp; &nbsp; mile</td>
-      <td class="tdc"><div>8,730</div></td>
-    </tr>
-    <tr>
-      <td class="left">&nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „ &nbsp; &nbsp; &nbsp; &nbsp; 5&nbsp; &nbsp; miles</td>
-      <td class="tdc"><div>5,250</div></td>
-    </tr>
-    <tr>
-      <td class="left">50,000&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „  &nbsp; &nbsp; &nbsp; &nbsp; 8&nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdc"><div>8,300</div></td>
-    </tr>
-    <tr>
-      <td class="left">&nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp;13½&nbsp; &nbsp;„</td>
-      <td class="tdc"><div>5,300</div></td>
-    </tr>
-  </table>
-
-  <p>General Abbot’s general conclusions are:—</p>
-
-  <p>1. A high magnifying power of telescope is essential in seismometric
-    observations.</p>
-
-  <p>2. The more violent the initial shock the higher is the velocity of
-    transmission.</p>
-
-  <p>3. This velocity diminishes as the general wave advances.</p>
-
-  <p>4. The movements of the earth’s crust are complex, consisting of many
-    short waves first, increasing and then decreasing in amplitude; and
-    with a detonating explosive the interval between the first wave and
-    the maximum<span class="pagenum" id="Page_65">65</span>
-    wave, at any station, is shorter than with a slow burning
-    explosive.</p>
-
-  <p><em>Results obtained in Japan.</em>—From some experiments made by the author
-    in the grounds of the Meteorological Department in Tokio, the following
-    results were obtained:—</p>
-
-    <table id="JapanResults" class="collapse" summary="Japan shockwave velocity">
-      <tr>
-        <th colspan="3" class="ball">No. of Explosion</th>
-        <th class="ball">Velocity in feet per second for the first 200 ft. (A to B)</th>
-        <th class="ball">Velocity in feet per second for the second 200 ft. (B to C)</th>
-        <th class="ball">Velocity in feet per second for 400 ft. (A to C)</th>
-        <th class="ball">Number of Cartridges of Dynamite (6 = 1 lb.)</th>
-      </tr>
-      <tr>
-        <td class="tdc bl" rowspan="4">Vertical vibrations</td>
-        <td class="tdr x400" rowspan="4"><div>{</div></td>
-        <td class="tdr0 br"><div>I.</div></td>
-        <td class="tdc br">464</td>
-        <td class="tdc br">186</td>
-        <td class="tdc br">265</td>
-        <td class="tdrx br"><div>8·3</div></td>
-      </tr>
-      <tr>
-        <td class="tdr0 br"><div>III.</div></td>
-        <td class="tdc br">&mdash;</td>
-        <td class="tdc br">211</td>
-        <td class="tdc br">&mdash;</td>
-        <td class="tdrx br"><div>10·1</div></td>
-      </tr>
-      <tr>
-        <td class="tdr0 br"><div>IV.</div></td>
-        <td class="tdc br">352</td>
-        <td class="tdc br">234</td>
-        <td class="tdc br">281</td>
-        <td class="tdrx br"><div>7·1</div></td>
-      </tr>
-      <tr>
-        <td class="tdr0 br"><div>V.</div></td>
-        <td class="tdc br">343</td>
-        <td class="tdc br">232</td>
-        <td class="tdc br">277</td>
-        <td class="tdrx br"><div>5·0</div></td>
-      </tr>
-      <tr>
-        <td class="tdc bl" rowspan="2">Normal vibrations</td>
-        <td class="tdr x200" rowspan="2"><div>{</div></td>
-        <td class="tdr0 br"><div>VI.</div></td>
-        <td class="tdc br">&mdash;</td>
-        <td class="tdc br">&mdash;</td>
-        <td class="tdc br">407</td>
-        <td class="tdrx br"><div>10·0</div></td>
-      </tr>
-      <tr>
-        <td class="tdr0 br"><div>VII.</div></td>
-        <td class="tdc br">&mdash;</td>
-        <td class="tdc br">&mdash;</td>
-        <td class="tdc br">516</td>
-        <td class="tdrx br"><div>12·5</div></td>
-      </tr>
-      <tr>
-        <td class="tdc bl bb">Transverse vibrations</td>
-        <td class="tdr x200 bb"><div>}</div></td>
-        <td class="tdr0 br bb"><div>VIII.</div></td>
-        <td class="tdc br bb">&mdash;</td>
-        <td class="tdc br bb">&mdash;</td>
-        <td class="tdc br bb">344</td>
-        <td class="tdrx br bb"><div>12·5</div></td>
-      </tr>
-    </table>
-
-  <p>The general results to be deduced from the above appear to be:—</p>
-
-  <p class="hang1">1. For vertical motion.</p>
-
-  <p class="hang2">(<i>a</i>) For the first 200 feet. The velocity depends upon the initial
-    force—the greater the charge of dynamite the greater the velocity.</p>
-
-  <p class="hang2">(<i>b</i>) For the second 200 feet. The above law only appears in
-    experiments IV. and V., but it must be remembered that the origins
-    of I. and III. were farther removed from A than IV. and V.</p>
-
-  <p class="hang1">The speed of the wave during the second 200 feet is always less
-    than during the first 200 feet.</p>
-
-  <p class="hang1">2. For normal vibrations.</p>
-
-  <p class="hang2">Here the speed between A and C is all that was measured, but we
-    again see that the greater the initial force, or the nearer we are
-    to the origin of the disturbance, the greater is the velocity. This
-    velocity is greater than the velocity of the vertical or transverse
-    vibrations.</p>
-
-  <p><span class="pagenum" id="Page_66">66</span></p>
-
-  <p class="hang1">3. For transverse vibrations.</p>
-
-  <p class="hang2">If we assume that the vertical vibrations are a component of the
-    transverse motions we see the same law as before—namely, that the
-    nearer we are to the origin of the disturbance the greater is the
-    speed with which that disturbance is propagated.</p>
-
-  <p>It will be observed that the chief law here enunciated respecting the
-    decrease in speed of earth vibrations is the same as that pointed
-    out by General Abbot, from which it only differs by its being in all
-    cases proved without the introduction of personal errors, for the same
-    explosion, along the same line of ground and for different kinds of
-    vibrations.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_V">
-    <span class="pagenum" id="Page_67">67</span>
-    <h2>CHAPTER V.<br />
-      <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION ON EARTHQUAKES.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Result of feelings—The direction of motion—Instruments as
-    indicators of direction—Duration of an earthquake—Period of
-    vibration—The amplitude of earth movements—Side of greatest
-    motion—Intensity of earthquakes—Velocity and acceleration of an
-    earth particle—Absolute intensity of an earthquake—Radiation of an
-    earthquake—Velocity of propagation.</div>
-
-  <p><em>Result of Feelings.</em>—As the result of our experiences, and by
-    observations upon the movements produced in various bodies, we can
-    say that an ordinary earthquake consists of a number of backward and
-    forward motions of the ground following each other in quick succession.
-    Sometimes these commence and die out so gently that those who have
-    endeavoured to time the duration of an earthquake have found it
-    difficult to say when the shock commenced and when it ended. This was
-    a difficulty which Mr. James Bissett in Yokohama, and the author in
-    Tokio, had to contend against when, in 1878, they commenced to time
-    shocks between these two places.</p>
-
-  <p>Sometimes these motions gradually increase to a maximum and then die
-    out as gradually as they commenced.</p>
-
-  <p>Sometimes the maximum comes suddenly, and at other times during an
-    earthquake our feelings distinctly tell us that there are several
-    maxima.</p>
-
-  <p><span class="pagenum" id="Page_68">68</span></p>
-
-  <p>These have been the experiences of many observers, and have been
-    recorded by writers since the earliest times. Mallet devotes a
-    chapter to a consideration of the tremulous motion that precedes and
-    follows a shock, and he tells us that a single shock is an absolute
-    impossibility. In speaking of earthquakes, he says: ‘The almost
-    universal succession of phenomena recorded in earthquakes is, first a
-    trembling, then a severe shock, or several in quick succession, and
-    then a trembling gradually but rapidly becoming insensible.’</p>
-
-  <p>A quantitative and exact knowledge of the nature of earthquake
-    motion has only been attained of late years. The chief results which
-    investigators have aimed at have been the measurement of the amplitude,
-    the period, the direction, and the duration of the motions which
-    constitute an earthquake. Attention has also been given to the velocity
-    with which a disturbance is propagated.</p>
-
-  <p><em>The Direction of Motion.</em>—One of the most ordinary observations
-    which are made about an earthquake is its direction. If we were to
-    ask the inhabitants of a town which had been shaken by an earthquake
-    the direction of the motion they experienced, it is not unlikely that
-    their replies would include all the points of the compass. Many,
-    in consequence of their alarm, have not been able to make accurate
-    observations. Others have been deceived by the motion of the building
-    in which they were situated. Some tell us that the motion had been
-    north and south, whilst others say that it was east and west. A certain
-    number have recognised several motions, and amongst the rest there will
-    be a few who have felt a wriggling or twisting. Leaving out exceptional
-    cases, the general result obtained from personal observation as to
-    the direction of an earthquake of moderate intensity is extremely
-    indefinite, and the only satisfactory information
-    <span class="pagenum" id="Page_69">69</span> to be obtained is
-    that derived from instruments or from the effects of the earthquake
-    exhibited in shattered buildings and bodies which had been overturned
-    or projected.</p>
-
-  <p>By the direction in which walls, columns, and other objects had been
-    overthrown or fractured, Mallet was enabled to determine the position
-    of the origin of the Neapolitan earthquake. Similar phenomena have many
-    times been taken advantage of by other investigators of earthquake
-    phenomena. Effects produced upon structures are, however, only to be
-    observed as the results of a destructive earthquake, at which time
-    cities may be regarded as collections of seismometers. (<em>See</em> chapter
-    on Effects in Buildings.)</p>
-
-  <p>To determine the direction of movement during a small earthquake, the
-    most satisfactory method appears to be an appeal to instruments.</p>
-
-  <p><em>Instruments as Indicators of Direction.</em>—The relative values of
-    different kinds of instruments, such as columns, pendulums, and the
-    like, as indicators of direction have already been discussed.</p>
-
-  <p>By the use of pendulum seismographs it has been shown that during an
-    earthquake the ground may move in one, two, or several directions (see
-    <a href="#Page_21">p. 21</a>); and it is, generally speaking, only in those cases where we
-    experience a decided shock in the disturbance that we can determine
-    with any confidence the direction in which the motion has been
-    propagated. Such directions are usually indicated by the major axis of
-    certain more or less elliptical figures which have been drawn, which
-    in themselves appear to indicate the combination of two rectilinear
-    movements.</p>
-
-  <p>Results similar to those indicated by the records of pendulum
-    seismographs have also been obtained upon moving plates with a double
-    bracket seismograph. Thus,<span class="pagenum" id="Page_70">70</span>
-    in the earthquake which shook Tokio at 6
-    <span class="smcap">a.m.</span> on July 5, 1881, there were indications of the following
-    motions:—</p>
-
-  <p>Near the commencement of the shock the motion was N. 112° E. One and a
-    half second after this, the direction of motion appears to have been
-    N. 50° E. In three-fourths of a second more it gradually changed to a
-    direction N. 145° E., and after a similar interval to N. 62° E. Half a
-    second after this it was N. 132° E., and four seconds later the motion
-    was again in the original direction—namely, N. 112° E.</p>
-
-  <p>These particular directions of motion have been selected because they
-    were so definitely indicated.</p>
-
-  <p>The commonest type of earthquake which is experienced in Japan, and
-    probably also in other earthquake-shaken districts, is the compound or
-    diastrophic form.</p>
-
-  <p>That earthquakes often have motions compounded of two sets of
-    vibrations, has also been proved by the analysis of the records
-    obtained from two component seismographs. From an analysis of a record
-    of this description, Professor Ewing has shown that in the earthquake
-    felt in Tokio on March 11, 1881, there were approximate circular
-    (somewhat spiral) movements.</p>
-
-  <p>This leads us to the consideration of the twisting and wriggling
-    motions which are said to be experienced by some observers. Motions
-    like these, which by the Italians and Mexicans are called <i xml:lang="es">vorticosi</i>,
-    are usually supposed to be the cause of objects like chimneys and
-    gravestones being rotated. These phenomena, it will be seen from what
-    is said in the chapter upon the effects produced in buildings, can be
-    more easily explained upon the supposition of a simple rectilinear
-    movement.</p>
-
-  <p>That at the time of an earthquake there may be motion in more than one
-    direction has been recognised since the time of Aristotle; and it is
-    possible that two sets<span class="pagenum" id="Page_71">71</span>
-    of rectilinear motion, as, for instance, the
-    normal and transverse movements, may have led observers to imagine that
-    there has been a twisting motion taking place, and this especially when
-    the two sets of movements have quickly succeeded each other.</p>
-
-  <p>Persons inside flexible buildings may possibly have experienced more
-    or less of a rotatory motion, although the shock was rectilinear; the
-    building assuming such a motion in consequence of its construction and
-    its position with regard to the direction of the shock.</p>
-
-  <p>In the case of destructive earthquakes, especially at points situated
-    practically above the origin, the universal testimony, Mallet tells
-    us, is that a twisting, wriggling motion in different planes, attended
-    by an up-and-down movement of greater range, is experienced. To such
-    disturbances the word <i xml:lang="es">sussultatore</i> is sometimes applied. Mallet has
-    given many elliptical and other closed curves to illustrate the nature
-    of such motions.</p>
-
-  <p><em>Duration of an Earthquake.</em>—When reading accounts of earthquakes
-    it is often difficult to determine the length of time a shaking was
-    continuous. In Japan, in <span class="smcap">a.d.</span> 745, there was a shaking which
-    is said to have lasted sixty hours; and in <span class="smcap">a.d.</span> 977 there were
-    a series of shakings lasting 300 days. Often we meet with records of
-    disturbances which have lasted from twenty to seventy days.</p>
-
-  <p>At San Salvador, in 1879, more than 600 shocks were felt within ten
-    days; in 1850, at Honduras, there were 108 shocks in a week; in 1746,
-    at Lima, 200 shocks were felt in twenty-four hours; at the island of
-    St. Thomas, in 1868, 283 shocks were felt during about ten hours.</p>
-
-  <p>Disturbances like these, which succeed each other with sufficient
-    rapidity to cause an almost continual trembling in the ground, may be
-    regarded as collectively forming one great seismic effort which may
-    last a minute, an hour,<span class="pagenum" id="Page_72">72</span>
-    a day, a week, or even several years. Strictly
-    speaking, they are a series of separate earthquakes, the resultant
-    vibrations of which more or less overlap. Whenever a large earthquake
-    occurs it is generally succeeded by a large number of smaller shocks.</p>
-
-  <p>The seismic disturbance as regards time is, as Mallet remarks, very
-    often ‘like an occasional cannonade during a continuous but irregular
-    rattle of musketry.’ In the New Zealand earthquake of 1848, shocks
-    continued for nearly five weeks, and during a large portion of the time
-    there were at least 1,000 shocks per day.<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a></p>
-
-  <p>The earthquake of Lisbon, which in five minutes destroyed the whole
-    town, was followed by a series of disturbances lasting over several
-    months. After Basle had, on October 18, 1356, been laid in ruins,
-    it is stated shocks followed each other for a period of a year. The
-    Calabrian earthquake was continued with considerable strength for a
-    year, and it is said that the earth did not come completely to rest for
-    ten years. During this cannonade the heavy shocks announced, as they
-    do in most earthquake countries at the present day, a series of weaker
-    disturbances. In certain exceptional cases this order of events has
-    been inverted, and slight shocks have announced the coming of heavy
-    ones. Fuchs gives an example of this in the earthquake of Broussa, when
-    the first shock was on February 28, 1855. On March 9 and 23 there were
-    heavier shocks, but the heaviest did not arrive until March 28.</p>
-
-  <p>Under certain conditions it is possible to have a sensible vibration
-    produced in the ground which is practically of unlimited duration;
-    thus, for instance, it has been noticed that the falling of water
-    at certain large waterfalls, by its continuous rhythmical impact on
-    the <span class="pagenum" id="Page_73">73</span>
-    rocks, produces in them tremors which are to be observed at
-    great distances. Of this the author convinced himself at the Falls
-    of Niagara, where he observed the reflected and ever-moving image of
-    the sun in a pool of water. Under favourable circumstances almost
-    continual condensation of steam might take place in volcanic foci, each
-    condensation giving rise to a blow sufficiently powerful to produce
-    vibrations in the surrounding ground. Those who have stood near a large
-    geyser, like the one in Iceland, when it makes an ineffectual effort
-    to erupt, will recognise how powerful such a cause might be. Humboldt
-    has remarked shocks on Vesuvius and Pichincha which were periodic,
-    occurring twenty to thirty seconds before each ejection of vapour and
-    ashes.</p>
-
-  <p>Earthquakes like these may be of vast extent, gradually spreading
-    further and further outwards. This spreading of earth vibrations may
-    be observed at a large factory containing heavy machinery or a steam
-    hammer. After the machinery comes to rest, it is probably some time
-    before the ground returns to rest. Examples of disturbances of this
-    nature are spoken of under the head of Earth Tremors.</p>
-
-  <p>The record of the duration of an ordinary earthquake as observed at a
-    given point is dependent upon the sensibility of our instruments.</p>
-
-  <p>Continuous motions perceptible to our senses without the aid of
-    instruments usually last from thirty seconds to about two or three
-    minutes. In Japan the shocks, as timed by watches, usually last from
-    twenty to forty seconds. Occasionally a continuous shaking is felt for
-    more than one and a half minutes, and cases have been recorded where
-    the motion has continued for as much as four minutes and thirty-three
-    seconds.</p>
-
-  <p>Seismometers having a multiplication of 6 to 12
-    <span class="pagenum" id="Page_74">74</span> usually indicate that
-    motion continues longer than is perceptible to the senses.</p>
-
-  <p><em>Period of Vibration.</em>—When an earthquake contains several prominent
-    vibrations which might be called the <em>shocks</em> of the disturbance, our
-    feelings tell us that these have occurred at unequal intervals.</p>
-
-  <p>About the time which is taken for the complete backward and forward
-    oscillation of the ground which constitutes the shock a little has
-    already been said. This was deduced from the records of disturbances as
-    drawn by seismographs. From the same sources we can readily obtain the
-    period of all the prominent vibrations in a disturbance.</p>
-
-  <p>In any given earthquake there are irregularities in period, and
-    different earthquakes differ from each other. About the early attempts
-    to determine the period of earth vibrations something has been said in
-    the chapter on Earthquake Instruments.</p>
-
-  <p>In the earthquake of March 11 (referred to on <a href="#Page_70">p. 70</a>) we find that both
-    components commenced with a series of small vibrations, about five or
-    six to the second; next came the shock, consisting of two complete
-    vibrations executed in two seconds. In this it is to be observed that
-    the motion eastwards was performed much more quickly than the motion
-    westwards. Next, by reference to the east and west component, it is
-    seen that there are a number of large vibrations, about one per second,
-    on which a number of smaller motions are superposed. As the motion
-    proceeds, these become less and less definitely pronounced and more
-    irregular in their intervals, until finally the motion dies away.</p>
-
-  <p>This earthquake, as recorded at the author’s house in Tokio, lasted
-    about one and a half minute.</p>
-
-  <p>The same earthquake, as recorded by Professor Ewing
-    <span class="pagenum" id="Page_75">75</span> at a station
-    situated about one and a half mile distant, but on flat ground, appears
-    to have lasted four and a half minutes. The largest wave had a period
-    of 0·7 second.</p>
-
-  <p>In the earthquake of March 8, 1881, there were on an average 1·4
-    vibrations per second. These vibrations were executed in a direction
-    transverse to the line joining the observing station and the locality
-    from which the disturbance must have originated as determined by time
-    observations. It can, therefore, be assumed that these vibrations,
-    having so slow a period, were transverse motions, this slowness or
-    sluggishness being due to the fact that the modulus for distortion
-    is less than the modulus which governs the propagation of normal
-    vibrations.</p>
-
-  <p><em>The Amplitude of Earth Movements.</em>—In making estimates of the
-    distances through which we are moved backward and forward at the time
-    of an earthquake, if we judge by our feelings, we may often be misled.
-    If a person is out of doors and walking, an earthquake may take place
-    sufficiently strong to cause chimneys to fall and unroof houses, which,
-    so far as the actual shaking of the ground is concerned, will be passed
-    by unnoticed. On the other hand, to persons indoors, especially on an
-    upper story, it is impossible even for a tremor to pass by without
-    creating considerable alarm by the angular movement that has been taken
-    up by the building.</p>
-
-  <p>Many observers have endeavoured to make actual measurements of the
-    maximum extent through which the earth moves at the time of an
-    earthquake. Among the reports of the British Association for 1841 is
-    the report of a committee which had been appointed ‘for obtaining
-    instruments and registers to record shocks of earthquakes in Scotland
-    and Ireland’. We read that in one<span class="pagenum" id="Page_76">76</span>
-    earthquake which had been measured
-    the displacement of the ground had been half an inch, and in another
-    it had been less than half an inch. The instruments used to make these
-    observations depended upon the inertia of pendulums which at the time
-    of the disturbance were supposed to remain at rest. Observations
-    similar to these have been made in Japan. One long series were made by
-    Mr. E. Knipping for Dr. Gr. Wagener. They extended from November 1878
-    to April 1880, and were as follows:—</p>
-
-  <table summary="Earthquake amplitude">
-    <tr>
-      <th>Number of<br />Earthquakes</th>
-      <th>Maximum horizontal<br />motion of the ground</th>
-    </tr>
-    <tr>
-      <td class="tdr2"><div>10</div></td>
-      <td>&nbsp; ·0&nbsp; &nbsp; to 0·15 mm.</td>
-    </tr>
-    <tr>
-      <td class="tdr2"><div>7</div></td>
-      <td>&nbsp; ·15&nbsp; „&nbsp; 0·5&nbsp; &nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdr2"><div>8</div></td>
-      <td>&nbsp; ·5&nbsp; &nbsp; „&nbsp; 2·5&nbsp; &nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdr2"><div>2</div></td>
-      <td>2·5&nbsp; &nbsp; „&nbsp; more &nbsp; „</td>
-    </tr>
-  </table>
-
-  <p>With his apparatus for vertical motion Dr. Wagener also made
-    observations on the absolute vertical motion. This seldom reached ·02
-    mm. The greatest value was that observed for the destructive shock of
-    Feb. 22, 1880, which was ·56 mm.</p>
-
-  <p>By means of a number of instruments distributed at various localities
-    round Tokio, the chief of which were pendulums with friction pointers
-    to render them ‘<em>dead beat</em>,’ and with magnifying apparatus to show
-    the actual motion of the ground, the author arrived at results similar
-    to those obtained by Dr. Wagener—namely, that the earth’s maximum
-    horizontal motion at the time of a small earthquake was usually only
-    the fraction of a millimetre, and it seldom exceeded three or four
-    millimetres. When we get a motion of five or six millimetres, we
-    usually find that brick and stone chimneys have been shattered.</p>
-
-  <p>The results obtained for vertical motion were also very small. In Tokio
-    it is seldom that vertical motion can be
-    <span class="pagenum" id="Page_77">77</span> detected, and when it is
-    recorded it is seldom more than a millimetre.</p>
-
-  <p>These results, which were put forward some years ago, have since
-    received confirmation by the use of a variety of instruments in the
-    hands of different observers.</p>
-
-  <p>Mallet, in his account of the Neapolitan earthquake of 1857,
-    approximated to the amplitude of an earth particle by observing the
-    width, at the level of the centre of gravity, of fissures formed
-    through and remaining in great masses of very inelastic masonry.</p>
-
-  <p>Taking stations situated on or very nearly on the same line passing
-    through the seismic vertical (<em>epicentrum</em>), Mallet observed the
-    amplitude increased as some function of the distance, as will be seen
-    from the following table:—</p>
-
-  <table id="amplitude2" summary="Earthquake amplitude per distance">
-    <tr>
-      <th>Station</th>
-      <th>Polla</th>
-      <th>La Sala</th>
-      <th>Certosa</th>
-      <th>Tramutola</th>
-      <th>Sarconi</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Distance from Seismic Vertical in geographical miles</td>
-      <td class="br">3·45</td>
-      <td class="br">11·60</td>
-      <td class="br">16·50</td>
-      <td class="br">20·60</td>
-      <td class="br">26·7</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br bb">Amplitude in inches</td>
-      <td class="br bb">2·5</td>
-      <td class="br bb">&nbsp; 3·5</td>
-      <td class="br bb">&nbsp; 4·0</td>
-      <td class="br bb">&nbsp; 4·5</td>
-      <td class="br bb">&nbsp; 4·75</td>
-    </tr>
-  </table>
-
-  <p>The possibility of a law such as this having an existence for places at
-    a distance from the seismic vertical comparable with the vertical depth
-    of the centrum will be shown farther on.</p>
-
-  <p>With regard to the maximum displacement of an earth particle. Mallet
-    was of opinion that there was evidence to show that it had in some
-    cases been over one foot. M. Abella, in an earthquake which occurred in
-    the Philippines in 1881, made a rough observation of the motion of the
-    earth to a distance of about <em>two metres</em>. This, as might be expected,
-    was beyond the elastic limits of the material, and caused fissures to
-    be formed, which were seen to open and shut.</p>
-
-  <p><em>Intensity of Earthquakes.</em>—In speaking of the strength
-    <span class="pagenum" id="Page_78">78</span> of an
-    earthquake, we usually employ terms like ‘weak,’ ‘strong,’ ‘violent,’
-    &amp;c. Although these expressions, accompanied by illustration of the
-    effects which an earth quake has produced, convey a general idea of
-    the strength of a shock as felt at some particular locality, our ideas
-    nevertheless wanting in definiteness; and if we endeavour to compare
-    one shock with another, as a whole, our want of exactness is augmented.
-    We have seen that Palmieri’s seismograph indicates intensity by a
-    certain number of degrees, which, to a certain extent, is a measure of
-    the violence of the motion as indicated at a particular locality. The
-    degrees, as before stated, refer to the height to which in consequence
-    of the shaking, a certain quantity of mercury was washed in a tube,
-    which is a function of the depth of mercury in the tube, and also of
-    the duration of the disturbance.</p>
-
-  <p>From this it seems possible that a very slow motion of small amplitude,
-    continuing over a sufficient period of time, might, if it agreed with
-    the period of the mercury, indicate an earthquake of many degrees of
-    intensity, whilst residents in the neighbourhood might not have noticed
-    the disturbance; and, on the other hand, a short but intense shock
-    creating considerable destruction might have been recorded as of only a
-    few degrees of intensity.</p>
-
-  <p>Although objections like these might be raised to such a method of
-    recording intensity, in practice it would appear that such results are
-    not pronounced, and the indications of the instrument usually give us
-    approximate indications of relative intensity.</p>
-
-  <p>In writing about the Neapolitan earthquake of 1857, Mallet says that
-    ‘area alone affords no test of seismic energy.’</p>
-
-  <p>The area over which a shock is felt will depend not only upon the
-    initial force of the disturbance, but also
-    <span class="pagenum" id="Page_79">79</span> upon the focal depth of
-    a shock, the form and position of that focus, the duration of the
-    disturbance, and the nature and arrangement of the materials which are
-    shaken.</p>
-
-  <p>From observations in Japan, it is clearly shown that massive mountain
-    ranges exert a considerable influence upon the extension of seismal
-    disturbances. On one side of a large range of mountains large cities
-    might be laid in ruins, whilst on the other side the disturbance
-    creating this destruction might not be noticed.</p>
-
-  <p><em>Velocity and Acceleration of an Earth Particle.</em>—We now pass on to
-    methods of determining the intensity of an earthquake which are less
-    arbitrary than those which have just been discussed. These methods have
-    already been discussed when speaking of artificial disturbances, where
-    it was shown that the intensity of an earthquake as measured by its
-    destructive effects greatly depended upon the suddenness with which the
-    backward and forward motions of the ground were commenced or ended.</p>
-
-  <p>Amongst the earlier investigators of seismic phenomena who observed
-    that there existed a connection between the distance to which bodies
-    had been projected during an earthquake and the suddenness or initial
-    velocity with which the ground had been moved beneath them, was
-    Professor Wenthrop of Cambridge, Massachusetts, who noted that bricks
-    from his chimneys had, by the New England earthquake of 1755, been
-    thrown thirty feet. From this and the known height of the chimney, he
-    calculates that the bricks had been projected with an initial velocity
-    of twenty-one feet per second.<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a></p>
-
-  <p>The calculations made by Mallet respecting the maximum velocity of
-    an earth particle at the time of the Neapolitan earthquake in 1857
-    depended upon the overthrow, projection, and fracture of bodies.</p>
-
-  <p><span class="pagenum" id="Page_80">80</span></p>
-
-  <p>The principles which guided him in making the calculations will be
-    understood from the following illustration.</p>
-
-  <div class="figleft">
-    <img src="images/i_p080.jpg" width="262" height="273" alt="" />
-    <div class="caption"><span class="smcap">Fig. 14.</span></div>
-  </div>
-
-  <p>If a column, <span class="smcap">a b c d</span>, receive a shock or be suddenly moved in
-    the direction of the arrow, the centre of gravity, <span class="smcap">g</span>, of this
-    column will revolve round the edge, and tend to describe the path <span class="smcap">g o</span>.
-    If it passes <span class="smcap">o</span>, the column will fall. The work done in
-    such a case as this is equal to lifting the column through the height
-    o <i>h</i>.</p>
-
-  <p>If <span class="smcap">g a</span> = <i>a</i>, the angle <span class="smcap">g a</span> <i>h</i> = <i>ϕ</i>, and the
-    weight of the body = <span class="smcap">w</span>, then the above work equals</p>
-
-  <p class="center"><span class="smcap">w</span><i>a</i> (1 − cos <i>ϕ</i>).</p>
-
-  <p>This must equal the work acquired—that is to say, the kinetic energy of
-    rotation of the body, or</p>
-
-  <div class="center">
-    <span class="smcap">w</span><i>a</i> (1 − cos <i>ϕ</i>)&nbsp;=&nbsp;
-    <span class="frac"><sup><span class="smcap">w</span><i>w</i><sup>2</sup><span class="smcap">k</span><sup>2</sup></sup>
-    <span>/</span><sub>2 <i>g</i></sub></span>.
-  </div>
-
-  <p>Where <i>w</i> is the angular velocity of the body at starting, <span class="smcap">k</span>
-    the radius of gyration round <span class="smcap">a</span>, and <i>g</i> the velocity acquired
-    by a falling body in one second. Whence</p>
-
-  <div class="center"><i>w</i><sup>2</sup> <span class="smcap">k</span><sup>2</sup> = 2 <i>ga</i> (1 − cos <i>ϕ</i>),</div>
-
-  <p class="noindent">but <i>w</i>, the angular velocity, is equal to the statical couple applied,
-    divided by the moment of inertia, or,</p>
-
-  <div class="center">
-    <i>w</i>&nbsp;=&nbsp;<span class="frac"><sup><span class="smcap">v</span><i>a</i> cos <i>ϕ</i></sup>
-    <span>/</span><sub><span class="smcap">k</span><sup>2</sup></sub></span>,
-  </div>
-
-  <p class="noindent">squaring and substituting</p>
-
-  <div class="center">
-    <span class="smcap">v</span><sup>2</sup>&nbsp;=&nbsp;2<i>g</i>&nbsp;×&nbsp;
-    <span class="frac"><sup><span class="smcap">k</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>&nbsp;×&nbsp;
-    <span class="frac"><sup>1 − cos <i>ϕ</i></sup><span>/</span><sub>cos<sup>2</sup> <i>ϕ</i></sub></span>,
-  </div>
-
-  <p><span class="pagenum" id="Page_81">81</span></p>
-
-  <p class="noindent">and since the length of the corresponding pendulum is <i>l</i>&nbsp;=&nbsp;
-    <span class="frac"><sup><span class="smcap">k</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>,
-  </p>
-
-  <div class="center">
-    <span class="smcap">v</span><sup>2</sup>&nbsp;= 2<i>gl</i>&nbsp;×&nbsp;<span class="frac"><sup>1 − cos <i>ϕ</i></sup>
-    <span>/</span><sub>cos<sup>2</sup> <i>ϕ</i></sub></span>,
-  </div>
-
-  <p>To apply this to any given case we must find the value of <i>l</i> or of&nbsp;
-    <span class="frac"><sup><span class="smcap">k</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>.
-  </p>
-
-  <p>Mallet finds these values for the cube, solid and hollow rectangular
-    parallelopipeds, solid and hollow cylinders, &amp;c. In these formulæ we
-    have a direct connection between the dimensions and form of a body and
-    the velocity with which the ground must move beneath it to cause its
-    overthrow.</p>
-
-  <div class="figright">
-    <img src="images/i_p081.jpg" width="221" height="212" alt="" />
-    <div class="caption"><span class="smcap">Fig. 15.</span></div>
-  </div>
-
-  <p>Not only is the case discussed for horizontal forces, but also for
-    forces acting obliquely. Similar reasonings are applied to the
-    productions of fractures in walls, but as there is uncertainty in our
-    knowledge of the co-efficient of force necessary to produce fracture
-    <em>through joints across</em> beds of masonry, the deductions ought not to
-    be applied as the measures of velocity. Where the fractures occur at
-    the base or in horizontal planes, or in those of the continuous beds of
-    the masonry, or through homogeneous bodies, the uncertainty is not so
-    great, and for cases like these Mallet gives several illustrations. The
-    distance to which bodies had been projected, as, for example, ornaments
-    from the tops of pedestals, coping-stones from the edges of roofs, were
-    also used as means of determining the angle at which the shock had
-    <span class="pagenum" id="Page_82">82</span>
-    emerged, or, if this be known, for determining the velocity.</p>
-
-  <p>Thus by a shock in the direction <span class="smcap">o c</span>, a ball, <span class="smcap">a</span>,
-    on the top of a pedestal would describe a trajectory to the point
-    <span class="smcap">c</span>. Let the angle which <span class="smcap">o c</span> makes with the horizon be
-    <i>e</i>, the vertical height through which the ball has fallen be <i>b</i>, and
-    the horizontal distance of projection be <i>a</i>; then</p>
-
-  <div class="center">
-    <i>b</i>&nbsp;=&nbsp;<i>a</i> tan <i>e</i> +
-    <span class="frac"><sup><i>a</i><sup>2</sup></sup>
-    <span>/</span><sub>4<span class="smcap">h</span> cos<sup>2</sup> <i>e</i></sub></span>,
-  </div>
-
-  <p class="noindent"><span class="smcap">h</span> being the height due to the velocity of projection. Whence</p>
-
-  <div class="center">
-    Tan <i>e</i>&nbsp;=&nbsp;
-    <span class="frac"><sup>2<span class="smcap">h</span> ± √<span class="sqrt">4<span class="smcap">h</span>(<span class="smcap">h</span> + <i>b</i>) − <i>a</i><sup>2</sup></span></sup>
-    <span>/</span><sub><i>a</i></sub></span>
-  </div>
-
-  <div class="center mt1">
-    <span class="smcap">v</span><sup>2</sup>&nbsp;=&nbsp;
-    <span class="frac"><sup><i>a</i><sup>2</sup> <i>g</i></sup>
-    <span>/</span><sub>2 cos<sup>2</sup> <i>e</i> (<i>b</i> − <i>a</i> tan <i>e</i>)</sub></span>.
-  </div>
-
-  <p>For the back motion or subnormal wave in the direction <span class="smcap">c o</span>,</p>
-
-  <div class="center">
-    Tan <i>e</i>&nbsp;=&nbsp;
-    <span class="frac"><sup>2<span class="smcap">h</span> ± √<span class="sqrt">4<span class="smcap">h</span>(<span class="smcap">h</span> + <i>b</i>) − <i>a</i><sup>2</sup></span></sup>
-    <span>/</span><sub><i>a</i></sub></span>
-  </div>
-
-  <div class="center mt1">
-    <span class="smcap">v</span><sup>2</sup>&nbsp;=&nbsp;
-    <span class="frac"><sup><i>a</i><sup>2</sup> <i>g</i></sup>
-    <span>/</span><sub>2 cos<sup>2</sup> <i>e</i> (<i>b</i> + <i>a</i> tan <i>e</i>)</sub></span>.
-  </div>
-
-  <p>A serious error which may enter into calculations of this description
-    when practically applied has been pointed out when speaking of columns
-    as seismometers. It was then shown that such bodies before being
-    overthrown may often be caused to rock, and therefore that their final
-    overthrow may not have any direct connection with the impulse of the
-    succeeding shock.</p>
-
-  <p>Another point to which attention must be drawn respecting the above
-    calculations is that if there was no friction or adherence between the
-    projected body and its<span class="pagenum" id="Page_83">83</span>
-    pedestal, in consequence of its inertia it
-    would be left behind by the forward motion of the shock, and simply
-    drop at the foot of its support. In the case of frictional adherence it
-    would be carried forward by the velocity acquired before this adherence
-    was broken, and thrown in a direction <em>opposite</em> to that given in the
-    figure—that is to say, in the direction of the
-    shock.<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a></p>
-
-  <p><em>The Absolute Intensity of the Force exerted by an Earthquake.</em>—No
-    doubt it has occurred to many who have experienced an earthquake
-    that the power which gave birth to such a disturbance must have been
-    enormously great. The estimates which we shall make of the absolute
-    amount of energy represented by an earthquake cannot, on account of the
-    nature of the factors with which we deal, be regarded as accurate. They
-    may, however, be of assistance in forming estimates of quantities about
-    which we have at present no conception. One method of obtaining the
-    result we seek is that which was employed by Mallet in his calculations
-    respecting the Neapolitan earthquake. Although disbelieving in the
-    general increment of temperature as we descend in the earth at an
-    average rate of 1° F. for every fifty or sixty feet of descent, for
-    want of better means. Mallet assumes this law to be true, and, knowing
-    from a variety of observations the depth of various parts of the cavity
-    from which the disturbance sprang, he calculates the temperatures of
-    this cavity in various parts as due to its depth beneath the surface.
-    Next, it is assumed that steam was suddenly admitted into this cavity,
-    which might exert the greatest possible pressure due to the maximum
-    temperature. This was calculated as being about 684 atmospheres.</p>
-
-  <p><span class="pagenum" id="Page_84">84</span></p>
-
-  <p>Next, he determined the column of limestone necessary to balance such a
-    pressure, which is about 8,550 feet in height. As the least thickness
-    of strata above this cavity was 16,700 feet, the pressure of 684
-    atmospheres was not sufficient to blow away its cover, but if suddenly
-    admitted or generated in the cavity it might have produced the wave of
-    impulse by the sudden compression of the walls of the cavity.</p>
-
-  <p>The pressure of 684 atmospheres is equivalent to about 4·58 tons on
-    the square inch, and, as the total area of the walls of the cavity is
-    calculated at twenty-seven square miles, the total accumulated pressure
-    would be more than 640,528 millions of tons. Mallet, however, shows
-    that it is probable that the temperature of the focal cavity was much
-    greater than that due to the hypogeal increment, and that therefore the
-    pressure may have been greater.</p>
-
-  <p>The capability of producing the earthquake impulse, however, depends
-    on the <em>suddenness</em> with which the steam is flashed off. According
-    to the experiments of Boutigny and others, Mallet tells us that the
-    most sudden production of steam would take place at a temperature of
-    500°-550° C., which is but a few degrees below that calculated for the
-    mean focal depth.</p>
-
-  <p>Assuming the above calculated pressure to be true, and knowing the
-    co-efficient of compression of the materials on which it acted, the
-    volume of the wave at a given moment near the instant of starting—that
-    is, at the focus—can be calculated, and from this the wave amplitude on
-    reaching the surface may be deduced.</p>
-
-  <p>Proceeding backwards, if we have observed the wave amplitude,
-    calculated the depth of the focus, and know the co-efficient of
-    expansion, then the total compression
-    <span class="pagenum" id="Page_85">85</span> may be calculated and the
-    temperature due to the pressure producing this may be arrived at. In
-    this way earthquakes may be used as a means of calculating subterranean
-    temperature at depths that can never be attained experimentally.</p>
-
-  <p>A method of proceeding which is probably more definite than that
-    adopted by Mallet would be the application of the method indicated when
-    speaking of the intensity of artificial disturbances.</p>
-
-  <p>If for a given earthquake the origin of which is known we have
-    determined by seismographs the mean acceleration of an earth particle
-    at two or more stations at different distances from that origin, we are
-    enabled to construct a curve of intensity the area between which and
-    its asymptotes was shown to be a measure of the total intensity of the
-    shock. Comparing this area with that of a unit disturbance produced,
-    say, by the explosion of a pound of dynamite, one may approximately
-    calculate in terms of this unit the initial intensity of the earthquake.</p>
-
-  <p><em>Radiation of an Earthquake.</em>—The tremors preceding the more violent
-    movements of an earthquake may be due, as Mallet has suggested,<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a>
-    to the free surface waves reaching a distant point before the direct
-    vibrations.</p>
-
-  <p>The fact that earth vibrations produced by striking a blow on or near
-    the surface of the ground are wholly obliterated in reaching a cutting
-    or valley, there being no underground waves of distortion to crop up
-    on the opposite side of the valley, indicates that the disturbance is
-    one that travels on the surface; the same fact is illustrated when we
-    endeavour to transmit vibrations through the side of a hill into a
-    tunnel.</p>
-
-  <p>In the tunnel, although the distance may be small,
-    <span class="pagenum" id="Page_86">86</span>no sensible effects
-    are produced, whilst the same disturbance may be recorded at a long
-    distance from its origin on the surface of the ground outside the
-    tunnel.</p>
-
-  <p>Lastly, we may refer to the experiences of miners underground.</p>
-
-  <p>Occasionally it has happened that miners when deep underground, as in
-    the Marienberg in the Saxon Erzgebirge, have felt shocks which have not
-    been noticed on the surface. These observations are rare, and it is
-    possible that they may be explained by the caving in of subterranean
-    excavations.</p>
-
-  <p>The usual experience is, that if a shock is felt underground it is also
-    felt on the surface, as for example in the lead mines in Derbyshire at
-    the time of the Lisbon disturbance (1755).</p>
-
-  <p>The most frequent observation, however, is that a shock may be felt on
-    the surface while it is not remarked by the miners beneath the surface,
-    as at Fahlun and Presburg in November, 1823.</p>
-
-  <p>At the Comstock Lode in Colorado about twelve years ago many
-    earthquakes were felt. On one particular day twenty-four were counted.
-    Superintendent Charles Foreman told the author when he visited Virginia
-    City in 1882, that special observations were made to determine whether
-    these shocks were felt as severely deep down in the mines as on the
-    surface, where they were on the verge of being destructive. The
-    universal testimony of many observers was that in most cases they were
-    not felt at all underground, and when a shock was felt it was extremely
-    feeble. At Takashima Colliery, in Japan, it is seldom that shocks are
-    felt underground.</p>
-
-  <p>The explanation of these latter observations appears to be either
-    that, in consequence of a smaller amplitude
-    <span class="pagenum" id="Page_87">87</span> of motion in the solid
-    rocks beneath the surface as compared with the extent of motion on
-    the surface, the disturbances are passed by unnoticed, or else the
-    disturbance is, at a distance from its origin, practically confined to
-    the surface.</p>
-
-  <p><em>Velocity of Propagation of an Earthquake.</em>—Although many have written
-    upon earthquakes and have endeavoured to give to us the velocity with
-    which they were propagated, the subject is one about which we have as
-    yet but little exact information.</p>
-
-  <p>The importance of this branch of investigation is undoubtedly great.
-    By knowing the velocity with which an earthquake has travelled in
-    various directions we are assisted in determining the locality of
-    its origin; we may possibly make important deductions respecting the
-    nature of the medium through which it has passed; perhaps also we
-    may learn something regarding the intensity of the disturbance which
-    created the earthquake. In the Report of the British Association for
-    1851 Mallet gives the table on next page, in which are placed together
-    the approximate rates of transit of shocks of several earthquakes
-    which he discusses. Some of these, it will be observed, are records of
-    disturbances which must have passed through or across the bed of the
-    ocean.</p>
-
-  <p>In Mallet’s British Association Report for 1858, he gives data compiled
-    by Mr. David Milne<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a> respecting the Lisbon earthquakes of 1755 and
-    1761, from which data the tables of velocities <a href="#Page_89">(p. 89)</a> have been
-    calculated, omitting those which Mr. Mallet has marked as uncertain.</p>
-
-  <p>The distances are marked in degrees of seventy English miles each, and
-    the time is reduced to Lisbon time.</p>
-
-  <p><span class="pagenum" id="Page_88">88</span></p>
-
-  <table id="propogation" class="collapse" summary="Propogation velocity">
-    <tr>
-      <th class="ball">Occasion and Place</th>
-      <th class="ball">Approx. rate in feet per second</th>
-      <th class="ball">Formation constituting Range on surface as far as known or conjectured</th>
-      <th class="ball">Authority</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Rev. John Mitchell’s guesses from the Lisbon earthquakes</td>
-      <td class="tdr br"><div>1,760</div></td>
-      <td class="tdl br">Sea bottom, probably on slates, secondary and crystalline rocks</td>
-      <td class="tdc br">Mitchell</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Von Humboldt’s guesses from South America</td>
-      <td class="tdr br"><div>1,760 to 2,464</div></td>
-      <td class="tdl br">From observations in various South American rocks in great part volcanic</td>
-      <td class="tdc br">Humboldt</td>
-    </tr>
-    <tr>
-      <td class="tdc bl br pt2"><i>Lisbon Earthquake of 1761.</i></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Lisbon to Corunna</td>
-      <td class="tdr br"><div>1,994</div></td>
-      <td class="tdl br">Transition, carboniferous and granitoid</td>
-      <td class="tdc br">‘Annual Register’</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Lisbon to Cork</td>
-      <td class="tdr br"><div>5,228</div></td>
-      <td class="tdl br">Transition, carboniferous crystalline slates and granitoid, probably, under sea bottom</td>
-      <td class="tdc br">„</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Lisbon to Santa Cruz</td>
-      <td class="tdr br"><div>3,261</div></td>
-      <td class="tdl br">The same with many alterations</td>
-      <td class="tdc br">„</td>
-    </tr>
-    <tr>
-      <td class="tdc bl br pt2"><i>Antilles.</i></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Pointe à Pitre to Cayenne (doubtful)</td>
-      <td class="tdr br"><div>6,586</div></td>
-      <td class="tdl br">Probably volcanic rocks under sea bottom</td>
-      <td class="tdc br">Stier and Perrey’s memorandum, Dijon</td>
-    </tr>
-    <tr>
-      <td class="tdc bl br pt2"><i>India.</i></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Cutch to Calcutta, 1819</td>
-      <td class="tdr br"><div>1,173</div></td>
-      <td class="tdl br">Alluvial, secondary, granitoid and later igneous rocks</td>
-      <td class="tdc br">‘Royal Asiatic Journal’</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">India, Nepauls, and basin of the Ganges, 1834:&mdash;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Rungpur to Arrah</td>
-      <td class="tdr1 br" rowspan="4">
-        <table summary="Propogation sub-table">
-          <tr>
-            <td class="tdr1">
-              <div>
-                2,314<br />
-                3,520<br />
-                990<br />
-                1,210
-              </div>
-            </td>
-            <td class="x400">}</td>
-          </tr>
-        </table>
-      </td>
-      <td class="tdl br" rowspan="4">Deep alluvia, with occasional transition, carboniferous, granitoid, and later igneous rocks</td>
-      <td class="tdc br" rowspan="4">‘Royal Asiatic Journal’</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Monghyr to Gorackpur</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Rungpur to Monghyr</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Rungpur to Calcutta</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br bb">Ships ‘Rambler’ and ‘Millwood,’ at sea, 1851; between lat. 16° 30′ N.L., 54° 30′ W., and lat. 23° 30′ N.L., 58° 0′ W.</td>
-      <td class="tdr br bb"><div>1,056</div></td>
-      <td class="tdl br bb">Sea bottom resting on unknown rock</td>
-      <td class="tdc br bb">‘Nautical Magazine’</td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_89">89</span></p>
-
-  <p class="center mt2"><span class="smcap">The Lisbon Earthquake on November 1, 1755.</span></p>
-
-  <table class="collapse" summary="Propogation velocity Lisbon 1755">
-    <tr>
-      <th class="ball">Localities</th>
-      <th class="ball" colspan="2">Moment observed of shock</th>
-      <th class="ball" colspan="2">Distance from presumed origin</th>
-      <th class="ball">Velocity in feet per second</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">&nbsp;</td>
-      <td class="tdr"><div>h.</div></td>
-      <td class="tdrx br"><div>m.</div></td>
-      <td class="tdr"><div>°</div></td>
-      <td class="tdrx br"><div>′</div></td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Presumed focus of shock, lat. 30°, long. 11° W.</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>23</div></td>
-      <td class="tdc br" colspan="2">&mdash;</td>
-      <td class="tdc br">&mdash;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">A ship at sea in lat. 38°, long. 10° 47′ W.</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>24</div></td>
-      <td class="tdr"><div>0</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>3,091</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Colares</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdr"><div>1</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>1,325</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Lisbon</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>32</div></td>
-      <td class="tdr"><div>1</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>1,030</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Oporto</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>38</div></td>
-      <td class="tdr"><div>2</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>1,030</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Ayamonte</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>50</div></td>
-      <td class="tdr"><div>4</div></td>
-      <td class="tdrx br"><div>0</div></td>
-      <td class="tdrx br"><div>916</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Cadiz</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>48</div></td>
-      <td class="tdr"><div>5</div></td>
-      <td class="tdrx br"><div>0</div></td>
-      <td class="tdrx br"><div>1,236</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Tangier and Tetuan</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>46</div></td>
-      <td class="tdr"><div>5</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>1,478</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Madrid</td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdrx br"><div>43</div></td>
-      <td class="tdr"><div>6</div></td>
-      <td class="tdrx br"><div>0</div></td>
-      <td class="tdrx br"><div>1,855</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Funchal</td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdrx br"><div>1</div></td>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>1,382</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Portsmouth</td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdrx br"><div>3</div></td>
-      <td class="tdr"><div>12</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>1,431</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Havre</td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdrx br"><div>23</div></td>
-      <td class="tdr"><div>13</div></td>
-      <td class="tdrx br"><div>0</div></td>
-      <td class="tdrx br"><div>1,339</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Reading</td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdrx br"><div>27</div></td>
-      <td class="tdr"><div>13</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>1,304</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Yarmouth</td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdrx br"><div>42</div></td>
-      <td class="tdr"><div>15</div></td>
-      <td class="tdrx br"><div>0</div></td>
-      <td class="tdrx br"><div>1,174</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Amsterdam</td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdrx br"><div>6</div></td>
-      <td class="tdr"><div>17</div></td>
-      <td class="tdrx br"><div>0</div></td>
-      <td class="tdrx br"><div>2,444</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br bb">Loch Ness</td>
-      <td class="tdr bb"><div>10</div></td>
-      <td class="tdrx br bb"><div>42</div></td>
-      <td class="tdr bb"><div>18</div></td>
-      <td class="tdrx br bb"><div>0</div></td>
-      <td class="tdrx br bb"><div>1,409</div></td>
-    </tr>
-  </table>
-
-  <p class="center mt2"><span class="smcap">The Lisbon Earthquake of March 31, 1761.</span><a
-    id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a></p>
-
-  <table class="collapse" summary="Propogation velocity Lisbon 1761">
-    <tr>
-      <th class="ball">Locality</th>
-      <th class="ball" colspan="2">Moment observed of shock</th>
-      <th class="ball" colspan="2">Distance from presumed origin</th>
-      <th class="ball">Velocity in feet per second</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">&nbsp;</td>
-      <td class="tdr"><div>h.</div></td>
-      <td class="tdrx br"><div>m.</div></td>
-      <td class="tdr"><div>°</div></td>
-      <td class="tdrx br"><div>′</div></td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Presumed focus, lat. 43°, long. 11° W.</td>
-      <td class="tdr"><div>11</div></td>
-      <td class="tdrx br"><div>51</div></td>
-      <td class="tdc br" colspan="2">&mdash;</td>
-      <td class="tdc br">&mdash;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Ship at sea in lat. 43°, many leagues from coast of Portugal</td>
-      <td class="tdr"><div>11</div></td>
-      <td class="tdrx br"><div>52</div></td>
-      <td class="tdr"><div>0</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>3,091</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Ship in lat. 44° 8′ and about 80 leagues from coast</td>
-      <td class="tdr"><div>11</div></td>
-      <td class="tdrx br"><div>54</div></td>
-      <td class="tdr"><div>1</div></td>
-      <td class="tdrx br"><div>45</div></td>
-      <td class="tdrx br"><div>3,607</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Corunna</td>
-      <td class="tdr"><div>11</div></td>
-      <td class="tdrx br"><div>51</div></td>
-      <td class="tdr"><div>2</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>2,576</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Ship lat. 44° 8′ and 80 leagues NNW. of Cape Finisterre</td>
-      <td class="tdr"><div>11</div></td>
-      <td class="tdrx br"><div>58</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>3,091</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Lisbon</td>
-      <td class="tdc br" colspan="2">noon</td>
-      <td class="tdr"><div>4</div></td>
-      <td class="tdrx br"><div>30</div></td>
-      <td class="tdrx br"><div>3,091</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Madeira</td>
-      <td class="tdr"><div>12</div></td>
-      <td class="tdrx br"><div>6</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdrx br"><div>0</div></td>
-      <td class="tdrx br"><div>4,122</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br bb">Cork</td>
-      <td class="tdr bb"><div>12</div></td>
-      <td class="tdrx br bb"><div>11</div></td>
-      <td class="tdr bb"><div>9</div></td>
-      <td class="tdrx br bb"><div>30</div></td>
-      <td class="tdrx br bb"><div>2,937</div></td>
-    </tr>
-  </table>
-
-  <p>These tables, owing to the nature of the materials
-    <span class="pagenum" id="Page_90">90</span>which Mallet had at
-    his disposal, are but rude approximations to the truth. Two interesting
-    facts are, however, observable: the first being that the velocities
-    for the earthquake of 1761 are much higher than those obtained for the
-    earthquake of 1755; and, secondly, that in both cases the velocities as
-    determined from the observations of ships at sea closely approximate
-    to each other, in all cases being nearly the same as that with which a
-    sound wave would travel through water.</p>
-
-  <p>The great differences in transit velocity obtained for different
-    earthquakes is a point worthy of attention.</p>
-
-  <p>Seebach’s velocity is a <em>true</em> transit velocity, and its determination
-    is dependent on the assumption that the shock radiated from the
-    <em>centrum</em> and not from the <em>epicentrum</em>, Seebach’s method is explained
-    when speaking about the determination of origins.</p>
-
-  <p>Some interesting observations on the velocity with which the earthquake
-    of October 7, 1874, was propagated, are given by M. S. di Rossi.<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a></p>
-
-  <p>One assumption is that the disturbance radiated from an origin
-    to surrounding points of observation, whilst another is that the
-    disturbance followed natural fractures, the direction of which is
-    derived from the crest of certain mountain ranges. These velocities are
-    as follows, Maradi being at or near the origin of the disturbance:—</p>
-
-  <table class="collapse" summary="Rossi's velocity results">
-    <tr>
-      <td colspan="2" class="tdc ball">Velocity in feet per second with direct radiation</td>
-      <td colspan="2" class="tdc ball">Velocity in feet per second by propagation along mountain chains</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Modigliana</td>
-      <td class="tdr br">820</td>
-      <td class="tdl">By the Valley of Marenzo</td>
-      <td class="tdr br"><div>1,080</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Bologna</td>
-      <td class="tdr br">656</td>
-      <td class="tdl">&nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Saveno</td>
-      <td class="tdr br"><div>1,080</div></td>
-    </tr>
-    <tr>
-    <td class="tdl bl">Forli</td>
-      <td class="tdr br">874</td>
-      <td class="tdl">&nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Montone</td>
-      <td class="tdr br"><div>1,080</div></td>
-    </tr>
-    <tr>
-      <td class="tdl vm bl">Modena</td>
-      <td class="tdr vm br">518</td>
-      <td class="tdl vm">&nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Panaro</td>
-      <td class="tdr0 pr0 br">
-        <div>
-          <table summary="maradi1" class="mr1">
-            <tr>
-              <td rowspan="2" class="tdr0 pr0"><div class="x200">{</div></td>
-              <td class="tdr0 pr1"><div>1,080</div></td>
-            </tr>
-            <tr>
-              <td class="tdr0 pr1"><div>984</div></td>
-            </tr>
-          </table>
-        </div>
-      </td>
-     </tr>
-     <tr>
-      <td class="tdl bl">Firenze</td>
-      <td class="tdr br">273</td>
-      <td class="tdl">&nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Sieve</td>
-      <td class="tdr br"><div>540</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl bb">Compiobbi</td>
-      <td class="tdr br bb">328</td>
-      <td class="tdl bb">&nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„</td>
-      <td class="tdr br bb"><div>540</div></td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_91">91</span></p>
-  <p>Another set of interesting results are those of P. Serpieri on the
-    earthquake of March 12, 1873. The curious manner in which this shock
-    radiated is described in the chapter on the Geographical Distribution
-    of Earthquakes (see <a href="#Page_231">p. 231</a>). Two large areas appear to have been
-    almost simultaneously struck, so that, there being no time for elastic
-    yielding, the velocities calculated between places situated on either
-    of the areas are exceedingly great.<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a></p>
-
-  <table summary="Serpieri's velocity results">
-    <tr>
-      <td class="tdl">From Ragusa to Venice the velocity was</td>
-      <td class="tdr"><div>2,734</div></td>
-      <td class="tdl">feet per second</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; „&nbsp; &nbsp; &nbsp; Spoleto&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </td>
-      <td class="tdr"><div>4,101</div></td>
-      <td class="tdl">&nbsp; &nbsp;&nbsp;„ &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; „&nbsp; &nbsp; &nbsp; Perugia to Orvieto&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;</td>
-      <td class="tdr"><div>601</div></td>
-      <td class="tdl">&nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; „ &nbsp; Ancona</td>
-      <td class="tdr"><div>1,640</div></td>
-      <td class="tdl">&nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; „ &nbsp; Rome</td>
-      <td class="tdr0 pr0">
-        <div>
-          <table summary="Serpieri sub-table">
-            <tr>
-              <td rowspan="2" class="tdr0 pr0"><span class="x200">{</span></td>
-              <td class="tdr0 pr0"><div>1,640</div></td>
-            </tr>
-            <tr>
-              <td class="tdr0 pr0"><div>or, 2,186</div></td>
-            </tr>
-          </table>
-        </div>
-      </td>
-      <td class="tdl">&nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„</td>
-    </tr>
-  </table>
-
-  <p>The following are examples of approximate earthquake velocities which
-    have been determined in Japan.</p>
-
-  <p><em>The Tokio Earthquake of October 25, 1881.</em>—From records respecting
-    this earthquake it appears to have been felt over the whole of Yezo
-    and the northern and eastern coast of Nipon, a little farther south
-    than Tokio. It was severest at Nemuro and Hakodate, and at the former
-    place a little damage was done. From these facts, together with the
-    indications of instruments recording direction of movement and a
-    general inspection of the time records, it seems that the disturbance
-    must have originated beneath the sea on the east coast of Yezo at a
-    very long distance to the north-east of Tokio, from which place it
-    passed in a practically direct line on to Yokohama.</p>
-
-  <p>As the disturbance was felt at Yokohama twenty-one seconds later than
-    at Tokio, and the distances between these two places is about sixteen
-    geographical miles, for this portion of its course the disturbance must
-    have travelled at a rate of at least 4,300 <em>feet per second</em>. If
-    <span class="pagenum" id="Page_92">92</span>we
-    assume that the shock, after having reached Hakodate, travelled on at
-    the same rate as it did between Tokio and Yokohama in order to reach
-    Saporo, where the shaking was felt eighteen seconds after Hakodate,
-    it must have had about thirteen geographical miles to travel after
-    Hakodate was shaken before Saporo felt its effect.</p>
-
-  <p>Drawing from Hakodate a tangent to the eastern side of a circle of
-    thirteen miles radius described round Saporo, the origin of the
-    disturbance must be on the line bisecting this tangent at right angles.
-    As it also lies on a line drawn through Tokio and Yokohama, it lies in
-    a position about 41 N. lat. and 144° 15′ E. long., which is a position
-    somewhat nearer to Nemuro than Hakodate, as we should anticipate. If
-    this be taken as approximately indicating the origin, then the shock,
-    after reaching Hakodate from the Hakodate <em>homoseist</em>, travelled about
-    218 miles to reach Tokio in 128 seconds, which gives a <em>velocity of
-      10,219 feet per second</em>.</p>
-
-  <p>The method here followed is equivalent to that of the hyperbola and one
-    direction (see <a href="#Page_204">p. 204</a>). The hyperbola is described on the assumption
-    that the velocity deduced from the time taken to travel between Tokio
-    and Yokohama is correct, and also that the earth waves travelled with
-    approximately the same velocity in the vicinity of Saporo as near
-    Tokio. The probability, however, is that they travelled more quickly.
-    If this be so, then the origin is thrown somewhat to the south-east and
-    the velocity between the Hakodate homoseist and Tokio reduced. Thus,
-    if the velocity in the Saporo district be double that observed in the
-    Tokio district, the origin is shifted about twenty-eight miles to the
-    south-west, and the last-mentioned velocity is reduced to about 9,000
-    feet per second.</p>
-
-  <p>If we work by the method of circles, and assume the velocity to have
-    been constant in all directions, then this
-    <span class="pagenum" id="Page_93">93</span> velocity must have been
-    about 6,000 feet per second. If we assume that the indications of
-    direction obtained from seismographs and other sources give to us by
-    this intersection a proper origin, the velocity in some directions may
-    have been as much as 17,000 feet per second.</p>
-
-  <p>An origin thus determined, or even if determined by the method of
-    circles, is in discord with the fact that places like Nemuro, in the
-    north-east of Yezo, were nearer to the origin than any of the other
-    places which have been mentioned.</p>
-
-  <p>The conclusion which we are therefore led to with regard to this shock,
-    assuming of course that the time observations are tolerably correct,
-    is that the velocity of propagation was variable, being greater when
-    measured between points near to the origin than between points at a
-    distance. The velocities estimated vary between 4,000 and 9,000 feet
-    per second.</p>
-
-  <p>In the case of the earthquake which has just been discussed, we have
-    an example of a disturbance which must have passed between Tokio and
-    Yokohama in what was almost a straight line from the origin. As this
-    direction ought to give the maximum time of transit if all earthquakes
-    are propagated with the same velocity, the following table is given of
-    the interval between the time of observation of several shocks at these
-    two stations:—</p>
-
-  <table class="collapse" summary="Yokohama to Tokio transit interval">
-    <tr>
-      <th><span class="smcap">From Yokohama to Tokio.</span></th>
-      <th><span class="smcap">From Tokio to Yokohama.</span></th>
-    </tr>
-    <tr>
-      <td class="tdl br">1880 December 20th&nbsp; &nbsp; 36 seconds</td>
-      <td>1882 October 25th&nbsp; 21 seconds</td>
-    </tr>
-    <tr>
-      <td class="tdl br">1881 January 7th&nbsp; &nbsp; 14–31&nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdl">1883 February 6th&nbsp; &nbsp;23&nbsp; &nbsp; &nbsp;„</td>
-    </tr>
-    <tr>
-      <td class="tdl br">&nbsp; &nbsp;„&nbsp; &nbsp; March&nbsp; 8th&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;60&nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdl">&nbsp; &nbsp;„&nbsp; &nbsp; March 1&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 53&nbsp; &nbsp; &nbsp;„</td>
-    </tr>
-    <tr>
-      <td class="tdl br">&nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; 17th.&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;66&nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdl">&nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 63&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl br">&nbsp; &nbsp;„&nbsp; &nbsp; November 15th&nbsp; &nbsp; 31&nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdl">&nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; 8th &nbsp; &nbsp; &nbsp; 27&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl br">1882 February 16th&nbsp; &nbsp; &nbsp; 22&nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdl">&nbsp; &nbsp;„&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; 11th&nbsp; &nbsp; &nbsp; &nbsp;26&nbsp; &nbsp; „</td>
-    </tr>
-  </table>
-
-  <p>As these are observations which have been made with the assistance of a
-    telegraphic signal daily employed to
-    <span class="pagenum" id="Page_94">94</span> correct and rate the clocks from
-    which the observations were obtained, they may be regarded as being
-    tolerably, correct.</p>
-
-  <p>The disturbance of February 6, the two shocks of March 1, appear, like
-    that of October 25, to have passed in almost a direct line from an
-    origin in the N.N.E, through Tokio on to Yokohama. Their velocities of
-    propagation as calculated from the above intervals are approximately
-    3,900, 1,900, and 1,400 feet per second. The shock of February 16
-    appears to have had its origin near to a point in Yedo Bay about eight
-    miles east of Yokohama. Assuming this to be the case, the shock between
-    the Yokohama homoseist and Tokio travelled at the rate of 2,454 feet
-    per second, but between the Tokio homoseist and Chiba at the rate
-    of 750 feet per second; that is to say, the velocity of propagation
-    rapidly decreased as the disturbance spread outwards.</p>
-
-  <p>At Yokohama it was recorded at 5.31.54, at Tokio at 5.32.16, and at
-    Chiba at 5.33.48. These times are given in Tokio mean time.</p>
-
-  <p>The shock of March 11, which was recorded at Tokio at 7.51.22
-    <span class="smcap">p.m.</span> and at Yokohama at 7.51.33 <span class="smcap">p.m.</span>, appears, from
-    the indications of instruments which were exceptionally definite in
-    their records, to have originated in the N.E. corner of Yedo Bay,
-    about nineteen miles S.S.W. from Chiba. This shock was rather severe,
-    fracturing several chimneys. From the Tokio homoseist it appears to
-    have travelled on to Yokohama at the rate of about 2,200 feet per
-    second. Assuming these observations to be <em>approximately</em> accurate, if
-    we take them with the records of previous observers they lead us to the
-    following conclusions:—</p>
-
-  <p>1. Different earthquakes, although they may travel across the same
-    country, have very variable velocities,
-    <span class="pagenum" id="Page_95">95</span> varying between several
-    hundreds and several thousands of feet per second.</p>
-
-  <p>2. The same earthquake travels more quickly across districts near to
-    its origin than it does across districts which are far removed.</p>
-
-  <p>3. The greater the intensity of the shock the greater is the velocity.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_VI">
-    <span class="pagenum" id="Page_96">96</span>
-    <h2>CHAPTER VI.<br />
-      <span class="subhead">EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    The destruction of buildings is not irregular—Cracks in
-    buildings—Buildings in Tokio—Relation of destruction to
-    earthquake motion—Measurement of relative motion of parts of a
-    building shaken by an earthquake—Prevention of cracks—Direction
-    of cracks—The pitch of roofs—Relative position of openings in a
-    wall—The last house in a row—The swing of buildings—Principle of
-    relative vibrational periods.</div>
-
-  <p class="noindent"><span class="smcap">The</span> subject of this
-    chapter is, from a practical point of view, one
-    of the most important with which a seismologist has to deal. We
-    cannot prevent the occurrence of earthquakes, and unless we avoid
-    earthquake-shaken regions, we have not the means of escaping from
-    them. What we can do, however, is in some degree to protect ourselves.
-    By studying the effects produced by earthquakes upon buildings of
-    different construction and variously situated, we are taught how to
-    avoid or at least to mitigate calamities which, in certain regions of
-    the world, are continually repeated. The subject is an extensive one,
-    and what is here said about it must be regarded only as a contribution
-    to the work of future writers who may give it the attention it
-    deservedly requires.</p>
-
-  <p><em>The Destruction produced by Earthquakes is not irregular.</em>—If we
-    were suddenly placed amongst the ruins of a large city which had
-    been shattered by an earthquake, it is doubtful whether we should at
-    once<span class="pagenum" id="Page_97">97</span>
-    recognise any law as to the relative position of the masses of
-    <i xml:lang="fr">débris</i> and the general destruction with which we are surrounded.
-    The results of observation have, however, shown us that, amongst the
-    apparently chaotic ruin produced by earthquakes, there is in many cases
-    more or less law governing the position of bodies which have fallen,
-    the direction and position of cracks in walls, and the various other
-    phenomena which result from such destructive disturbances.</p>
-
-  <p>Mallet, at the commencement of his first volume, describing the
-    Neapolitan earthquake of 1857, discusses the general effect produced by
-    various shocks upon differently constructed buildings. First he shows
-    that, if we have a rectangular building, the walls at right angles to
-    the shock will be more likely to be overthrown than those which are
-    parallel to it. Experience teaches a similar lesson. Thus Darwin, when
-    speaking of the earthquake at Concepcion in 1835,<a id="FNanchor_20"
-    href="#Footnote_20" class="fnanchor">[20]</a> tells us that
-    the town was built in the usual Spanish fashion, with all the streets
-    running at right angles to each other. One set ranged S.W. by W. and
-    N.E. by E. and the other N.W. by N. and S.E. by S. The walls in the
-    former direction certainly stood better than those in the latter. The
-    undulations came from the S.W.</p>
-
-  <p>In Caraccas it is said that every house has its <i xml:lang="es">laga securo</i>, or safe
-    side, where the inhabitants place their fragile property. This <i xml:lang="es">laga
-    securo</i> is the north side, and it was chosen because about two out of
-    every three destructive shocks traversed the city from west to east,
-    so that the walls in these sides of a building have been stricken
-    broadside on.<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a></p>
-
-  <p><span class="pagenum" id="Page_98">98</span></p>
-
-  <p><em>Cracks in Buildings.</em>—Results like the above come from destructive
-    earthquakes rather than from movements such as those we have to deal
-    with ordinarily. When a building is subjected to a slight movement
-    it is assumed that the walls at right angles to the direction of the
-    shock move backwards and forwards as a whole, and there is little or
-    no tendency for them to be fractured at their weaker parts, these
-    weaker parts being those over the various openings. The walls, however,
-    which are parallel to the direction of the movement are, so to speak,
-    extended and contracted along their length, and in consequence they may
-    be expected to give way over the various openings. This tendency for
-    extension and contraction of a wall along its length may be supposed,
-    for instance, to be due to the different portions of a wall, owing to
-    differences in dimensions and elasticity, having different periods of
-    natural vibration, or possibly for two portions of a long line of wall
-    to be simultaneously affected by portions of waves in different phases.</p>
-
-  <p>As an illustration of the giving way of a building in the manner here
-    suggested we may take the case of a large brick structure which was
-    recently being erected in Tokio. This building, at the time of the
-    earthquake, was only some fourteen or fifteen feet above the surface
-    of the ground. The length of the building stretched from N.W. to S.E.,
-    and it was intersected by many walls at right angles to this direction.
-    Through all the walls of this building there were many arched openings.
-    In the central part of the transverse walls, which walls were fully
-    five feet in thickness, the arches which joined them together were 4
-    feet 4 inches in thickness. The arches therefore formed a comparatively
-    lightly constructed link between heavy masses of brickwork.</p>
-
-  <p>On March 3, 1879, at 4.43 <span class="smcap">p.m.</span>, an earthquake was
-    <span class="pagenum" id="Page_99">99</span> felt
-    throughout Tokio, the strength of which, as judged by our feelings,
-    was above that of an average shock. As registered by one of Palmieri’s
-    instruments, it had a direction S.S.W. to N.N.E. and an intensity of
-    11°. On the same day there were several smaller shocks having the same
-    direction, and these were succeeded by others on the 9th of the month.</p>
-
-  <p>Immediately after these shakings it was discovered that almost every
-    arch in the internal walls of the building here referred to had been
-    cracked across the crown in a direction about N. 40° W. All the
-    other arches of the building, of which there were a great number in
-    walls at right angles to the direction of the shock, were found not
-    to have sustained any injury. To this statement, however, there was
-    one exception, which was subsequently proved to have been due to a
-    settlement taking place.</p>
-
-  <p>After examining these cracks the only cause to which they could be
-    attributed was the series of shakings which they had just experienced.
-    It seemed as if the heavy walls right and left of the arches had been
-    in vibration without synchronism in their periods, and as a consequence
-    the arches which connected them had been torn asunder.</p>
-
-  <p>Although the time at which the cracks were formed and the peculiar
-    positions in which they were only to be found pointed distinctly to
-    their origin, to be certain that they were not due to settlement of the
-    foundations, horizontal lines were ruled upon the brickwork and from
-    time to time subsequently observed.</p>
-
-  <p>The points to which the various cracks extended were also marked and
-    observed. Beneath the walls as foundations there were beds of concrete
-    about three feet thick and about ten feet in width. These had been
-    under the pressure of the partially built walls for two years before
-    <span class="pagenum" id="Page_100">100</span>
-    the arches had been put in. As these foundations were unusually strong,
-    being intended to carry so very much greater weight than that to which
-    they had been subjected, if any settlement had been detected it would
-    have been a matter of surprise.</p>
-
-  <p>Some weeks after the formation of these cracks it was observed that
-    they gradually closed. This was probably due to the gradual falling
-    inwards of the two broken portions of the arch, their position when
-    open being one of instability.</p>
-
-  <div class="figleft">
-    <img src="images/i_p100.jpg" width="195" height="296" alt="" />
-    <div class="caption"><span class="smcap">Fig. 16.</span>—Cracks in a corner house,<br />Belluno, June 29, 1873
-    (Bittner).</div>
-  </div>
-
-  <p>Had this building been more complete at the time of the shock, and
-    the heavy walls been tied together at higher points, although the
-    archways would have been points of weakness, it is quite possible
-    that fracture would not have taken place. This illustration shows us
-    that when a building is shaken in a definite direction there will be
-    some rule as to the positions in which fractures occur. As another
-    example, we may take the observations of Alexander Bittner upon the
-    buildings of Belluno after the shock of June 29, 1873 (see Beiträge
-    zur Kenntniss des Erdbebens von Belluno am 29 Juni, 1873, p. 40. Von
-    Alexander Bittner. Aus dem LXIX. Bande der Sitzungsb. der K. Akad. der
-    Wissensch., II. Abth., April-Heft. Jahrg. 1874).</p>
-
-  <p>Speaking generally, he remarks that ‘Houses similarly situated have
-    suffered in corresponding walls and corners in a similar manner. In
-    Belluno there is a certain kind of damage which is repeated everywhere,
-    making a peculiar<span class="pagenum" id="Page_101">101</span>
-    system of splits in the S.W. and N.E. corners of the
-    houses.’ This is well shown in the accompanying sketch, which evidently
-    illustrates the effect of a shock oblique to the direction of two walls
-    at right angles to each other.</p>
-
-  <div class="figright">
-    <img src="images/i_p101.jpg" width="345" height="268" alt="" />
-    <div class="caption"><span class="smcap">Fig. 17.</span>—Brick buildings<br />in Tokio, showing
-    fractures.</div>
-  </div>
-
-  <p><em>Buildings in Tokio.</em>—For the purpose of finding out what has been the
-    effect produced by earthquakes upon the buildings of Tokio, and at
-    the same time for ascertaining whether blocks of buildings ranging in
-    different directions suffered to the same extent, the author examined,
-    in company with Mr. Josiah Conder, a large number of foreign-built
-    houses in the district of the Ginza. The chief reason for choosing
-    this was because it was the only district where a large number of
-    <em>similar</em> buildings could be found. By examining houses or buildings of
-    different constructions, the effects produced upon them by earthquakes
-    are very often likely to show so many differences that it becomes
-    almost an impossibility to determine what the general effect has
-    been—unsymmetrical construction involving unsymmetrical ruin.</p>
-
-  <p>A number of similarly constructed buildings in one locality may be
-    regarded as a number of seismographs, the effect upon any one of them
-    being judged of by the average of the general effect which has been
-    produced upon the whole. The general form of two of these houses
-    which were examined is shown in fig. 17. In this figure
-    <span class="pagenum" id="Page_102">102</span> the general
-    character of the fractures which have been produced can also be seen.
-    The houses are built of brick, and are in many cases faced with a thin
-    coat of white plaster. Projecting from the level of the upper floor
-    there is a balcony fronted by a low balustrade. This is supported by
-    small beams which at their outer extremity are carried on a row of
-    cylindrical columns. This forms a covered way in front of each row of
-    houses. The roofs are covered with thick tiles. It will be observed
-    that the arches of the upper windows spring <em>sharply</em> from their
-    abutments, and at their crown they carry a heavy key-stone. The lower
-    openings, which have a span of 9 feet, have evidently been constructed
-    in imitation of the open front of an ordinary Japanese house. These
-    archways curve out <em>gently</em> from their abutments. The outside walls
-    have a thickness of 13½ inches.</p>
-
-  <p>The results obtained from a careful examination of 174 houses in
-    streets running N.E. and 156 houses in streets running N.W., all of
-    these houses being similar, were as follows:—</p>
-
-  <p>1. In the upper windows nearly all the cracks ran from the springing,
-    which formed an angle with the abutment.</p>
-
-  <p>2. In the lower arches, which <em>curved</em> into the abutments, not a single
-    crack was observed at the springway. The cracks in these arches were
-    near the crown, where beams projected to carry the balcony. In many
-    instances the cracks proceeded from such beams, even if there were
-    no arch beneath. That cracks should occur in peculiar positions, as
-    is here indicated, is shown in the illustrations which accompany the
-    accounts of many earthquakes.</p>
-
-  <p>3. The houses which were most cracked were in the streets running
-    parallel to the direction in which the greater number and most powerful
-    set of shocks cross the city.</p>
-
-  <p><span class="pagenum" id="Page_103">103</span></p>
-
-  <p>The results showed that, in order to avoid the effects of small shocks,
-    all walls containing principal openings should be placed as nearly
-    as possible at right angles to the direction in which the shocks of
-    the districts usually travel. The blank walls, or those containing
-    unimportant openings, would then be parallel to the direction of the
-    shocks—that is, presuming our building to be made up of two sets of
-    walls at right angles to each other.</p>
-
-  <p>Another point of importance would be to build archways <em>curving</em> into
-    the supporting buttresses; the archways over doors and windows which we
-    find in earthquake countries do not appear to be in any way different
-    from those which are built in countries free from earthquakes. In
-    the one country these structures have simply to withstand vertical
-    pressures applied statically; in the other, they have to withstand more
-    or less horizontal stresses, applied suddenly.</p>
-
-  <p><em>Relation of Destruction to Earthquake Motion.</em>—The relations which
-    exist between the overturning and projection of bodies and the motion
-    of the ground have already been discussed. It may be interesting
-    to call attention to the fact that in the formulæ showing three
-    relationships, it was the <em>shape</em> rather than the <em>weight</em> of a body
-    which determined whether it should be overturned or projected by a
-    motion at its base.</p>
-
-  <p>As an interesting proof that light bodies may be overturned as easily
-    as heavy ones. Mallet refers to the overturning of several large
-    haystacks as one of the results of the Neapolitan earthquake.</p>
-
-  <p>If masses of material are displaced or fractured, then Mallet remarks
-    that the maximum velocity will exceed √<span class="sqrt3">2<i>gh</i></span>, where <i>h</i> is the
-    amplitude of the wave. Should the maximum velocity be less than this
-    quantity, the masses which are acted upon will be simply raised and
-    lowered,<span class="pagenum" id="Page_104">104</span>
-    and there will be no relative displacements even if the
-    emergence of the wave be nearly or quite vertical.</p>
-
-  <p>When we get a vertical wave acting upon an irregular mass of masonry,
-    the heavier portions of the masonry, by their inertia, tend to descend
-    relatively to the remaining portions, and in this way vertical fissures
-    will be produced. For this reason it would not be advisable to use
-    heavy materials above archways, heavy roofs, or heavy floors. The
-    vertical fissures, Mallet remarks, would have their widest opening at
-    the base.</p>
-
-  <p>In considering cases of fracture produced by earthquake motion, it
-    must be remembered that these are due to stresses applied <em>suddenly</em>,
-    and that if the same amount of stress had been <em>slowly</em> applied to a
-    building, fractures might not have occurred.</p>
-
-  <p>If a disturbance is horizontal, and has a direction parallel to the
-    length of a wall, the wall is carried forward at its foundations. This
-    motion is opposed by the inertia of the upper portion of the wall and
-    the various loads it carries. The wall being elastic, distortion takes
-    place, and cracks, which are widest at the top, will be formed. In a
-    uniform wall the two most prominent fissures ought to be near the ends.</p>
-
-  <p>If the horizontal backward and forward movement has a direction
-    oblique to the plane of the wall, the wall will be either overthrown,
-    fractured, or have a triangular fragment thrown off towards the origin
-    from the end last reached.</p>
-
-  <p>Should the wave emerge steeply, diagonal fissures at right angles to
-    the direction of transit will be formed, or else triangular pieces will
-    be projected.</p>
-
-  <p>The accompanying figures are reduced from Mallet’s ‘Account of the
-    Neapolitan Earthquake of 1857.’</p>
-
-  <div class="figcenter">
-    <span class="pagenum" id="Page_105">105</span>
-    <img src="images/i_p105.jpg" width="700" height="297" alt="" />
-    <div class="caption"><span class="smcap">Fig. 18.</span>—Cathedral Church, Potenza (Mallet).</div>
-  </div>
-
-  <p><span class="pagenum" id="Page_106">106</span></p>
-
-  <p>Taking <i>a b</i> as the general direction of the fractures in
-    fig. 18, then <i>c d</i> will represent the direction in which the shock
-    emerged, which is at an angle of 23°·20′ to the horizon. It might be
-    argued that the direction of these fractures was due to the direction
-    in which surface undulations had travelled, or to the relative
-    strengths and proportions of different portions of the building. The
-    directions of cracks in a building are undoubtedly due to a complexity
-    of causes, but for buildings situated in the region of shock the
-    impulsive effect of the shock is probably<span class="pagenum" id="Page_107">107</span>
-    the most important function
-    to be considered. The method of applying the directions of emergence,
-    deduced from observations on fractures, to determine the origin of a
-    disturbance will be referred to in Chapter X.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p106.jpg" width="533" height="600" alt="" />
-    <div class="caption"><span class="smcap">Fig. 19.</span>—The Cathedral, Paterno (Mallet).
-    Neapolitan Earthquake of 1857.</div>
-  </div>
-
-  <p>Mallet observed that, although two ends of a building might be nearly
-    the same, the fissures and joints do not occur at equal distances from
-    the ends, nor are they equally opened.</p>
-
-  <p>The end where the joints are the most opened is that which was first
-    acted upon, and this phenomenon may be sufficiently well pronounced
-    to indicate the direction in which we must look to find the origin
-    of a disturbance. Amongst possible explanations for this disposition
-    of fractures in a wall. Mallet suggests that they may be due to real
-    differences in the two semiphases of the wave of shock, the second
-    semiphase being described with a somewhat slower velocity than the
-    first. This, it will be observed, is contrary to the indications of
-    seismographs.</p>
-
-  <p>Fig. 19, of the cathedral at Paterno, shows the effect of a subnormal
-    shock striking a wall obliquely and projecting one of its corners.</p>
-
-  <h3>MEASUREMENTS OF THE RELATIVE MOTION OF PARTS OF A BUILDING AT THE TIME
-    OF AN EARTHQUAKE.</h3>
-
-  <p>In 1880 a series of observations was made in Tokio to determine whether
-    at the time of an earthquake the various parts of the arched openings
-    which we see in many buildings synchronised in their vibrations, or,
-    for want of synchronism, were caused to approach and recede from each
-    other. The arches experimented on were heavy brick arches forming the
-    two corridors of the Imperial College of Engineering. The direction of
-    one set of these corridors is N. 40° E. and that of the other N. 50° W.</p>
-
-  <p><span class="pagenum" id="Page_108">108</span></p>
-
-  <p>The thickness of the walls in which these arches are placed is 1 ft.
-    11 in. They are built of Japanese bricks bound together with ordinary
-    lime. The span of the arches is 8 ft. 3 in., and the height of the arch
-    from the springing-line to the crown 4 ft. 1 in. The height of the
-    abutments is 7 ft. 1½ in. The voussoirs of the arch are formed of a
-    light grey soft volcanic rock, and on their faces show a depth of 12
-    inches. The width of the intermediate columns between the arches is 4
-    ft. 6⅞ in.</p>
-
-  <p>To determine whether at the time of an earthquake there was any
-    variation in the dimensions of these arches, a light stiff deal
-    rod, about 2 in. by ½ in. in cross section, was placed across the
-    springing-line of the arch. One end of this was firmly fixed to the
-    top of one abutment by means of a spike; on the other end, which was
-    to indicate any horizontal movement if the abutments approached each
-    other, there was fixed a pointer made out of a piece of steel wire.
-    This rested on a piece of smoked glass fixed to the ledge on which
-    the loose end of the rod was resting. If the abutments approached or
-    receded from each other a line would be drawn measuring the extent of
-    the motion. As a further indication of motion, a second smoked glass
-    plate was fixed on the transverse rod, which plate was marked on by a
-    pointer attached to a vertical rod hanging down from the crown of the
-    arch.</p>
-
-  <p>As a general result of these experiments it may be said that the
-    portions of the building which were examined usually either did not
-    move at all, or else they practically synchronised in their movements.
-    When they did move, the extent of motion was small, and the small
-    differences in movement which were observed were in every probability
-    far within the elastic limits of the structure.</p>
-
-  <p><em>Observations on Cracks.</em>—To determine whether the walls of a building
-    which have once been cracked, when<span class="pagenum" id="Page_109">109</span>
-    subjected to a series of shocks,
-    similar to those which they experienced before being cracked, still
-    continued to give way, the extremities of a considerable number of
-    cracks in the N.E. end of the museum buildings of the Engineering
-    College were marked with pencil. Although since the time of marking
-    there had been many severe shocks, these cracks did not visibly extend.
-    These marks were made on the outside wall of the building. On the
-    inside, one of these same cracks showed itself as a fissure about ¼
-    inch in width. Across this crack a horizontal steel wire pointer was
-    placed. One end of this wire was fixed in the wall; the other end,
-    which was pointed, rested on the surface of a smoked glass plate placed
-    on the other side of the crack. After small earthquakes there was no
-    indication of motion having taken place, but after a shock on February
-    21, as indicated by a line upon the smoked glass plate, it was seen
-    that the sides of the crack had approached and receded from each other
-    through a distance of about ¹/₁₆ inch.</p>
-
-  <p>By similar contrivances placed on cracks in a neighbouring building
-    exactly similar results were obtained, namely, that during small
-    earthquakes the two sides of the crack had retained their relative
-    positions, but at the time of a large shock this position had been
-    changed.</p>
-
-  <p>In this building it was also observed that the cracks in many instances
-    increased their length.</p>
-
-  <p>By attaching levers to the end of the pointers to multiply any motion
-    that might take place, no doubt the indications would be more frequent
-    and more definite. It would also be easier to note the relative
-    distances of motion in two directions, namely, how far the cracks had
-    closed and how far they had opened. As to whether motion would occur or
-    not, much would no doubt depend upon the direction of the earthquake.</p>
-
-  <p><span class="pagenum" id="Page_110">110</span></p>
-
-  <p><em>Prevention of Fractures.</em>—One conclusion which may perhaps be drawn
-    from these observations is, that a cracked building at the time of an
-    earthquake shows a certain amount of flexibility. Whether a building
-    which had been designed with cracks or joints between those parts
-    which were likely to have different periods of vibration would be more
-    stable, so far as earthquake shakings are concerned, than a similar
-    building put up in an ordinary manner, is a matter to be decided by
-    experiment. Certainly some of the cracks which have been examined
-    indicate that if they had not existed, the strain upon the portion of
-    the building where they occur would have been extremely great.</p>
-
-  <p><em>Direction of Cracks.</em>—In looking at the cracks produced by small
-    earthquakes it is interesting to note the manner of their extension.
-    The basements of the buildings which have been most carefully examined
-    are, for a height of two or three feet, built of large rectangular
-    blocks of a greyish-coloured volcanic rock. In these parts the cracks
-    pass in and out between the joints of the stone, indicating that
-    the stones have evidently been stronger than the mortar which bound
-    them together, and as a consequence the latter had to give way.
-    Above this basement when the cracks enter the brickwork, they no
-    longer exclusively confine themselves to the joints, but run in an
-    irregular line through all they meet with, sometimes across the bricks
-    and sometimes through the mortar joints. In places where they have
-    traversed the brickwork, we can say that the mortar has been stronger
-    than the bricks. This traversing of the bricks rather than the joints
-    is, I think, the general rule for the direction of the cracks in the
-    brickwork of Tokio buildings.</p>
-
-  <p><em>The Pitch of Roofs.</em>—From observation of the effects
-    <span class="pagenum" id="Page_111">111</span> produced by
-    earthquakes, it appears to us that the houses which lost the greater
-    number of tiles appear to be those with the steepest pitch, and those
-    where the tiles were simply laid upon the roof and not in any manner
-    fastened down. It would seem that destruction of this sort might to a
-    great extent be obviated by giving the roofs a less inclination and
-    fixing the tiles with nails. It was also noticed that the greatest
-    disturbance amongst the tiles was upon the ridges of the roofs.
-    Destruction of this sort might be overcome by giving especial attention
-    to these portions during the construction of the roof.</p>
-
-  <div class="figleft">
-    <img src="images/i_p112.jpg" width="330" height="236" alt="" />
-    <div class="caption"><span class="smcap">Fig. 20. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Fig. 21.</span></div>
-  </div>
-
-  <p><em>Relative Position of Openings in Walls.</em>—From what has been said about
-    the fractures in the buildings of Tokio it will have been seen that,
-    with but few exceptions, they have all taken place above openings
-    like doorways and windows. If architecture demands that openings like
-    arches should be placed one above another in heavy walls of this kind,
-    as in fig. 17, there will be lines of weakness running through the
-    openings parallel to the dotted lines. As arches are only intended
-    to resist vertical thrusts, special construction must be adopted to
-    make them strong enough to resist horizontal pulls. For instance, a
-    flat arch would offer more resistance to horizontal pulls than an
-    arch put together with ordinary voussoirs, there being in the former
-    case more friction to prevent the component parts sliding over each
-    other. Or again, above each arch an iron girder or wooden lintel might
-    be inserted in the brick or stone arch. It was suggested to me by my
-    colleague, Mr. Perry, that the best form calculated to give a wall
-    uniform strength, would be to build it so that the openings of each
-    tier would occupy alternate positions, that is to say, along lines
-    parallel to the struts and ties of a girder. In this way we should have
-    our materials so arranged that they would offer the same resistance to
-    horizontal<span class="pagenum" id="Page_112">112</span>
-    as to vertical movements. Such a wall is shown in fig. 20:
-    the dotted lines running through the openings, and all similar lines
-    parallel to the former, representing lines of weakness. If we compare
-    this with fig. 21, we shall see that in the case of a horizontal
-    movement <i>a b</i> or of a vertical movement <i>c d</i>, we should rather expect
-    to find fractures in a house built like fig. 21 than in one built
-    like fig. 20. If, however, these two buildings were shaken by a shock
-    which had an angle of emergence of about 45° in the direction <i>e f</i>,
-    the effects might be reversed. Usually, however, and always in a town
-    like Tokio which is visited by shocks originating at a distance, the
-    movements are practically horizontal ones, and, therefore, buildings
-    erected on the principles illustrated by fig. 20 should be much
-    superior, so far as resisting earthquakes is concerned, to buildings
-    constructed in the ordinary manner, as in fig. 21. Fractures following
-    a vertical line of weakness are shown in the accompanying drawing,
-    fig. 22, of the Church of St. Augustin, at Manilla, shattered by the
-    earthquakes of 1880.</p>
-
-  <p><em>The last House in a Row.</em>—When an earthquake shock enters a line
-    of buildings, and proceeds in a direction coincident with that of
-    the buildings, we should expect that the last of these houses, being
-    unsupported on one side, would be in the position of the last person in
-    Tyndall’s row of boys. From this it would seem that the end house in a
-    row would show the greatest tendency to fly away from its neighbours.
-    If the last house stood upon the<span class="pagenum" id="Page_113">113</span>
-    edge of a deep canal or a cliff,
-    there would be a layer of ground, equal in thickness to the depth of
-    the canal or to the height of the cliff, as the case may be, which
-    would also be in a position to be thrown forward. The effect which is
-    sometimes produced upon an end building is shown in fig. 23, which is
-    taken from the photograph of a house shattered in 1868 at San Francisco.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p113.jpg" width="558" height="700" alt="" />
-    <div class="caption"><span class="smcap">Fig. 22.</span>—Church of St. Augustin, Manilla.
-    Earthquakes of July 18–20, 1880.</div>
-  </div>
-
-  <p><span class="pagenum" id="Page_114">114</span></p>
-
-  <p><em>The Swing of Buildings.</em>—The distance through which buildings are
-    moved at the time of an earthquake depends partly on their construction
-    and partly on the extent, nature, and duration of the movement
-    communicated to them at their foundations. By violent shocks buildings
-    may be completely overthrown. In the case of small earthquakes, the
-    upper portion of a house may frequently move through a much greater
-    distance than the ground at its foundation. For instance, during the
-    Yokohama earthquake of February, 1880, when the maximum amplitude of
-    the earth’s motion was probably under ¾ of an inch, from the slow
-    swing of long Japanese pictures, from three to six feet in length,
-    which oscillated backwards and forwards on the wall, it is very
-    probable that the extent through which the upper portion of houses
-    moved was very considerable. In some instances these pictures seem to
-    have swung as much as two feet, and from the manner in which they swung
-    they evidently synchronised with the natural swing of the house.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p114.jpg" width="536" height="336" alt="" />
-    <div class="caption"><span class="smcap">Fig. 23.</span>—Webber House, San Francisco.&nbsp; &nbsp; Oct. 21,
-    1868.</div>
-  </div>
-
-  <p><span class="pagenum" id="Page_115">115</span></p>
-
-  <p>From this it would seem that such a house must have rocked from side to
-    side one foot out of its normal perpendicular position. That the motion
-    was great is testified by nearly all who tried to stand at the time of
-    the shock, it having been impossible to walk steadily across the floor
-    of a room in an upper story. The houses here referred to are either
-    those which are purely Japanese, or else those which are framed of wood
-    and built on European models, a class of building which is very common
-    in Tokio and Yokohama.</p>
-
-  <p>Perry and Ayrton calculated the period of a complete natural vibration
-    of different structures. For a square house whose outer and inner
-    sections were respectively 30 and 26 feet, and whose height was 30
-    feet, the period calculated would be about ·06 second.</p>
-
-  <p>At the time of the above earthquake many houses seem to have moved like
-    inverted pendulums. On the morning after the shock my neighbour, who
-    was living upstairs in a tall wooden house with a tile roof, told me
-    that he endeavoured to count the vibrations, and was of the impression
-    that to make a complete swing it took about 2 seconds.</p>
-
-  <p>Assuming now that the distance through which the top of a wooden house
-    moved was about 1 foot, and the number of vibrations which it made per
-    second was about ·5, then the greatest velocity of a point on the top
-    of such a house must have been about 6 feet per second.</p>
-
-  <p>Mallet, who made observations upon the vibrations of various
-    structures, tells us that Salisbury spire moves to and fro in a gale
-    more than 3 inches. A well-constructed brick and mortar wall, 40 feet
-    high and 1 foot 6 inches thick, was observed to vibrate in a gale 2
-    feet transversely before it fell.</p>
-
-  <p>An octagonal chimney with a heavy granite capping,
-    <span class="pagenum" id="Page_116">116</span> 160 feet high, was
-    observed instrumentally to vibrate at the top nearly 5 inches.<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a></p>
-
-  <p>At the time of a severe earthquake it does not seem impossible but that
-    a building may be swung completely over. The accompanying illustration,
-    fig. 24, taken from a photograph,<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a> apparently indicates a movement
-    of description.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p116.jpg" width="534" height="337" alt="" />
-    <div class="caption"><span class="smcap">Fig. 24.</span>—Stud Mill at Haywards,
-      California.&nbsp; &nbsp; Oct. 21, 1868.</div>
-  </div>
-
-  <p><em>Principle of relative Vibrational Period.</em>—If a lath or thin pole
-    loaded at one end with a weight fixed to the ground, so as to stand
-    vertically, be shaken by an earthquake it will be caused to rock to
-    and fro like an inverted pendulum. The period of its swing will be
-    chiefly dependent on its dimensions, its elasticity, and its load. In
-    a building we have to consider the vibration of a number of parts, the
-    periods of which, if they were <span class="pagenum" id="Page_117">117</span>
-    independent of each other, would be
-    different. On account of this difference in period, whilst one portion
-    of a building is endeavouring to move towards the right, another
-    is pulling towards the left, and, in consequence, either the bonds
-    which join them or else they themselves are strained or broken. This
-    was strikingly illustrated by many of the chimneys in the houses at
-    Yokohama, which by the earthquake of February 20, 1880, were shorn off
-    just above the roof. The chimneys were shafts of brick, and probably
-    had a slower period of vibration than the roof through which they
-    passed, this latter vibrating with the main portion of the house, which
-    was framed of wood.</p>
-
-  <p>A particularly instructive example of this kind which came under my
-    notice is roughly sketched in fig. 25.</p>
-
-  <div class="figright">
-    <img src="images/i_p117.jpg" width="142" height="219" alt="" />
-    <div class="caption"><span class="smcap">Fig. 25.</span></div>
-  </div>
-
-  <p>This is a chimney standing alone, which, for the sake of support, was
-    strapped by an iron band to an adjoining building. It would seem that
-    at the time of the shock, the building moving one way and the chimney
-    another, the swing of the heavy building gave the chimney a sharp jerk
-    and cut it off. The upper portion, being then loose upon the lower
-    part, rotated under the influence of the oscillations in manner similar
-    to that in which gravestones are rotated.</p>
-
-  <p>Mallet made observations similar to these in Italy. He tells us that a
-    buttress may often not have time to transmit its stability to a wall.
-    The wall and the buttress have different periods of vibration, and
-    therefore they exert impulsive actions on each other. Effects like
-    these were strikingly observable in many of the rural
-    <span class="pagenum" id="Page_118">118</span> Italian churches
-    where the belfry tower is built into one of the quoins of the main
-    rectangular building.</p>
-
-  <p>Not only have we to consider the relative vibrations of the various
-    parts of a building amongst themselves, but we have to consider the
-    relation of the natural vibrations of any one of them or the vibration
-    of the building as a whole, with regard to the earth, the vibrations of
-    which it must be remembered are not strictly periodic.</p>
-
-  <p>Some of the more important results dependent upon the principle of
-    ‘relative vibrational periods’ may be understood from the following
-    experiments:—</p>
-
-  <div class="figleft">
-    <img src="images/i_p118.jpg" width="258" height="155" alt="" />
-    <div class="caption"><span class="smcap">Fig. 26.</span></div>
-  </div>
-
-  <p>In fig. 26 <span class="smcap">a</span>, <span class="smcap">b</span>, and <span class="smcap">c</span> are three flat
-    springs made out of strips of bamboo, and loaded at the top with pieces
-    of lead. At the bottom they are fixed into a piece of board <span class="smcap">d e</span>,
-    and the whole rests on a table <span class="smcap">f g</span>. The legs of this
-    table being slightly loose, by placing the fingers on the top of it, a
-    quick short backward and forward movement can be produced. The weights
-    on <span class="smcap">a</span> and <span class="smcap">b</span> are the same, but they are larger than
-    the weight on <span class="smcap">c</span>. Consequently the periods of <span class="smcap">a</span> and
-    <span class="smcap">b</span> are the same, but different to the period of <span class="smcap">c</span>.
-    The dimensions of these springs are as follows: height, 18 inches;
-    <span class="smcap">a</span> and <span class="smcap">b</span> each carry weights equal to 320 grammes, and
-    they make one vibration per second; <span class="smcap">c</span> has a weight of 199
-    grammes, and makes 0·75 vibrations per second.</p>
-
-  <p><em>First Experiment.</em>—It will be found that by giving the table a gentle
-    backward and forward movement, the extent of which movement may be
-    so small that it will be difficult to detect it with the eye, either
-    <span class="smcap">a</span> and <span class="smcap">b</span> may be
-    <span class="pagenum" id="Page_119">119</span> made to oscillate violently whilst
-    <span class="smcap">c</span> remains still; or <i xml:lang="la">vice versâ</i>, <span class="smcap">c</span> may be caused to
-    oscillate whilst <span class="smcap">a</span> and <span class="smcap">b</span> remain still. In the one
-    case the period of shaking will have been synchronous with the natural
-    period of <span class="smcap">a</span> and <span class="smcap">b</span>, whilst in the latter it will have
-    been synchronous with that of <span class="smcap">c</span>. This would seem to show us
-    that if the natural period of vibration of a house, or of parts of
-    it, at any time agree with the period of the shock, it may be readily
-    thrown into a state of oscillation which will be dangerous for its
-    safety.</p>
-
-  <p><em>Second Experiment.</em>—Bind <span class="smcap">a</span> and <span class="smcap">b</span> together with a
-    strip of paper pasted between them. (The paper used was three-eighths
-    of an inch broad and would carry a weight of nearly three pounds.)
-    If the table be now shaken as before, <span class="smcap">a</span> and <span class="smcap">b</span> will
-    always have similar movements, and tend to remain at the same distance
-    apart, and as a consequence the strip of paper will not be broken. From
-    this experiment it would seem that so long as the different portions
-    of a building have almost the same periods of vibration, there will be
-    little or no strain upon the tie-rods or whatever contrivance may be
-    used in connecting the different parts.</p>
-
-  <p><em>Third Experiment.</em>—Join <span class="smcap">a</span> and <span class="smcap">c</span>, or <span class="smcap">b</span>
-    and <span class="smcap">c</span> with a strip of paper in a manner similar to the last
-    experiment. If the table be now shaken with a period approximating
-    either to that of <span class="smcap">a</span> and <span class="smcap">b</span>, or with that of
-    <span class="smcap">c</span>, the paper will be suddenly snapped.</p>
-
-  <p>This indicates that if we have different portions of a building of
-    such heights and thicknesses that their natural periods of vibration
-    are different, the strain upon the portions which connect such parts
-    is enormous, and it would seem, as a consequence, that either the
-    vibrators themselves, or else their connections, must, of a necessity,
-    give way. This was very forcibly illustrated in the Yokohama
-    <span class="pagenum" id="Page_120">120</span>
-    earthquake of February 1880 by the knocking over of chimneys. The
-    particular case of the chimneys is, however, better illustrated by the
-    next experiment.</p>
-
-  <p><em>Fourth Experiment.</em>—Take a little block of wood three-quarters of
-    an inch square and about one inch high, and place it on the top of
-    <span class="smcap">a</span>, <span class="smcap">b</span>, or <span class="smcap">c</span>. It will be found that, although
-    the spring on which it stands is caused to swing backwards and forwards
-    through a distance of three inches, the little block will retain its
-    position.</p>
-
-  <p>This little block we may regard as the upper part of a chimney standing
-    on a vibrating stack, and we see that, so long as this upper portion is
-    light, it has no tendency to fall.</p>
-
-  <p><em>Fifth Experiment.</em>—Repeat the fourth experiment, having first placed a
-    small leaden cap on the top of the block representing the chimney. (The
-    cap used only weighed a few grammes.) When vibration commences it will
-    be found that the block quickly falls. This would seem to indicate that
-    chimneys with heavy tops are more likely to fall than light ones.</p>
-
-  <p><em>Sixth Experiment.</em>—Bind <span class="smcap">a</span> and <span class="smcap">b</span> together with a
-    strip of paper and stand the little block upon the top of either. It
-    will be found that the block will stand as in the fourth experiment.</p>
-
-  <p><em>Seventh Experiment.</em>—Bind <span class="smcap">a</span> and <span class="smcap">c</span>, or <span class="smcap">b</span>
-    and <span class="smcap">c</span> together, and place the block upon the top of either of
-    them. When vibration commences, although the paper may not be broken,
-    the little block will quickly fall.</p>
-
-  <p><em>Eighth Experiment.</em>—Take two pencils or pieces of glass tube and place
-    them under the board <span class="smcap">d e</span>. If the table <span class="smcap">F G</span> be now
-    shaken in the direction <span class="smcap">d e</span>, it will be found that the springs
-    will not vibrate.</p>
-
-  <p>In a similar manner if a house or portion of a house were carried on
-    balls or rollers, as has already been suggested,
-    <span class="pagenum" id="Page_121">121</span> it would seem that
-    the house might be saved from much vibration.</p>
-
-  <p><em>Ninth Experiment.</em>—Set any of the springs in violent vibration by
-    gently shaking <span class="smcap">d e</span> instead of the table, and then suddenly
-    cease the actuating motion. It will be observed that at the moment
-    of cessation the board and the springs will have a sudden and very
-    decided motion of translation in the same direction as that in which
-    the springs were last moving, and although the springs were at the time
-    swinging through a considerable arc, all motion will suddenly cease.</p>
-
-  <p>This shows, that if a house is in a state of vibration the strain at
-    the foundations must be very great.</p>
-
-  <p>It would not be difficult to devise other experiments to illustrate
-    other phenomena connected with the principle of relative vibrational
-    periods, but these may perhaps be sufficient to show to those who have
-    not considered this matter its great importance in the construction
-    of buildings. Perhaps the greater portion of what is here said may by
-    many be regarded as self-evident truisms hardly worth the trouble of
-    demonstration. Their importance, however, seems to be so great that I
-    hope that their discussion has not been altogether out of place.</p>
-
-  <p>I may remark that in the rebuilding of chimneys in Yokohama the
-    principles here enunciated were taken advantage of by allowing the
-    chimneys to pass freely through the roofs without coming in contact
-    with any of the main timbers.</p>
-
-  <p>In putting up buildings to resist the effects of an earthquake, besides
-    the idea of making everything strong because the earthquake is strong,
-    there are several principles which, like the one just enunciated, might
-    advantageously be followed which as yet appear to have received but
-    little attention.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_VII">
-    <span class="pagenum" id="Page_122">122</span>
-    <h2>CHAPTER VII.<br />
-      <span class="subhead">EFFECTS PRODUCED UPON BUILDINGS (continued).</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Types of buildings used in earthquake countries—In Japan,
-    in Italy, in South America, in Caraccas—Typical houses
-    for earthquake countries—Destruction due to the nature of
-    underlying rocks—The swing of mountains—Want of support on
-    the face of hills—Earthquake shadows—Destruction due to the
-    interference of waves—Earthquake bridges—Examples of earthquake
-    effects—Protection of buildings—General conclusions.</div>
-
-  <p><em>Types of buildings used in earthquake countries.</em>—In Japan there
-    are excellent opportunities of studying various types of buildings.
-    The Japanese types, of course, form the majority of the buildings.
-    The ordinary Japanese house consists of a light framework of 4 or
-    5 inch scantling, built together without struts or ties, all the
-    timbers crossing each other at right angles. The spaces are filled
-    in with wattle-work of bamboo, and this is plastered over with mud.
-    This construction stands on the top of a row of boulders or of square
-    stones, driven into the surface soil to a distance varying from a few
-    inches to a foot. The whole arrangement is so light that it is not an
-    uncommon thing to see a large house rolled along from one position to
-    another on wooden rollers. In buildings such as these after a series of
-    small earthquake shocks, we could hardly expect to find more fractures
-    than in a wicker basket.</p>
-
-  <p>The larger buildings, such as temples and pagodas, are
-    <span class="pagenum" id="Page_123">123</span> also built of
-    timber. These are built up of such a multitude of pieces and framed
-    together in such an intricate manner that they also are capable of
-    yielding in all directions. The European buildings are, of course,
-    made of brick and stone with mortar joints. Some of these, as the
-    buildings of the Ginza in Tokio, are not designed for great strength.
-    On the other hand, others have thick and massive walls and are equal in
-    strength to those we find in Europe.</p>
-
-  <p>The third type of buildings are those which are built in blocks; and
-    these blocks being bound together with iron rods traversing the walls
-    in various directions are especially designed to withstand earthquakes.
-    A system somewhat similar to this has been patented in America, and
-    examples of these so-called earthquake-proof buildings are to be found
-    in San Francisco.</p>
-
-  <p>Speaking of Japanese buildings, Mr. R. H. Brunton, who has devoted
-    especial attention to them says that,<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a> ‘to imagine that slight
-    buildings, such as are seen here (i.e. in Japan), are the best
-    calculated to withstand an earthquake shock is an error of the most
-    palpable kind.’ After describing the construction of a Japanese house
-    in pretty much the same terms as we have used, he says ‘that with its
-    unnecessarily heavy roof and weak framework it is a structure of all
-    others the worst adapted to withstand a heavy shock.’ He tells us,
-    further, that these views are sustained by the truest principles of
-    mechanics. In order to render buildings to some extent proof against
-    earthquakes, some of the heavy roofs in Tokio have been so constructed
-    that they are capable of sliding on the walls. Mr. Brunton mentions
-    a design for a house, the upper <span class="pagenum" id="Page_124">124</span>
-    part of which is to rest on balls,
-    which roll on inverted cups fixed on the lower part of the building,
-    which is to be firmly embedded in the earth. A similar design was,
-    at the suggestion of Mallet, used to support the tables carrying the
-    apparatus of some of the lighthouses erected in Japan by Mr. Brunton.
-    The very existence of these designs seems to indicate that the ordinary
-    European house, however solidly and strongly it may be built, is not
-    sufficient to meet the conditions imposed upon it. What is required,
-    is something that will give way—an approximation to the timber frame
-    of a Japanese house, so strongly condemned by Mr. Brunton and others.
-    The crucial test of the value of the Japanese structure, as compared
-    with the modern buildings of brick and stone, is undoubtedly to be
-    found by an appeal to the buildings themselves. So far as my own
-    experience has gone, I must say that I have never seen any signs in
-    the Japanese timber buildings which could be attributed to the effects
-    of earthquakes, and His Excellency Yamao Yozo, Vice Minister of Public
-    Works, who has made the study of the buildings of Japan a speciality,
-    told me that none of the temples and palaces, although many of them
-    are several centuries old, and although they have been shaken by small
-    earthquakes and also by many severe ones, show any signs of having
-    suffered. The greatest damage wrought by large earthquakes appears to
-    have resulted from the influx of large waves or from fires. In every
-    case where an earthquake has been accompanied by great destruction,
-    by consulting the books describing the same, it can be seen, from the
-    illustrations in these books portraying conflagrations, that this
-    destruction was chiefly due to fire. When we remember that nearly
-    all Japanese houses are constructed of materials that are readily
-    inflammable, it is not hard to imagine how destruction of
-    <span class="pagenum" id="Page_125">125</span> this kind
-    has come about. To a Japanese, living as he does in a house which has
-    been compared to a tinder-box, fire is one of his greatest enemies,
-    and in a city like Tokio it is not at all uncommon to see during the
-    winter months many fires which sweep away from 100 to 500 houses. In
-    one winter I was a spectator of three fires, each of which was said to
-    have destroyed upwards of 10,000 houses.</p>
-
-  <p>Although it would appear that the smaller earthquakes of Japan produce
-    no visible effect upon the native buildings, it is nevertheless
-    probable that small effects may have been produced, the observation
-    of which is rendered difficult by the nature of the structure. If we
-    look at buildings of foreign construction, by which are meant buildings
-    of brick and stone, the picture before us is quite different, and
-    everywhere the effects of earthquakes are palpable even to the most
-    casual observer. Of these effects numerous examples have already been
-    given. Not only are these buildings damaged by the cracking of walls
-    and the overturning of chimneys, but they also appear to be affected
-    internally. For instance, in the timbers of the roof of the museum
-    attached to the Imperial College of Engineering in Tokio, there are a
-    number of diagonal faces acting as struts or ties intended to prevent
-    more or less horizontal movements taking place. Those which are rigidly
-    joined together with bolts and angle irons have apparently suffered
-    from their rigidity, being twisted and bent into various forms. The
-    buildings in Tokio, which are strongly put together, being especially
-    designed to withstand earthquakes, appear to have suffered but little.
-    I know only one example which at the time of the severe shock of 1880
-    had several of its chimneys damaged.</p>
-
-  <p>The ordinary houses in Italy, though built of stone and
-    <span class="pagenum" id="Page_126">126</span> mortar, are
-    but poorly put together, and, as Mallet has remarked, are in no way
-    adapted to withstand the frightful shakings to which they are subjected
-    from time to time.</p>
-
-  <p>In the large towns, like Naples, Rome, and Florence, where happily
-    earthquakes are of rare occurrence, although the building may be
-    better than that found in the country, the height of the houses and
-    the narrowness of the streets are sufficient to create a shudder, when
-    we think of the possibility of the occurrence of a moderately severe
-    earthquake.</p>
-
-  <p>In South America, although many buildings are built with brick
-    and stone, the ordinary houses, and even the larger edifices, are
-    specially built to withstand earthquakes. In Mr. James Douglas’s
-    account of a ‘Journey Along the West Coast of South America,’ we read
-    the following<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a>: ‘The characteristic building material of Guayaquil
-    is bamboo, which grows to many inches in thickness, and which, when
-    cut partially through longitudinally at distances of an inch or so,
-    and once quite through, can be opened out into fine elastic boards of
-    serviceable width. Houses, and even churches, of a certain primitive
-    beauty are built of such reeds, so bound together with cords that few
-    nails enter into the construction, and which, therefore, yield so
-    readily to the contortions of the earth during an earthquake as to be
-    comparatively safe.’</p>
-
-  <p>Here we have a house, which, so far as earthquakes are concerned, is
-    an exaggerated example of the principles which are followed in the
-    construction of an ordinary Japanese dwelling.</p>
-
-  <p>Another plan adopted in South America can be gathered from the same
-    author’s writings upon Lima, about which he says, ‘To build high houses
-    would be to erect structures for the first earthquake to make sport of,
-    <span class="pagenum" id="Page_127">127</span>
-    and, therefore, in order to obtain space, safety, and comfort, the
-    houses of the wealthy surround court after court, filled with flowers,
-    and cooled with fountains, connected one with another with wide
-    passages which give a vista from garden to garden.’</p>
-
-  <p>History would indicate that houses of this type have been arrived at as
-    the results of experience, for it is said that when the inhabitants of
-    South America first saw the Spaniards building tall houses, they told
-    them they were building their own sepulchres.<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a></p>
-
-  <p>In Jamaica, we find that even as early as 1692 experience had taught
-    the Spaniards to construct low houses, which withstood shakings better
-    than the tall ones.<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a></p>
-
-  <p>In Caraccas, which has been called the city of earthquakes, it is
-    said that the earthquakes cause an average yearly damage amounting to
-    the equivalent of a <i xml:lang="la">per capita</i> tax of four dollars. To reduce this
-    impost to a minimum much attention is paid to construction. ‘Projecting
-    basement corners (giving the house a slightly pyramidal appearance)
-    have been found better than absolutely perpendicular walls; mortised
-    corner-stones and roof beams have saved many lives when the central
-    walls have split from top to bottom; vaults and key-stone arches, no
-    matter how massive, are more perilous than common wooden lintels, and
-    there are not many isolated buildings in the city. In many streets
-    broad iron girders, riveted to the wall, about a foot above the house
-    door, run from house to house along the front of an entire square.
-    Turret-like brick chimneys, with iron top ornaments, would expose the
-    architect to the vengeance of an excited mob; the roofs are flat,
-    or flat terraced; the chimney flues terminate near the eaves in a
-    perforated lid.’<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a></p>
-
-  <p><span class="pagenum" id="Page_128">128</span></p>
-
-  <p><em>Typical houses for earthquake countries.</em>—From what has now been said
-    about the different buildings found in earthquake countries, it will be
-    seen that if we wish to put up a building able to withstand a severe
-    shaking, we have before us structures of two types. One of these types
-    may be compared with a steel box, which, even were it rolled down a
-    high mountain, would suffer but little damage; and the other, with a
-    wicker basket, which would equally withstand so severe a test. Both
-    of these types may be, to some extent, protected by placing them upon
-    a loose foundation, so that but little momentum enters them at their
-    base. One suggestion is to place a building upon iron balls. Another
-    method would be to place them upon two sets of rollers, one set resting
-    upon the other set at right angles. The Japanese, we have seen, place
-    their houses on round stones. The solid type of building is expensive,
-    and can only be approached partially, whilst the latter is cheap, and
-    can be approached closely. In the case of a solid building it would be
-    a more difficult matter to support it upon a movable foundation than in
-    the case of a light framework. Such a building is usually firmly fixed
-    on the ground, and consequently at the time of an earthquake, as has
-    already been shown by experiment, must be subjected to stresses which
-    are very great. In consequence also of the greater weight of the solid
-    structure, more momentum will enter it at its base than in the case of
-    the light structure. Also, we must remember that the rigidity favours
-    the transmission of momentum, and with rigid walls we are likely to
-    have ornaments, coping-stones, and the comparatively freer portions
-    forming the upper part of a building displaced; whilst, with flexible
-    walls absorbing momentum in the friction of their various parts, such
-    disturbances would not be so likely. Mr. T. Ronaldson, referring to
-    <span class="pagenum" id="Page_129">129</span>
-    this, says, that in 1868, at San Francisco, the ornamental stone work
-    in stone and cement buildings was thrown from its position, whilst
-    similar ornaments in neighbouring brick buildings stood.</p>
-
-  <p>To reduce the top weight of a building, hollow bricks might be
-    employed. To render a building more homogeneous and elastic, the
-    thickness of bricks might be reduced. Inasmuch as the elasticity
-    of brick and timber are so different, the two ought to be employed
-    separately. For internal decorations plaster mouldings might be
-    replaced by <i xml:lang="fr">papier mâché</i> and <i xml:lang="fr">carton-pierre</i>, the elastic yielding of
-    which is comparatively great.<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a> Houses, whether of brick and stone,
-    or of timber, ought to be broad and low, and the streets three or four
-    times as wide as the houses. The flatter the roofs the better.</p>
-
-  <p>One of the safest houses for an earthquake country would probably be a
-    one-storied strongly framed timber house, with a light flattish roof
-    made of shingles or sheet-iron, the whole resting on a quantity of
-    small cast-iron balls carried on flat plates bedded in the foundations.
-    The chimneys might be made of sheet-iron carried through holes free of
-    the roof. The ornamentation ought to be of light materials.</p>
-
-  <p>At the time of severe earthquakes many persons seek refuge from their
-    houses by leaving them. In this case accidents frequently happen from
-    the falling of bricks and tiles. Others rush to the doorways and
-    stand beneath the lintels. Persons with whom the author has conversed
-    have suggested that strongly constructed tables and bedsteads in
-    their rooms would give protection. To see persons darting beneath
-    tables and bedsteads would undoubtedly give rise to humiliating and
-    ludicrous exhibitions. This latter idea is not without a value, and
-    most <span class="pagenum" id="Page_130">130</span>
-    certainly, if applied in houses of the type described, would be
-    valuable.</p>
-
-  <p>The great danger of fire may partially be obviated by: the use of
-    ‘earthquake lamps,’ which are so constructed that before they overturn
-    they are extinguished. It is said that in South America some of the
-    inhabitants are ready at any moment to seek refuge in the streets,
-    and they have coats prepared, stocked with provisions and; other
-    necessaries, which, if occasion demands, will enable them to spend the
-    night in the open air. These coats, called ‘earthquake coats,’ might
-    also, with properly constructed houses, be rendered unnecessary.</p>
-
-  <p><em>Destruction due to the nature of the underlying rocks.</em>—That the
-    nature of the ground on which a building stands is intimately related
-    with the severity of the blow it receives is a fact which has often
-    been demonstrated.</p>
-
-  <p>One cause of destruction is due to placing a building on foundations
-    which are capable of receiving the full effects of a shock, and
-    transmitting it to the buildings standing on them.</p>
-
-  <p>For instance, the reason why a soft bed might possibly make a good
-    foundation, is, as has been pointed out by Messrs. Perry and Ayrton,
-    because the time of transmission of momentum is increased; in fact, the
-    soft bed is very like a piece of wood interposed between a nail and
-    the blows of a hammer—it lengthens the duration of impact. For this
-    reason we are told that a quaking bog will make a good foundation. When
-    a shock enters loose materials its waves will be more crowded, and it
-    is possible that a line of buildings may rest on more than one wave
-    during a shock. There are many examples on record of the stability of
-    buildings which rested on beds of particular material at the time of
-    destructive earthquakes. As<span class="pagenum" id="Page_131">131</span>
-    the observations which have been made
-    by various writers on this subject appear to point in a contrary
-    direction, I give the following examples:—</p>
-
-  <p>In the great Jamaica earthquake of 1692, the portions of Port
-    Royal which remained standing were situated on a compact limestone
-    foundation; whilst those on sand and gravel were destroyed (‘Geological
-    Observer,’ p. 426). Again, on p. 148 of the same work, we read,
-    ‘According to the observations made at Lisbon, in 1737, by Mr. Sharpe,
-    the destroying effects of this earthquake were confined to the tertiary
-    strata, and were most violent on the blue clay, on which the lower part
-    of the city is constructed. Not a building on the secondary limestone
-    or on the basalt was injured.’</p>
-
-  <p>In the great earthquakes of Messina, those portions of the town
-    situated on alluvium, near the sea, were destroyed, whilst the high
-    parts of the town, on granite, did not suffer so much. Similar
-    observations were made in Calabria, when districts consisting of
-    gravel, sand, and clay became, by the shaking, almost unrecognisable,
-    whilst the surrounding hills of slate and granite were but little
-    altered. At San Francisco, in 1868, the chief destruction was in the
-    alluvium and made ground.</p>
-
-  <p>At Talcahuano, in 1835, the only houses which escaped were the
-    buildings standing on rocky ground; all those resting on sandy soil
-    were destroyed.</p>
-
-  <p>From the results of observations like these, it would seem the harder
-    rocks form better foundations than the softer ones. The explanation of
-    this, in many cases, appears to lie in the fact that the soft strata
-    were in a state of unstable equilibrium, and by shaking, they were
-    caused to settle. Observations like the following, however, point out
-    another reason why soft strata may sometimes afford a bad foundation.</p>
-
-  <p><span class="pagenum" id="Page_132">132</span></p>
-
-  <p>‘Humboldt observed that the Cordilleras, composed of gneiss and
-    mica-slate, and the country immediately at their foot, were more shaken
-    than the plains.’<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a></p>
-
-  <p>‘Some writers have asserted that the wave-like movements (of the
-    Calabrian earthquake in 1783) which were propagated through recent
-    strata from west to east, became very violent when they reached the
-    point of junction with the granite, as if a reaction was produced when
-    the undulatory movement of the soft strata was suddenly arrested by the
-    more solid rocks.’</p>
-
-  <p>Dolomieu when speaking of this earthquake says, the usual effect ‘was
-    to disconnect from the sides of the Apennines all those masses (of sand
-    and clay) which either had not sufficient bases for their bulk, or
-    which were supported only by lateral adherence.’</p>
-
-  <p>These intensified actions taking place at and near to lines of
-    junction between dissimilar strata is probably due to the phenomena of
-    reflection and refraction.</p>
-
-  <p>When referring to the question as to whether buildings situated on
-    loose materials suffered more or less than those on solid rocks,
-    Mallet, in his description of the Neapolitan earthquake of 1857,
-    remarks: ‘We have in this earthquake, towns such as Saponara and
-    Viggiano, situated upon solid limestone, totally prostrated; and we
-    have others such as Montemarro, to a great extent based upon loose
-    clays, totally levelled. We have examples of almost complete immunity
-    in places on plains of deep clay as that of Viscolione, and in places
-    on solid limestone, like Castelluccio, or perched on mountain tops like
-    Petina.’<a id="FNanchor_31" href="#Footnote_31" class="fnanchor">[31]</a></p>
-
-  <p>After reading the above, we see that the probable reason why, in
-    several cases, beds of soft materials have not made
-    <span class="pagenum" id="Page_133">133</span>good foundations,
-    consists in the fact that they have either been of small extent or else
-    have been observed only in the neighbourhood of lines which divided
-    them from other formations, which lines are always those of great
-    disturbances.</p>
-
-  <p>At the end of his description of the Neapolitan earthquake of 1857,
-    Mallet says that more buildings were destroyed on the rock than on the
-    loose clay. This, however, he remarks, is hardly a fact from which we
-    can draw any valuable deductions, because it so happened that more
-    buildings were constructed on the hills than on the loose ground.<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">[32]</a></p>
-
-  <p>Professor D. S. Martin, writing on the earthquake of New England in
-    1874, remarks that in Long Island the shock was felt where there was
-    gneiss between the drift. Around portions to the east the observations
-    were few and far between. He also remarks that generally the shocks
-    were felt more strongly and frequently on rocky than on soft ground.<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">[33]</a></p>
-
-  <p>From these examples, it would appear that the hard ground, which
-    usually means the hills, forms a better foundation than the softer
-    ground, which is usually to be found in the valleys and plains. Other
-    examples, however, point to a different conclusion. For instance, a
-    civil engineer, writing about the New Zealand earthquake of 1855, when
-    all the brick buildings in Wellington were overthrown, says that ‘it
-    was most violent on the sides of the hills at those places, and least
-    so in the centre of the alluvial plains.’<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">[34]</a></p>
-
-  <p>In this example it must be noticed that the soft alluvium here referred
-    to was of large extent, and not loose material resting on the flanks of
-    rocks, from which it was <span class="pagenum" id="Page_134">134</span>
-    likely to be shaken down, as in most of the previous examples.</p>
-
-  <p>The results of my own observations on this subject point as much in one
-    direction as in the other. In Tokio, from instrumental observations
-    upon the slopes and tops of hills, the disturbance appears to be
-    very much less than it is in the plains. Thus, at my house, situated
-    on the slope of a hill about 100 feet in height, for the earthquake
-    of March 11, 1882, I obtained a maximum amplitude of motion of from
-    three to four millimètres only, whilst Professor Ewing, with a similar
-    instrument, situated on the level ground at about a mile distant,
-    found a motion of fully seven millimètres. This calculation has been
-    confirmed by observations on other earthquakes. Thus, for instance, in
-    the destructive earthquake of 1855, when a large portion of Tokio was
-    devastated, it was a fact, remarked by many, that the disturbance was
-    most severe on the low ground and in the valleys, whilst on the hills
-    the shock had been comparatively weak. As another illustration, I may
-    mention that within three-quarters of a mile from my house in Tokio
-    there is a prince’s residence which has so great a reputation for the
-    severity of the shakings it receives that its marketable value has been
-    considerably depreciated, and it is now untenanted.</p>
-
-  <p>In Hakodadi, which is a town situated very similarly to Gibraltar,
-    partly built on the slope of a high rocky mountain and partly on a
-    level plain, from which the mountain rises, the rule is similar to
-    that for Tokio, namely, that the low, flat ground is shaken more
-    severely than the high ground. At Yokohama, sixteen miles south-west
-    from Tokio, the rule is reversed, as was very clearly demonstrated
-    by the earthquake of February 1880, when almost every house upon the
-    high ground lost its chimney, whilst on the low ground there was
-    scarcely any damage<span class="pagenum" id="Page_135">135</span>
-    done; the only places on the low ground which
-    suffered were those near to the base of the hills. The evidence as to
-    the relative value of hard ground as compared with soft ground, for
-    the foundation of a building, is very conflicting. Sometimes the hard
-    ground has proved the better foundation and sometimes the softer, and
-    the superiority of one over the other depends, no doubt, upon a variety
-    of local circumstances.</p>
-
-  <p>These latter observations open up the inquiry as to the extent to which
-    the intensity of an earthquake may be modified by the topography of the
-    disturbed area.</p>
-
-  <p><em>The swing of mountains.</em>—If an earthquake wave is passing through
-    ground the surface of which is level, so long as this ground is
-    homogeneous, as the wave travels further and further we should expect
-    its energy to become less and less, until, finally, it would insensibly
-    die out. If, however, we have standing upon this plain a mountain,
-    judging from Mallet’s remarks, this mountain would be set in a state
-    of vibration much in the same way as a house is set in vibration,
-    and it would tend to oscillate backward and forward with a period of
-    vibration dependent upon the nature of its materials, size, and form.
-    The upper portion of this mountain would, in consequence, swing through
-    a greater arc than the lower portion, and buildings situated on the top
-    of it would swing to and fro through a greater arc than those which
-    were situated near its foot. This explanation why buildings situated
-    on the top of a mountain should suffer more than those situated on
-    a plain, is one which was offered by Mallet when writing of the
-    Neapolitan earthquake. He tells us that towns on hills are ‘rocked as
-    on the top of masts,’ and if we accept this explanation it would, in
-    fact, be one reason why the houses situated on the Bluff at
-    <span class="pagenum" id="Page_136">136</span> Yokohama
-    suffered more than those situated in the settlement. This explanation
-    is given on account of the great authority it claims as a consequence
-    of its source. It is not clear how the statement can be supported,
-    as different portions of the mountain receive momentum in opposite
-    directions at the same time.</p>
-
-  <p><em>Want of support on the faces of hills.</em>—When a wave of elastic
-    compression is propagated through a medium, we see that the energy of
-    motion is being continually transmitted from particle to particle of
-    that medium. A particle, in moving forwards, meets with an elastic
-    resistance of the particles towards which it moves, but, overcoming
-    these resistances, it causes these latter particles to move, and in
-    turn to transmit the energy to others further on. So long as the medium
-    in which this transfer of energy is continuous, each particle has a
-    limit to its extent of motion, dependent on the nature of the medium.
-    When, however, the medium, which we will suppose to be the earth, is
-    not continuous, but suddenly terminates with a cliff or scarp, the
-    particles adjacent to this cliff or scarp, having no resistance offered
-    to their forward motion, are shot forward, and, consequently, the
-    ground here is subjected to more extensive vibrations than at those
-    places where it was continuous. This may be illustrated by a row of
-    marbles lying in a horizontal groove; a single marble rolled against
-    one end of this row will give a concussion which will run through the
-    chain, like the bumping of an engine against a row of railway cars, and
-    as a result, the marble at the opposite end of the row, being without
-    support, will fly off. Tyndall illustrates the same thing with his
-    well known row of boys, each one standing with his arms stretched out
-    and his hands resting upon the shoulders of the boy before him. A push
-    being given to the boy at the back,<span class="pagenum" id="Page_137">137</span>
-    the effect is to transmit a push
-    to the first boy, who, being unsupported, flies forward.</p>
-
-  <p>In the case of some earthquakes, most disastrous results have occurred
-    which seem only to admit of an explanation such as this. A remarkable
-    instance of this kind occurred when the great earthquake of 1857 ‘swept
-    along the Alps from Geneva to the east-north-east, and its crest
-    reached the edge of the deep glen between Zermatt and Visp. Then the
-    upper part of the wave-movement, a thousand or two thousand feet in
-    depth from the surface, came to an end; the forward pulsation acted
-    like the breaker of the sea, and heavy falls of rock encumbered the
-    western side of the valley.’</p>
-
-  <p><em>Earthquake shadows.</em>—If a mountain stands upon a plain through which
-    an elastic wave is passing, which is almost horizontal, the mountain
-    is, so to speak, in the <em>shadow</em> of such a wave. If we only consider
-    the normal motion of this wave, we see that the only motion which the
-    mountain can obtain will be a wave of elastic distortion produced
-    by a shearing force along the plain of the base. Should, however,
-    the wave approach the mountain from below, and emerge into it at a
-    certain angle, only the portion of the mountain on the side from
-    which the wave advanced could remain in shadow, whilst the portion
-    on the opposite side would be thrown into a state of compression and
-    extension. Portions in shadow, however, would be subject to waves
-    of elastic distortion. In a manner similar to this we may imagine
-    that certain portions of the bluff, so far as the advancing wave was
-    concerned, were in shadow, and thus saved from the immediate influence
-    of the direct shock. A hypothetical case of such a shadow is shown in
-    the accompanying section, illustrating the contour of the ground at
-    Yokohama. The situation which might be in the shadow of
-    <span class="pagenum" id="Page_138">138</span> one shock,
-    however, it is quite possible might not be in that of another. We
-    must also remember that a place in shadow for a direct shock might be
-    affected by reflected waves, and also by the transverse vibrations of
-    the direct shock. These effects are over and above the effects produced
-    by the waves of elastic distortion just referred to. It might be asked
-    whether whole countries, like England, which are but seldom shaken, are
-    in shadow.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p138.jpg" width="700" height="247" alt="" />
-    <div class="caption"><span class="smcap">Fig. 27.</span>—Hypothetical section at Yokohama.</div>
-  </div>
-
-  <p><em>Destruction due to the interference of waves.</em>—Referring to the
-    section of the ground at Yokohama (Fig. 27), it will be seen that
-    both the settlement and the bluff stand upon beds of gravel capping
-    horizontal beds of grey tuff. The gravel of that portion of the
-    settlement on the seaboard originally<span class="pagenum" id="Page_139">139</span>
-    formed the line of a shingle
-    beach. That portion of the settlement back from the sea stands upon
-    ground which was originally marshy. In the central portions of the
-    settlement this bed of gravel is very thick, perhaps 100 feet or so,
-    but as you near the edge of the bluff it probably becomes thinner,
-    until it finally dies out upon the flanks of the scarps.</p>
-
-  <p>On the top of the bluff, the beds of gravel will, in every probability,
-    be generally thinner than they are upon the lower level. The beds of
-    tuff, which is a soft grey-coloured clay-like rock, produced by the
-    solidification of volcanic mud, appear, when walking on the seaboard,
-    to be horizontally stratified. If there is a dip inland, it is in all
-    probability very slight. Here and there the beds slightly faulted.
-    Taken as a whole we may consider these beds as being tolerably
-    homogeneous, and an earthquake in passing through them would meet
-    with but little reflection or refraction. At the junction of these
-    beds with the overlying gravels, both reflection and refraction would
-    comparatively be very great.</p>
-
-  <p>On entering the gravel, as the wave would be passing into a less
-    elastic medium, the direction of the wave would be bent towards the
-    perpendicular to the line of junction, and the angle of emergence at
-    the surface would consequently be augmented. At the surface certain
-    reflection would also take place, but the chief reflections would be
-    those at the junction of the tuff and the alluvium.</p>
-
-  <p>Under the settlement it is probable that all the reflections which took
-    place would be single. Thus wave fronts like <span class="smcap">a</span><sub>1</sub> advancing
-    in a direction parallel to the line <i>a</i><sub>1</sub>; would be reflected in
-    a direction <i>a</i><sub>2</sub> and give rise to a series of reflected waves
-    <span class="smcap">a</span><sub>2</sub>. These are shown by thicker lines. Similarly all the
-    <span class="pagenum" id="Page_140">140</span>neighbouring waves to the right and left of
-    <span class="smcap">a</span><sub>1</sub> would give
-    rise to a series of reflected waves. If the lines drawn representing
-    wave fronts are districts of compression, then, where two of the
-    lines cross each other, there would be double energy in producing
-    compression. Similarly, districts of rarefaction might accord, and,
-    again, compression of one wave might meet with the rarefaction
-    of another and a neutralisation of effect take place. A diagram
-    illustrating concurrence and interference of this description is given
-    in Le Conte’s ‘Elements of Geology,’ p. 115. The interference which has
-    been spoken of, however, is not the greatest which would occur. The
-    greatest would probably be beneath the bluff and the scarps which run
-    down to join the level ground below. This would be the case because it
-    is a probability that there might not only be cases of interference of
-    single reflected waves, but also of waves which had been not only twice
-    but perhaps thrice reflected. For example, a wave like <span class="smcap">b</span><sub>1</sub>
-    (which is parallel to <span class="smcap">a</span><sub>1</sub> of the first supposition),
-    advancing in a direction parallel to <i>b</i><sub>1</sub> might be reflected
-    along the line <i>b</i><sub>2</sub> giving rise to waves like <span class="smcap">b</span><sub>2</sub>,
-    which in turn might be reflected along <i>b</i><sub>3</sub> giving rise to waves
-    like <span class="smcap">b</span><sub>3</sub>. The number of districts where there would be
-    concurrence and interference would, in consequence of the number of
-    times waves might be reflected, be augmented. Here the violence of
-    the shock would, at certain points, be considerably increased, but
-    as a general result energy must be lost, so that even if some of the
-    reflected waves found their way into the portion we have regarded as
-    being in shadow, their intensity would not be so great as if they had
-    entered it directly.</p>
-
-  <p>The shaking down of loose materials from the sides of hills may be
-    partially explained on the assumption of an increased disturbance due
-    to interference.</p>
-
-  <p><em>Earthquake bridges.</em>—In certain parts of South<span class="pagenum" id="Page_141">141</span> America there appear
-    to exist tracts of ground which are practically exempt from earthquake
-    shocks, whilst the whole country around is sometimes violently shaken.
-    It would seem as if the shock passes beneath such a district as water
-    passes beneath a bridge, and for this reason these districts have been
-    christened ‘bridges.’</p>
-
-  <p>This phenomenon appears to depend upon the nature of the underlying
-    soil. When an elastic wave passes from one bed of rock to another of a
-    different character, a certain portion of the wave is reflected, while
-    the remainder of it is transmitted and refracted, and ‘bridges’ we
-    may conceive of as occurring where the phenomenon of total reflection
-    occurs.</p>
-
-  <p>In the instances given of soft materials having proved good
-    foundations, it was assumed that they had chiefly acted as absorbers
-    of momentum. They have also acted as reflecting surfaces, and where no
-    effects have been felt by those residing on them, this may have been
-    the result of total reflection, and the soft beds thus have played the
-    part of bridges.</p>
-
-  <p>Fuchs gives an example taken from the records of the Syrian earthquake
-    of 1837, where not only neighbouring villages suffered differently, but
-    even neighbouring houses. In one case a house was entirely destroyed,
-    whilst in the next house nothing was felt.</p>
-
-  <p>In Japan, at a place called Choshi, about 55 miles east of the capital,
-    earthquakes are but seldom felt, although the surrounding districts may
-    be severely shaken.</p>
-
-  <p>From descriptions of this place it would appear that there is a large
-    basaltic boss rising in the midst of alluvial strata. The immunity from
-    earthquakes in this district has probably given rise to the myth of
-    the Kanam rock, which is a stone supposed to rest upon the
-    <span class="pagenum" id="Page_142">142</span> head of a
-    monstrous catfish (Namadzu), which by its writhings causes the shakings
-    so often felt in this part of the world.<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">[35]</a></p>
-
-  <p>Prof. D. S. Martin, writing on the earthquake of New England in 1874,
-    says that it was felt at four points; it was felt in the heart of
-    Brooklyn all within a circle of half a mile across; ‘and this fact
-    would suggest that a ridge of rock perhaps approaches the surface at
-    that point, though none is known to appear.’<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">[36]</a></p>
-
-  <p>The subject of special districts, which are more or less protected from
-    severe shakings, will be again referred to, and it will be seen that
-    after a seismic survey has been made even of a country like Japan,
-    where there are on the average at least two earthquakes per day, it
-    is possible to choose a place to build in as free from earthquakes as
-    Great Britain.</p>
-
-  <p><em>General examples of earthquake effects.</em>—The following examples of
-    earthquake effects are drawn from Mallet’s account of the Neapolitan
-    earthquake of 1857.</p>
-
-  <p>At a town called Polla there was great destruction. Judging from
-    the fissures in the parts that remained standing it seemed that the
-    emergence of the shock had been more vertical in the upper part of the
-    town than in the lower, proving that whatever had been the angle below,
-    the hill had itself vibrated, which, being horizontal, had modified the
-    angle of the fissures.</p>
-
-  <p>Diano suffered but little, partly because it was well built, and partly
-    on account of its situation, which was such that before the shock
-    reached it the disturbance had to pass from beds of clay into nearly
-    vertically placed beds of limestone. Also a great portion of the shock
-    was cut off by the Vallone del Raccio to the north and north-west
-    <span class="pagenum" id="Page_143">143</span> of
-    the town. Here the effects of the partial extinction of the wave on the
-    ‘free outlaying stratum’ were visible in the masses of projected rock.</p>
-
-  <p>Castellucio did not suffer because its well buttressed knoll was end
-    on to the direction of shock, and on account of a barrier of vertical
-    breccia beds protecting it upon the east.</p>
-
-  <p>Pertosa stands on a mound. The destruction was least in the southern
-    part of the town. From the relation of the beds of breccia on which the
-    town stands, and the direction of the wave-path, it is evident that the
-    southern part of the town received the force of the shock through a
-    greater thickness of the breccia beds than the other parts did.</p>
-
-  <p>Petina, standing on a level limestone spur jutting out from a mountain
-    slope, suffered nothing, whilst Anletta five miles to the south-west,
-    and Pertosa six miles distant, were in great part prostrated. (1) The
-    terrace did not vibrate, and (2) between Petina and Anletta there is
-    almost 6,000 feet of piled up limestone, so that any shock emergent at
-    a steep angle had to pass up transversely through these beds.</p>
-
-  <p><em>Protection of buildings.</em>—In addition to giving proper construction
-    to our buildings, choosing proper foundations and positions for them,
-    something might possibly be done to ward off the destructive effects of
-    an earthquake. We read that the Temple of Diana at Ephesus was built on
-    the edge of a marsh, in order to ward off the effect of earthquakes.
-    Pliny tells us that the Capitol of Rome was saved by the Catacombs,
-    and Elisée Reclus<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">[37]</a> says that the Romans and Hellenes found out that
-    caverns, wells, and quarries retarded the disturbance of the earth,
-    and protected edifices in their neighbourhood.
-    <span class="pagenum" id="Page_144">144</span> The tower of Capua was
-    saved by its numerous wells. Vivenzis asserts that in building the
-    Capitol the Romans sunk wells to weaken the effects of terrestrial
-    oscillations. Humboldt relates the same of the inhabitants of San
-    Domingo.</p>
-
-  <p>Quito is said to receive protection from the numerous cañons in the
-    neighbourhood, whilst Lactacunga, fifteen miles distant, has often been
-    destroyed.</p>
-
-  <p>Similarly, it is extremely probable that many portions of Tokio have
-    from time to time been protected more or less from the severe shocks of
-    earthquakes by the numerous moats and deep canals which intersect it.</p>
-
-  <p>Although we are not prepared to say how far artificial openings of this
-    description are effectual in warding off the shocks of earthquakes,
-    from theoretical considerations, and from the fact that their use has
-    been discovered by persons who, in all probability, were without the
-    means of making theoretical deductions, the suggestions which they
-    offer are worthy of attention.</p>
-
-  <p><em>General conclusions.</em>—The following are a few of the more important
-    results which may be drawn from the preceding chapter:—</p>
-
-  <p>1. In choosing a site for a house find out by the experience of others
-    or experimental investigation the localities which are least disturbed.
-    In some cases this will be upon the hills, in others in the valleys and
-    on the plains.</p>
-
-  <p>2. A wide open plain is less likely to be disturbed than a position on
-    a hill.</p>
-
-  <p>3. Avoid loose materials resting on harder strata.</p>
-
-  <p>4. If the shakings are definite in direction, place the blank walls
-    parallel to such directions, and the walls with many openings in them
-    at right angles to such directions.</p>
-
-  <p><span class="pagenum" id="Page_145">145</span></p>
-
-  <p>5. Avoid the edges of scarps or bluffs, both above and below.</p>
-
-  <p>6. So arrange the openings in a wall, that for horizontal stresses the
-    wall shall be of equal strength for all sections at right angles.</p>
-
-  <p>7. Place lintels over flat arches of brick or stone.</p>
-
-  <p>8. To withstand destructive shocks either rigidly follow one or other
-    of the two systems of constructing an earthquake-proof building. The
-    light building on loose foundations is the cheaper and probably the
-    better.</p>
-
-  <p>9. Let all portions of a building have their natural periods of
-    vibration nearly equal.</p>
-
-  <p>10. If it is a necessity that one portion of a building should have a very
-    different period of vibration to the remainder, as for instance a
-    brick chimney in a wooden house, it would seem advisable either to let these
-    two portions be sufficiently free to have an independent motion,
-    or else they must be bound together with great strength.</p>
-
-  <p>11. Avoid heavy topped roofs and chimneys. If the foundations were free
-    the roof might be heavy.</p>
-
-  <p>12. In brick or stone work use good cement.</p>
-
-  <p>13. Let archways curve into their abutments.</p>
-
-  <p>14. Let roofs have a low pitch, and the tiles, especially those upon
-    the ridges, be well secured.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_VIII">
-    <span class="pagenum" id="Page_146">146</span>
-    <h2>CHAPTER VIII.<br />
-      <span class="subhead">EFFECTS OF EARTHQUAKES ON LAND.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    1. Cracks and fissures—Materials discharged from
-    fissures—Explanation of fissure phenomena. 2. Disturbances in
-    lakes, rivers, springs, wells, fumaroles, &amp;c.—Explanation of
-    these latter phenomena. 3. Permanent displacement of ground—On
-    coast lines—Level tracts—Among mountains—Explanation of these
-    movements.</div>
-
-  <p><em>Cracks and fissures formed in the ground.</em>—Almost all large
-    earthquakes have produced cracks in the ground. The cracks which were
-    found in the ground at Yokohama (February 22, 1880) were about two or
-    three inches wide, and from twenty to forty yards in length. They could
-    be best seen as lines along a road running near the upper edge of some
-    cliffs which overlook the sea at that place. The reason that cracks
-    should have occurred in such a position rather than in others was
-    probably owing to the greater motion at such a place, due to the face
-    of the cliff being unsupported, and there being no resistance opposed
-    to its forward motion. It often happens that earthquake cracks are many
-    feet in width. At the Calabrian earthquake of 1783, one or two of the
-    crevasses which were formed were more than 100 feet in width and 200
-    feet in depth. Their lengths varied from half a mile to a mile.<a
-    id="FNanchor_38" href="#Footnote_38" class="fnanchor">[38]</a>
-    Besides these large cracks, many smaller ones of one or two feet in
-    breadth and of great length were formed. In
-    <span class="pagenum" id="Page_147">147</span>the large fissures many
-    houses were engulfed. Subsequent excavations showed that by the closing
-    of the fissures these had been jammed together to form one compact
-    mass. These cracks are usually more or less parallel, and at the same
-    time parallel to some topographical feature, like a range of mountains.
-    For example, the cracks which were formed by the Mississippi earthquake
-    of 1812 ran from north-east to south-west parallel to the Alleghanies.
-    By succeeding shocks these crevasses are sometimes closed and sometimes
-    opened still wider. Their permanency will also depend upon the nature
-    of the materials in which they are made.</p>
-
-  <p>During an earthquake large cracks may suddenly open and shut.</p>
-
-  <p>During the convulsions of 1692 which destroyed Port Royal, it is said
-    that many of the fissures which were formed, opened and shut. In some
-    of these, people were entirely swallowed up and buried. In others they
-    were trapped by the middle, and even by the neck, where if not killed
-    instantaneously they perished slowly. Subsequently their projecting
-    parts formed food for dogs.<a id="FNanchor_39" href="#Footnote_39"
-    class="fnanchor">[39]</a></p>
-
-  <p>The earthquake which, July 18, 1880, shook the Philippines caused many
-    fissures to be found, which in some places were so numerous that the
-    ground was broken up into steps. Near to the village of San Antonio the
-    soil was so disturbed that the surface of a field of sugar-canes was
-    so altered that in some cases the top of one row of full grown plants
-    was on a level with the roots of the next. Into one such fissure a boat
-    disappeared, and into another, a child.</p>
-
-  <p>Subsequently the child was excavated, and its body, which was found a
-    short distance below the surface, was completely crushed.<a id="FNanchor_40"
-    href="#Footnote_40" class="fnanchor">[40]</a></p>
-
-  <p><span class="pagenum" id="Page_148">148</span></p>
-
-  <p>At the time of the Riobamba earthquake, not only were men engulfed,
-    but animals, like mules, also sank into the fissures which were formed.</p>
-
-  <p>The fissures which were formed at the time of the Owen’s Valley
-    earthquake in 1872 extended for miles nearly parallel to the
-    neighbouring Sierras. In some places the ground between the fissures
-    sank twenty or thirty feet, and at one place about three miles east of
-    Independence, a portion of the road was carried eighteen feet to the
-    south by a fissure.<a id="FNanchor_41" href="#Footnote_41"
-    class="fnanchor">[41]</a></p>
-
-  <p>Speaking generally, it may be said that all large earthquakes are
-    accompanied by the formation of fissures. The Japanese have a saying
-    that at the time of a large earthquake persons must run to a bamboo
-    grove.</p>
-
-  <p>The object of this is to escape the danger of being engulfed in
-    fissures, the ground beneath a bamboo grove being so netted together
-    with fine roots that it is almost impossible for it to be rent open.</p>
-
-  <p><em>Materials discharged from fissures.</em>—Together with the opening of
-    cracks in the earth it often has happened that water, mud, vapours,
-    gases, and other materials, have been ejected.</p>
-
-  <p>At the time of the Mississippi earthquake water, mixed with sand and
-    mud, was thrown out with such violence that it spurted above the tops
-    of the highest trees. In Italy such phenomena have often been repeated.</p>
-
-  <p>From the fissures which were formed in 1692 at the time of the
-    earthquakes in Sicily, water issued which in some instances was
-    salt.<a id="FNanchor_42" href="#Footnote_42" class="fnanchor">[42]</a></p>
-
-  <p>By the Cachar earthquake (January 10, 1869) numerous fissures were
-    formed parallel to the banks of a river, from this water and mud were
-    ejected. Dr. Oldham, who describes this earthquake, says that the
-    <span class="pagenum" id="Page_149">149</span>
-    first shot of dry mud or sand was mistaken for smoke or steam. The
-    water was foul, and hotter than surface water at the time, but only
-    slightly so; and the sulphurous smell was nothing more than you would
-    perceive in stirring up the mud at the bottom of any stagnant pool
-    which had lain undisturbed for some time.<a id="FNanchor_43"
-    href="#Footnote_43" class="fnanchor">[43]</a></p>
-
-  <p>In 1755, when Tauris was destroyed, boiling water issued from the
-    cracks which were formed. Similar phenomena were witnessed at a place
-    eight miles from La Banca in Mexico, in the year 1820. Part of this hot
-    water was pure and part was muddy.</p>
-
-  <p>Sometimes the water which has been ejected has been so muddy that the
-    mud has been collected to form small hills. This was the case at the
-    time of the Riobamba earthquake. The mud in this case consisted partly
-    of coal, fragments of augite, and shells of infusoria.</p>
-
-  <p>At the time of the Jamaica earthquake men who had fallen into crevices
-    were in some cases thrown out again by issuing water.</p>
-
-  <p>Sometimes, as has already been mentioned, vapour, gases, and even
-    flames issue from fissures. Vapour of sulphur appears to be exceedingly
-    common. Kluge says that many fish were killed in consequence of the
-    sulphurous vapours which rose in the sea near to the coast of New
-    Zealand in 1855.</p>
-
-  <p>On December 14, 1797, an insupportable smell of sulphur was observed to
-    have accompanied the earthquake which at that time shook Cumana, which
-    was greatest when the disturbance was greatest.</p>
-
-  <p>Sulphurous fumes which were combustible were belched out of the
-    earth at the time of the Jamaica earthquake in 1692. The smell which
-    accompanied this <span class="pagenum" id="Page_150">150</span>
-    was so powerful that it caused a general sickness
-    which swept away about 3,000 persons.<a id="FNanchor_44" href="#Footnote_44"
-    class="fnanchor">[44]</a></p>
-
-  <p>From the fissures formed at Concepcion in 1835, water, which was black
-    and fœtid, issued.<a id="FNanchor_45" href="#Footnote_45" class="fnanchor">[45]</a></p>
-
-  <p>The earthquakes of New England in 1727 were accompanied by the
-    formation of fissures, from which sand and water boiled out in
-    sufficient quantity to form a quagmire. In some places ash and sulphur
-    are said to have been ejected.</p>
-
-  <p>At one house the stink of sulphur accompanying the earthquake was so
-    great that the family could not bear to remain in doors.<a id="FNanchor_46"
-    href="#Footnote_46" class="fnanchor">[46]</a></p>
-
-  <p>Emanations of gas sometimes appear to have burst out from submarine
-    sources.</p>
-
-  <p>Thus the earthquake at Lima, in March, 1865, was accompanied with great
-    agitation of the water and an odour of sulphuretted and carburetted
-    hydrogen. This former gas was developed to such an extent that the
-    white paint of the U.S. ship ‘Lancaster’ was blackened.<a id="FNanchor_47"
-    href="#Footnote_47" class="fnanchor">[47]</a> With the
-    smell, flames have sometimes been observed, as, for instance, at the
-    time of the Lisbon earthquake.</p>
-
-  <p>At the time of the earthquakes of 1811 and 1813, in the Mississippi
-    valley, steam and smoke issued from some of the fissures which were
-    formed.</p>
-
-  <p>Instances are recorded where stones have been shot up from fissures
-    unaccompanied by water, as, for instance, at the earthquake of Pasto
-    (January, 1834). It is imagined that the propelling power must have
-    been the sudden expansion of escaping gases.</p>
-
-  <p>It has been suggested that flames seen above fissures
-    <span class="pagenum" id="Page_151">151</span>might perhaps
-    be due to the burning of materials like sulphur. Mr. D. Forbes, who
-    examined the effects of the earthquakes of Mendoza, which were felt for
-    a distance of 1,200 miles, says that where the hard rock came to the
-    surface there were no traces of fissures, these being entirely confined
-    to the alluvium. The rumours of fire and smoke having appeared at some
-    of the fissures were without foundation, the presumed smoke being
-    nothing but dust.<a id="FNanchor_48" href="#Footnote_48" class="fnanchor">[48]</a></p>
-
-  <p>In addition to flames lights appear often to have been observed, the
-    origin of which cannot be easily explained.</p>
-
-  <p>The earthquake of November 22, 1751, at Genoa is said to have been
-    accompanied by a light like that of a prodigious fire which seemed to
-    arise out of the ground.<a id="FNanchor_49" href="#Footnote_49" class="fnanchor">[49]</a></p>
-
-  <p><em>Explanation of fissure phenomena.</em>—The manner in which fissures are
-    formed has already been explained when referring to the want of support
-    in the face of hills (page 136).</p>
-
-  <p>Similar remarks may be applied to the banks of rivers and all
-    depressions, whether natural or artificial, which have a steep slope.
-    At such places the wave of shock emerges on a free surface, which,
-    being unsupported in the direction of its motion, tends to tear itself
-    away from the material behind, and form a fissure parallel to the face
-    of the free surface. The distance of the fissure from the face of the
-    free surface will, theoretically, be equal to half the amplitude of the
-    wave of motion, one half tending to move forwards, and the other half
-    backwards. The reason that water and other materials rush forth from
-    fissures has been explained by Schüler as being due to cracks having
-    been opened through impervious <span class="pagenum" id="Page_152">152</span>
-    strata, which, before the earthquake,
-    by their continuity prevented the rising of subterranean water under
-    hydrostatic pressure.<a id="FNanchor_50" href="#Footnote_50" class="fnanchor">[50]</a></p>
-
-  <p>Kluge explains the coming up of the waters as being due to the same
-    causes which he considers may be the origin of disturbances in the sea.</p>
-
-  <p>The most reasonable explanations of the eruption of water, mud, sand,
-    and gas through fissures are those given by Oldham and Mallet in their
-    account of the Cachar earthquake.</p>
-
-  <p>In the case of a horizontal shock passing through a bed of ooze or
-    water-bearing strata, the elastic wave will tend to pack up the water
-    during the forward motion to such an extent that it will flow or
-    spout up through any aperture communicating with the surface. By the
-    repetition of these movements causing ejections, sand or mud cones,
-    like those produced by a volcanic eruption, may be formed, and by a
-    similar action water may be shot violently up out of wells, as was the
-    case in Jamaica in 1692.</p>
-
-  <p>If an emergent wave acts through a water-bearing bed upon a
-    superincumbent layer of impervious material, this upper layer is,
-    during the upward motion, by its inertia suddenly pressed down upon the
-    latter.</p>
-
-  <p>This pressure is equal to that which would raise the upper layer to
-    a height equal to the amplitude of the motion of an earth particle,
-    and with a velocity at least equal to the mean velocity of the earth
-    particle resolved in the vertical direction.</p>
-
-  <p>For a moment the water-bearing strata receive an enormous squeeze, and
-    the water or mud starts up through any crevice which may be formed
-    leading to the surface.</p>
-
-  <p><span class="pagenum" id="Page_153">153</span></p>
-
-  <p>From this we see that liquids may rise far beyond the level due to
-    hydrostatic pressure.<a id="FNanchor_51" href="#Footnote_51" class="fnanchor">[51]</a></p>
-
-  <p>Volger has attributed the origin of lights or flames appearing above
-    fissures to the friction which must take place between various rocky
-    materials at the time when the fissures are opened. As confirmatory of
-    this he refers to instances where similar phenomena have been observed
-    at the time of landslips. At the time of these landslips the heat
-    developed by friction has been sufficiently intense to convert water
-    into steam, the tension of which threw mud and earth into the air like
-    the explosion of a mine.<a id="FNanchor_52" href="#Footnote_52" class="fnanchor">[52]</a></p>
-
-  <p>The gas eruptions which occasionally take place with earthquakes are
-    probably due to the opening of fissures communicating with reservoirs
-    or strata charged with products of natural distillation, or chemical
-    action, which previously had accumulated beneath impervious strata. Of
-    the existence of such gases we have abundant evidence. In coal mines we
-    have fire damp which escapes in increased quantities with a lowering of
-    the barometrical pressure. In volcanic regions we have many examples of
-    natural springs of carbon dioxide.</p>
-
-  <p>These various gases sometimes escape in quantity, or erupt without
-    the occurrence of earthquakes. Rossi mentions an instance where a few
-    years ago quantities of fish were killed by the eruption of gas in the
-    Tiber, near Rome. Another instance is one which occurred at Follonica
-    on April 6, 1874. On the morning of that day many of the streets and
-    roads were covered with the dead bodies of rats and mice. It seemed as
-    if it had rained rats. From the facts that the bodies of the creatures
-    seemed healthy, <span class="pagenum" id="Page_154">154</span>
-    that the destruction had happened suddenly, and not
-    come on gradually like an epidemic, it was supposed that they had been
-    destroyed by an emanation of carbon dioxide. The fact that many of them
-    lay in long lines suggested the idea that they had been endeavouring
-    to escape at the time of the eruption.<a id="FNanchor_53" href="#Footnote_53" class="fnanchor">[53]</a> If we can suppose sudden
-    developments of gas like this to have occasionally accompanied
-    earthquakes, we may sometimes have the means of accounting for the
-    sickness which has been felt.</p>
-
-  <p><em>Disturbances in lakes.</em>—It has often been observed that, at the time
-    of large earthquakes, lakes have been thrown into violent agitation,
-    and their waters have been raised or lowered. At the time of the great
-    Lisbon earthquake, not only were the waters of European lakes thrown
-    into a state of oscillation, but similar effects were produced in the
-    great lakes of North America. In some instances, as in the case of
-    small ponds, these movements may be produced by the horizontal backward
-    and forward motion of the ground. At other times they are probably due
-    to an actual tipping of a portion of their basins. Movements like these
-    latter will be again referred to in the chapter on Earth Pulsations.
-    On January 27, 1856, there was a shock of earthquake at Bailyborough,
-    Ireland, which occasioned an adjacent lough to overflow its banks
-    and rush into the town with great impetuosity. In returning it swept
-    away two men, leaving behind a great quantity of pike and eels of a
-    prodigious growth.<a id="FNanchor_54" href="#Footnote_54" class="fnanchor">[54]</a></p>
-
-  <p><em>Disturbances in rivers.</em>—Just as lakes have been disturbed, so also
-    have there been sudden disturbances in rivers. Sometimes these have
-    overflowed their banks, whilst at other times they have been suddenly
-    dried up. In certain cases the reason that a portion of a river should
-    <span class="pagenum" id="Page_155">155</span>have become dry has been very apparent, as, for instance, at the time
-    of the Zenkoji earthquake in Japan in 1847, when the Shikuma-gawa
-    became partly dry in consequence of the large masses of earth which
-    had been shaken down from overhanging cliffs damming a portion of
-    its course, and thus forming, first, lakes, and subsequently, new
-    water-courses. As another example, out of the many which might be
-    quoted, may be mentioned the sudden drying up of the river Aboat,
-    a tributary of the Magat, in the Philippine Islands, on July 27,
-    1881, shortly after a severe shock of earthquake. The water of this
-    river ceased to flow for two hours, after which it reappeared with
-    considerable increase of volume and of a reddish colour. Signor E. A.
-    Casariego, who describes this, remarks that the phenomenon could easily
-    be explained through the slipping down of the steep banks in narrow
-    parts of its upper valley, by which means its flow had been obstructed
-    until the water had time to accumulate and pass over or demolish the
-    obstruction.</p>
-
-  <p>After the earthquake of Belluno (June 29, 1873), the torrent Tesa,
-    which is ordinarily limpid, became very muddy.<a id="FNanchor_55" href="#Footnote_55" class="fnanchor">[55]</a> Similar phenomena
-    have been observed even in Britain, as, for instance, in 1787, when, at
-    the time of a shock which was felt in Glasgow, there was a temporary
-    stoppage in the waters of the Clyde. Again, in 1110, there was a
-    dreadful earthquake at Shrewsbury and Nottingham, and the Trent became
-    so low at Nottingham that people walked over it.</p>
-
-  <p>The earthquake of 1158, which was felt in many parts of England, was
-    accompanied by the drying up of the Thames, which was so low that it
-    could be crossed on foot even at London.<a id="FNanchor_56" href="#Footnote_56" class="fnanchor">[56]</a></p>
-
-  <p><span class="pagenum" id="Page_156">156</span></p>
-
-  <p>Facts analogous to these are mentioned in the accounts of many large
-    earthquakes. Sometimes rivers only come muddy or change their colour.
-    In an account of the Lisbon earthquake we read that some of the rivers
-    near Neufchâtel suddenly became muddy.<a id="FNanchor_57" href="#Footnote_57" class="fnanchor">[57]</a></p>
-
-  <p>At other times large waves are formed. Thus the earthquake of Kansas
-    (April 24, 1867) apparently created a disturbance in the rivers at
-    Manhattan, which rolled in a heavy wave from the north to the south
-    bank.<a id="FNanchor_58" href="#Footnote_58" class="fnanchor">[58]</a></p>
-
-  <p>Sometimes curious phenomena have happened with regard to rivers without
-    the occurrence of earthquakes. Thus, for instance, on November 27,
-    1838, there was a simultaneous stoppage of the Teviot, Clyde, and Nith.</p>
-
-  <p>In these rivers similar phenomena have been observed in previous years.</p>
-
-  <p>Again, on January 1, 1755, there was a sudden sinking of the river
-    Frooyd, near Pontypool. This appears to have been due to the water
-    sinking into chasms which were suddenly opened.<a id="FNanchor_59" href="#Footnote_59" class="fnanchor">[59]</a></p>
-
-  <p><em>Effects produced in springs, wells, fumaroles, &amp;c.</em>—Springs also are
-    often affected by earthquakes. Sometimes the character of their waters
-    change; those which were pure become muddy, whilst those which were hot
-    have their temperature altered.</p>
-
-  <p>Sometimes springs have been dried up, whilst at other times new springs
-    have been formed.</p>
-
-  <p>This latter was the case in New England (October 27, 1727). In some
-    places springs were formed, whilst at other places they were either
-    entirely or partly dried up.<a id="FNanchor_60" href="#Footnote_60" class="fnanchor">[60]</a></p>
-
-  <p>At and near Lisbon, in 1755, some fountains became
-    <span class="pagenum" id="Page_157">157</span>muddy, others
-    decreased, others increased, and others dried up. At Montreux, Aigle,
-    and other places, springs became turbid.</p>
-
-  <p>The baths at Toplitz, in Bohemia, which were discovered in
-    <span class="smcap">a.d.</span> 762, were seriously affected by the same earthquake.
-    Previous to the earthquake it is said that they had always given a
-    constant supply of hot water. At this time, however, the chief spring
-    sent up vast quantities of water and ran over. One hour before this
-    it had grown turbid and flowed muddy. After this it stopped for about
-    one minute, but recommenced to flow with prodigious violence, driving
-    before it considerable quantities of reddish ochre. Finally, it settled
-    back to its original clear state and flowed as before.<a id="FNanchor_61" href="#Footnote_61" class="fnanchor">[61]</a></p>
-
-  <p>In 1855, at the earthquake of Wallis, many new springs burst forth, and
-    some of these in Nicolai Thale were so rich in iron that they quickly
-    formed a deposit of ochre.</p>
-
-  <p>At the time of the Belluno earthquake (June 29, 1873), a hot spring, La
-    Vena d’Oro, suddenly became red.<a id="FNanchor_62" href="#Footnote_62" class="fnanchor">[62]</a></p>
-
-  <p>The following examples of like changes are taken from the writings of
-    Fuchs.<a id="FNanchor_63" href="#Footnote_63" class="fnanchor">[63]</a></p>
-
-  <p>In 1738 the hot springs of St. Euphema rose considerably in their
-    temperature.</p>
-
-  <p>During the earthquake of October, 1848, the hot springs of Ardebil,
-    which usually had a temperature of from 44° to 46° C., rose so high
-    that their temperature was sufficient to cause scalding.</p>
-
-  <p>At the time of the earthquake of Wallis, in 1855, the temperature of
-    hot springs rose 7°, and the quantity of water increased three times.</p>
-
-  <p>During the earthquake of 1835 in Chili, the springs
-    <span class="pagenum" id="Page_158">158</span>of Cauquenes fell
-    from 118° to 92° F. Subsequently, however, they again rose.</p>
-
-  <p>Fumaroles are similarly disturbed. Thus, at the time of the earthquakes
-    of Martinique (September, 1875), the fumaroles there showed an abnormal
-    activity.<a id="FNanchor_64" href="#Footnote_64" class="fnanchor">[64]</a></p>
-
-  <p>Wells often appear to be acted upon in the same manner as springs.</p>
-
-  <p>At the time of the California earthquake (April, 1855), the level of
-    the water in certain wells was raised ten to twelve feet.</p>
-
-  <p>A consequence of the earthquake at Neufchâtel, in 1749, was to fill
-    some of the wells with mud.<a id="FNanchor_65" href="#Footnote_65" class="fnanchor">[65]</a> At Constantinople, on September 2,
-    1754, wells became dry.<a id="FNanchor_66" href="#Footnote_66" class="fnanchor">[66]</a></p>
-
-  <p><em>Explanation of the above phenomena.</em>—That the water in springs and
-    wells should be caused to rise at the time of an earthquake, admits of
-    explanation on the supposition of compressions taking place similar
-    to those which cause the rise of water in fissures. That the water in
-    wells and springs should be rendered turbid, is partly explained on the
-    supposition of more or less dislocation taking place in the earthy or
-    rocky cavities in which they are contained or through which they flow.</p>
-
-  <p>At the time of a large earthquake it is extremely probable that there
-    is a general disturbance in the lines of circulation of subterranean
-    waters and gases throughout the shaken area. By these disturbances, new
-    waters may be brought to the surface, two or more lines of circulation
-    may be united, and the flow of a spring or supply of a well be
-    augmented. Fissures, through which waters reached the surface, may be
-    closed, wells may become dry, or springs may cease to flow, hot springs
-    may have their <span class="pagenum" id="Page_159">159</span>
-    temperature lowered by the additions of cold water
-    from another source, and, in a similar manner, waters may be altered
-    in their mineralisation. An important point to be remembered in this
-    consideration is the mutual dependence of various underground water
-    supplies, and the area over which any given supply may circulate. A
-    well in the higher part of Lincoln Heath is said to be governed by the
-    river Trent, which is ten miles distant; when the river rises the well
-    rises in proportion, and when the river falls the water in the well
-    falls.<a id="FNanchor_67" href="#Footnote_67" class="fnanchor">[67]</a></p>
-
-  <p>The change which is usually observed in hot springs is, that before or
-    with earthquakes they increase in temperature, but afterwards sink back
-    to their normal state. This increase in temperature may possibly be due
-    to communication being opened with new or deeper centres of volcanic
-    activity, or a temporarily increased rate of flow.</p>
-
-  <p>That the water issuing from newly formed fissures or springs should
-    be hot, might be explained on the supposition of its arising from a
-    considerable depth, or from some volcanic centre. It might also be
-    attributed to the heat developed by friction at the opening of the
-    fissures. These changes which earthquakes produce upon the underground
-    circulation of waters are phenomena deserving especial attention.
-    Although we know much about the circulation of surface water, it is but
-    little that we yet know about the movement of the streams hidden from
-    view, from which these surface waters have their sources. Earthquakes
-    may be regarded as gigantic experiments on the circulatory system of
-    the earth, which, if properly interpreted, may yield information of
-    scientific and utilitarian value.</p>
-
-  <p>The sudden elevations, depressions, or lateral shifting of large tracts
-    of country at the time of destructive earthquakes
-    <span class="pagenum" id="Page_160">160</span> are phenomena
-    with which all students of geology are familiar. In most cases these
-    displacements have been permanent; and evidences of many of the
-    movements which occurred within the memory of man, remain as witnesses
-    of the terrible convulsions with which they were accompanied.</p>
-
-  <p><em>Movements on coast lines and level tracts.</em>—At the time of the
-    great earthquake of Concepcion, on February 20, 1835, much of the
-    neighbouring coast line was suddenly elevated four or five feet
-    above sea level. This, however, subsequently sank until it was only
-    two feet. A rocky flat, off the island of Santa Maria, was lifted
-    above high-water mark, and left covered with ‘gaping and putrefying
-    mussel-shells, still attached to the bed on which they had lived.’ The
-    northern end of the island itself was raised ten feet and the southern
-    extremity eight feet.<a id="FNanchor_68" href="#Footnote_68" class="fnanchor">[68]</a></p>
-
-  <p>By the earthquake of 1839, the island of Lemus, in the Chonos
-    Archipelago, was suddenly elevated eight feet.<a id="FNanchor_69" href="#Footnote_69" class="fnanchor">[69]</a></p>
-
-  <p>Of movements like these, especially along the western shores of South
-    America, Darwin, who paid so much attention to this subject, has given
-    many examples. In 1822, the shore near Valparaiso was suddenly lifted
-    up, and Darwin tells us that he heard it confidently asserted ‘that a
-    sentinel on duty, immediately after the shock, saw a part of a fort
-    which previously was not within the line of his vision, and this would
-    indicate that the uplifting was not vertical.’<a id="FNanchor_70" href="#Footnote_70" class="fnanchor">[70]</a> That the large areas
-    of land should be shifted permanently in horizontal directions, as well
-    as vertically, we should anticipate from the observations which we are
-    able to make upon large fissures which are caused by earthquakes.</p>
-
-  <p>Another remarkable example of sudden movement in
-    <span class="pagenum" id="Page_161">161</span>the rocky crust is
-    that which took place during the earthquakes of 1811–12 in the valley
-    of the Mississippi, near to the mouth of the Ohio, which was convulsed
-    to such a degree, that lakes, twenty miles in extent, were formed
-    in the course of an hour. This country, which is called the ‘sunk
-    country,’ extends some seventy to eighty miles north and south, and
-    thirty miles east and west.<a id="FNanchor_71" href="#Footnote_71" class="fnanchor">[71]</a></p>
-
-  <p>In the ‘Gentleman’s Magazine’ we read of the little territory of
-    Causa Nova, in Calabria, being sunk twenty-nine feet into the earth
-    by an earthquake, without throwing down a house. The inhabitants,
-    being warned by a noise, escaped into the fields, and only five were
-    killed.<a id="FNanchor_72" href="#Footnote_72" class="fnanchor">[72]</a></p>
-
-  <p>Other examples of these permanent dislocations of strata are to be
-    found in almost every text-book on geology.</p>
-
-  <p><em>Geological changes produced.</em>—Passing over the accounts of earth
-    movements which are more or less fictitious, and confining our
-    attention to the well authenticated facts, we see at once the important
-    part which earthquakes have played as agents working geological
-    changes. Even in the nineteenth century long tracts of coast, as in
-    Chili and New Zealand, have been raised, whilst other areas, like the
-    Delta of the Indus, have been sunk. Sir H. Bartle Frere, speaking about
-    the disturbance which took place in his latter region in 1819, remarks
-    that all the canals drawn from the Fullalee River ceased to run for
-    about three days, probably indicating a general upheaval of the lower
-    part of the canal. In consequence of the earthquakes in former times
-    it is not unlikely that water-courses have ceased to flow, water has
-    decreased in wells, and districts have been depopulated.<a id="FNanchor_73" href="#Footnote_73" class="fnanchor">[73]</a></p>
-
-  <p><span class="pagenum" id="Page_162">162</span></p>
-
-  <p>Sometimes these changes have taken place gradually and sometimes with
-    violence. Mountains have been toppled over, valleys have been filled,
-    cities have been submerged or buried.</p>
-
-  <p>With the records of these convulsions before us, we see that seismic
-    energy yet exhibits a terrible activity in changing the features of the
-    globe.</p>
-
-  <p><em>Reason of these movements.</em>—To formulate a single reason for these
-    catastrophes would be difficult. Where they are of the nature of
-    landslips, or materials have been dislodged from mountain sides, the
-    cause is evidently the sudden movement of this ground acting upon
-    strata not held together in a sufficiently stable condition. A similar
-    explanation may be given for the sudden elevations or depressions of
-    strata in a district removed from the centre where the disturbance had
-    its origin. The seismic effort exhibits itself in a certain area round
-    its origin as a sudden push, and by this push, strata are fractured and
-    caused to move relatively to each other.</p>
-
-  <p>At or near to the origin of an earthquake it might be argued that it
-    was the sudden falling of rocky strata towards a position of stable
-    equilibrium that caused the shaking, and in such a case the movements
-    referred to may be regarded as the cause rather than the effect of an
-    earthquake.</p>
-
-  <p>A subject closely connected with the sudden dislocation of strata,
-    is the production of secondary or consequent earthquakes, due to the
-    disturbance of ground in a critical state (see p. 248).</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_IX">
-    <span class="pagenum" id="Page_163">163</span>
-    <h2>CHAPTER IX.<br />
-      <span class="subhead">DISTURBANCES IN THE OCEAN.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Sea vibrations—Cause of vibratory blows—Sea waves: Preceding
-    earthquakes; Succeeding earthquakes—Magnitude of waves—Waves
-    as recorded in countries distant from the origin—Records on
-    tide gauges—Waves without earthquakes—Cause of waves—Phenomena
-    difficult of explanation—Velocity of propagation—Depth of the
-    ocean—Examples of calculations—Comparison of velocities of
-    earthquake waves with velocities which ought to exist from the
-    known depth of the ocean.</div>
-
-  <p><em>Sea vibrations.</em>—Whilst residing in Japan I have had many
-    opportunities of conversing with persons who had experienced
-    earthquakes when on board ships, and it has often happened that these
-    same earthquakes have been recorded on the shore. For example, at the
-    time of every moderately severe earthquake which has shaken Yokohama,
-    the same disturbance has been felt on board the ships lying in the
-    adjoining harbour. In some cases the effect had been as if the ship
-    was grounding; in others, as if a number of sharp jerks were being
-    given to the cable. The effect produced upon a man-of-war lying in the
-    Yokohama harbour on the evening of March 11, 1881, was described to me
-    as a ‘violent irresistible shaking.’ Vessels eighty miles at sea have
-    recorded and timed shocks which were felt like sudden blows. These were
-    accompanied by a noise described as a ‘dull rattle like thunder.’</p>
-
-  <p><span class="pagenum" id="Page_164">164</span></p>
-
-  <p>In none of the cases here quoted was any disturbance of the water
-    observed.</p>
-
-  <p>The great earthquake of Lisbon was felt by vessels on the Atlantic,
-    fifty miles away from shore.</p>
-
-  <p>On February 10, 1716, the vessels in the harbour of New Pisco were so
-    violently shaken that both ropes and masts were broken, and yet no
-    motion in the water was observed. Some have described these shocks like
-    those which would be produced by the sudden dropping of large masses of
-    ballast in the hold of the vessel. Other cases are known where rigging
-    was damaged, and even cannon have been jerked up and down from the
-    decks on which they rested.</p>
-
-  <p><em>Cause of vibratory blows.</em>—From the rattling sound which has
-    accompanied some of these submarine shocks, many of which, it may be
-    remarked, have never been recorded as earthquakes upon neighbouring
-    shores, it does not seem improbable that they may have been the result
-    of the sudden condensation of volumes of steam produced by submarine
-    volcanic eruptions.</p>
-
-  <p>As confirmatory of this supposition we have the fact, that many of
-    the marine disturbances which might be called ‘sea-quakes,’ have been
-    observed in places which are close to, or in the line of, volcanic
-    vents. Thus, M. Daussy, who has paid special attention to this subject,
-    has collected evidence to show that a large number of shocks have been
-    felt by vessels in that portion of the Atlantic between Cape Palamas,
-    on the west coast of Africa, and Cape St. Roque, on the east coast of
-    South America.<a id="FNanchor_74" href="#Footnote_74" class="fnanchor">[74]</a></p>
-
-  <p>Some of the vessels only felt shocks and tremblings, but others saw
-    smoke, and some even collected floating ashes. In considering the
-    submarine shocks of this particular
-    <span class="pagenum" id="Page_165">165</span> area, we must bear in mind that it
-    lies in the line of Iceland, the west coast of Scotland, the Azores,
-    Canaries, Cape de Verd Islands, St. Helena, and other places, all of
-    which, if not at present in volcanic activity, shew evidence of having
-    been so within recent times. The connection between volcanic action and
-    earthquakes will be again referred to.</p>
-
-  <p><em>Sea waves.</em>—Although in the above-mentioned instances sea waves have
-    not been noticed, it is by no means uncommon to find that destructive
-    earthquakes have been accompanied by waves of an enormous size, which,
-    if the earthquake has originated beneath the sea, have, subsequently
-    to the shaking, rolled in upon the land, to create more devastation
-    than the actual earthquake. It may, however, be mentioned that a few
-    exceptional cases exist when it is said that the sea wave has preceded
-    the earthquake, as, for example, at Smyrna, on September 8, 1852.</p>
-
-  <p>Again, at the earthquake in St. Thomas, in 1868, it is said that the
-    water receded shortly <em>before</em> the first shock. When it returned, after
-    the second shock, it was sufficient to throw the U.S. ship ‘Monagahela’
-    high and dry.<a id="FNanchor_75" href="#Footnote_75" class="fnanchor">[75]</a></p>
-
-  <p>Another American ship, the ‘Wateree,’ was also lost in 1868 by being
-    swept a quarter of a mile inland by the sea wave which inundated
-    Arequippa.</p>
-
-  <p>Much of the great destruction which occurred at the time of the great
-    Lisbon earthquake was due to a series of great sea waves, thirty to
-    sixty feet higher than the highest tide, which swamped the town. These
-    came in about an hour after the town had been shattered by the motion
-    of the ground.</p>
-
-  <p>The first motion in the waters was their withdrawal, which was
-    sufficient to completely uncover the bar at the mouth of the Tagus. At
-    Cadiz, the first wave, which was<span class="pagenum" id="Page_166">166</span>
-    the greatest, is said to have been sixty feet in height. Fortunately
-    the devastating effect which this would have produced was partially
-    warded off by cliffs.</p>
-
-  <p>At the time of the Jamaica earthquake (1692) the sea drew back for a
-    distance of a mile.</p>
-
-  <p>In South America sea waves are common accompaniments of large
-    earthquakes, and they are regarded with more fear than the actual
-    earthquakes.</p>
-
-  <p>On October 28, 1724, Lima was destroyed, and on the evening of that day
-    the sea rose in a wave eighty feet over Callao. Out of twenty-three
-    ships in the harbour, nineteen were sunk, and four others were carried
-    far inland. The first movement which is usually observed is a drawing
-    back of the waters, and this is so well known to precede the inrush of
-    large waves, that many of the inhabitants in South America have used it
-    as a timely warning to escape towards the hills, and save themselves
-    from the terrible reaction which, on more than one occasion, has so
-    quickly followed.</p>
-
-  <p>At Caldera, near to Copiapo, on May 9, 1877, which was the time when
-    Iquique was devastated, the first motion which was observed in the sea
-    was that it silently drew back for over 200 feet, after which it rose
-    as a wave over five feet high. At some places the water came in as
-    waves from twenty to eighty feet in height.</p>
-
-  <p>At Talcahuano, on the coast of Chili, in 1835, there was a repetition
-    of the phenomena which accompanied the destruction of Penco in 1730
-    and 1751. About forty minutes after the first shock, the sea suddenly
-    retired. Soon afterwards, however, it returned in a wave twenty feet
-    high, the reflex of which swept everything towards the sea. These
-    phenomena were repeated three times.<a id="FNanchor_76" href="#Footnote_76" class="fnanchor">[76]</a></p>
-
-  <p>When Callao and Lima were destroyed, in 1746, the
-    <span class="pagenum" id="Page_167">167</span>sea first drew back,
-    then came in as waves, four or five minutes after the earthquake.
-    Altogether, between October 28 and February 24, 451 shocks were
-    counted. At one time the sea came in eighty feet above its usual
-    level. One account says that the large waves came in forty-one and a
-    half hours after the first shock, and seventeen and a half hours after
-    comparative tranquillity had prevailed.<a id="FNanchor_77" href="#Footnote_77" class="fnanchor">[77]</a></p>
-
-  <p>At the eruption of Monte Nuovo, near Naples, in September 27, 1538, the
-    water drew back forty feet, so that the whole gulf of Baja became dry.</p>
-
-  <p>In 1696, at the time of the Catanian earthquake, the sea is said to
-    have gone back 2,000 fathoms. Instances are recorded where the sea has
-    receded several miles.</p>
-
-  <p>The time taken for the flowing back of the sea is usually very
-    different. Sometimes it has only been five or six minutes, whilst at
-    other times over half an hour, and there are records where the time is
-    said to have been still longer.</p>
-
-  <p>Thus, at the earthquake of Santa (June 17, 1678), the sea is stated to
-    have gone as far back as the eye could reach, and did not rise again
-    for twenty-four hours, when it flooded everything.</p>
-
-  <p>In 1690 at Pisco the sea went back two miles, and did not return for
-    three hours. When it returns it does so with violence, and examples of
-    the heights to which it may reach have been given. The greatest sea
-    wave yet recorded, according to Fuchs, is one which, on October 6,
-    1737, broke on the coast of Lupatka, 210 feet in height.</p>
-
-  <p>There are, however, cases known where the sea has returned as gradually
-    as it went out. Thus, on December
-    <span class="pagenum" id="Page_168">168</span>4, 1854, when Acapulco was
-    destroyed, the sea is said to have returned as gently as it went out.</p>
-
-  <p>When sea waves have travelled long distances from their origin, as,
-    for instance, whenever a South American wave crosses the Pacific to
-    Japan, the phenomena which are observed are like those which were
-    observed at Acapulco; the sea falls and rises, at intervals of from ten
-    minutes to half an hour, to heights of from six to ten feet, without
-    the slightest appearance of a wave. Its phenomenon is like that of an
-    unusually high tide, which repeats itself several times per hour. Even
-    if we watch distant rocks with a telescope, although the surface of the
-    ocean may be as smooth as the surface of a mirror, there is not the
-    slightest visible evidence of what is popularly called a wave. The sea
-    being once set in motion it continues to move as waves of oscillation
-    for a considerable time. In 1877, as observed in Japan, the motion
-    continued for nearly a whole day. The period and amplitude of the rise
-    and fall were variable, usually it quickly reached a maximum, and then
-    died out gradually. As observed in a self-recording tide gauge at San
-    Francisco, the disturbance lasted for about four days. A diagram of
-    this is here given. In its general appearance it is very similar to the
-    records of other earthquake waves. The large waves represent the usual
-    six hours rise and fall of the tides; usually these are fairly smooth
-    curves. Superimposed on the large waves are the smaller zigzag curves
-    of the earthquake disturbance, lasting with greater or less intensity
-    for several days. As these curves are drawn to scale—horizontally for
-    hours, and vertically one fifth inch to the foot, to show the extent of
-    the rise and fall—they will be easily understood.</p>
-
-  <p>Sometimes, as in the present example, the first movement in the
-    waters is that of an incoming wave. In many
-    <span class="pagenum" id="Page_169">169</span> instances, however, this
-    observation may be due to the slow and more gentle phenomena of the
-    previous drawing out of the water, which in a steep waste, or when the
-    water is rough, would be difficult to observe, not having been remarked.</p>
-
-  <p>The distance to which these sea waves have extended has usually been
-    exceedingly great.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p169.jpg" width="692" height="517" alt="" />
-    <div class="caption"><span class="smcap">Fig. 28.</span>—Record of Tide Gauge at Port Point,
-    San Francisco.&nbsp; &nbsp; Showing Earthquake Waves of May 1877.</div>
-  </div>
-
-  <p>The sea wave of the Iquique earthquake of May 9, 1877, like many of its
-    predecessors, was felt across the basin of the whole Pacific, from New
-    Zealand in the south, to Japan and Kamschatka in the north. And but for
-    the intervention of the Eurasian and American continents
-    <span class="pagenum" id="Page_170">170</span> would have
-    made itself appreciable over the surface of the whole of our globe. At
-    places on the South American coast, it has been stated that the height
-    of the waves varied from twenty to eighty feet. At the Samoa Islands
-    the heights varied from six to twelve feet. In New Zealand the sea rose
-    and fell from three to twenty feet. In Australia the heights to which
-    the water oscillated were similar to those observed in New Zealand. In
-    Japan it rose and fell from five to ten feet. In this latter country,
-    the phenomena of sea waves which follow a destructive earthquake on the
-    South American coast are so well known that old residents have written
-    to the local papers announcing the probability of such occurrences
-    having taken place some twenty-five hours previously in South America.
-    In this way news of great calamities has been anticipated, details of
-    which only arrived some weeks subsequently. Just as the destructive
-    earthquakes of South America have announced themselves in Japan, in
-    a like manner the destructive earthquakes of Japan have announced
-    themselves upon the tide gauges of California. Similarly, but not so
-    frequently, disturbances shake the other oceans of the world.</p>
-
-  <p>For example, the great earthquake of Lisbon propagated waves to the
-    coasts of America, taking on their journey nine and a half hours.</p>
-
-  <p><em>Sea waves without earthquakes.</em>—Sometimes we get great sea waves
-    like abnormal tides occurring without any account of contemporaneous
-    earthquakes. Although earthquakes have not been recorded, these ill
-    understood phenomena are usually attributed to such movements.</p>
-
-  <p>Several examples of these are given by Mallet. Thus, at 10
-    <span class="smcap">a.m.</span> on March 2, 1856, the sea rose and fell for a
-    considerable distance at many places on the coast of Yorkshire. At
-    Whitby, the tide was described ebbing and flowing six times per hour,
-    and this to such a distance<span class="pagenum" id="Page_171">171</span>
-    that a vessel entering the harbour was alternately afloat and aground.</p>
-
-  <p>In 1761, on July 17, a similar phenomena was observed at the same place.</p>
-
-  <p>A like occurrence took place at Kilmore, in the county of Wexford, on
-    September 16, 1864, when the water ebbed and flowed seven times in the
-    course of two hours and a half. These tides, which appear to have taken
-    about five minutes to rise and five minutes to fall, were seen by an
-    observer approaching from the west as six distinct ridges of water. The
-    general character of the phenomena appears to have been very similar to
-    that which was produced at the same place by the Lisbon earthquake of
-    1755; and the opinion of those who saw and wrote about their occurrence
-    was that it was due to an earthquake disturbance. Such phenomena are
-    not uncommon on the Wexford coast, where they are popularly known as
-    ‘death waves,’ probably in consequence of the lives which have been
-    lost by these sudden inundations.</p>
-
-  <p>They have also been observed in other parts of Ireland, the north-east
-    coast of England, and in many parts of the globe. They will be again
-    referred to under the head of earth pulsations.</p>
-
-  <p><em>Cause of sea waves.</em>—Mallet, who in his report to the British
-    Association in 1858, writes upon this last-mentioned occurrence at
-    considerable length, whilst admitting that many may have originated
-    from earthquakes, he thinks it scarcely probable that an earthquake
-    blow, sufficiently powerful to have produced waves like those observed
-    at Kilmore, should not have been felt generally throughout the south of
-    Ireland. He, therefore, suggests that sometimes waves like the above
-    might be produced by an underwater slippage of the material forming
-    the face of a submarine bank, the slope of which by degradation and
-    <span class="pagenum" id="Page_172">172</span>
-    deposition, produced by currents, had reached an angle beyond the
-    limits of repose of the material of which it was formed. Mallet does
-    not insist upon the existence of these submarine landslips, but only
-    suggests their existence as a means of explaining certain abnormal sea
-    waves which do not appear to have been accompanied by earthquakes.</p>
-
-  <p>In the generality of cases sea waves are accompanied by earthquakes,
-    but it may often happen that the connection between the two is
-    difficult to clearly establish. One simple explanation for the origin
-    of waves occurring with earthquakes, is, that in consequence of the
-    earthquake a large volume of water suddenly finds its way into cavities
-    which have been opened, the disturbance produced by the inrush giving
-    rise to waves.</p>
-
-  <p>A second explanation is, that the land along a shore is caused by an
-    earthquake to oscillate upwards, the water running off to regain its
-    level. A supposition like this is negatived by the fact that these
-    disturbances are felt far away from the chief disturbance, on small
-    islands. Also, it may be added, that the whole disturbance appears to
-    approach the land from the sea, and not in the opposite direction.
-    Thus, in the earthquake of Oahu (February 18, 1871), it was remarked
-    that the shock was first felt by the ships farthest from the land.<a id="FNanchor_78" href="#Footnote_78" class="fnanchor">[78]</a></p>
-
-  <p>Another suggestion is that the waves are due to a sudden heaving up
-    of the bottom of the ocean. If this lifting took place slowly, then
-    the first result would be that the water situated over the centres of
-    disturbance would flow away radially in all directions from above the
-    area of disturbance.</p>
-
-  <p>If, however, the submarine upheaval took place with great rapidity, say
-    by the sudden evolution of a large
-    <span class="pagenum" id="Page_173">173</span>volume of steam developed by the
-    entry of water into a volcanic vent, as the water was heaped above the
-    disturbed area, water might run in radially towards this spot.</p>
-
-  <p>Supposing a primary wave to be formed in the ocean by any such causes,
-    then the falling of this will cause a second wave to be formed,
-    existing as a ring round the first one. The combined action of the
-    first and second wave will form a third one, and so the disturbance,
-    starting from a point, will radiate in broadening circles. During the
-    up and down motion of these waves, the energy which is imparted to any
-    particle of water will, on account of the work which it has to do in
-    displacing its neighbours, by frictional resistance, gradually grow
-    less and less, until it finally dies away. The waves which are the
-    result of this motion will also grow less and less.</p>
-
-  <p>If a series of sea waves were produced by a single disturbance, we see
-    that these will be of unequal magnitude. Now, for small waves, the
-    velocity with which they travel depends upon the square root of their
-    lengths; but with large waves, like earthquake waves, the velocity
-    depends upon the square root of the depth of water, and these latter
-    travel more quickly than the former.</p>
-
-  <p>If, therefore, we have a series of disturbances of unequal magnitude
-    producing sea waves, which, from the series of shocks which have
-    been felt upon shores subsequently invaded by waves, seems in all
-    probability often to have been the case, it is not unlikely that the
-    waves of an early disturbance may be overtaken and interfered with by a
-    series which followed.</p>
-
-  <p>These considerations help us to understand the appearance of the
-    records on our tide gauges, and also the phenomena observed by those
-    who have recorded tidal waves as they swept inwards upon the land.
-    For instance, we understand the reason why sea waves, as observed at
-    <span class="pagenum" id="Page_174">174</span>
-    places at different distances from the origin of a disturbance, should
-    be of different heights. We also see an explanation for the fact that
-    small waves should sometimes appear to be interpolated between large
-    ones, and that these should occur at varying intervals.</p>
-
-  <p>The fact that whenever a wave is produced, a certain quantity of water
-    must be drawn from the level which surrounds it, in order that it
-    should be formed, explains the phenomena that the sea is often observed
-    first to draw back. Out in the open ocean it is drawn from the hollow
-    between two waves. As has been pointed out by Darwin, it is like the
-    drawing of the water from the shore of a river by a passing steamer.</p>
-
-  <p>The difference in the height of waves, as observed at places lying
-    close to each other, is probably due to the configuration of the coast,
-    the interference of outlying islands, reefs, &amp;c.—causes which would
-    produce similar effects in the height of tide.</p>
-
-  <p>As a wave approaches shallow water it gradually increases in height,
-    its front slope becomes steep, and its rear slope gentle, until finally
-    it topples over and breaks. This increasing in height of waves is
-    no doubt connected with the destruction of Talcahuano and Callao,
-    which are situated at the head of shallow bays. Valparaiso, which is
-    on the edge of deep water, has never been overwhelmed.<a id="FNanchor_79" href="#Footnote_79" class="fnanchor">[79]</a> Another
-    case tending to produce anomalies in the character of waves would be
-    their reflection and mutual interference, the reflections due to the
-    configuration of the ocean bed and coast lines.</p>
-
-  <p>The complete phenomena which may accompany a violent submarine
-    disturbance are as follows:—</p>
-
-  <p>By the initial impulse of explosion or lifting of the ground, a ‘great
-    sea wave’ is generated, which travels
-    <span class="pagenum" id="Page_175">175</span>shorewards with a velocity
-    dependent upon its size and the depth of the ocean. At the same
-    instant, a ‘sound wave’ may be produced in the air, which travels at a
-    quicker rate than the ‘great sea wave.’ A third wave which is produced,
-    is an ‘earth wave,’ which will reach the shore with a velocity
-    dependent on the intensity of the impulse and the elasticity of the
-    rocks through which it is propagated. This latter, which travels the
-    fastest, may carry on its back a small ‘forced sea wave.’ On reaching
-    the shore and passing inland, this ‘earth wave’ will cause a slight
-    recession of the water as the ‘forced sea wave’ slips from its back.</p>
-
-  <p>As these ‘forced sea waves’ travel they will give blows to ships
-    beneath which they may pass, being transmitted from the bottom of the
-    ocean to the bottom of the ships like sound waves in water. At the
-    time of small earthquakes, produced, for example, by the explosion of
-    small quantities of water entering volcanic fissures, or by the sudden
-    condensation of steam from such a fissure entering the ocean, aqueous
-    sound waves are produced, which cause the rattling and vibrating jars
-    so often noticed on board ships.</p>
-
-  <p><em>Phenomena difficult of explanation.</em>—Although we can in this way
-    explain the origin and phenomena of sea waves, we must remember, as
-    Kluge has pointed out, that it is not the simple backward and forward
-    movement of the ground which produces sea waves, and that the majority
-    of earthquakes which have occurred in volcanic coasts have been
-    unaccompanied by such phenomena. Out of 15,000 earthquakes observed on
-    coast lines, only 124 were accompanied by sea waves.<a id="FNanchor_80" href="#Footnote_80" class="fnanchor">[80]</a> Out of 1,098
-    earthquakes catalogued by Perrey for the west coast of South America,
-    only nineteen are said to have been accompanied
-    <span class="pagenum" id="Page_176">176</span>by movements in the
-    waters. According to the ‘Geographical Magazine’ (August 1877, p. 207),
-    it would seem that out of seventy-one severe earthquakes which have
-    occurred since the year 1500 upon the South American coast many have
-    been accompanied by sea waves. Darwin also remarks, when speaking of
-    South America, that almost every large earthquake has been accompanied
-    by considerable agitation in the neighbouring sea.<a id="FNanchor_81" href="#Footnote_81" class="fnanchor">[81]</a></p>
-
-  <p>On April 2, 1851, when many towns in Chili were destroyed, the sea was
-    not disturbed. At the time of the great earthquake of New Zealand (June
-    23, 1855), although all the shocks came from the sea, yet there was no
-    flood. The small shock of February 14, however, was accompanied by a
-    motion in the sea.</p>
-
-  <p>To these examples, which have been chiefly drawn from the writings of
-    Fuchs, must be added the fact that the greater number of disturbances
-    which are felt in the north-eastern part of Japan, although they
-    emanate from beneath the sea, do not produce any visible sea waves.
-    They are, however, sufficient to cause a vibratory motion on board
-    ships situated near their origin.</p>
-
-  <p>Another point referred to by Fuchs, as difficult of explanation,
-    is, that the water, when it draws back, often does so with extreme
-    slowness, and farther, in some instances, it has not returned to its
-    original level. That the sea might be drawn back for a period of
-    fifteen or thirty minutes is intelligible, when we consider the great
-    length of the waves which are formed. Cases where it has retired for
-    several hours or days, and when its original level is altered, appear
-    only to be explicable on the assumption of more or less permanent
-    changes in the levels of the ground. For example, in the earthquake
-    of 1855 which shook New Zealand, the whole southern portion of the
-    northern island was raised several feet.</p>
-
-  <p><span class="pagenum" id="Page_177">177</span></p>
-
-  <p>These sudden alterations in the levels of coast lines have already
-    been referred to.</p>
-
-  <p>Other points which are difficult to understand are the occurrence
-    of disturbances in the sea at the time of feeble earthquakes, and
-    with earthquakes occurring in distant places. As examples of such
-    occurrences, Fuchs quotes the following: ‘On May 16, 1850, at 4.28
-    <span class="smcap">a.m.</span>, an earthquake took place in Pesth, and at 7.30 a
-    motion was observed in the sea at Livorno. Again, at the time of the
-    earthquake of December 19, 1850, which shook Heliopolis, a flood
-    suddenly came in upon Cherbourg.’ May not these phenomena be the result
-    of an earth pulsation, which produced an earthquake at one point, and a
-    sea wave at another?</p>
-
-  <p>Equally difficult to understand are the observations when the
-    disturbance in the sea has occurred several hours after an earthquake;
-    as, for instance, at Batavia, in 1852, when there was an interval of
-    two hours; and to this must be added the observations where the motion
-    of the sea has preceded that of the earthquake—as, for instance, in
-    1852, at Smyrna. Whilst recognising the fact that it is possible to
-    suggest explanations for many of these anomalies, we must also bear
-    in mind that they are, generally speaking, exceptional, and, in some
-    instances, may possibly be due to errors in observations.</p>
-
-  <p><em>Velocity of propagation of sea waves, and depth of the ocean.</em>—It
-    has long been known to physical science that the velocity with which
-    a given wave is propagated along a trough of uniform depth, holds a
-    relation to the depth of the trough.</p>
-
-  <p>If <i>v</i> is the velocity of the wave, and <i>h</i> the depth of the trough,
-    this relation may be expressed as follows:—</p>
-
-  <div class="center">
-    <i>h</i> = <span class="frac"><sup><i>v</i><sup>2</sup></sup><span>/</span><sub><i>g</i></sub></span>
-    or <i>h</i> = <span class="x200b">(</span><span class="frac"><sup><i>v</i></sup><span>/</span><sub><i>k</i></sub></span><span class="x200b">)</span><sup>2</sup>
-  </div>
-
-  <p class="center">
-    <span class="pagenum" id="Page_178">178</span>
-    Where <i>g</i> = 32·19 and <i>k</i> = 5·671.
-  </p>
-
-  <p>It will be observed that these two formulæ (the first of which is
-    known as Russell’s formula, and the second as Airy’s) are practically
-    identical.</p>
-
-  <p>The apparent difference is in the average value assigned to the
-    constant.</p>
-
-  <p>For large waves such as we have to deal with, it would be necessary,
-    if we were desirous of great accuracy, to increase the value of <i>h</i>
-    by some small fraction of itself. We might also make allowance for
-    the different values of <i>g</i>, according to our position on the earth’s
-    surface. With these formulæ at our disposal it is an easy matter, after
-    having determined the velocity with which a wave was propagated, to
-    determine the average depth of the area over which it was transmitted.</p>
-
-  <p>In making certain earthquake investigations the reverse problem is
-    sometimes useful—namely, determining the velocity with which a sea wave
-    has advanced upon a shown line, from a knowledge of the depth of the
-    water in which it has been propagated.</p>
-
-  <p>Calculations of the average depths of the Pacific, dependent on the
-    velocity with which earthquake waves have been propagated, have been
-    made by many investigators.</p>
-
-  <p>In most cases, however, in consequence of having assumed the wave to
-    have originated on a coast line, when the evidence clearly showed it
-    to have originated some distance out at sea, the calculations which
-    have been made are open to criticism. The average depths which I
-    obtained for various lines across the Pacific appear to be somewhat
-    less than the average depth as given by actual soundings. We must,
-    however, remember that the common error in actual soundings is that
-    they are usually too great, it being difficult in deep-sea sounding
-    <span class="pagenum" id="Page_179">179</span>
-    to determine when the lead actually reaches the bottom. Until oceans
-    have been more thoroughly surveyed with the improved forms of sounding
-    apparatus, we shall not be able to verify the truth of the results
-    which have been given to us by earthquake waves.</p>
-
-  <h3><span class="smcap">Examples of Calculations on Sea Waves.</span></h3>
-
-  <p>1. <em>The wave of 1854.</em>—This wave originated near Japan, and it was
-    recorded on tide gauges at San Francisco, San Diego, and Astoria.</p>
-
-  <p>On December 23, at 9.15. <span class="smcap">a.m.</span>, a strong shock was felt at
-    Simoda in Japan, which, at 10 o’clock, was followed by a large wave
-    thirty feet in height. The rising and falling of the water continued
-    until noon. Half an hour after, the movement became more violent than
-    before. At 2.15 <span class="smcap">p.m.</span> this agitation decreased, and at 3
-    <span class="smcap">p.m.</span> it was comparatively slow. Altogether there were five
-    large waves.</p>
-
-  <p>On December 23 and 25, unusual waves were recorded upon the
-    self-registering tide gauges at San Francisco, San Diego, and Astoria.</p>
-
-  <p>At San Francisco three sets of waves were observed. The average time of
-    oscillation of one of the first set was thirty-five minutes, whilst one
-    of the second and third sets was almost thirty-one minutes.</p>
-
-  <p>At San Diego three series of waves were also shown, but with average
-    times of oscillation of from four to two minutes shorter than the waves
-    at San Francisco.</p>
-
-  <p>The San Francisco waves appear to indicate a recurrence of the same
-    phenomena.</p>
-
-  <p>The record at San Diego shows what was probably the effect of a
-    series of impulses, the heights increasing to the
-    <span class="pagenum" id="Page_180">180</span> third wave, then
-    diminishing, then once more renewed, after which it died away.</p>
-
-  <p>The result of calculations based on these data were:—</p>
-
-  <table class="collapse" summary="Simoda sea waves">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th class="ball">Distance<br />geographical<br />miles</th>
-      <th colspan="2" class="ball">Time of<br />transmission</th>
-      <th class="ball">Velocity in<br />feet per sec.</th>
-      <th class="ball">Depth of ocean<br />in fathoms</th>
-    </tr>
-    <tr>
-      <th class="bl">&nbsp;</th>
-      <th class="bl">&nbsp;</th>
-      <th class="bl">h.</th>
-      <th class="br">m.</th>
-      <th class="br">&nbsp;</th>
-      <th class="br">&nbsp;</th>
-    </tr>
-    <tr>
-      <td class="tdl bl">Simoda to San Diego</td>
-      <td class="tdc bl">4917</td>
-      <td class="tdc bl">12</td>
-      <td class="tdc br">13</td>
-      <td class="tdc br">545</td>
-      <td class="tdc br">2100</td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl">Simoda to San Francisco</td>
-      <td class="tdc bl">4527</td>
-      <td class="tdc bl">12</td>
-      <td class="tdc br">39</td>
-      <td class="tdc br">528</td>
-      <td class="tdc br">2500 or 2230</td>
-    </tr>
-  </table>
-
-  <p>The difference for the depths in the San Francisco path depends whether
-    the length of the waves is reckoned at 210 or 217 miles. The length of
-    the waves on the San Diego path were 186 or 192 miles.<a id="FNanchor_82" href="#Footnote_82" class="fnanchor">[82]</a></p>
-
-  <p><em>The wave of 1868.</em>—On August 11, 1868, a sea wave ruined many cities
-    on the South American coast, and 25,000 lives were lost. This wave,
-    like all the others, travelled the length and breadth of the Pacific.</p>
-
-  <p>In Japan, at Hakodate, it was observed by Captain T. Blakiston, R.A.,
-    who very kindly gave me the following account:</p>
-
-  <p>On August 15, at 10.30 <span class="smcap">a.m.</span>, a series of bores or tidal waves
-    commenced, and lasted until 3 <span class="smcap">p.m.</span> In ten minutes there was
-    a difference in the sea level of ten feet, the water rising above
-    high water and falling below low water mark with great rapidity. The
-    ordinary tide is only two and a half to three feet. The disturbance
-    producing these waves originated between Iquique and Arica, in about
-    lat. 18.28 S. at about 5 <span class="smcap">p.m.</span> on August 13. In Greenwich time
-    this would be about 13h. 9m. 40s. August 13. The arrival of the wave
-    at Hakodate in Greenwich time would be about 14h. 7m. 6s. August 14:
-    that is to say, the wave took about 24h. 57m. to travel about 8,700
-    miles, which <span class="pagenum" id="Page_181">181</span>
-    gives us an average rate of about 511 feet per second.
-    These waves were felt all over the Pacific. At the Chatham Islands they
-    rushed in with such violence that whole settlements were destroyed. At
-    the Sandwich Islands the sea oscillated at intervals of ten minutes for
-    three days.</p>
-
-  <p>Comparing this wave with the one of 1877 we see that one reached
-    Hakodate with a velocity of 511 feet per second, whilst the other
-    travelled the same distance at 512 feet per second.</p>
-
-  <p>An account of this earthquake wave has been given by F. von Hochstetter
-    (‘Über das Erdbeben in Peru am 13. August 1868 und die dadurch
-    veranlassten Fluthwellen im Pacifischen Ocean,’ Sitzungsberichte
-    der Kaiserl. Akademie der Wissenschaften, Wien 58. Bd., 2. Abth.
-    1868). From an epitome of this paper given in ‘Petermann’s Geograph.
-    Mittheil.’ 1869, p. 222, I have drawn up the following table of the
-    more important results obtained by F. von Hochstetter.</p>
-
-  <p>The wave is assumed to have originated near Arica.</p>
-
-  <table class="collapse" summary="Hochstetter sea waves">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th class="ball">Distance<br />sea miles<br />from Arica</th>
-      <th colspan="2" class="ball">Time<br />taken by wave</th>
-      <th class="ball">Velocity<br />in feet per<br />second</th>
-      <th class="ball">Depth of<br />ocean in feet</th>
-    </tr>
-    <tr>
-      <th class="bl">&nbsp;</th>
-      <th class="bl">&nbsp;</th>
-      <th class="tdr bl"><div>h.</div></th>
-      <th class="tdr br"><div>m.</div></th>
-      <th class="br">&nbsp;</th>
-      <th class="tdr br">&nbsp;</th>
-    </tr>
-    <tr>
-      <td class="tdl bl">Valdivia</td>
-      <td class="tdc bl">1,420</td>
-      <td class="tdr bl"><div>5</div></td>
-      <td class="tdr br"><div>0</div></td>
-      <td class="tdc br">479</td>
-      <td class="tdr br"><div>7,140</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Chatham Islands</td>
-      <td class="tdc bl">5,520</td>
-      <td class="tdr bl"><div>15</div></td>
-      <td class="tdr br"><div>19</div></td>
-      <td class="tdc br">608</td>
-      <td class="tdr br"><div>11,472</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Lyttleton</td>
-      <td class="tdc bl">6,120</td>
-      <td class="tdr bl"><div>19</div></td>
-      <td class="tdr br"><div>18</div></td>
-      <td class="tdc br">533</td>
-      <td class="tdr br"><div>8,838</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Newcastle</td>
-      <td class="tdc bl">7,380</td>
-      <td class="tdr bl"><div>22</div></td>
-      <td class="tdr br"><div>28</div></td>
-      <td class="tdc br">538</td>
-      <td class="tdr br"><div>9,006</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Apia (Samoa)</td>
-      <td class="tdc bl">5,760</td>
-      <td class="tdr bl"><div>16</div></td>
-      <td class="tdr br"><div>2</div></td>
-      <td class="tdc br">604</td>
-      <td class="tdr br"><div>11,346</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Rapa</td>
-      <td class="tdc bl">4,057</td>
-      <td class="tdr bl"><div>11</div></td>
-      <td class="tdr br"><div>11</div></td>
-      <td class="tdc br">611</td>
-      <td class="tdr br"><div>11,598</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Hilo</td>
-      <td class="tdc bl">5,400</td>
-      <td class="tdr bl"><div>14</div></td>
-      <td class="tdr br"><div>25</div></td>
-      <td class="tdc br">555</td>
-      <td class="tdr br"><div>9,568</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl">Honolulu</td>
-      <td class="tdc bl">5,580</td>
-      <td class="tdr bl"><div>12</div></td>
-      <td class="tdr br"><div>37</div></td>
-      <td class="tdc br">746</td>
-      <td class="tdr br"><div>17,292</div></td>
-    </tr>
-  </table>
-
-  <p>Calculations on the same disturbance are also given by J. E.
-    Hilgard.<a id="FNanchor_83" href="#Footnote_83" class="fnanchor">[83]</a></p>
-
-  <p>Assuming the origin of the wave to have been at Arica, his results are
-    as follows:</p>
-
-  <p><span class="pagenum" id="Page_182">182</span></p>
-
-  <table class="collapse" summary="Hilgard sea waves">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th class="ball">Distance<br />from Africa</th>
-      <th colspan="2" class="ball">Time of<br />transmission</th>
-      <th class="ball">Nautical miles<br />per hour</th>
-      <th class="ball">Mean depth<br />of ocean</th>
-    </tr>
-    <tr>
-      <th class="bl">&nbsp;</th>
-      <th class="bl">miles</th>
-      <th class="tdr bl"><div>h.</div></th>
-      <th class="tdr br"><div>m.</div></th>
-      <th class="br">&nbsp;</th>
-      <th class="br">feet</th>
-    </tr>
-    <tr>
-      <td class="tdl bl">San Diego</td>
-      <td class="tdc bl">4,030</td>
-      <td class="tdr bl"><div>10</div></td>
-      <td class="tdr br"><div>55</div></td>
-      <td class="tdc br">369</td>
-      <td class="tdr br"><div>12,100</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Fort Point</td>
-      <td class="tdc bl">4,480</td>
-      <td class="tdr bl"><div>12</div></td>
-      <td class="tdr br"><div>56</div></td>
-      <td class="tdc br">348</td>
-      <td class="tdr br"><div>10,800</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Astoria</td>
-      <td class="tdc bl">5,000</td>
-      <td class="tdr bl"><div>18</div></td>
-      <td class="tdr br"><div>51</div></td>
-      <td class="tdc br">265</td>
-      <td class="tdr br"><div>6,200</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Kodiak</td>
-      <td class="tdc bl">6,200</td>
-      <td class="tdr bl"><div>22</div></td>
-      <td class="tdr br"><div>00</div></td>
-      <td class="tdc br">282</td>
-      <td class="tdr br"><div>7,000</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Rapa</td>
-      <td class="tdc bl">4,057</td>
-      <td class="tdr bl"><div>10</div></td>
-      <td class="tdr br"><div>54</div></td>
-      <td class="tdc br">372</td>
-      <td class="tdr br"><div>12,200</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Chatham Islands</td>
-      <td class="tdc bl">5,520</td>
-      <td class="tdr bl"><div>15</div></td>
-      <td class="tdr br"><div>01</div></td>
-      <td class="tdc br">368</td>
-      <td class="tdr br"><div>12,100</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Hawaii</td>
-      <td class="tdc bl">5,460</td>
-      <td class="tdr bl"><div>14</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdc br">385</td>
-      <td class="tdr br"><div>13,200</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Honolulu</td>
-      <td class="tdc bl">5,580</td>
-      <td class="tdr bl"><div>12</div></td>
-      <td class="tdr br"><div>18</div></td>
-      <td class="tdc br">454</td>
-      <td class="tdr br"><div>18,500</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Samoa</td>
-      <td class="tdc bl">5,760</td>
-      <td class="tdr bl"><div>15</div></td>
-      <td class="tdr br"><div>38</div></td>
-      <td class="tdc br">368</td>
-      <td class="tdr br"><div>12,100</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Lyttelton</td>
-      <td class="tdc bl">6,120</td>
-      <td class="tdr bl"><div>19</div></td>
-      <td class="tdr br"><div>01</div></td>
-      <td class="tdc br">322</td>
-      <td class="tdr br"><div>9,200</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Newcastle</td>
-      <td class="tdc bl">9,380</td>
-      <td class="tdr bl"><div>22</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdc br">332</td>
-      <td class="tdr br"><div>9,800</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl">Sydney</td>
-      <td class="tdc bl">7,440</td>
-      <td class="tdr bl"><div>23</div></td>
-      <td class="tdr br"><div>41</div></td>
-      <td class="tdc br">314</td>
-      <td class="tdr br"><div>8,800</div></td>
-    </tr>
-  </table>
-
-  <p><em>The wave of 1877.</em>—Two sets of calculations have been made upon the
-    wave of 1877 by Dr. E. Geinitz of Rostock.<a id="FNanchor_84" href="#Footnote_84" class="fnanchor">[84]</a></p>
-
-  <p>The following table is taken from Dr. Geinitz’s second paper, in which
-    there are several modifications of his first results. The origin of the
-    disturbance is assumed to have been near Iquique.</p>
-
-  <table class="collapse" summary="Geinitz sea waves">
-    <tr>
-      <th class="ball">Observation stations</th>
-      <th class="ball">Distance<br />from Iquique<br />geol. miles</th>
-      <th colspan="3" class="ball">Arrival of wave</th>
-      <th colspan="2" class="ball">Time taken<br />by wave</th>
-      <th class="ball">Velocity in<br />feet per<br />second</th>
-      <th class="ball">Mean depth<br />of ocean<br />in fathoms</th>
-    </tr>
-    <tr>
-      <th class="bl">&nbsp;</th>
-      <th class="bl">&nbsp;</th>
-      <th class="tdr bl"><div>h.</div></th>
-      <th class="tdr"><div>m.</div></th>
-      <th class="br">&nbsp;</th>
-      <th class="tdr"><div>h.</div></th>
-      <th class="tdr br"><div>m.</div></th>
-      <th class="br">&nbsp;</th>
-      <th class="br">&nbsp;</th>
-    </tr>
-    <tr>
-      <td class="tdl bl">Taiohāc (Marquesa Islands)</td>
-      <td class="tdc bl">4,086</td>
-      <td class="tdr bl"><div>8</div></td>
-      <td class="tdr"><div>40</div></td>
-      <td class="tdc br"><span class="smcap">a.m.</span></td>
-      <td class="tdr"><div>12</div></td>
-      <td class="tdr br"><div>15</div></td>
-      <td class="tdc br">563·8</td>
-      <td class="tdc br">1,647</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Apia (Samoa)</td>
-      <td class="tdc bl">5,740</td>
-      <td class="tdr bl"><div>12</div></td>
-      <td class="tdr"><div>0</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>15</div></td>
-      <td class="tdr br"><div>30</div></td>
-      <td class="tdc br">610·4</td>
-      <td class="tdc br">1,930</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Hilo (Sandwich Islands)</td>
-      <td class="tdc bl">5,526</td>
-      <td class="tdr bl"><div>10</div></td>
-      <td class="tdr "><div>24</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>0</div></td>
-      <td class="tdc br">667·9</td>
-      <td class="tdc br">2,310</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Kahuliu&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdc bl">5,628</td>
-      <td class="tdr bl"><div>10</div></td>
-      <td class="tdr"><div>30</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>5</div></td>
-      <td class="tdc br">675·2</td>
-      <td class="tdc br">2,361</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Honolulu&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;„</td>
-      <td class="tdc bl">5,712</td>
-      <td class="tdr bl"><div>10</div></td>
-      <td class="tdr"><div>50</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>25</div></td>
-      <td class="tdc br">669·7</td>
-      <td class="tdc br">2,319</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Wellington (New Zealand)</td>
-      <td class="tdc bl">5,657</td>
-      <td class="tdr bl"><div>2</div></td>
-      <td class="tdr"><div>40</div></td>
-      <td class="tdc br"><span class="smcap">p.m.</span></td>
-      <td class="tdr"><div>18</div></td>
-      <td class="tdr br"><div>15</div></td>
-      <td class="tdc br">524·2</td>
-      <td class="tdc br">1,430</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Lyttelton&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdc bl">5,641</td>
-      <td class="tdr bl"><div>2</div></td>
-      <td class="tdr"><div>48</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>18</div></td>
-      <td class="tdr br"><div>23</div></td>
-      <td class="tdc br">519·8</td>
-      <td class="tdc br">1,400</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Newcastle (Australia)</td>
-      <td class="tdc bl">6,800</td>
-      <td class="tdr bl"><div>2</div></td>
-      <td class="tdr"><div>32</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>18</div></td>
-      <td class="tdr br"><div>7</div></td>
-      <td class="tdc br">633·0</td>
-      <td class="tdc br">2,075</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Sydney&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdc bl">6,782</td>
-      <td class="tdr bl"><div>2</div></td>
-      <td class="tdr"><div>35</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>18</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdc br">631·4</td>
-      <td class="tdc br">2,065</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Kamieshi (Japan)</td>
-      <td class="tdc bl">8,790</td>
-      <td class="tdr bl"><div>7</div></td>
-      <td class="tdr"><div>20</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>22</div></td>
-      <td class="tdr br"><div>55</div></td>
-      <td class="tdc br">649·0</td>
-      <td class="tdc br">2,182</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Hakodate&nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdc bl">8,760</td>
-      <td class="tdr bl"><div>9</div></td>
-      <td class="tdr"><div>25</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>25</div></td>
-      <td class="tdr br"><div>0</div></td>
-      <td class="tdc br">592·5</td>
-      <td class="tdc br">1,818</td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl">Kadsusa&nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdc bl">8,939</td>
-      <td class="tdr bl"><div>9</div></td>
-      <td class="tdr"><div>50</div></td>
-      <td class="tdc br">„</td>
-      <td class="tdr"><div>25</div></td>
-      <td class="tdr br"><div>15</div></td>
-      <td class="tdc br">604·9</td>
-      <td class="tdc br">1,895</td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_183">183</span></p>
-
-  <p>The mean depths represent a mean of two sets of calculations, one made
-    with the aid of Airy’s formula, and the other by Scott-Russell’s
-    formula. The result of my own investigation about this disturbance, the
-    origin of which, by several methods of calculation, is shown to have
-    been beneath the ocean, near 71° 5′ west long., and 21° 22′ south lat.,
-    are given on next page.</p>
-
-  <p>Dr. Geinitz considers that his calculated depths of the ocean and those
-    obtained by actual soundings are in accordance, a result which is
-    diametrically opposed to that which I have obtained.</p>
-
-  <p>This difference between my calculations and those of Dr. Geinitz,
-    Hochstetter, and others, chiefly rests on the origin we have assigned
-    for the sea waves. Dr. Geinitz, for instance, although he says that
-    the origin of the 1877 earthquake was not on the continent but to
-    the west in the ocean, bases all his calculations on the assumption
-    that the <em>centrum</em> was at or near to Iquique, and the time at which
-    that city was disturbed was the time at which the waves commenced to
-    spread across the ocean. This time is 8.25 <span class="smcap">p.m.</span> At this time,
-    however, it appears that the waves must have been more than double
-    the distance between the true origin and Iquique, from Iquique on
-    their way towards the opposite side of the Pacific. Introducing this
-    element into the various calculations which have been made respecting
-    the depth of the Pacific Ocean as derived from observations on
-    earthquake waves—which element, insomuch as the waves appear to have
-    come in to inundate the land some time after the shock, needs to be
-    introduced—we reduce the velocity of transit of the earthquake wave
-    and, consequently, the resultant depths of the ocean.</p>
-
-  <p><span class="pagenum" id="Page_184">184</span></p>
-
-  <table class="collapse" summary="Geinitz sea waves 2">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th colspan="3" class="ball">Longitude</th>
-      <th colspan="3" class="ball">Arrival of<br />wave in<br />Greenwich<br /> mean time</th>
-      <th colspan="2" class="ball">Time taken<br />by wave</th>
-      <th class="ball">Distance<br />from the<br />origin<br />in miles<br />(calculated<br />in great<br />circles)</th>
-      <th class="ball">Velocity<br />in feet<br />per second</th>
-      <th class="ball">Depth of<br />the ocean<br />in feet</th>
-      <th class="ball">Height<br />of waves</th>
-      <th class="ball">Interval<br />between waves<br />in minutes</th>
-    </tr>
-    <tr>
-      <th class="bl">&nbsp;</th>
-      <th class="tdr bl"><div>°</div></th>
-      <th class="tdr"><div>′</div></th>
-      <th class="tdr br"><div>&nbsp;</div></th>
-      <th class="tdr bl"><div>day</div></th>
-      <th class="tdr"><div>h.</div></th>
-      <th class="tdr br"><div>m.</div></th>
-      <th class="tdr bl"><div>h.</div></th>
-      <th class="tdr br"><div>m.</div></th>
-      <th class="br">&nbsp;</th>
-      <th class="br">&nbsp;</th>
-      <th class="br">&nbsp;</th>
-      <th class="br">&nbsp;</th>
-      <th class="br">&nbsp;</th>
-    </tr>
-    <tr>
-      <td class="tdl bl">Origin of wave</td>
-      <td class="tdr bl"><div>71</div></td>
-      <td class="tdr"><div>5</div></td>
-      <td class="tdr br"><div>W.</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>12</div></td>
-      <td class="tdr br"><div>59</div></td>
-      <td class="tdr"><div>&nbsp;</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="br"><div>&nbsp;</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br"><div>&nbsp;</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">San Francisco</td>
-      <td class="tdr bl"><div>122</div></td>
-      <td class="tdr"><div>32</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>2</div></td>
-      <td class="tdr br"><div>28</div></td>
-      <td class="tdr"><div>13</div></td>
-      <td class="tdr br"><div>29</div></td>
-      <td class="tdr br"><div>4,578</div></td>
-      <td class="tdc br">498</td>
-      <td class="tdr br"><div>7,721</div></td>
-      <td class="br">9 in.</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Callao</td>
-      <td class="tdr bl"><div>77</div></td>
-      <td class="tdr"><div>15</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>17</div></td>
-      <td class="tdr br"><div>9</div></td>
-      <td class="tdr"><div>4</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdr br"><div>658</div></td>
-      <td class="tdc br">231</td>
-      <td class="tdr br"><div>1,657</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Iquique</td>
-      <td class="tdr bl"><div>70</div></td>
-      <td class="tdr"><div>14½</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>13</div></td>
-      <td class="tdr br"><div>21</div></td>
-      <td class="tdr"><div>0</div></td>
-      <td class="tdr br"><div>22</div></td>
-      <td class="tdr br"><div>87</div></td>
-      <td class="tdc br">348</td>
-      <td class="tdr br"><div>3,770</div></td>
-      <td class="br">20 ft.</td>
-      <td class="br">22</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Cobija</td>
-      <td class="tdr bl"><div>70</div></td>
-      <td class="tdr"><div>21</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>13</div></td>
-      <td class="tdr br"><div>19</div></td>
-      <td class="tdr"><div>0</div></td>
-      <td class="tdr br"><div>20</div></td>
-      <td class="tdr br"><div>80</div></td>
-      <td class="tdc br">352</td>
-      <td class="tdr br"><div>3,857</div></td>
-      <td class="br">30 „</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Mejillones</td>
-      <td class="tdr bl"><div>70</div></td>
-      <td class="tdr"><div>35</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>13</div></td>
-      <td class="tdr br"><div>27</div></td>
-      <td class="tdr"><div>0</div></td>
-      <td class="tdr br"><div>28</div></td>
-      <td class="tdr br"><div>108</div></td>
-      <td class="tdc br">339</td>
-      <td class="tdr br"><div>3,587</div></td>
-      <td class="br">35 „</td>
-      <td class="br">15 or 45</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Chanaral</td>
-      <td class="tdr bl"><div>71</div></td>
-      <td class="tdr"><div>34</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>15</div></td>
-      <td class="tdr br"><div>26</div></td>
-      <td class="tdr"><div>2</div></td>
-      <td class="tdr br"><div>27</div></td>
-      <td class="tdr br"><div>455</div></td>
-      <td class="tdc br">272</td>
-      <td class="tdr br"><div>2,309</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">10</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Coquimbo</td>
-      <td class="tdr bl"><div>71</div></td>
-      <td class="tdr"><div>24</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>15</div></td>
-      <td class="tdr br"><div>15</div></td>
-      <td class="tdr"><div>2</div></td>
-      <td class="tdr br"><div>16</div></td>
-      <td class="tdr br"><div>508</div></td>
-      <td class="tdc br">328</td>
-      <td class="tdr br"><div>3,363</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">30</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Valparaiso</td>
-      <td class="tdr bl"><div>71</div></td>
-      <td class="tdr"><div>38</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>16</div></td>
-      <td class="tdr br"><div>16</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdr br"><div>17</div></td>
-      <td class="tdr br"><div>695</div></td>
-      <td class="tdc br">310</td>
-      <td class="tdr br"><div>3,000</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Concepcion</td>
-      <td class="tdr bl"><div>73</div></td>
-      <td class="tdr"><div>5</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>9</div></td>
-      <td class="tdr"><div>16</div></td>
-      <td class="tdr br"><div>52</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdr br"><div>53</div></td>
-      <td class="tdr br"><div>928</div></td>
-      <td class="tdc br">350</td>
-      <td class="tdr br"><div>3,824</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">12 to 15</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Honolulu</td>
-      <td class="tdr bl"><div>157</div></td>
-      <td class="tdr"><div>55</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdr br"><div>52</div></td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>53</div></td>
-      <td class="tdr br"><div>5,694</div></td>
-      <td class="tdc br">561</td>
-      <td class="tdr br"><div>9,807</div></td>
-      <td class="br">34 to 54 ft.</td>
-      <td class="br">25</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Hilo</td>
-      <td class="tdr bl"><div>155</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdr br"><div>5</div></td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>6</div></td>
-      <td class="tdr br"><div>5,506</div></td>
-      <td class="tdc br">563</td>
-      <td class="tdr br"><div>10,217</div></td>
-      <td class="br">30 or 8 „</td>
-      <td class="tdr0 pl0 br">
-        <table summary="Geinitz sub-table">
-          <tr>
-            <td class="x200 pl0">{</td>
-            <td class="tdr0 pr0 pl0"><div>3 or 15<br />18 or 27</div>
-            </td>
-          </tr>
-        </table>
-      </td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Kahuliu</td>
-      <td class="tdr bl"><div>156</div></td>
-      <td class="tdr"><div>43</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdr br"><div>12</div></td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>13</div></td>
-      <td class="tdr br"><div>5,611</div></td>
-      <td class="tdc br">579</td>
-      <td class="tdr br"><div>10,437</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Samoa</td>
-      <td class="tdr bl"><div>171</div></td>
-      <td class="tdr"><div>41</div></td>
-      <td class="tdr br"><div>W.</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>3</div></td>
-      <td class="tdr br"><div>57</div></td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>58</div></td>
-      <td class="tdr br"><div>5,773</div></td>
-      <td class="tdc br">566</td>
-      <td class="tdr br"><div>9,972</div></td>
-      <td class="br">12 ft.</td>
-      <td class="br">10</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Taurauga</td>
-      <td class="tdr bl"><div>176</div></td>
-      <td class="tdr"><div>11</div></td>
-      <td class="tdr br"><div>E.</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdr br"><div>15</div></td>
-      <td class="tdr"><div>19</div></td>
-      <td class="tdr br"><div>16</div></td>
-      <td class="tdr br"><div>5,615</div></td>
-      <td class="tdc br">427</td>
-      <td class="tdr br"><div>5,697</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Wellington</td>
-      <td class="tdr bl"><div>174</div></td>
-      <td class="tdr"><div>30</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>7</div></td>
-      <td class="tdr br"><div>22</div></td>
-      <td class="tdr"><div>18</div></td>
-      <td class="tdr br"><div>23</div></td>
-      <td class="tdr br"><div>5,574</div></td>
-      <td class="tdc br">445</td>
-      <td class="tdr br"><div>6,168</div></td>
-      <td class="br">11 „</td>
-      <td class="br">10</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Akaroa</td>
-      <td class="tdr bl"><div>172</div></td>
-      <td class="tdr"><div>59</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>7</div></td>
-      <td class="tdr br"><div>28</div></td>
-      <td class="tdr"><div>18</div></td>
-      <td class="tdr br"><div>29</div></td>
-      <td class="tdr br"><div>5,542</div></td>
-      <td class="tdc br">440</td>
-      <td class="tdr br"><div>6,031</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Lyttelton</td>
-      <td class="tdr bl"><div>172</div></td>
-      <td class="tdr"><div>45</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>7</div></td>
-      <td class="tdr br"><div>29</div></td>
-      <td class="tdr"><div>18</div></td>
-      <td class="tdr br"><div>30</div></td>
-      <td class="tdr br"><div>5,558</div></td>
-      <td class="tdc br">441</td>
-      <td class="tdr br"><div>6,055</div></td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Kameishi</td>
-      <td class="tdr bl"><div>140</div></td>
-      <td class="tdr"><div>50</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>12</div></td>
-      <td class="tdr br"><div>37</div></td>
-      <td class="tdr"><div>23</div></td>
-      <td class="tdr br"><div>38</div></td>
-      <td class="tdr br"><div>8,844</div></td>
-      <td class="tdc br">549</td>
-      <td class="tdr br"><div>9,378</div></td>
-      <td class="br">6 „</td>
-      <td class="br">15</td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl">Hakodate</td>
-      <td class="tdr bl"><div>140</div></td>
-      <td class="tdr"><div>50</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr"><div>10</div></td>
-      <td class="tdr"><div>14</div></td>
-      <td class="tdr br"><div>7</div></td>
-      <td class="tdr"><div>25</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>8,778</div></td>
-      <td class="tdc br">512</td>
-      <td class="tdr br"><div>8,169</div></td>
-      <td class="br">7 „</td>
-      <td class="br">20</td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_185">185</span></p>
-
-  <p>In Dr. Geinitz’s paper there are also some slight differences in the
-    times at which the earthquake phenomena were observed at various localities. These, however, are but
-    of minor importance. At the end of the paper by Dr. Geinitz two
-    interesting tide gauge records are introduced, one from Sydney and the
-    other from Newcastle. These appear to show a marked difference in the
-    periods of the sea waves at these two places.<a id="FNanchor_85" href="#Footnote_85" class="fnanchor">[85]</a></p>
-
-  <p><em>Comparison of velocities of wave-transit which have been actually
-    observed, with velocities which ought to exist from what we know of
-    the depth of the Pacific by actual soundings.</em>—From a chart given in
-    ‘Petermann’s Geograph. Mittheilungen,’ Band xxiii. p. 164, 1877, it is
-    possible to draw approximate sections on lines in various directions
-    across the bed of the Pacific.</p>
-
-  <p>From the origin of the shock to Japan (Kameishi) the line would be as
-    follows:—</p>
-
-  <table summary="Wave travel depth to Japan">
-    <tr>
-      <td class="tdr"><div>about 7,441</div></td>
-      <td>miles</td>
-      <td class="tdr"><div> 15,000</div></td>
-      <td>feet deep</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>1,100</div></td>
-      <td class="tdc">„</td>
-      <td class="tdr"><div>18,000</div></td>
-      <td class="tdc">„</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>160</div></td>
-      <td class="tdc">„</td>
-      <td class="tdr"><div>27,000</div></td>
-      <td class="tdc">„</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>80</div></td>
-      <td class="tdc">„</td>
-      <td class="tdr"><div>12,000</div></td>
-      <td class="tdc">„</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>60</div></td>
-      <td class="tdc">„</td>
-      <td class="tdr"><div>6,000</div></td>
-      <td class="tdc">„</td>
-    </tr>
-  </table>
-
-  <p>On account of the Tuscarora and Belkap Deeps this would be the most
-    irregular line over which the wave had to travel.</p>
-
-  <p>From the origin to New Zealand (Wellington) the line would be</p>
-
-  <table summary="Wave travel depth to New Zealand">
-    <tr>
-      <td class="tdr"><div>about 5,274</div></td>
-      <td>miles</td>
-      <td class="tdr"><div> 15,000</div></td>
-      <td>feet deep</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>„ &nbsp; &nbsp; &nbsp; 300</div></td>
-      <td class="tdc">„</td>
-      <td class="tdr"><div>12,000</div></td>
-      <td class="tdc">„</td>
-    </tr>
-  </table>
-
-  <p>From the origin to Samoa the line would be</p>
-
-  <table summary="Wave travel depth to Samoa">
-    <tr>
-      <td class="tdr"><div>about 5,773</div></td>
-      <td>miles</td>
-      <td class="tdr"><div> 15,000</div></td>
-      <td>feet deep</td>
-    </tr>
-  </table>
-
-  <p>From the origin to the Sandwich Islands (Honolulu) the line would be</p>
-
-  <p><span class="pagenum" id="Page_186">186</span></p>
-
-  <table summary="Wave travel depth to Honolulu">
-    <tr>
-      <td class="tdr"><div>almost 5,634</div></td>
-      <td>miles</td>
-      <td class="tdr"><div> 15,000</div></td>
-      <td>feet deep</td>
-    </tr>
-    <tr>
-      <td class="tdr"><div>and&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 60</div></td>
-      <td class="tdc">„</td>
-      <td class="tdr"><div> 12,000</div></td>
-      <td class="tdc">„</td>
-    </tr>
-  </table>
-
-  <p>By Scott-Russell’s rule, or, what is almost identically the same, by
-    Airy’s general formula, we can calculate how long it would take such
-    waves as we have been speaking about to travel over the different
-    portions of each of these lines, and by adding these times together we
-    obtain the time taken to travel across any one line. I have made these
-    calculations, but as I get in every case answers which are too small, I
-    think it unnecessary to give them.</p>
-
-  <p>The actual times taken to travel the distances just referred to were,</p>
-
-  <table summary="Wave travel time">
-    <tr>
-      <td class="tdl">To Japan (Kameishi)</td>
-      <td class="tdl">23 hr. 38 min.</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;„&nbsp; New Zealand (Wellington)</td>
-      <td class="tdl">18&nbsp; „&nbsp; &nbsp;23&nbsp; &nbsp;„</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;„&nbsp; Samoa</td>
-      <td class="tdl">14&nbsp; „&nbsp; &nbsp;58&nbsp; &nbsp;„</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;„&nbsp; Sandwich Islands (Honolulu)</td>
-      <td class="tdl">14&nbsp; „&nbsp; &nbsp;53&nbsp; &nbsp;„</td>
-    </tr>
-  </table>
-
-  <p>From San Francisco to Simoda the line is almost 3,567 miles, 3,000
-    fathoms deep, 840 miles, 2,500 fathoms deep, and 120 miles 1,000
-    fathoms deep. This gives an average depth of about 2,854 fathoms. Bache
-    calculated the depth at 2,500 fathoms.</p>
-
-  <p>If we are to consider that, because the sea wave at Simoda came in
-    some time after the land shock had been felt, the origin of this
-    earthquake, instead of being at Simoda, was some distance out at sea,
-    this calculated depth would be reduced.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_X">
-    <span class="pagenum" id="Page_187">187</span>
-    <h2>CHAPTER X.<br />
-      <span class="subhead">DETERMINATION OF EARTHQUAKE ORIGINS.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Approximate determination of an Origin—Earthquake-hunting
-    in Japan—Determinations by direction of motion—Direction
-    indicated by destruction of buildings—Direction determined by
-    rotation—Cause of rotation—The use of time observations—Errors
-    in such observations—Origin determined by the method of straight
-    lines—The method of circles, the method of hyperbolas, the method
-    of co-ordinates—Haughton’s method—Difference in time between
-    sound, earth, and water waves—Method of Seebach.</div>
-
-  <p class="noindent"><span class="smcap">One</span> of the most practical problems which can be suggested to the
-    seismologist is the determination of the district or districts in any
-    given country from which earthquake disturbances originate. With a
-    map of a country before us, shaded with tints of different intensity
-    to indicate the relative frequency of seismic disturbance in various
-    districts, we at once see the localities where we might dwell with
-    the least disturbance, and those we should seek if we wish to make
-    observational seismology a study. Before erecting observatories for
-    the systematic investigation of earthquakes in a country, it would
-    be necessary for us, in some way or other, to examine the proposed
-    country to find out the most suitable district. The special problem
-    of determining <em>approximately</em> the origin or origins of a set of
-    earthquakes would be given to us. Having made this preliminary
-    investigation, the next point is, by means of observatories so
-    arranged that<span class="pagenum" id="Page_188">188</span>
-     they could always work in conjunction with each other,
-    to determine the origins more accurately. By knowing the origin from
-    which a set of shocks spring we know the general direction in which
-    we may expect the most violent disturbances, and we can arrange our
-    seismometers accordingly.</p>
-
-  <p><em>Approximate determination of origins.</em>—In 1880 I obtained a tolerably
-    fair idea of the distribution of seismic energy throughout Japan, by
-    compiling the facts obtained from some hundreds of communications
-    received from various parts of the country respecting the number of
-    earthquakes that had been felt.</p>
-
-  <p>The communications were replies to letters sent to various residents
-    in the country and to a large number of public officers. By taking
-    these records, in conjunction with the records made by instruments, it
-    was ascertained that in Japan alone there were certainly 1,200 shocks
-    felt during the year, that is to say, three or four shocks per day.
-    The greater number of these shocks were felt along the eastern coast,
-    commencing at Tokio, in the south, and going northwards to the end of
-    the main island. These shocks were seldom felt on the west coast. It
-    appeared as if the central range of mountains formed a barrier to their
-    progress. Similarly, ranges of mountains to the south-west of Tokio
-    prevented the shocks from travelling southwards. Proceeding in this way
-    the conclusion was arrived at that the west coast, the southern part
-    of Japan, and the islands of Shikoku and Kiushiu, had their own local
-    earthquakes.</p>
-
-  <p><em>Earthquake-hunting.</em>—These preliminary enquiries having shown that
-    the northern part of Japan was a better district for seismological
-    observations than the southern half, the next step was to subject the
-    northern half to a closer analysis. This analysis was commenced by
-    sending<span class="pagenum" id="Page_189">189</span>
-    to all the important towns, from thirty to one hundred miles
-    distant from Tokio, bundles of postcards. These were entrusted to the
-    local government offices with a request that each week one of these
-    cards would be returned to Tokio stating the number of shocks felt.
-    In this way it was quickly discovered that the majority of shakings
-    emanated from the north and east, and seldom, if ever, passed a heavy
-    range of mountains to the south. The barricade of postcards was then
-    extended farther northwards, with the result of surrounding the origin
-    of certain shocks amongst the mountains, whilst others were traced to
-    the sea shore. By systematically pursuing earthquakes it was seen that
-    many shocks had their origin beneath the sea—they shook all the places
-    on the north-east coast, but it was seldom that they crossed through
-    the mountains, forming the backbone of the island, to disturb the
-    places on the west coast.</p>
-
-  <p>The actual results obtained in three months by this method of working
-    are shown in the accompanying map, which embraces the northern half of
-    the main island of Nipon and part of Yezo. The shaded portion of the
-    map indicates the mountainous districts, which are traversed by ranges
-    varying in height from about 2,000 and 7,000 feet. The dotted lines
-    show the boundaries of the more important groups of earthquakes which
-    were recorded.</p>
-
-  <p>I. is the western boundary of earthquakes, which at places to the
-    eastward are usually felt somewhat severely. Some of these have been
-    felt the most severely at or near Hakodadi, whilst farther south their
-    effects have been weak. Occasionally the greatest effect has been near
-    to Kameishi. Sometimes these earthquakes terminate along the western
-    boundaries of III. or IV., not being able to pass the high range of
-    mountains which separate the plain of Musashi from Kofu.</p>
-
-  <div class="figcenter">
-    <span class="pagenum" id="Page_190">190</span>
-    <img src="images/i_p190.jpg" width="378" height="700" alt="" />
-    <div class="caption"><span class="smcap">Fig. 29.</span>—Northern Japan.&nbsp; &nbsp; Mountainous districts
-      shaded with oblique lines.</div>
-  </div>
-
-  <p><span class="pagenum" id="Page_191">191</span></p>
-
-  <p>II. is the boundary of a shock confined to the plain which surrounds
-    Kofu. These earthquakes are evidently quite local. Many of the
-    disturbances have evidently originated beneath the ocean, having
-    come in upon the land in the direction of the arrows <span class="smcap">a</span> or
-    <span class="smcap">b</span>.</p>
-
-  <p>III. This line indicates the boundary of a group of shocks which
-    are often experienced in Tokio. These may come in the directions
-    <span class="smcap">d</span>, <span class="smcap">e</span>, or
-    <span class="smcap">f</span>. It is probable that some of
-    them originate to the eastward of Yokohama, on or near to the opposite
-    peninsula.</p>
-
-  <p>IV. V. and VI. The earthquakes bounded by these lines probably
-    originate in the directions <span class="smcap">c</span> or
-    <span class="smcap">d</span>.</p>
-
-  <p>VII. The earthquakes bounded by this line probably come from the
-    direction <span class="smcap">e</span>.</p>
-
-  <p>VIII. This line gives us the boundary of earthquakes which may come
-    from the direction <span class="smcap">b</span>.</p>
-
-  <p>The above boundaries sometimes do not extend so far to the westward as
-    they are shown. At other times, groups like V. and VI. extend farther
-    to the south-west. These earthquake boundaries, which so clearly show
-    the effects of high mountains in preventing the extension of motion,
-    have been drawn up, not from single earthquakes, but from a large
-    series of earthquakes which have been plotted upon blank maps, and are
-    now bound together to form an atlas. To give an idea of the material
-    upon which I have been working, I may state that between March 1 and
-    March 10, 1882, I received records of no less than thirty-four distinct
-    shocks felt in districts between Hakodate and Tokio, and for each of
-    these it is quite possible to draw a map. In addition to the boundaries
-    of disturbances given in the accompanying map, other boundaries might
-    be drawn for shocks which were more local in their character. The
-    groups which contained the greatest number of shocks are III., IV., V.,
-    VI., and VII. By<span class="pagenum" id="Page_192">192</span>
-    work of this description it was found that a very
-    important group of earthquakes might be studied by a line of stations
-    commencing at Saporo in the north, passing through Hakodate, down the
-    east coast of the main island, to Tokio or Yokohama in the south. A
-    further aid to the study of this group, together with the study of
-    an important local group, might be effected with the help of a few
-    additional stations properly distributed on the plain of Musashi, which
-    surrounds Tokio. With this example before us it will be recognised that
-    the choice of sites for a connected set of seismological observatories
-    will often be more or less a special problem. If earthquake stations
-    were to be placed in different directions around Tokio without
-    preliminary investigation, it is quite possible that some of them might
-    be so situated that they would seldom if ever work in conjunction with
-    the remaining observatories, and therefore be of but little value.
-    And this remark must equally apply to districts in other portions
-    of the globe. The method is crude, and, so far as actual earthquake
-    origins are concerned, it only yields results which are approximate.
-    The crudeness and the want of absoluteness in the results is, however,
-    more than counterbalanced by the certainty with which we are enabled
-    to express ourselves with regard to such results as are obtained. Even
-    when working with the best instruments we have at our command, unless
-    we are employing some elaborate system, this method of working gives a
-    most valuable check upon our instrumental records, and enables us to
-    interpret them with greater confidence.</p>
-
-  <p><em>Determination of earthquake origins from the direction of motion.</em>—If
-    we assume that an earthquake is propagated from a centre as a series
-    of waves, in which normal vibrations are conspicuous, and obtain at
-    two localities, not in the same straight line with the origin,<span class="pagenum" id="Page_193">193</span> and
-    sufficiently far separated from each other, the direction of movement
-    of these normal motions, by drawing lines parallel to these directions
-    through our two stations, the lines would intersect at a point above
-    the required origin. If instead of two points we had three, or, better
-    still, a large number, the results we should obtain ought to be
-    still more certain. Unfortunately, it seems that earthquakes seldom
-    originate from a given point, and, further, normal motions are not
-    always (sufficiently) prominent. Sometimes, as has already been shown
-    in the chapters on earthquake motion, they may be non-existent. It is
-    probable, however, that difficulties of this sort are more usually
-    associated with non-destructive earthquakes. Mallet regards the
-    destructive effects of an earthquake as almost solely due to normal
-    motions. If this be true, for destructive earthquakes, the problem is
-    shorn of many of its difficulties. In cases where normal vibrations
-    are not prominent, where we have only transverse vibrations, motions
-    due to the interference of normal or transverse motions, or directions
-    of motions due to the topographical or geological nature through which
-    the disturbance has passed, the determination of the origin of an
-    earthquake by observations on the direction in which the ground has
-    been moving appears to be a problem which is practically without a
-    solution. We will, therefore, only consider the determination of the
-    origin of those earthquakes which have predominating directions in
-    their movements, which directions we will consider as normal ones.
-    The question which is, then, before us, is the determination of the
-    direction of these normal movements. First of all we may take the
-    evidence of our senses. In exceptional cases these have given results
-    which closely approximate to the truth, but in the majority of cases
-    such results are not to be relied upon, as the inhabitants of a town
-    will, for the<span class="pagenum" id="Page_194">194</span>
-    same shock, give directions corresponding to all points
-    of the compass. Much, no doubt, depends upon the situation of the
-    observer, and much, perhaps, upon his temperament. If he is sitting
-    in a room alone, and is accustomed to making observations on an
-    earthquake, on feeling the earthquake, if he concentrates his attention
-    on the direction in which he is being moved, his observations may be
-    of value. If, however, he is not so situated, and his attention is
-    not thus concentrated, his opinions, unless the motion has been very
-    decided in its character, are usually of but little worth.</p>
-
-  <p><em>Direction determined from destruction of buildings.</em>—When an observer
-    first sees a town that has been partially shattered by an earthquake,
-    all appears to be confusion, and it is difficult to imagine that in
-    such apparent chaos we are able to discover laws. If, however, we take
-    a general view of this destruction and compare together similarly
-    built buildings, it is possible to discover that similar and similarly
-    situated structures have suffered in a similar manner. By carefully
-    analysing the destruction we are enabled to infer the direction in
-    which the destroying forces have acted. It was chiefly by observing the
-    cracks in buildings, and the direction in which bodies were overthrown
-    or projected, that Mallet determined the origin of the Neapolitan
-    earthquake. From the observations given in Chapter VII. it would appear
-    that, with destructive earthquakes, walls which are transverse to the
-    direction of motion are most likely to be overturned, whilst, with
-    small earthquakes, these walls are the least liable to be fractured.</p>
-
-  <p>From a critical examination of the <em>general</em> nature of the damage
-    done on the buildings of a town, earthquake observers have shown that
-    the direction of a shock may often be approximately determined. The
-    direction in<span class="pagenum" id="Page_195">195</span>
-    which a body having a regular form like a prismatic
-    gravestone or a cylindrical column is overturned sometimes gives the
-    means by which we can determine the direction from which a movement
-    came.</p>
-
-  <p><em>The rotation of bodies.</em>—It has often been observed that almost all
-    large earthquakes have caused objects like tombstones, obelisks,
-    chimneys, &amp;c., to rotate.</p>
-
-  <p>One of the most natural and at the same time most simple explanations
-    is to suppose that during the shock there had been a twisting, or
-    backward and forward screw-like motion in the ground. Amongst the
-    Italians and the Mexicans earthquakes producing an effect like this are
-    spoken of as ‘vorticosi.’ In the Calabrian earthquake, not only were
-    bodies like obelisks twisted on their bases, but straight rows of trees
-    seem to have been left in interrupted zigzags. These latter phenomena
-    have been explained upon the assumption of the interference of direct
-    waves and reflected waves, the consequence of which being that points
-    in close proximity might be caused to move in opposite directions.
-    Reflections such as these would be most likely to occur near to the
-    junction of strata of different elasticity, and it may be remarked that
-    it is often near such places that much twisting has been observed.</p>
-
-  <p>Another way in which it is possible for twisting to have taken place
-    would be by the interference of the normal and transverse waves which
-    probably always exist in an earthquake shock, or by the meeting of
-    the parts of the normal wave itself, one having travelled in a direct
-    line from the origin, whilst the other, travelling through variable
-    material, has had its direction changed.</p>
-
-  <p>Mallet, however, has shown that the rotation may have been in many
-    cases brought about without the supposition of any actual twisting
-    motion of the earth—<span class="pagenum" id="Page_196">196</span>a
-    simple backward and forward motion being quite
-    sufficient. If one block of stone rests upon another, and the two are
-    shaken backwards and forwards in a straight line, and if the vertical
-    through the centre of gravity of the upper block does not coincide with
-    the point where there is the greatest friction between the blocks,
-    rotation must take place. If the vertical through the centre of gravity
-    falls on one side of the centre of friction, the rotation would be in
-    one direction, whilst, if on the other side, the rotation would be in
-    the opposite direction.</p>
-
-  <p>Although the above explanation is simple, and also in many cases
-    probably true, it hardly appears sufficient to account for all the
-    phenomena which have been observed.</p>
-
-  <p>Thus, for instance, if the stones in the Yokohama cemetery, at the time
-    of the earthquake of 1880, had been twisted in consequence of the cause
-    suggested by Mallet, we should most certainly have found that some
-    stones had turned in one direction whilst others had been twisted in
-    another. By a careful examination of the rotated stones, I found that
-    every stone—the stones being in parallel lines—had <em>revolved in the
-    same direction</em>, namely in a direction opposite to that of the hands of
-    a watch.</p>
-
-  <p>As it would seem highly improbable that the centre of greatest friction
-    in all these stones of different sizes and shapes should have been at
-    the same side of their centres of gravity, an effect like this could
-    only be explained by the conjoint action of two successive shocks, the
-    direction of one being transverse to the other.</p>
-
-  <p>Although fully recognising the sufficiency of two transverse shocks
-    to produce the effects which have been observed in Yokohama, I will
-    offer what appears to me to be the true explanation of this phenomenon:
-    it was first suggested by my colleague, Mr. Gray, and appears to be
-    simpler than any with which I am acquainted.</p>
-
-  <p><span class="pagenum" id="Page_197">197</span></p>
-
-  <div class="figright">
-    <img src="images/i_p197.jpg" width="180" height="228" alt="" />
-    <div class="caption"><span class="smcap">Fig. 30.</span></div>
-  </div>
-
-  <p>If any columnar-like object, for example a prism which the basal
-    section is represented by <span class="smcap">a b c d</span> (see fig. 30), receives a
-    shock at right angles to <span class="smcap">b c</span>, there will be a tendency for
-    the inertia of the body to cause it to overturn on the edge <span class="smcap">b c</span>.
-    If the shock were at right angles to <span class="smcap">d c</span>, the tendency
-    would be to overturn on the edge <span class="smcap">d c</span>. If the shock were in
-    the direction of the diagonal <span class="smcap">c a</span>, the tendency would be to
-    overturn on the point <span class="smcap">c</span>. Let us, however, now suppose the
-    impulse to be in some direction like <span class="smcap">e g</span>, where <span class="smcap">g</span>
-    is the centre of gravity of the body. For simplicity we may imagine
-    the overturning effect to be an impulse given through <span class="smcap">g</span> in an
-    opposite direction—that is, in the direction <span class="smcap">g e</span>. This force
-    will tend to tip or make the body bear heavily on <span class="smcap">c</span>, and at
-    the same time to whirl round <span class="smcap">c</span> as an axis, the direction of
-    turn being in the direction of the hands of a watch. If, however, the
-    direction of impulse had been <span class="smcap">e′ g</span>, then, although the turning
-    would still have been round <span class="smcap">c</span>, the direction would have been
-    <em>opposite</em> to that of the hands of a watch.</p>
-
-  <p>To put these statements in another form, imagine <span class="smcap">g e′</span> to
-    be resolved into two components, one of them along <span class="smcap">g c</span> and
-    the other at right angles, <span class="smcap">g f</span>. Here the component of the
-    direction <span class="smcap">g c</span> tends to make the body tip on <span class="smcap">c</span>, whilst
-    the other component along <span class="smcap">g f</span> causes revolution. Similarly
-    <span class="smcap">g e</span> may be resolved into its two components <span class="smcap">g c</span> and
-    <span class="smcap">g f′</span>, the latter being the one tending to cause revolution.</p>
-
-  <p>From this we see that if a body has a rectangular section, so long as
-    it is acted upon by a shock which is parallel to its sides or to its
-    diagonals, there ought not to<span class="pagenum" id="Page_198">198</span>
-     be any revolution. If we divide our
-    section <span class="smcap">a b c d</span> up into eight divisions by lines through these
-    directions, we shall see that any shock the direction of which passes
-    through any of the octants which are shaded will cause a <em>positive</em>
-    revolution in the body—that is to say, a revolution corresponding in
-    its direction to that of the movements of the hands of a watch; whilst
-    if its direction passes through any of the remaining octants the
-    revolution will be <em>negative</em>, or opposite to that of the hands of a
-    watch. From the direction in which any given stone has turned, we can
-    therefore give two sets of limits between one of which the shock must
-    have come.</p>
-
-  <p>Further, it will be observed that the tendency of the turning is to
-    bring a stone, like the one we are discussing, broadside on to the
-    shock; therefore, if a stone with a rectangular cross section has
-    turned sufficiently, the direction of a shock will be parallel to one
-    of its faces, but if it has not turned sufficiently it will be more
-    nearly parallel to its faces in their new position than it was to its
-    faces when in their original position.</p>
-
-  <p>If a stone receives a shock nearly parallel with its diagonal, on
-    account of its instability it may turn either positively or negatively
-    according as the friction on its base or some irregularity of surface
-    bearing most influence. Similarly, if a stone receives a shock parallel
-    to one of its faces, the twisting may be either positive or negative,
-    but the probability is that it would only turn slightly; whereas in the
-    former case, where the shock was nearly parallel to a diagonal, the
-    turning would probably be great.</p>
-
-  <p><em>Determination of direction from instruments.</em>—When speaking about
-    earthquakes it was shown, as the result of many observations, that
-    the same earthquake in the space of a few seconds, although it may
-    sometimes have only one direction of motion, very often has many
-    <span class="pagenum" id="Page_199">199</span>
-    directions of motion. In certain cases, therefore, our records, if we
-    assume the most permanent motions to be normal ones, give definite and
-    valuable results. In other cases it is necessary to carefully analyse
-    the records, comparing those taken at one station with those taken at
-    another.</p>
-
-  <p>One remarkable fact which has been pointed out in reference to
-    artificial earthquakes produced by exploding charges of gunpowder or
-    dynamite, and also with regard to certain earthquakes, is that the
-    greatest motion of the ground is <em>inwards</em>, towards the point from
-    which the disturbance originated. Should this prove the rule, it gives
-    a means of determining, not only the direction of earthquake, but the
-    side from which it came.</p>
-
-  <p><em>Determination of earthquake origins by time observations.</em>—The times
-    at which an earthquake was felt at a number of stations are among the
-    most important observations which can be made for the determination of
-    an earthquake origin. The methods of making time observations, and the
-    difficulties which have to be overcome, have already been described.
-    When determining the direction from which a shock has originated, or
-    determining the origin of the shock by means of time observations,
-    it has been usual to assume that the velocity of propagation of the
-    shock has been uniform from the origin. The errors involved in this
-    assumption appear to as follows:—</p>
-
-  <p>1. We know from observations on artificial earthquakes that the
-    velocity of propagation is greater between stations near to the origin
-    of the shock than it is between more remote stations; and also the
-    velocity of propagation varies with the initial force which produced
-    the disturbance. If our points of observation are sufficiently close
-    together as compared with their distance from the
-    <span class="pagenum" id="Page_200">200</span> origin of the
-    disturbance, it is probable that errors of this description are small
-    and will not make material differences in the general results.</p>
-
-  <p>2. We have reasons for believing that the transit velocity of an
-    earthquake is dependent on the nature of the rocks through which it
-    is propagated. Errors which arise from causes of this description
-    will, however, be practically eliminated if our observation points are
-    situated on an area sufficiently large, so that the distribution of the
-    causes tending to alter the velocity of a shock balance each other. It
-    must be remarked, that causes of this description may also produce an
-    alteration in the direction of our shock.</p>
-
-  <p>Other errors which may sometimes enter into our results, when
-    determining the origin of shocks by means of observations on
-    velocities, are the assumptions that the disturbance has travelled
-    along the surface from the <em>epicentrum</em> and not in a direct line from
-    the <em>centrum</em>. Again, it is assumed that the origin is a point, whereas
-    it may possibly be a cavity or a fissure. Lastly, if we desire extreme
-    accuracy, we must make due allowance for the sphericity of the earth
-    and the differences of elevation of the observing stations.</p>
-
-  <p>I. <em>The method of straight lines.</em>—Given a number of pairs of points
-    <span class="smcap">a</span><sub>0</sub>, <span class="smcap">a</span><sub>1</sub>, <span class="smcap">b</span><sub>0</sub>, <span class="smcap">b</span><sub>1</sub>,
-    <span class="smcap">c</span><sub>0</sub>, <span class="smcap">c</span><sub>1</sub>, &amp;c., at each of which the shock was
-    felt simultaneously, to determine the origin.</p>
-
-  <p>Theoretically if we bisect the line which joins <span class="smcap">a</span><sub>0</sub>
-    and <span class="smcap">a</span><sub>1</sub> by a line at right angles to <span class="smcap">a</span><sub>0</sub>,
-    <span class="smcap">a</span><sub>1</sub>, and similarly bisect the lines <span class="smcap">b</span><sub>0</sub>,
-    <span class="smcap">b</span><sub>1</sub>, <span class="smcap">c</span><sub>0</sub>, <span class="smcap">c</span><sub>1</sub>, all these bisecting
-    lines <i>a</i><sub>0</sub>, <i>a</i><sub>1</sub>, <i>b</i><sub>0</sub>, <i>b</i><sub>1</sub>, <i>c</i><sub>0</sub>, <i>c</i><sub>1</sub>, &amp;c., ought
-    to intersect in a point, which point will be the <em>epicentrum</em> or the
-    point above the origin.</p>
-
-  <p>This method will fail, first, if <span class="smcap">a</span><sub>0</sub>, <span class="smcap">a</span><sub>1</sub>,
-    <span class="smcap">b</span><sub>0</sub>, <span class="smcap">b</span><sub>0</sub>, <span class="smcap">c</span><sub>0</sub>, <span class="smcap">c</span><sub>1</sub> form a
-    continuous straight line, or if they form a series of parallel lines.</p>
-
-  <p><span class="pagenum" id="Page_201">201</span></p>
-
-  <p>Hopkins gives a method based on a principle similar to the one which
-    is here employed—namely, given that a shock arrives simultaneously
-    at <em>three</em> points to determine, the centre. In this case, the
-    relative positions of the three points, where the time of arrival was
-    simultaneous, must be accurately known, and these three points must
-    not lie in a straight line, or the method will fail. For practical
-    application the problem must be restricted to the case of three points
-    which do not lie nearly in the same straight line.</p>
-
-  <p>II. <em>The method of circles.</em>—Given the times <i>t</i><sub>0</sub>, <i>t</i><sub>1</sub>, <i>t</i><sub>2</sub>,
-    &amp;c., at which a shock arrived at a number of places <span class="smcap">a</span><sub>0</sub>,
-    <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>, &amp;c., to determine the position from
-    which the shock originated.</p>
-
-  <p>Suppose <span class="smcap">a</span><sub>0</sub> to be the place which the shock reached first,
-    and that it reached <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>, <span class="smcap">a</span><sub>3</sub>,
-    &amp;c., successively afterwards.</p>
-
-  <p>
-    Let&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <i>t</i><sub>1</sub> - <i>t</i><sub>0</sub> = <i>a</i><br />
-    &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<i>t</i><sub>2</sub> - <i>t</i><sub>0</sub> = <i>b</i><br />
-    &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<i>t</i><sub>3</sub> - <i>t</i><sub>0</sub> = <i>c</i>, &amp;c.
-  </p>
-
-  <p>With <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>, <span class="smcap">a</span><sub>3</sub>, &amp;c. as centres,
-    describe circles with radii proportional to the known qualities <i>a</i>,
-    <i>b</i>, <i>c</i>, &amp;c., and also a circle which passes through <span class="smcap">a</span><sub>0</sub>
-    and touches these circles. The centre of the last circle will be the
-    <em>epicentrum</em>. The radii proportional to <i>a</i>, <i>b</i>, <i>c</i>, &amp;c. may be
-    represented by the quantities <i>ax</i>, <i>bx</i>, <i>cx</i>, &amp;c., where <i>x</i> is the
-    velocity of propagation of the shock.</p>
-
-  <p>It will be observed that the times at which the shock arrived at three
-    places might alone be sufficient. If, instead of taking the times of
-    arrival of a shock, the arrival of a sea wave be taken, the result
-    would be a closer approximate to the absolute truth.</p>
-
-  <p>It will be observed that this method is not a direct one, but is one
-    of trial. If, however, an imaginary case
-    <span class="pagenum" id="Page_202">202</span> be taken, and three given
-    points of observation, <span class="smcap">a</span><sub>0</sub>,
-    <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>,
-    be plotted on a piece of paper, it will be found that it is not a
-    difficult matter to determine two numbers proportional to <i>a</i> and <i>b</i>
-    which will allow you to draw two circles so that they may be touched
-    by a third circle drawn through <span class="smcap">a</span><sub>0</sub>. This problem has
-    practically been applied in the case of the arrival of a sea wave
-    at a number of places on the South American coast, at the time of
-    the earthquake of May 9, 1877. This is illustrated as follows. The
-    places which were chosen were Huanillos, Tocopilla, Cobija, Iquique,
-    Mejillones.</p>
-
-  <p>In the following table the first column gives the times at which the
-    sea wave arrived at each of these places in Iquique time; in the second
-    column the difference between these times and the time at which it
-    reached Huanillos is given; in the third column the distances through
-    which a sea wave, propagated at the rate of 350 feet per second, could
-    travel during the intervals noted in the second column is given.</p>
-
-  <table class="collapse" summary="South America sea waves">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th colspan="2" class="ball">Arrival of<br /> sea wave</th>
-      <th class="ball">Time after<br />arrival at<br />Huanillos</th>
-      <th class="ball">Distance<br />at 350 feet<br />per second</th>
-    </tr>
-    <tr>
-      <th class="bl">&nbsp;</th>
-      <th class="tdr bl"><div>h.</div></th>
-      <th class="tdr br"><div>m.</div></th>
-      <th class="tdr br"><div>minutes</div></th>
-      <th class="tdr br"><div>miles</div></th>
-    </tr>
-    <tr>
-      <td class="tdl bl">Huanillos</td>
-      <td class="tdr bl"><div>8</div></td>
-      <td class="tdr br"><div>30</div></td>
-      <td class="tdr br"><div>0</div></td>
-      <td class="tdr br"><div>0</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Tocopilla</td>
-      <td class="tdr bl"><div>8</div></td>
-      <td class="tdr br"><div>32</div></td>
-      <td class="tdr br"><div>2</div></td>
-      <td class="tdr br"><div>8</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Cobija</td>
-      <td class="tdr bl"><div>8</div></td>
-      <td class="tdr br"><div>38</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>32</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl">Iquique</td>
-      <td class="tdr bl"><div>8</div></td>
-      <td class="tdr br"><div>40</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdr br"><div>40</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl">Mejillones</td>
-      <td class="tdr bl"><div>8</div></td>
-      <td class="tdr br"><div>46</div></td>
-      <td class="tdr br"><div>16</div></td>
-      <td class="tdr br"><div>64</div></td>
-    </tr>
-  </table>
-
-  <p>The distances marked in the third column are used as radii of the
-    circles drawn round the places to which they respectively refer.</p>
-
-  <p>The centre of the circle drawn to touch the circles of the first
-    column, and at the same time to pass through Huanillos, is marked
-    <span class="smcap">c</span>.</p>
-
-  <p><span class="pagenum" id="Page_203">203</span></p>
-
-  <p>The position from which the shock originated appears therefore to have
-    occurred very near to a place lying in Long. 7° 15′ W. and Lat. 21° 22′ S.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p203.jpg" width="549" height="680" alt="" />
-    <div class="caption"><span class="smcap">Fig. 31.</span></div>
-  </div>
-
-  <p>The actual operations which were gone through in making the
-    accompanying map were as follows. First, the places with which we had
-    to deal were represented on a map in orthographical projection, the
-    centre of projection<span id="Page_204" class="pagenum">204</span>
-    being near to the centre of the map. This was
-    done so that the measurements which were made upon the map might be
-    more correct than those we should obtain from an ordinary chart where
-    this portion of the world was not the centre of projection. Next, a
-    number was taken as equal to the velocity with which the sea wave had
-    travelled. The first velocity taken was about 400 feet per second—this
-    being about the velocity with which, theoretically, it must have
-    travelled in an ocean having a depth equal to that indicated upon the
-    charts—also it seemed to have travelled at this rate from the various
-    times of arrival as recorded at places along the coast. Circles were
-    then drawn round Tocopilla, Cobija, Iquique, and Mejillones with radii
-    equal to 2, 8, 10, and 15, each multiplied by (60 × 400). It was then
-    seen <em>by trial</em> that it was impossible to draw a single circle which
-    should touch four circles and also pass through Huanillos. These four
-    circles were, in fact, too large. Four new but smaller circles, which
-    are shown in the map, were next drawn, their radii being respectively
-    equal to the numbers 2, 8, 10, and 16, each multiplied by (60 × 350),
-    and it was found that a circle, with a centre <span class="smcap">c</span>, could be
-    drawn which would practically touch the four circles, and at the same
-    time would pass through Huanillos.</p>
-
-  <p>III. <em>The method of hyperbolas.</em>—The method which I call that of
-    hyperbolas is only another form of the method of circles. It is,
-    however, useful in special cases, as, for instance, where we have the
-    times of arrival of earthquakes at only two stations. Between Tokio
-    and Yokohama, at which places I frequently obtain tolerably accurate
-    time records, the method has been applied on several occasions with
-    advantage. In the preceding example let us suppose that the only time
-    records which we had were for Huanillos and Mejillones, and that the
-    wave was felt at the<span class="pagenum" id="Page_205">205</span>
-     latter place sixteen minutes or 960 seconds after
-    it was experienced at the former. Calling these places <span class="smcap">h</span> and
-    <span class="smcap">m</span> respectively, round <span class="smcap">m</span> draw a circle equal to the
-    960 multiplied by the velocity with which the wave was propagated. It
-    is then evident that the origin of this disturbance must be the centre
-    of a circle which passes through <span class="smcap">h</span> and touches the circle
-    drawn round <span class="smcap">m</span>. Join <span class="smcap">h m</span>, cutting the circle round
-    <span class="smcap">m</span> in <span class="smcap">y</span>. Bisect <span class="smcap">y h</span> in <span class="smcap">v</span>. It is
-    evident that <span class="smcap">v</span> is one possible origin for the disturbance.
-    Next, from <span class="smcap">m</span>, in the direction of <span class="smcap">h</span>, draw any line
-    <span class="smcap">m z p</span>; join <span class="smcap">z h</span>; bisect <span class="smcap">z h</span> at right angles
-    by the line <span class="smcap">o p n</span>. Because <span class="smcap">ph</span> = <span class="smcap">pz</span>, it is
-    evident that <span class="smcap">p</span> is a second possible origin. Proceeding in this
-    way a series of points lying to the right and left of <span class="smcap">v</span> on
-    the curve <span class="smcap">r v t</span> may be found, and we may therefore say that
-    the origin lies somewhere in the curve <span class="smcap">r v t</span>. By increasing
-    or decreasing our velocity we vary the position of the curve <span class="smcap">r v t</span>,
-    and, instead of a line on which our origin may be, we obtain a
-    band. As the assumed velocity increases, the circle round <span class="smcap">m</span>
-    becomes larger, and has its limit when it passes through <span class="smcap">h</span>,
-    where the two arms of the curve <span class="smcap">r v t</span> will close together
-    and form a prolongation of the line <span class="smcap">m y h</span> as the assumed
-    velocity diminishes. The circle round <span class="smcap">m</span> becomes smaller
-    until it coincides with the point <span class="smcap">m</span>. At such a moment the
-    curve <span class="smcap">r v t</span> opens out to form a straight line bisecting
-    <span class="smcap">m h</span> at right angles. The curve <span class="smcap">r v t</span> is a hyperbola
-    with a vertex <span class="smcap">v</span> and foci <span class="smcap">h</span> and <span class="smcap">m</span>. Inasmuch
-    as <span class="smcap">pm</span> - <span class="smcap">ph</span> = a constant quantity. If we have the
-    time given at which the shock or wave arrived at a third station as
-    at Iquique, it is evident that a second hyperbola <span class="smcap">r′ v′ t′</span>
-    might be drawn with Iquique and Huanillos as foci, and that the mutual
-    intersection of these two hyperbolas with a third hyperbola, having for
-    its foci Iquique and Mejillones, would give the origin of the wave.
-    <span class="pagenum" id="Page_206">206</span>
-    The obtaining of a mutual intersection would depend on the assumed
-    velocity, and the accuracy of the result, like that of the method
-    of circles, would depend upon the trials we made. The method here
-    enunciated may be carried farther by describing hyperboloids instead
-    of hyperbolas, the mutual intersection of which surfaces would, in the
-    case of an earth wave, give the actual origin or <em>centrum</em> rather than
-    the point above the origin or <em>epicentrum</em>.</p>
-
-  <p>IV. <em>The method of co-ordinates.</em>—Given the times at which a shock
-    arrived at five or more places, the position of which we have marked
-    upon a map, or chart, to determine the position on the map of the
-    centre of the shock, its depth, and the velocity of propagation.</p>
-
-  <p>Commencing with the place which was last reached by the shock, call
-    these places <i>p</i>, <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, and <i>p</i><sub>4</sub>, and let the
-    times taken to reach these places from the origin be respectively <i>t</i>,
-    <i>t</i><sub>1</sub>, <i>t</i><sub>2</sub>, <i>t</i><sub>3</sub>, and <i>t</i><sub>4</sub>.</p>
-
-  <p>Through <i>p</i> draw rectangular co-ordinates, and with a scale measure the
-    co-ordinates of <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, and <i>p</i><sub>4</sub>, and let these
-    respectively be <i>a</i><sub>1</sub>, <i>b</i><sub>1</sub>; <i>a</i><sub>2</sub>, <i>b</i><sub>2</sub>; <i>a</i><sub>3</sub>, <i>b</i><sub>3</sub>;
-    <i>a</i><sub>4</sub>, <i>b</i><sub>4</sub>. Then if <i>x</i>, <i>y</i>, and <i>z</i> be the co-ordinates of the
-    origin of the shock, <i>d</i>, <i>d</i><sub>1</sub>, <i>d</i><sub>2</sub>, <i>d</i><sub>3</sub>, and <i>d</i><sub>4</sub>, the
-    respective distances of <i>p</i>, <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, and <i>p</i><sub>4</sub>
-    from this origin, and <i>v</i> the velocity of the shock, we have</p>
-
-  <div class="list-container">
-    <ol>
-      <li><i>x</i><sup>2</sup> + <i>y</i><sup>2</sup> + <i>z</i><sup>2</sup> = <i>d</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sup>2</sup></li>
-      <li>(<i>a</i><sub>1</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>1</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>1</sub><sup>2</sup></li>
-      <li>(<i>a</i><sub>2</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>2</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>2</sub><sup>2</sup></li>
-      <li>(<i>a</i><sub>3</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>3</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>3</sub><sup>2</sup></li>
-      <li>(<i>a</i><sub>4</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>4</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>4</sub><sup>2</sup></li>
-    </ol>
-  </div>
-
-  <p>Because we know the actual times at which the waves arrived at the
-    places <i>p</i>, <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, <i>p</i><sub>4</sub>, we know the values
-    <i>t</i>—<i>t</i><sub>1</sub>, <i>t</i>—<i>t</i><sub>2</sub>, <i>t</i>—<i>t</i><sub>3</sub>, <i>t</i>—<i>t</i><sub>4</sub>. Call these
-    respectively <i>m</i>, <i>p</i>, <i>q</i>, and <i>r</i>. Suppose <i>t</i> known, then</p>
-
-  <p><span class="pagenum" id="Page_207">207</span></p>
-
-  <div class="list-container">
-    <ul>
-      <li><i>t</i><sub>1</sub> = <i>t</i> - <i>m</i></li>
-      <li><i>t</i><sub>2</sub> = <i>t</i> - <i>p</i></li>
-      <li><i>t</i><sub>3</sub> = <i>t</i> - <i>q</i></li>
-      <li><i>t</i><sub>4</sub> = <i>t</i> - <i>r</i>.</li>
-    </ul>
-  </div>
-
-  <p>Subtracting equation No. 1 from each of the equations 2, 3, 4, and 5,
-    we obtain,</p>
-
-  <div class="list-container">
-    <ul>
-      <li><i>a</i><sub>1</sub><sup>2</sup> + <i>b</i><sub>1</sub><sup>2</sup> - 2<i>a</i><sub>1</sub> <i>x</i> - 2<i>b</i><sub>1</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>1</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>m</i><sup>2</sup> - 2<i>t</i> <i>m</i>)</li>
-      <li><i>a</i><sub>2</sub><sup>2</sup> + <i>b</i><sub>2</sub><sup>2</sup> - 2<i>a</i><sub>2</sub> <i>x</i> - 2<i>b</i><sub>2</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>2</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>p</i><sup>2</sup> - 2<i>t</i> <i>p</i>)</li>
-      <li><i>a</i><sub>3</sub><sup>2</sup> + <i>b</i><sub>3</sub><sup>2</sup> - 2<i>a</i><sub>3</sub> <i>x</i> - 2<i>b</i><sub>3</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>3</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>q</i><sup>2</sup> - 2<i>t</i> <i>q</i>)</li>
-      <li><i>a</i><sub>4</sub><sup>2</sup> + <i>b</i><sub>4</sub><sup>2</sup> - 2<i>a</i><sub>4</sub> <i>x</i> - 2<i>b</i><sub>4</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>4</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>r</i><sup>2</sup> - 2<i>t</i> <i>r</i>)</li>
-    </ul>
-  </div>
-
-  <p>Now let <i>v</i><sup>2</sup> = <i>u</i>, and 2<i>v</i><sup>2</sup> <i>t</i> = <i>w</i>.</p>
-
-  <p>Then</p>
-
-  <div class="list-container">
-    <ol>
-      <li>2<i>a</i><sub>1</sub> <i>x</i> + 2<i>b</i><sub>1</sub> <i>y</i> + <i>u</i> <i>m</i><sup>2</sup> - <i>n</i> <i>m</i> = <i>a</i><sub>1</sub><sup>2</sup> + <i>b</i><sub>1</sub><sup>2</sup></li>
-      <li>2<i>a</i><sub>2</sub> <i>x</i> + 2<i>b</i><sub>2</sub> <i>y</i> + <i>u</i> <i>p</i><sup>2</sup> - <i>n</i> <i>p</i> = <i>a</i><sub>2</sub><sup>2</sup> + <i>b</i><sub>2</sub><sup>2</sup></li>
-      <li>2<i>a</i><sub>3</sub> <i>x</i> + 2<i>b</i><sub>3</sub> <i>y</i> + <i>u</i> <i>q</i><sup>2</sup> - <i>n</i> <i>q</i> = <i>a</i><sub>3</sub><sup>2</sup> + <i>b</i><sub>3</sub><sup>2</sup></li>
-      <li>2<i>a</i><sub>4</sub> <i>x</i> + 2<i>b</i><sub>4</sub> <i>y</i> + <i>u</i> <i>r</i><sup>2</sup> - <i>n</i> <i>r</i> = <i>a</i><sub>4</sub><sup>2</sup> + <i>b</i><sub>4</sub><sup>2</sup></li>
-    </ol>
-  </div>
-
-  <p>We have here four simple equations containing the four unknown
-    quantities <i>x</i>, <i>y</i>, <i>u</i>, and <i>w</i>.</p>
-
-  <p><i>x</i> and <i>y</i> determine the origin of the shock. The absolute velocity
-    <i>v</i> equals √ <i>u</i>. From <i>v</i> and <i>w</i> we obtain <i>t</i>. Substituting
-    <i>x</i>, <i>y</i>, <i>v</i>, and <i>t</i> in the first equation we obtain <i>z</i>.</p>
-
-  <p>We have here assumed that the points of observation have all about the
-    same elevation above sea level.</p>
-
-  <p>If it is thought necessary to take these elevations into account, a
-    sixth equation may be introduced.</p>
-
-  <p>If the propagation of the wave is considered as a horizontal one, as
-    would be done when calculating the position of the <em>epicentrum</em> or
-    point above the origin, by means of the times of arrival of a sea wave,
-    the ordinate <i>z</i> of the first five equations would be omitted. Working
-    in this way the resulting four equations, viz.</p>
-
-  <div class="center">
-    2<i>a</i><sub>1</sub> <i>x</i> + 2<i>b</i><sub>1</sub> <i>y</i> + <i>u</i><i>m</i><sup>2</sup> - <i>w</i><i>m</i><sup>2</sup> = <i>a</i><sub>1</sub><sup>2</sup> + <i>b</i><sub>1</sub><sup>2</sup><br />
-    &amp;c.&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &amp;c.&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &amp;c.
-  </div>
-
-  <p class="noindent">remained unchanged.</p>
-
-  <p>Applying this method to the same example as that
-    <span class="pagenum" id="Page_208">208</span> used as illustration
-    for the two previous methods, we obtain for the co-ordinates of
-    Mejillones, Iquique, Cobija, Tocopilla, and Huanillos, measured in
-    geographical miles, and the times in Iquique time at which the wave
-    reached each, as given in the following table; <i>ox</i> and <i>oy</i> being,
-    drawn through Mejillones.</p>
-
-  <table class="collapse" summary="South America sea waves 2">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th colspan="2" class="ball">Co-ordinates</th>
-      <th colspan="3" class="ball">Time of arrival</th>
-    </tr>
-    <tr>
-      <th class="bl br">&nbsp;</th>
-      <th class="br">OX</th>
-      <th class="br">OY</th>
-      <th class="tdr"><div>h.</div></th>
-      <th class="tdr "><div>m.</div></th>
-      <th class="br">&nbsp;</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Mejillones</td>
-      <td class="tdr br"><div><i>a</i>&nbsp; or&nbsp; &nbsp; &nbsp;0</div></td>
-      <td class="tdr br"><div><i>b</i>&nbsp; or&nbsp; &nbsp;0</div></td>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdr"><div>46</div></td>
-      <td class="br">p. m.</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Iquique</td>
-      <td class="tdr br"><div><i>a<sub>1</sub></i> or 150</div></td>
-      <td class="tdr br"><div><i>b<sub>1</sub></i> or 96</div></td>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdr"><div>40</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Cobija</td>
-      <td class="tdr br"><div><i>a<sub>2</sub></i> or&nbsp; &nbsp;36</div></td>
-      <td class="tdr br"><div><i>b<sub>2</sub></i> or 14</div></td>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdr"><div>38</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Tocopilla</td>
-      <td class="tdr br"><div><i>a<sub>3</sub></i> or&nbsp; &nbsp;66</div></td>
-      <td class="tdr br"><div><i>b<sub>3</sub></i> or 31</div></td>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdr"><div>32</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl br">Huanillos</td>
-      <td class="tdr br"><div><i>a<sub>4</sub></i> or 102</div></td>
-      <td class="tdr br"><div><i>b<sub>4</sub></i> or 58</div></td>
-      <td class="tdr"><div>8</div></td>
-      <td class="tdr"><div>30</div></td>
-      <td class="br">„</td>
-    </tr>
-  </table>
-
-  <p>From this data we find the co-ordinates <i>x</i> and <i>y</i> of this origin to
-    be 85·8 and 56·7; whilst the velocity of propagation = 45 feet per
-    second.</p>
-
-  <p>Measuring these ordinates upon the map, we obtain a centre lying very
-    near Long. 71° 5′ W. and Lat. 21° 22′ S., a position which is very near
-    to that which has already been obtained by other methods.</p>
-
-  <p>If instead of Huanillos we substitute the ordinates and time of arrival
-    of the sea wave for Pabalon de Pica, another point for the origin will
-    be obtained lying farther out at sea. To obtain the best result, the
-    method to be taken will evidently be, first to reject those places
-    at which it seems likely that some mistake has been made with the
-    time observations, and then with the remaining places to form as many
-    equations as possible, and from these to obtain a mean value. This
-    is a long and tedious process, and as the time observations of this
-    particular earthquake are probably one and all more or less inaccurate,
-    it is hardly worth while to follow the investigation farther.</p>
-
-  <p>In this example, as in the preceding ones, it will be
-    <span class="pagenum" id="Page_209">209</span> observed that
-    it has been sea waves that have been dealt with, rather than earth
-    vibrations. It is evident, however, that these latter vibrations may be
-    dealt with in a similar manner.</p>
-
-  <p>In these determinations it will also have been observed that it
-    is assumed that the disturbance has radiated from a centre, and,
-    therefore, approached the various stations in different directions.
-    If we assume that we have three stations very near to each other as
-    compared with their distances from the origin, so that we can assume
-    that the wave fronts at the various stations were parallel, the
-    determination of the direction in which the wave advanced appears to be
-    much simplified. The determination of the direction in which a wave has
-    passed across three stations was first given by Professor Haughton.</p>
-
-  <p><em>Haughton’s method.</em>—Given, the time of an earthquake shock at three
-    places, to determine its horizontal velocity and coseismal line.</p>
-
-  <p>The solution of this is contained in the formula</p>
-
-  <div class="center">
-    tan <i>ϕ</i> = <span class="frac"><sup><i>a</i> (<i>t<sub>2</sub></i> - <i>t<sub>1</sub></i>)
-    sin β</sup><span>/</span><sub><i>c</i> (<i>t<sub>3</sub></i> - <i>t<sub>2</sub></i>) +
-    <i>a</i> (<i>t<sub>2</sub></i> - <i>t<sub>1</sub></i>) cos β</sub></span>.
-  </div>
-
-  <p>When <span class="smcap">a</span>, <span class="smcap">b</span>, and <span class="smcap">c</span> are three stations at
-    which a shock is observed at the times <i>t<sub>1</sub></i>, <i>t<sub>2</sub></i>, and <i>t<sub>3</sub></i>;
-    <i>a</i>, <i>b</i>, and <i>c</i> are the distances between <span class="smcap">a</span>, <span class="smcap">b</span>,
-    and <span class="smcap">c</span>, and <i>ϕ</i> is the angle made by the coseismal lines <i>x</i>
-    <span class="smcap">a</span> <i>x</i>, <i>y</i> <span class="smcap">b</span> <i>y</i>, and the line <span class="smcap">a b</span>, which
-    are assumed to be parallel.</p>
-
-  <p>This I applied in the case of the Iquique earthquake, but owing to
-    the smallness of the angles between the three stations <span class="smcap">a</span>,
-    <span class="smcap">b</span>, and <span class="smcap">c</span>, the result was unsatisfactory. The problem
-    ought to be restricted, first, to places which are a long distance
-    away from a centre, and, secondly, to places which are not nearly in a
-    straight line. This problem may be solved more readily by geometrical
-    methods.<span class="pagenum" id="Page_210">210</span>
-    Plot the three stations <span class="smcap">a</span>, <span class="smcap">b</span>, and
-    <span class="smcap">c</span> on a map, join the two stations between which there was the
-    greatest difference in the time observation. Let these, for example,
-    be <span class="smcap">a</span> and <span class="smcap">c</span>. Divide the line <span class="smcap">a c</span> at point
-    <span class="smcap">d</span>, so that <span class="smcap">a d</span> : <span class="smcap">d c</span> as the interval between
-    the shock felt at <span class="smcap">a</span> and <span class="smcap">b</span> is to the interval between
-    the shock felt at <span class="smcap">b</span> and <span class="smcap">c</span>. The line <span class="smcap">b d</span> will
-    be parallel to the direction in which the wave advanced.</p>
-
-  <p><em>The difference in time of the arrival of two disturbances.</em>—In the
-    various calculations which have been made to determine an origin
-    based on the assumption of a known or of a constant velocity, we have
-    only dealt with a single wave, which may have been a disturbance in
-    the earth or in the water. A factor which has not yet been employed
-    in this investigation is the difference in time between the arrival
-    of two disturbances; one propagated, for instance, through the
-    earth, and the other, for example, through the ocean. The difference
-    in the times of the arrival of two waves of this description is a
-    quantity which is so often recorded that it is well not to pass it
-    by unnoticed. To the waves mentioned we might also add sound waves,
-    which so frequently accompany destructive earthquakes, and, in some
-    localities, as, for instance, in Kameishi, in North Japan, are also
-    commonly associated with earthquakes of but small intensity. It was by
-    observing the difference in time between the shaking and the sound in
-    different localities that Signor Abella was enabled to come to definite
-    conclusions regarding the origin of the disturbances which affected the
-    province of Neuva Viscoya in the Philippines, in 1881; the places where
-    the interval of time was short, or the places where the two phenomena
-    were almost simultaneous, being, in all probability, nearer to the
-    origin than when the intervals were comparatively large. I myself
-    applied the method with considerable success
-    <span class="pagenum" id="Page_211">211</span> when seeking for the
-    origin of the Iquique earthquake of 1877. The assumptions made in that
-    particular instance were, first, that the velocity of the disturbance
-    through the earth was known, and, secondly, that the velocity with
-    which a sea wave was propagated was also known.</p>
-
-  <p>A method similar to the above was first suggested by Hopkins. It
-    depended on the differences of velocity with which normal and
-    transversal waves are propagated.<a id="FNanchor_86" href="#Footnote_86" class="fnanchor">[86]</a></p>
-
-  <p><em>Seebach’s method.</em>—To determine the true velocity of an earthquake,
-    the time of the first shock, and the depth of the centre.</p>
-
-  <div class="figright">
-    <img src="images/i_p211.jpg" width="315" height="295" alt="" />
-    <div class="caption"><span class="smcap">Fig. 32.</span></div>
-  </div>
-
-  <p>Let the straight line <span class="smcap">m</span>, <i>m<sub>1</sub></i>, <i>m<sub>2</sub></i>, <i>m<sub>3</sub></i> represent
-    the surface of the earth shaken by an earthquake. For small
-    earthquakes, to consider the surface of the earth as a plane will not
-    lead to serious errors.</p>
-
-  <p>If an earthquake originates at <span class="smcap">c</span>, then to reach the surface at
-    <span class="smcap">m</span> it traverses a distance <i>h</i> in the time <i>t</i>. To reach the
-    surface at <span class="smcap">m</span><sub>1</sub> it traverses a distance <i>h</i> + <i>x<sub>1</sub></i> in a
-    time <i>t<sub>2</sub></i>. If <i>v</i> equals the velocity of propagation,</p>
-
-  <div class="center">
-    then <i>t</i> = <span class="frac"><sup><i>h</i></sup><span>/</span><sub><i>v</i></sub></span>,&nbsp;
-    <i>t</i><sub>1</sub> = <span class="frac"><sup><i>h</i> + <i>x</i><sub>1</sub></sup><span>/</span><sub><i>v</i></sub></span>,
-  </div>
-
-  <div class="center">
-    <i>t</i><sub>2</sub> = <span class="frac"><sup><i>h</i> + <i>x</i><sub>2</sub></sup><span>/</span><sub><i>v</i></sub></span>, &amp;c.
-  </div>
-
-  <p>Seebach now says that <em>if we have given the position of</em> <span class="smcap">m</span>
-    <em>or epicentrum of the shock</em>, and draw through it rectangular axes
-    like <span class="smcap">m</span> <i>m</i><sub>3</sub> and <span class="smcap">m</span> <span class="smcap">t</span><sub>3</sub>, and lay down
-    <span class="pagenum" id="Page_212">212</span>on <span class="smcap">m</span>
-    <i>m<sub>3</sub></i> in miles the distances from M of the various
-    stations which have been shaken, and in equal divisions for minutes
-    lay down on <span class="smcap">m</span> <span class="smcap">t</span><sub>3</sub> the differences of time at which
-    <span class="smcap">m</span>, <i>m</i><sub>1</sub>, <i>m</i><sub>2</sub>, &amp;c.
-    were shaken, then <span class="smcap">m</span><sub>1</sub>
-    <span class="smcap">t</span><sub>1</sub>, <span class="smcap">m</span><sub>2</sub>
-    <span class="smcap">t</span><sub>2</sub>, &amp;c. are the co-ordinates
-    of points on an hyperbola. The degree of exactness with which this
-    hyperbola is in any given case constructed is a check upon the accuracy
-    of the time observations and the position of the <em>epicentrum</em>. The apex
-    of the hyperbola is the <em>epicentrum</em>.</p>
-
-  <p>The intersection of the asymptote with the ordinate axis is the time
-    point of the first shock, which, because the scale for time and
-    for space were taken as equal, gives the absolute position of the
-    <em>centrum</em>. This intersection is shown by dotted lines. Knowing the
-    position of the <em>centrum</em>, we can directly read from our diagram how
-    far the disturbance has been propagated in a given time.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XI">
-    <span class="pagenum" id="Page_213">213</span>
-    <h2>CHAPTER XI.<br />
-      <span class="subhead">THE DEPTH OF AN EARTHQUAKE CENTRUM.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    The depth of an earthquake centrum—Greatest possible depth of an
-    earthquake—Form of the focal cavity.</div>
-
-  <p><em>Depth of centrum.</em>—The first calculations of the depth at which an
-    earthquake originated were those made by Mallet for the Neapolitan
-    earthquake of 1857. These were made on the assumption that the earth
-    wave radiated in straight lines from the origin, and, therefore, at
-    points at different distances from the <em>epicentrum</em> it had different
-    angles of emergence. These angles of emergence were chiefly calculated
-    from the inclination of fissures produced in certain buildings, which
-    were assumed to be at right angles to the direction of the normal
-    motion. If we have determined the <em>epicentrum</em> of an earthquake and
-    the muzoseismal circle, and make either the assumption that the angle
-    of emergence in this circle has been 45° or 54° 44′ 9″ (see page
-    54), it is evidently an easy matter by geometrical construction to
-    determine the depth of the <em>centrum</em>. Höfer followed this method when
-    investigating the earthquake of Belluno.</p>
-
-  <p>Other methods of calculation which have been employed are based on time
-    observations, as, for instance, the method of Seebach, the method of
-    co-ordinates, the method of hyperboloids or spheres (see pages 200–212).</p>
-
-  <p>By means of a number of lines parallel to twenty-six
-    <span class="pagenum" id="Page_214">214</span> angles of
-    emergence, drawn in towards the seismic vertical, Mallet found that
-    twenty-three of these intersected at a depth of 7⅛ geographical
-    miles. The maximum depth was 8⅛ geographical miles, and the minimum
-    depth 2¾ geographical miles.</p>
-
-  <p>The mean depth was taken at a depth of 5¾ geographical miles where,
-    within a range of 12,000 feet, eighteen of the wave paths intersected
-    the seismic vertical.</p>
-
-  <p>The point where these wave paths start thickest is at a depth not
-    greater than three geographical miles, and this is considered to be the
-    vertical depth of the focal cavity itself.</p>
-
-  <p>For the Yokohama earthquake of 1880, from the indications of
-    seismometers, and by other means, certain angles of emergence were
-    obtained, leading to the conclusion that the depth of origin of that
-    earthquake might be between 1½ and 5 miles.</p>
-
-  <p>Possibly, perhaps, the earthquake may have originated from a fissure
-    the vertical dimensions of which was comprised between these depths.</p>
-
-  <p>A source of error in a calculation of this description is that the
-    vertical motions may have been a component of transverse motions or
-    perhaps due to the slope of surface waves.</p>
-
-  <p>The following table of the depths at which certain earthquakes have
-    originated has been compiled from the writings of several observers.</p>
-
-  <table class="collapse" summary="Earthquake depths">
-    <tr>
-      <th colspan="2" class="ball">&nbsp;</th>
-      <th colspan="3" class="ball">In feet</th>
-    </tr>
-    <tr>
-      <th class="bl br">&nbsp;</th>
-      <th class="br">&nbsp;</th>
-      <th class="br">Minimum</th>
-      <th class="br">Mean</th>
-      <th class="br">Maximum</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Rhineland</td>
-      <td class="tdl br">1846 (Schmidt)</td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr br"><div>127,309</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Sillien</td>
-      <td class="tdl br">1858 (Schmidt)</td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-      <td class="tdr br"><div>86,173</div></td>
-      <td class="tdr br"><div>&nbsp;</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Middle Germany</td>
-      <td class="tdl br">1872 (Seebach)</td>
-      <td class="tdr br"><div>47,225</div></td>
-      <td class="tdr br"><div>58,912</div></td>
-      <td class="tdr br"><div>70,841</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Herzogenrath</td>
-      <td class="tdl br">1873 (Lasaulx)</td>
-      <td class="tdr br"><div>16,553</div></td>
-      <td class="tdr br"><div>36,516</div></td>
-      <td class="tdr br"><div>56,477</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Neapolitan</td>
-      <td class="tdl br">1857 (Mallet)</td>
-      <td class="tdr br"><div>16,705</div></td>
-      <td class="tdr br"><div>34,930</div></td>
-      <td class="tdr br"><div>49,359</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl br">Yokohama</td>
-      <td class="tdl br">1880 (Milne)</td>
-      <td class="tdr br"><div>7,920</div></td>
-      <td class="tdr br"><div>17,260</div></td>
-      <td class="tdr br"><div>26,400</div></td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_215">215</span></p>
-
-  <p>A table similar to this has been compiled by Lasaulx.<a id="FNanchor_87" href="#Footnote_87" class="fnanchor">[87]</a></p>
-
-  <p>With the exception of the determination for the two last disturbances
-    these calculations have been made with the assistance of the method
-    of Seebach, which depends, amongst other things, on the assumptions
-    of exact time determinations, direct transmission of waves from the
-    centrum, and a constant velocity of propagation.</p>
-
-  <p>Admitting that our observations of time are practically accurate, it
-    appears that the other assumptions may often lead to errors of such
-    magnitude that our results may be of but little value.</p>
-
-  <p>From what has been said respecting the velocity with which earth
-    disturbances are propagated, it seems that these velocities may vary
-    between large limits, being greatest nearest to the origin.</p>
-
-  <p>If we refer to Seebach’s method, we shall see that a condition of this
-    kind would tend to make the differences in time between various places,
-    as we recede from the <em>epicentrum</em>, greater than that required for the
-    construction of the hyperbola. The curve which is obtained would, in
-    consequence, have branches steeper than that of the hyperbola, and the
-    resultant depth, obtained by the intersection of the asymptotes of this
-    curve with the seismic vertical, indicates an origin which may be much
-    too great.</p>
-
-  <p>Another point worthy of attention, which is common to the method
-    of Mallet as well as to that of Seebach, is the question whether
-    the shock radiates directly from the origin, or is propagated from
-    the origin more or less vertically to the surface, and then spreads
-    horizontally. We know that earthquakes, both natural and artificial,
-    may be propagated as undulations on the surface of the ground, and
-    that the vertical motion of the latter, as
-    <span class="pagenum" id="Page_216">216</span>testified by the records
-    of well-constructed instruments, has no practical connection with the
-    depth from which the disturbance originated.</p>
-
-  <p>In cases like these, the direction of cracks in buildings, and other
-    phenomena usually accredited to a normal radiation, may in reality be
-    due to changes in inclination of the surface on which the disturbed
-    objects rested. When our points of observation are at a distance from
-    the <em>epicentrum</em> of the disturbance which, as compared with the depth
-    of the same, is not great, calculations or observations based on the
-    assumption of a direct radiation of the disturbance may possibly lead
-    to results which are tolerably correct. The calculations of Mallet
-    for the Neapolitan earthquake appear to have been made under such
-    conditions.</p>
-
-  <p>For smaller earthquakes, and for places at a distance from the seismic
-    vertical of a destructive earthquake, the results which are deduced
-    from the observations on shattered buildings, and all observations
-    based upon the assumption of direct radiation, we must accept with
-    caution.</p>
-
-  <p>Another error which may enter into calculations of this description
-    is one which has been discussed by Mallet at some length. This is the
-    effect which the form and the position of the focal cavity may have
-    upon the transmission of waves.</p>
-
-  <p>Should the impulse originate from a point or spherical cavity, then
-    we might, in a homogeneous medium perhaps, regard the isoseismals as
-    concentric circles, and expect to find that equal effects had been
-    produced at equal distances from the <em>epicentrum</em>. Should, however,
-    this cavity be a fissure, it is evident that even in a homogeneous
-    medium the inclination of the plane of such a cavity will have
-    considerable effect upon the form of the waves which would radiate from
-    its two walls.</p>
-
-  <p><span class="pagenum" id="Page_217">217</span></p>
-
-  <p>For example, let it be assumed that the first impulse of an earthquake
-    is due to the sudden formation of a fissure, rent open from its centre,
-    and that the waves leave the walls at all points normal to its surface.
-    Then, as Mallet points out, it is evident that the disturbance will
-    spread out in ellipsoidal waves, the greatest axis of which will be
-    perpendicular to the plane of the fissure.</p>
-
-  <p>By taking a number of cases of fissures lying in various directions
-    and drawing the ellipsoidal waves which would result from an elastic
-    pressure, like that of steam suddenly admitted into such cavities, the
-    differences in effect which would be simultaneously produced by these
-    waves on reaching the surface can be readily understood. The following
-    example of an investigation on this subject will serve as an example to
-    illustrate the general nature of the many other cases which might be
-    taken.</p>
-
-  <div class="figright">
-    <img src="images/i_p217.jpg" width="369" height="295" alt="" />
-    <div class="caption"><span class="smcap">Fig. 33.</span></div>
-  </div>
-
-  <p>Let a disturbance simultaneously originate from all points of the
-    fissure <i>f</i> <i>f</i>. This will spread outwards in ellipsoidal shells to
-    the surface of the earth <i>e</i> <i>e</i>. The major axis of these ellipsoidal
-    shells will be the direction of greatest effect. In the direction <i>c</i>
-    <i>d</i> the waves will plunge into the earth, and places to the right side
-    of the fissure will, to use an expression due to Stokes, when speaking
-    of analogous phenomena connected with sound, be in <em>earthquake shadow</em>.
-    The same expression has been<span class="pagenum" id="Page_218">218</span>
-    employed, somewhat differently, when
-    speaking of the effects produced on buildings.</p>
-
-  <p>For places, like <i>s</i> and <i>p</i>, situated at equal distances from the
-    seismic vertical, it is evident that the intensity of the shock will be
-    different, and also its time of arrival. It will also be observed that
-    the isoseismals will take the form of ovals or distorted ellipses, the
-    larger or fuller end of which being to the left of the fissure.</p>
-
-  <p>Other cases, like those just given, which are discussed by Mallet in
-    his account of the Neapolitan earthquake, are where the fissure forms
-    the division between materials of different elasticities. In the hard
-    and more elastic material the waves will be more crowded, the velocity
-    of a wave particle will be greater, and the transit will be quicker
-    than in the less elastic medium.</p>
-
-  <p>The result is that the distance of equal effect from the seismic
-    vertical will be greatest in the direction of the more compressible
-    material.</p>
-
-  <p>Unless these considerations are kept carefully before the mind when
-    investigating the depth and, we may add, the position and form of the
-    centrum of an earthquake, serious errors may arise.</p>
-
-  <p><em>Greatest depth of an earthquake origin.</em>—A curious but instructive
-    calculation which Mallet made was a determination of the greatest
-    possible depth at which an earthquake may occur. This calculation is
-    based upon the idea that the impulsive effect of an earthquake has an
-    intimate relationship with the height of neighbouring volcanoes, the
-    column of lava supported on a volcanic cone being a measure of the
-    internal pressure tending to rupture the adjacent crust of the earth.</p>
-
-  <p>Mitchell, in 1700, virtually propounded this idea, when he suggested that
-    the velocity of propagation of an earthquake was related to the height
-    of such a column.<a id="FNanchor_88" href="#Footnote_88" class="fnanchor">[88]</a></p>
-
-  <p><span class="pagenum" id="Page_219">219</span></p>
-
-  <p>Mallet showed that there was probably considerable truth in such a
-    supposition by appealing to the results of actual observation. The
-    pressure gauge of the Neapolitan district would be Vesuvius, the height
-    of which has in round numbers varied between 3,500 to 4,000 feet. One
-    of the most destructive earthquakes in this district—namely, the one of
-    1857—projected bodies with an initial velocity of about fifteen feet
-    per second. The Riobamba earthquake, which projected bodies with an
-    initial velocity of eighty feet per second, appears to have been the
-    most violent earthquake, so far as its impulsive effort is concerned,
-    of which we have any record. It occurred amongst the Andes, where there
-    are volcanoes from 16,000 to 21,000 feet in height.</p>
-
-  <p>Comparing these two earthquakes together, we see that the Riobamba
-    shock had a destructive power 5·33 times that of the Neapolitan shock,
-    and we also see that the Riobamba volcanoes were about 5·33 times
-    higher than Vesuvius. The accordance in these quantities is certainly
-    interesting, and tends to substantiate the idea that volcanoes are
-    barometrical-like pressure gauges of a district.</p>
-
-  <p>Carrying the argument still further. Mallet says that if the depth of
-    origin of earthquakes were the same, then the <em>area of disturbance</em>
-    would, for like formations and configuration of surface, be a measure
-    of the earthquake effort, and also some function of the velocity of the
-    wave. From this we may generally infer ‘that earthquakes, like that of
-    Lisbon, which have a <em>very great area</em> of sensible disturbance, have
-    also a very deep seismal focus, and also the greatest depth of seismal
-    focus within our planet is probably not greater than that ascertained
-    for this Neapolitan earthquake, multiplied by the ratio that the
-    velocity of the Riobamba wave bears to that of its wave,
-    <span class="pagenum" id="Page_220">220</span> or, what is
-    the same thing, by the ratio of the altitudes of the volcanoes of the
-    Andes to that of Vesuvius.’</p>
-
-  <p>Now, as the depth of the Neapolitan shock may be taken at 34,930
-    feet, the greatest probable depth of origin of any earthquake impulse
-    occurring in our planet is limited to 5·333 × 4,930 feet, or 30·64
-    geographical miles.</p>
-
-  <p>Ingenious as this argument is, we can hardly admit it without certain
-    qualifications.</p>
-
-  <p>First, we are called upon to admit the identity of the originating
-    cause of the volcano and the earthquake—as to what may be the
-    originating cause of earthquakes we have yet to refer, but certainly in
-    the case of particular earthquakes, as, for instance, those which occur
-    in countries like Scotland, Scandinavia, and portions of Siberia, the
-    direct connection between these phenomena are not at first sight very
-    apparent.</p>
-
-  <p>Secondly, even if we admit the identity of the origin of these
-    phenomena, it is not difficult to imagine that the fluid pressure
-    brought to bear upon certain portions of the crust of the earth may
-    possibly in many instances be infinitely greater than that indicated
-    by the height of the column of liquid lava in the throat of a volcano,
-    the true height of which we are unable to obtain. Further, in certain
-    instances such a column only appears to be a measure of the pressure
-    upon the crust of the earth in the immediate vicinity of the cone.</p>
-
-  <p>Thus, in the Sandwich Islands, we have lava standing in the throat
-    of the volcano of Mauna Loa 10,000 feet higher than it stands in the
-    crater Kilauea, only twenty miles distant. That these columns should be
-    measures of the same pressure, originating in a general subterranean
-    liquid layer with which they are connected, is a supposition difficult
-    to satisfactorily substantiate.</p>
-
-  <p>Another measure of the impulsive efforts which subterranean
-    <span class="pagenum" id="Page_221">221</span>terranean forces
-    may exert upon the crust above them is evidently the height to which
-    volcanoes eject materials. Cotopaxi is said to have hurled a 200-ton
-    block of stone nine miles. Sir W. Hamilton tells us that in 1779
-    Vesuvius shot up a column of ashes 10,000 feet in height; and Judd
-    tells us that this same mountain in 1872 threw up vapours and rock
-    fragments to the enormous height of 20,000 feet. This would indicate an
-    initial velocity of 1,131 feet per second.</p>
-
-  <p>Notwithstanding Mallet’s calculation that thirty miles is the limiting
-    depth for the origin of an earthquake, the origin of the Owen’s
-    Valley earthquake of March 1872 was estimated as being at least fifty
-    miles.<a id="FNanchor_89" href="#Footnote_89" class="fnanchor">[89]</a></p>
-
-  <p><em>Form of the focal cavity.</em>—Among the various problems which are put
-    before those who study the physics of the interior of our earth it
-    would at first sight appear that there was none more difficult than the
-    attempt to determine the form of the cavity, if it be a cavity, from
-    which an earthquake originates. Almost all investigators of seismology
-    have recognised that the birthplace of an earthquake is not a point,
-    and have made suggestions about its general nature. The ordinary
-    supposition is that the earthquake originates from a fissure, and if
-    the focus of a disturbance could be laid bare to us it would have the
-    appearance of a fault such as we so often see exposed on the faces of
-    cliffs.</p>
-
-  <p>A strong argument, tending to demonstrate that some of the shakings
-    which are felt in Japan are due to the production of such fissures,
-    is the fact that the vibrations which are recorded are transverse to
-    a line joining the point of observation and the district from which,
-    by time observations, we know the shock to have originated. The most
-    probable explanation of this phenomena appears to
-    <span class="pagenum" id="Page_222">222</span>be that one mass
-    of rock has been sliding across another mass, giving rise to shearing
-    strains, and producing waves of distortion.</p>
-
-  <p>The first seismologist who attacked the problem of finding out the
-    dimensions and position of such a fissure was Mallet, when working on
-    the Neapolitan earthquake of 1857. The reasons that the origin should,
-    in the first place, have been a fissure, rather than any other form of
-    cavity, was that such a supposition seemed to be <i xml:lang="la">a priori</i> the most
-    probable, and, further, that it afforded a better explanation of the
-    various phenomena which were observed, than that obtained from any
-    other assumption.</p>
-
-  <p>The method on which Mallet worked to determine the form and position of
-    the assumed fissure, which method was subsequently more or less closely
-    followed by other investigators, was as follows:—</p>
-
-  <p>From an observation of the various phenomena produced upon the surface
-    of the disturbed area, a map of isoseismals was constructed. These were
-    seen, as has been the case with many earthquakes, not to distribute
-    themselves in circles round the <em>epicentrum</em>, but as distorted oval or
-    elliptical figures, the major axes of which roughly coincided with each
-    other. Further, the <em>epicentrum</em>, did not lie in the centre of these
-    ovals, but was near to the narrow end where they converged.</p>
-
-  <p>This at once showed, if the reasoning respecting the manner in which
-    waves are propagated from an inclined fissure be correct, that the
-    fissure was at right angles to the major axis of the curves, dipping
-    from their narrow end downwards, in the direction of their larger
-    widespread ends.</p>
-
-  <p>The next weapon which Mallet employed to attack this problem was the
-    sound which was heard at different points round about the focus. These
-    sounds appear to have<span class="pagenum" id="Page_223">223</span>
-    been of the nature of sudden explosive reports
-    accompanied by rushing, rolling sounds. The form of the area in which
-    these sounds were heard was closely similar to that of the first two
-    isoseismals. Except in the central area of great disturbance, no sound
-    was heard to accompany the shock.</p>
-
-  <p>Those at the northern and southern extremity of the sound area all
-    described what they heard as a ‘low, grating, heavy, sighing rush, of
-    twenty to sixty seconds’ duration.’ Those in the middle and towards the
-    east and west boundaries of this area described a sound of the same
-    tone, but shorter and more abrupt, and accompanied with more rumbling.</p>
-
-  <p>The nature of the arguments which were followed in discussing the sound
-    observations will be found in the chapter relating to these phenomena.</p>
-
-  <p>A portion of the argument which it is difficult to follow relates to
-    the maximum rate at which it can be supposed possible for a fissure to
-    be rent in rocks, which rate depends on the density and elasticity of
-    these rocks and other constant factors.</p>
-
-  <p>Next it was observed that the paths of the waves drawn on the surface,
-    although generally intersecting in a point, did not do so absolutely,
-    but along a line passing through the main focus some 7½ miles in
-    length. This, coupled with the observations of sounds, led to the
-    supposition that the centre of disturbance, considered horizontally,
-    originated along a curved line passing through the chief focus and the
-    various intersections of the wave paths.</p>
-
-  <p>The last phenomena brought forward to assist in the solution of this
-    interesting problem were a study of the tremulous movements that
-    preceded and followed the shock, and their relation to the sound
-    phenomena.</p>
-
-  <p><span class="pagenum" id="Page_224">224</span></p>
-
-  <p>If the earthquake originated by the formation of a fissure, after the
-    rending has gone on for a certain time the focal cavity is enlarged
-    to a certain extent, and the great shock takes place. This would be
-    followed by concluding tremulous waves. A succession of phenomena like
-    those accompanied the shock about which Mallet writes.</p>
-
-  <p>By observations such as these, coupled with what has been said about
-    the maximum and mean depths of the focal cavity, Mallet came to the
-    conclusion that the focal cavity was a fissure, the rending open of
-    which produced the earthquake. The vertical dimensions of this cavity
-    were not more than 5·3 miles, but were probably limited to three miles.</p>
-
-  <p>From the intersection of the wave paths upon the surface and the
-    observed emergences, this fissure followed horizontally a curve of
-    double flexure, about nine geographical miles in length. The area of
-    this fissure was twenty-seven geographical miles. The time of rending
-    it open in Apennine limestone would be about 7½ seconds, which
-    should be the same as the period during which tremors were felt. The
-    time actually recorded was six or eight seconds.</p>
-
-  <p>Briefly, this is, then, the line of reasoning which was followed by
-    Mallet in an investigation the results of which are as interesting
-    as they are startling. Since the line of investigation has been
-    opened, and the existence of new problems has been indicated, other
-    investigators, although not exactly following Mallet’s method in
-    all their details, have, when endeavouring to attain the same ends,
-    employed similar weapons.</p>
-
-  <p>Thus, for example, Seebach, when determining the depth and nature of
-    the origin of the earthquake of Middle Germany, reasoned somewhat as
-    follows:—</p>
-
-  <p><span class="pagenum" id="Page_225">225</span></p>
-
-  <p>Had the origin been more or less of a spherical cavity, then the region
-    of most violent disturbance upon the surface would, according to a
-    theorem we have already mentioned, have been upon or near a circle of
-    about 8·8 miles in radius round the <em>epicentrum</em>. This region, however,
-    was found by observation to lie along a curved band about forty miles
-    in length, altogether on one side of the <em>epicentrum</em>.</p>
-
-  <p>To explain this anomaly Seebach followed Mallet, and assumed that the
-    origin was not a spherical cavity, but a fissure.</p>
-
-  <p>The depth and strike of this fissure was determined by the observation
-    that the area of greatest disturbance was along a curved line lying
-    radial to the <em>epicentrum</em>. Such a condition it was assumed indicated
-    that the fissure of origin must be inclined towards this area of
-    greatest disturbance. A line was then drawn from this area to the
-    <em>centrum</em>. A second line at right angles to this one gave the dip of
-    the fissure.</p>
-
-  <p>Höfer, when working on the earthquake of Belluno, came to the
-    conclusion that the disturbance originated from two faults meeting
-    each other at an angle of 60°. In this determination he was chiefly
-    influenced by the form of a certain homoseist which was of the form of
-    an elongated ellipse met on one side by a second ellipse, the principal
-    axes of the two ellipses giving the strike of the two faults.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XII">
-    <span class="pagenum" id="Page_226">226</span>
-    <h2>CHAPTER XII.<br />
-      <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    General distribution of earthquakes—Occurrence along lines—Examples
-    of distribution—Italian earthquake of 1873—In Tokio—Extension
-    of earthquake boundaries—Seismic energy in relation to
-    geological time; to historical time—Relative frequency of
-    earthquakes—Synchronism of earthquakes—Secondary earthquakes.</div>
-
-  <p><em>General distribution of earthquakes.</em>—The records of earthquakes
-    collected by various seismologists lead us to the conclusion that
-    at some time or other every country and every ocean in the world
-    has experienced seismic disturbances. In some countries earthquakes
-    are felt daily, and from what will be said in the chapter on earth
-    pulsations it is not unlikely that every large earthquake might with
-    proper instrumental appliances be recorded at any point on the land
-    surfaces of our globe. The area over which any given earthquake extends
-    is indeterminate. The area over which an earthquake is sensible is
-    sometimes very great. The Lisbon shock of 1755 is estimated as having
-    been sensible over an area of 3,300 miles long and 2,700 miles wide,
-    but in the form of tremors and pulsations it may have shaken the whole
-    globe.</p>
-
-  <p>The regions in which earthquakes are frequent are indicated in the
-    accompanying map, which, to a great extent, is a reproduction of a map
-    drawn by Mallet. The regions coloured with the darkest tint are those
-    where<span class="pagenum" id="Page_227">227</span>
-    great earthquakes are the most frequent. The actual number of
-    earthquakes which have been felt in the differently coloured areas are
-    given, when speaking of the relation of seismic energy to season.</p>
-
-  <p>When looking at this chart it must be remembered that if we were
-    to make a detailed map of any one of the different countries where
-    earthquakes are frequent, we should find in it all the differences that
-    we observe in the general chart. For instance, one portion of Japan,
-    where perhaps sixty shocks are felt per year, would be coloured with
-    a dark tint, whilst other portions of the same country, where there
-    is only one slight shaking felt every few years, would be left almost
-    uncoloured. The black dots indicating the position of volcanic vents
-    are even more general in their signification than the tinted areas.
-    Professor Haughton gives for the world a list of 407 volcanoes, 225 of
-    which are active. These numbers are the same as those given by A. von
-    Humboldt. Of the active volcanoes 172 are on the margin of the Pacific,
-    and of the total number eight are in Japan. From my own observations
-    in Japan independently of the Kurile Islands, I have enumerated
-    fifty-three volcanoes which are either active or have been active
-    within a recent period. In a few years’ time this list will probably be
-    increased. I mention this fact to show how very imperfect our knowledge
-    is respecting the number of volcanic vents existing on our globe. If we
-    were in a position to indicate the volcanoes which had been in eruption
-    during the last 4,000 years, the probability is that they would number
-    several thousands rather than four or five hundred.</p>
-
-  <p>An inspection of the map shows that earthquakes chiefly occur in
-    volcanic and mountainous regions. The most earthquake-shaken regions of
-    the world form the<span class="pagenum" id="Page_228">228</span>
-    boundaries of the Pacific ocean. It may be remarked
-    that these boundaries slope beneath the neighbouring ocean at a much
-    steeper angle than the boundaries of countries where earthquakes occur
-    but seldom. The coasts of South America, Kamschatka, the Kuriles,
-    Japan, and the Sandwich Islands, for example, have slopes beneath the
-    Pacific from one in twenty to one in thirty. The coasts of Australia,
-    Scandinavia, and the eastern parts of South America, where earthquakes
-    are practically unknown, have slopes from one in fifty to one in two
-    hundred and fifty. Many earthquakes have taken place in mid-ocean. In
-    the Atlantic Ocean M. Perrey has given about 140 instances of such
-    occurrences.</p>
-
-  <p>The majority of the earthquakes which shake Japan appear to have their
-    origin in the neighbouring ocean. If we could draw a map of earthquake
-    origins, it is probable that the greater number of the marks indicating
-    these origins would be found to be suboceanic and along lines parallel
-    to the shores of continents and islands which rise steeply from the
-    bed of deep oceans. In countries like Switzerland and India, our marks
-    would hold a relationship to the mountains of these countries.<a id="FNanchor_90" href="#Footnote_90" class="fnanchor">[90]</a>
-    Looking at the broad features of the globe, we see on its surface
-    many vast depressions. Some of these saucer-like hollows form land
-    surfaces, as in central Asia. The majority of these, however, are
-    occupied by the oceans. Active volcanoes chiefly occur near the rim of
-    the hollows which have the steepest slopes. The majority of earthquakes
-    probably have their origin on or near the bottom of these slopes. To
-    these, however, there are exceptions, as for instance the earthquakes
-    in the Alps, in the hills of <span class="pagenum" id="Page_229">229</span>
-    Scotland, and the shakings which are
-    occasionally felt in countries like Egypt. The earthquakes which shake
-    the borders of the Pacific have their origins in, and their effects are
-    almost exclusively felt on, the sides of the bounding ridge facing this
-    ocean. In Japan it is the eastern sides of the islands which suffer,
-    the western side being almost as free from these convulsions as England.</p>
-
-  <p>Similar remarks may be made about the eastern side of South America,
-    especially the southern portion of the continent. At Buenos Ayres, for
-    example, there has been no disturbance since Mendoza was destroyed,
-    some twenty years ago. In British Guiana slight shocks are occasionally
-    felt in the low delta which forms the settled portion of the colony,
-    but they are extremely rare.</p>
-
-  <p><em>Disturbances in lines or zones.</em>—It has often been observed that
-    disturbances are propagated along the length of mountains or valleys,
-    and it is but seldom that earthquakes cross them transversely. Thus the
-    valleys of the Rhone, the Rhine, and the Danube are lines along which
-    disturbances travel.</p>
-
-  <p>The major axes of the elliptical areas of disturbances which have
-    shaken India have a general direction parallel to the valley of the
-    Ganges along the flanks of the Himalayas.</p>
-
-  <p>The disturbances which have shaken London appear to have been chiefly
-    east and west, or along the valley of the Thames. In South America the
-    line of disturbance is along the western sides of the Andes. Another
-    line is along the northern coast of the continent through Andalusia
-    and Caraccas towards the Antilles and Trinidad. The shocks of the
-    Pyrenees are chiefly felt along the southern side of these mountains.
-    In the middle and on the northern side they are but seldom felt. This
-    propagation in lines or zones may in certain cases be apparent rather
-    <span class="pagenum" id="Page_230">230</span>
-    than real. Thus the north and south ranges of mountains in Japan are
-    mountains almost simultaneously shaken along their eastern flanks,
-    giving the impression that an earthquake had originated simultaneously
-    from a fissure parallel to this line, or else, starting at one end,
-    had run down their lengths. Time observations have, however, shown
-    that such disturbances had their origin at some distance in the ocean,
-    and, travelling inwards, had reached all points on the flanks of these
-    mountains almost simultaneously. The same explanation will probably
-    hold for the so-called linear disturbances of western South America.</p>
-
-  <p>All earthquake disturbances have probably a tendency to radiate from
-    their source, and are only prevented from doing so by meeting with
-    heavy mountainous districts, which by their mass and structure absorb
-    the energy communicated to them. Much energy is also lost by emergence
-    on the open flanks of a range of mountains. Rather than say that
-    high mountains often bound the extension of an earthquake, or that
-    earthquakes appear to run along the flanks of such mountains, we might
-    say that earthquakes have boundaries parallel to the strike of the
-    rocks in a given district, that such a direction is the one in which
-    the propagation is the easier.</p>
-
-  <p>Rossi is of opinion that volcanic fractures play an important part in
-    governing the distribution of seismic disturbances. When a volcano is
-    formed, a series of starlike fractures are formed round the central
-    crater. Secondary craters may indicate the line of these fissures. The
-    mountains about Rome are regarded as typical of this radial structure.
-    The more distant the secondary craters are from the centre of the
-    system, the smaller will they be, and also the younger. If two fissures
-    intersect we get a larger crater at the junction. Earthquakes are
-    propagated along the direction of these fissures, whilst the
-    <span class="pagenum" id="Page_231">231</span> rising
-    and falling of these lips throw off transverse waves. Rossi adduces
-    observations which appear to meet with explanation on such suppositions.</p>
-
-  <p>Suess, who has written upon the earthquakes of lower Austria, shows how
-    the majority of the disturbances have had their origin along certain
-    lines which form a break in the continuity of the Alps. One line runs
-    north-east from Bruck towards Vienna. Near Wiener Neustadt, where the
-    greatest number and heaviest shocks have occurred, this line is met by
-    a north-north-west line crossing the Danube and following the valley
-    of the river Kamp.<a id="FNanchor_91" href="#Footnote_91" class="fnanchor">[91]</a> Hoeffer has drawn similar lines from the head
-    of the Adriatic, one set running north-north-east to intersect near
-    Litschau, and the other north-north-west to intersect near Frankfort in
-    the valley of the Rhine.<a id="FNanchor_92" href="#Footnote_92" class="fnanchor">[92]</a></p>
-
-  <p><em>Examples of distribution.</em>—A curious example of the distribution of
-    seismic movement is that of the earthquake of March 12, 1873, worked
-    out by Professor P. A. Serpieri. This earthquake appears to have been
-    simultaneously felt on the Dalmatian coast and in central Italy, in a
-    region lying north-east from Rome and south-east from Florence. In both
-    of these areas the motion was from south-east to north-west. The shock
-    then radiated from the central Italian regions, so that at places on
-    the western shore of the Adriatic it was felt after it had been felt on
-    the Dalmatian coast.</p>
-
-  <p>Many explanations might be offered for this peculiar distribution
-    of seismic activity. Possibly the shock originated at a great depth
-    beneath the bed of the southern part of the Adriatic, and by following
-    existing lines of weakness simultaneously reached the surface of the
-    earth in central Italy and Dalmatia.</p>
-
-  <p><span class="pagenum" id="Page_232">232</span></p>
-
-  <p>In Tokio, which is built partly on a flat plain, partly in valleys
-    denuded from a low tableland, and partly on the spurs of the tableland
-    itself, the distribution of earthquakes is a subject yet requiring
-    attention. Sometimes it has happened that persons in one house have
-    been sufficiently alarmed to escape into the open air, whilst others,
-    not more than a mile distant, have not been aware that the city had
-    been shaken.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p232.jpg" width="302" height="430" alt="" />
-    <div class="caption"><span class="smcap">Fig. 34.</span><br />
-      <div class="list-container">
-        <div class="left nowrap">
-          <div>
-            Areas almost simultaneously struck from S.E. to N.W.&nbsp;&nbsp;&nbsp;
-            <img class="iglyph-a" src="images/i_p232a.png" alt="" width="40" height="40" />
-          </div>
-          <div>
-            Subsequent radial disturbance&nbsp;&nbsp;&nbsp;
-            <img class="iglyph-b" src="images/i_p232b.png" width="36" height="60" alt="" />
-          </div>
-        </div>
-      </div>
-    </div>
-  </div>
-
-  <p><em>Extension of earthquake boundaries.</em>—Natural obstructions which may be
-    sufficient to retard small earthquakes may in certain instances not be
-    found sufficient to retard the larger disturbances. Thus the shocks of
-    <span class="pagenum" id="Page_233">233</span>
-    Calabria are usually only felt on the western side of the Apennines,
-    but instances have occurred when they have crossed this barrier. In
-    1801 the earthquake of Cumana crossed a branch of the coast range.</p>
-
-  <p>Sometimes earthquake boundaries give way, and countries which they
-    sheltered subsequently become exposed to all disturbances. The true
-    explanation of this is probably in a shifting of the centre of seismic
-    activity. Thus up to December 14, 1797, although Cumana was often
-    devastated, the peninsula of Araya was not hurt. On this date Araya
-    commenced to suffer, and has continued to suffer ever since.</p>
-
-  <p>Fuchs gives an example of the movement of a seismic centre in the case
-    of the Calabrian earthquake. The first shock commenced near Oppiedo,
-    the second shock commenced four or five miles farther to the north, and
-    the third shock had its origin five or six miles still farther, near to
-    Girifalco.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XIII">
-    <span class="pagenum" id="Page_234">234</span>
-    <h2>CHAPTER XIII.<br />
-      <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</span>
-    </h2>
-  </div>
-
-  <p><em>Seismic energy in relation to geological time.</em>—If we admit that
-    seismic energy is only a form of volcanic energy, it must also be
-    admitted that any cause tending to produce a general decrease in the
-    amount of the latter will also produce an alteration in the amount of
-    the former.</p>
-
-  <p>The nebular hypothesis of Laplace tells us that the solar system is the
-    result of the whirling of a heated gaseous mass, which as it cooled
-    continually contracted and consequently whirls the faster. With this
-    hypothesis before us, we understand why all the planets and their
-    satellites have a similarity in the directions of their movements, why
-    they revolve nearly in the same plane, in orbits nearly circular, why
-    some have a flattened figure and are surrounded by rings or belts, why
-    the exterior planets should have a greater velocity of rotation, a
-    greater number of satellites, and a less density as compared with the
-    interior planets, the similarity of the elements in meteoric stones,
-    the sun, the stars, and those found upon our earth, and lastly why
-    there should be an increase in temperature as we descend into our
-    earth.<a id="FNanchor_93" href="#Footnote_93" class="fnanchor">[93]</a> This increase in temperature as we descend into the earth
-    <span class="pagenum" id="Page_235">235</span>
-    as deduced from many observations appears to be about 1° F. for every
-    fifty or sixty feet of descent.</p>
-
-  <p>To explain this and other kindred phenomena it is assumed that the
-    earth was once very much hotter than it is at present, and to reach its
-    present stage it has been gradually cooling. As the laws of cooling are
-    perfectly known, to calculate how many years it must have taken a body
-    like our earth to cool down to its present temperature is a definite
-    problem. Sir William Thomson, starting with the temperature of 7,000°
-    F., when all the rocks of the earth must have been molten and a skin or
-    crust upon the surface, such as is so quickly produced upon the surface
-    of molten lava, finds by calculation that the time taken to reach the
-    present temperature must have been about one hundred million years.
-    Into this period he and other physicists desire to compress the history
-    of all the stratified deposits. Geologists find this period too short.
-    Others seeking to reconcile the views of physicists and geologists
-    endeavour to show that the various agencies engaged in degrading rocks
-    and accumulating sediments in former ages are not to be judged of by
-    the agencies we now see around us; in former times they were more
-    active. At one period the elastic tides in the earth may have been so
-    great that they resulted in the fracturing off from our planet its
-    satellite the moon, and subsequently the moon, acting on the waters of
-    the earth, may, even as late as 150,000 years ago, have produced every
-    three hours tides 150 feet in height.</p>
-
-  <p>Whatever may be the value of the figures here quoted, reasonings like
-    these bring us to the conclusions that the various agencies which we
-    now know to be acting upon our earth were once far more potent than
-    they are at present, and if the moon, as a producer of elastic tides,
-    has any influence upon the occurrence of earthquakes,
-    <span class="pagenum" id="Page_236">236</span> it must have had
-    a much greater influence in bygone times.</p>
-
-  <p>We might speak similarly with regard to the internal heat of the earth.</p>
-
-  <p>From the present heat gradient of our globe it is possible to calculate
-    how much heat flows from the earth every year.</p>
-
-  <p>This is equivalent to a quantity which would raise a layer of water ·67
-    centimetres thick, covering the whole of our globe, from a temperature
-    of 0° to 100° C.</p>
-
-  <p>Similarly, we might calculate the quantity of heat which would be lost
-    when the average heat gradient, instead of being 1° F. for fifty feet
-    of descent, was 1° F. for twenty-five feet of descent.</p>
-
-  <p>We might also calculate how many years ago it was since such a gradient
-    existed.</p>
-
-  <p>The general result which we should arrive at would be that in past
-    ages the loss of heat was more rapid than it is at present. Now the
-    contraction of a body as it cools is for low temperatures proportional
-    to its loss of heat, and this law is also probably true for contraction
-    as it takes place from high temperatures.</p>
-
-  <p>Contraction being more rapid, it is probable that phenomena like
-    elevations and depressions would be more rapid than they are at
-    present, and generally all changes due to plutonic action, as has
-    already been pointed out by Sir William Thomson, must have been more
-    active.</p>
-
-  <p>We have, therefore, every reason to imagine that earthquakes which
-    belong to the category of phenomena here referred to were also numerous
-    and occurred on a grander scale during the earlier stages of the
-    world’s history than they do at present, and seismic and volcanic
-    energy, when considered in reference to long periods of time, is
-    probably a decreasing energy.</p>
-
-  <p><span class="pagenum" id="Page_237">237</span></p>
-
-  <p>In making this statement we must not overlook the fact that in
-    geological time, as testified by the records of our rocks, volcanic
-    action, and with it probably seismic action, has been continually
-    shifting, first appearing in one area and then in another, and even
-    in the same area we have evidence to show that these have periods
-    of activity and repose successively succeeding each other. Thus in
-    Britain, during the Palæozoic times, we have many evidences of an
-    intense volcanic activity. During the Mesozoic or Secondary period
-    volcanic energy appears to have subsided, to wake up with renewed
-    vigour in the Cainozoic or Tertiary period.</p>
-
-  <p>During this latter period it is not at all improbable that Scotland
-    was in past times as remarkable for its earthquakes as Japan is at the
-    present day.</p>
-
-  <p>Later on it will also be shown that earthquakes are concomitant
-    phenomena, with those elevatory processes which we have reason to
-    believe are slowly going on in certain portions of the earth’s crust.
-    If, therefore, we are able by the examination of the rocks which
-    constitute the accessible portions of our globe to determine which
-    periods were characterised by elevation, we may assume that such
-    periods were also periods of seismic activity.</p>
-
-  <p>Amongst these periods we have those in which various mountain ranges
-    appeared. Thus the Grampians, and the mountains of Scandinavia, were
-    probably produced before the deposition of the Old Red sandstone. The
-    Urals were upheaved prior to Permian times. The chief upheaval in the
-    Alps took place after Eocene times. The Rigi and other sub-Alpine
-    mountains were formed after the deposition of the Miocene beds. About
-    this same time the Himalayas were upheaved.<a id="FNanchor_94" href="#Footnote_94" class="fnanchor">[94]</a></p>
-
-  <p>The earthquakes which from time to time shake those
-    <span class="pagenum" id="Page_238">238</span>newer mountains
-    apparently indicate that the process of mountain-making is hardly ended.</p>
-
-  <p><em>Seismic energy in relation to historical time.</em>—The distribution
-    of seismic energy with regard to historical time is a subject which
-    has been very carefully examined by Mallet, who collected together
-    a catalogue of between six and seven thousand earthquakes, embraced
-    between the periods <span class="smcap">b.c.</span> 1606 and <span class="smcap">a.d.</span> 1850. The
-    earthquake of <span class="smcap">b.c.</span> 1606 was on the occasion of the delivery of
-    the law at Mount Sinai. Between <span class="smcap">b.c.</span> 1604 and <span class="smcap">b.c.</span>
-    1586 an earthquake probably occurred in Arabia, when Korah, Dathan,
-    and Abiram were swallowed up. Another biblical record is that of
-    <span class="smcap">b.c.</span> 1566, when the walls of Jericho were overthrown.</p>
-
-  <p>The earliest records from China is in <span class="smcap">b.c.</span> 595; in Japan
-    <span class="smcap">b.c.</span> 285; in India <span class="smcap">a.d.</span> 894.</p>
-
-  <p>By using the number of earthquakes which have been recorded in each
-    century as ordinates, Mallet constructed a curve, which apparently
-    shows a continual increase in seismic energy, especially during recent
-    times. This, Mallet remarks, contradicts all the analogies of the
-    physics of the globe, and points out that the rapid increase in the
-    number of earthquakes in latter years is chiefly due to the greater
-    number of records which have been made, and the increase of the area
-    of observation. No doubt many of the records made by the ancients have
-    been lost.</p>
-
-  <p>If we compare Mallet’s records, as he invites us to do, with the
-    great outlines of human progress, we see that the two increase
-    simultaneously, and we come to the conclusion that, taken as a whole,
-    during the historical period the seismic activity of the world has been
-    tolerably constant.</p>
-
-  <p>These conclusions, based on the evidence at our
-    <span class="pagenum" id="Page_239">239</span> command, are not to
-    be confuted. If, however, instead of considering the seismic energy of
-    the whole world, we consider the seismic energy of particular areas,
-    it seems reasonable to expect that in certain instances sometimes a
-    decrease and sometimes an increase in this energy might be discovered,
-    especially, perhaps, in areas which are highly volcanic.</p>
-
-  <p>In France we know that volcanic activity ceased at a period closely
-    bordering on historical times, and it is not unlikely that seismic
-    activity may have ceased at a corresponding time.</p>
-
-  <p>In a country like Japan, it is possible that in one district seismic
-    energy may be on the increase, whilst in another upon the decrease.</p>
-
-  <p>In a country like England, it is probable that the seismic state is
-    constant, and, whatever changes may be now occurring, they are taking
-    place at so slow a rate that, even if our records of the historical
-    period were complete, we could hardly be expected to find these changes
-    sufficiently marked to be observable.</p>
-
-  <p>For purposes of reference, and also for examining the present question,
-    the table, page 240, has been compiled. The earthquakes given are
-    chiefly those which have been recorded in histories as being more or
-    less destructive.</p>
-
-  <p>In the second column of this table will be seen the number of
-    earthquakes which have occurred in Japan during each century, the
-    centuries being marked in the first column. In columns 3 to 18
-    inclusive are given the number of earthquakes which have occurred
-    during different centuries in the various countries and districts
-    mentioned at the head of each column. These latter, which are taken
-    from the writings of Mallet, are given for the sake of comparison with
-    the Japanese earthquakes. If we commence with the seventh century in
-    the column for Japan, we see that a great increase in the number of earthquakes,
-    as we come towards the present time, is not so observable as it is in
-    the other columns.</p>
-
-  <p><span class="pagenum" id="Page_240">240</span></p>
-
-  <div class="left hang2">Key:</div>
-
-  <table class="ml5 mb2 lh1" summary="Earthquakes per country key">
-    <tr><td class="tdr"><div>1</div></td> <td class="tdr">Centuries</td></tr>
-    <tr><td class="tdr"><div>2</div></td> <td>Japan</td></tr>
-    <tr><td class="tdr"><div>3</div></td> <td>Scandinavia and Iceland</td></tr>
-    <tr><td class="tdr"><div>4</div></td> <td>British Isles and Northern Isles</td></tr>
-    <tr><td class="tdr"><div>5</div></td> <td>Spanish Peninsula</td></tr>
-    <tr><td class="tdr"><div>6</div></td> <td>France, Belgium, Holland</td></tr>
-    <tr><td class="tdr"><div>7</div></td> <td>Rhine Basin</td></tr>
-    <tr><td class="tdr"><div>8</div></td> <td>Switzerland and Rhine Basin</td></tr>
-    <tr><td class="tdr"><div>9</div></td> <td>Danube Basin</td></tr>
-    <tr><td class="tdr"><div>10</div></td> <td>Italy, Sicily, Sardinia, and Malta</td></tr>
-    <tr><td class="tdr"><div>11</div></td> <td>Supplemental table for Italy, Sardinia, and Malta</td></tr>
-    <tr><td class="tdr"><div>12</div></td> <td>Turco-Hellenic Territory, Syria, Ægean Isles, and Levant</td></tr>
-    <tr><td class="tdr"><div>13</div></td> <td>United States and Canada</td></tr>
-    <tr><td class="tdr"><div>14</div></td> <td>Mexico and Central America</td></tr>
-    <tr><td class="tdr"><div>15</div></td> <td>Antilles</td></tr>
-    <tr><td class="tdr"><div>16</div></td> <td>Cuba</td></tr>
-    <tr><td class="tdr"><div>17</div></td> <td>Chili and La Plata Basin</td></tr>
-    <tr><td class="tdr"><div>18</div></td> <td>Northern Zone of Asia</td></tr>
-    <tr><td class="tdr"><div>19</div></td> <td>Approximate Intensity in the Kioto District of Japan</td></tr>
-  </table>
-
-  <table class="collapse" summary="Earthquakes per country">
-    <tr class="bb">
-      <th class="ball">1</th>
-      <th class="ball">2</th>
-      <th class="ball">3</th>
-      <th class="ball">4</th>
-      <th class="ball">5</th>
-      <th class="ball">6</th>
-      <th class="ball">7</th>
-      <th class="ball">8</th>
-      <th class="ball">9</th>
-      <th class="ball">10</th>
-      <th class="ball">11</th>
-      <th class="ball">12</th>
-      <th class="ball">13</th>
-      <th class="ball">14</th>
-      <th class="ball">15</th>
-      <th class="ball">16</th>
-      <th class="ball">17</th>
-      <th class="ball">18</th>
-      <th class="ball">19</th>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>I.</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>II.</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>III.</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>IV.</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>6</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>23</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>V.</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>5</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>19</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>VI.</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>6</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td rowspan="10" class="tdr br"><div>19</div></td>
-      <td class="tdr br"><div>3</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>27</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>VII.</div></td>
-      <td class="tdr br"><div>12</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>15</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>VIII.</div></td>
-      <td class="tdr br"><div>11</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>2</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>12</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>17</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>IX.</div></td>
-      <td class="tdr br"><div>40</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>21</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>19</div></td>
-      <td class="tdr br"><div>6</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>7</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>60</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>X.</div></td>
-      <td class="tdr br"><div>17</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>2</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>2</div></td>
-      <td class="tdr br"><div>3</div></td>
-      <td class="tdr br"><div>3</div></td>
-      <td class="tdr br"><div>5</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>24</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XI.</div></td>
-      <td class="tdr br"><div>20</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>3</div></td>
-      <td class="tdr br"><div>16</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>9</div></td>
-      <td class="tdr br"><div>7</div></td>
-      <td class="tdr br"><div>5</div></td>
-      <td class="tdr br"><div>18</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>28</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XII.</div></td>
-      <td class="tdr br"><div>18</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>11</div></td>
-      <td class="tdr br"><div>4</div></td>
-      <td class="tdr br"><div>12</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>18</div></td>
-      <td class="tdr br"><div>22</div></td>
-      <td class="tdr br"><div>23</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>20</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XIII.</div></td>
-      <td class="tdr br"><div>16</div></td>
-      <td rowspan="5" class="tdr br"><div>28</div></td>
-      <td class="tdr br"><div>15</div></td>
-      <td class="tdr br"><div>3</div></td>
-      <td class="tdr br"><div>9</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>3</div></td>
-      <td class="tdr br"><div>15</div></td>
-      <td class="tdr br"><div>26</div></td>
-      <td class="tdr br"><div>13</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>16</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XIV.</div></td>
-      <td class="tdr br"><div>19</div></td>
-      <td class="tdr br"><div>4</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>21</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>18</div></td>
-      <td class="tdr br"><div>20</div></td>
-      <td class="tdr br"><div>51</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>25</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XV.</div></td>
-      <td class="tdr br"><div>36</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>4</div></td>
-      <td class="tdr br"><div>14</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>12</div></td>
-      <td class="tdr br"><div>18</div></td>
-      <td class="tdr br"><div>47</div></td>
-      <td class="tdr br"><div>11</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>29</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XVI.</div></td>
-      <td class="tdr br"><div>17</div></td>
-      <td class="tdr br"><div>8</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdr br"><div>61</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdr br"><div>52</div></td>
-      <td class="tdr br"><div>35</div></td>
-      <td class="tdr br"><div>32</div></td>
-      <td class="tdr br"><div>5</div></td>
-      <td class="tdr br"><div>22</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>6</div></td>
-      <td class="tdr br"><div>1</div></td>
-      <td class="tdr br"><div>4</div></td>
-      <td class="tdr br"><div>5</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>17</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XVII.</div></td>
-      <td class="tdr br"><div>26</div></td>
-      <td class="tdr br"><div>14</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdr br"><div>91</div></td>
-      <td class="tdr br"><div>29</div></td>
-      <td class="tdr br"><div>120</div></td>
-      <td class="tdr br"><div>31</div></td>
-      <td class="tdr br"><div>121</div></td>
-      <td class="tdr br"><div>9</div></td>
-      <td class="tdr br"><div>53</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdr br"><div>7</div></td>
-      <td class="tdr br"><div>16</div></td>
-      <td class="tdr br"><div>4</div></td>
-      <td class="tdr br"><div>9</div></td>
-      <td class="tdr br"><div>&mdash;</div></td>
-      <td class="tdr br"><div>11</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XVIII.</div></td>
-      <td class="tdr br"><div>31</div></td>
-      <td class="tdr br"><div>111</div></td>
-      <td class="tdr br"><div>63</div></td>
-      <td class="tdr br"><div>93</div></td>
-      <td class="tdr br"><div>237</div></td>
-      <td class="tdr br"><div>71</div></td>
-      <td class="tdr br"><div>141</div></td>
-      <td class="tdr br"><div>88</div></td>
-      <td class="tdr br"><div>438</div></td>
-      <td class="tdr br"><div>20</div></td>
-      <td class="tdr br"><div>124</div></td>
-      <td class="tdr br"><div>88</div></td>
-      <td class="tdr br"><div>24</div></td>
-      <td class="tdr br"><div>85</div></td>
-      <td class="tdr br"><div>2</div></td>
-      <td class="tdr br"><div>10</div></td>
-      <td class="tdr br"><div>32</div></td>
-      <td class="tdr br"><div>8</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdr bl br"><div>XIX.</div></td>
-      <td class="tdr br"><div>27</div></td>
-      <td class="tdr br"><div>113</div></td>
-      <td class="tdr br"><div>110</div></td>
-      <td class="tdr br"><div>85</div></td>
-      <td class="tdr br"><div>211</div></td>
-      <td class="tdr br"><div>81</div></td>
-      <td class="tdr br"><div>173</div></td>
-      <td class="tdr br"><div>145</div></td>
-      <td class="tdr br"><div>390</div></td>
-      <td class="tdr br"><div>88</div></td>
-      <td class="tdr br"><div>194</div></td>
-      <td class="tdr br"><div>51</div></td>
-      <td class="tdr br"><div>30</div></td>
-      <td class="tdr br"><div>145</div></td>
-      <td class="tdr br"><div>50</div></td>
-      <td class="tdr br"><div>170</div></td>
-      <td class="tdr br"><div>57</div></td>
-      <td class="tdr br"><div>8</div></td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_241">241</span></p>
-
-  <p>The explanation for this probably lies in the fact that Japan has
-    practised civilised arts for a longer period than many of the European
-    and other countries mentioned in the table.</p>
-
-  <p>In Japan, no doubt, the records of later years have been more perfect
-    than they were in early times, but this, although so potent an effacer
-    of what was probably the true state of natural phenomena in the case of
-    Europe, has not quite obliterated the truth in Japan; for instead of an
-    apparent increase of seismic energy since early times it shows a slight
-    decrease.</p>
-
-  <p>To draw up a table of earthquakes such as the one which has just been
-    given, and then, after the inspection of it, draw conclusions as to
-    whether there has been an increase or decrease in seismic energy, is,
-    however, hardly a just method of reasoning. The earthquakes, taken as
-    they are, for the whole of Japan, represent a collection of places some
-    of which are 1,000 miles apart. When we consider that many earthquakes
-    which occurred at one end of this line were never felt at the other
-    end, in order to justly estimate the periodicity of seismic phenomena
-    it would seem that we ought either to take some particular seismic area
-    or else the whole world.</p>
-
-  <p>The particular area which has been taken is that of Kioto in Central
-    Japan, and the earthquakes which have been felt there are enumerated in
-    the table.</p>
-
-  <p>In order to show the variation in seismic activity of this district a
-    curve has been plotted, fig. 35, with ordinates equal to the values
-    given for the Kioto earthquakes during succeeding centuries. The upper
-    points of these ascending and descending lines are
-    <span class="pagenum" id="Page_242">242</span> joined by a free
-    curve. The lower points are similarly joined. The points of bisection
-    of ordinates drawn between these two curves are taken as points in a
-    curve to show the true secular change in seismic energy.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p242.jpg" width="700" height="209" alt="" />
-    <div class="caption"><span class="smcap">Fig. 35.</span>—Curve of Seismic Intensity for Kioto.</div>
-  </div>
-
-  <p>By looking at this wavy line it will be seen that the intervals between
-    maxima and minima are closer together in early times than they are
-    later on.</p>
-
-  <p>Thus, between the eighth century and the ninth century, points of
-    maximum and minimum seismic efforts occurred at times a century apart,
-    whilst later on, from the eleventh to the fifteenth century, they were
-    at intervals of 300 years apart.</p>
-
-  <p>By inspecting either the wavy line or the resultant curve, it will be
-    seen that since the ninth century down to the present time there has
-    been a decided decrease in seismic energy. From the ninth century down
-    to the fifteenth century this decrease is represented by a
-    <span class="pagenum" id="Page_243">243</span> regular
-    curve. At this point, however, the decrease becomes slightly more
-    rapid, and is represented by a second curve. If, instead of calculating
-    ordinates for my curve, in which intensity has been considered, simply
-    the number of earthquakes are counted, a similar result is obtained.
-    From this it appears that the rate at which seismic energy decreased
-    during the last 500 years was about the same as that at which it
-    decreased during the 500 years previous to this period.</p>
-
-  <p>If the lists for the Italian and Turco-Hellenic districts could be
-    similarly analysed, and the earthquakes of any particular district
-    picked out from the others, it is very probable that a similar decrease
-    or alteration in seismic energy might be observed.</p>
-
-  <p>Provided that we have at our disposal records of the various
-    earthquakes which have occurred in any given district during a
-    sufficiently long period of time, one conclusion that we may expect
-    to arrive at is that we shall be able to trace some variation in
-    the seismic activity of that district. For the Kioto area, it has
-    been shown that there is a diminution in seismic activity, In other
-    districts, however, there may possibly be an increase.<a id="FNanchor_95" href="#Footnote_95" class="fnanchor">[95]</a></p>
-
-  <p><em>Relative frequency of earthquakes.</em>—A question which is of great
-    interest to those who dwell in shaken districts is as to how often
-    disturbances may be expected to occur.</p>
-
-  <p>From a general examination of this question, considering the
-    earthquakes of the whole world. Mallet arrived at the following
-    conclusions:—</p>
-
-  <p><span class="pagenum" id="Page_244">244</span></p>
-
-  <p>1. While the smallest or minimum paroxysmal intervals may be a year
-    or two, the average interval is from five to ten years of comparative
-    repose.</p>
-
-  <p>2. The shorter intervals are in connection with periods of fewer
-    earthquakes—not always with those of least intensity, but usually so.</p>
-
-  <p>3. The alternations of paroxysm and of repose appear to follow no
-    absolute law deducible from these curves.</p>
-
-  <p>4. Two marked periods of extreme paroxysm are observable in each
-    century, one greater than the other—that of greatest number and
-    intensity occurring about the middle of each century, the other towards
-    the end of each.</p>
-
-  <p>The form of the curves which Mallet has drawn seem to indicate that
-    seismic energy came in sudden bursts, and then subsided, gradually
-    gathering strength for another exhibition. This is continually seen
-    in the shocks experienced in various seismic areas—a large shock, or
-    the maximum of the activity dying out by repeated small shocks on
-    succeeding days.</p>
-
-  <p>Mr. I. Hattori, writing on the large earthquakes of Japan, remarks that
-    on the average there has been one large earthquake every ten years.
-    They, however, occur in groups, as shown in the following table.</p>
-
-  <table class="collapse" summary="Earthquake intervals">
-    <tr>
-      <th class="ball">No. of shocks</th>
-      <th colspan="2" class="ball">Period</th>
-      <th colspan="2" class="ball">Interval</th>
-    </tr>
-    <tr>
-      <td class="bl br">6</td>
-      <td class="pr0">A.D.</td>
-      <td class="br">827–836</td>
-      <td class="tdr pr0"><div>10</div></td>
-      <td class="br">years</td>
-    </tr>
-    <tr>
-      <td class="bl br">6</td>
-      <td class="pr0">„</td>
-      <td class="br">880–890</td>
-      <td class="tdr pr0">10<div>10</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr>
-      <td class="bl br">4</td>
-      <td class="pr0">„</td>
-      <td class="br">1040–1043</td>
-      <td class="tdr pr0"><div>4</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr>
-      <td class="bl br">5</td>
-      <td class="pr0">„</td>
-      <td class="br">1493–1507</td>
-      <td class="tdr pr0"><div>5</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr>
-      <td class="bl br">4</td>
-      <td class="pr0">„</td>
-      <td class="br">1510–1513</td>
-      <td class="tdr pr0"><div>4</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr>
-      <td class="bl br">5</td>
-      <td class="pr0">„</td>
-      <td class="br">1645–1650</td>
-      <td class="tdr pr0"><div>6</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr>
-      <td class="bl br">5</td>
-      <td class="pr0">„</td>
-      <td class="br">1662–1664</td>
-      <td class="tdr pr0"><div>3</div></td>
-      <td class="br">„</td>
-    </tr>
-    <tr class="bb">
-      <td class="bl br">4</td>
-      <td class="pr0">„</td>
-      <td class="br">1853–1856</td>
-      <td class="tdr pr0"><div>4</div></td>
-      <td class="br">„</td>
-    </tr>
-  </table>
-
-  <p>Dr. E. Naumann, who has also written on the earthquakes
-    <span class="pagenum" id="Page_245">245</span>quakes in Japan,
-    remarks that if periods of seismic activity do not occur every 490
-    years, there is a repetition of the cycle after 980 years, but there
-    is much variability. A period of 68 years is very marked. On the
-    average, large earthquakes have occurred every 5·9 years. Fuchs
-    gives some interesting examples of the repetition of earthquakes at
-    definite intervals, of which the following are examples. Sometimes
-    earthquakes appear to have repeated themselves after 100 years. One
-    remarkable example of this is that of Lima, on June 17, 1578, which was
-    repeated on the same day in the year 1678. In Copiapo it is believed
-    that earthquakes occur every twenty-three years, and examples of such
-    repetitions are found in the years 1773, 1796, and 1819. In Canada,
-    near to Quebec, earthquakes lasting forty days are said to occur every
-    twenty-five years. The plateau of Ardebil is said to be regularly
-    shaken by earthquakes every two years.</p>
-
-  <p>A. Caldcleugh, writing on the earthquake of Chili, in 1835,<a id="FNanchor_96" href="#Footnote_96" class="fnanchor">[96]</a> remarks
-    that the Spaniards first had the idea that a great earthquake occurs
-    every century. Afterwards they thought the period was every fifty
-    years. As a matter of fact, however, there were large earthquakes in
-    1812, at Caracas; in 1818, at Copiapo; in 1822, at Santiago; in 1827,
-    at Bogota; in 1828, at Lima; in 1829, at Santiago; and in 1832, at
-    Huasco.</p>
-
-  <p>The average period of seismic disturbances in any country probably
-    depends upon the subterranean volcanic activity of that country.
-    When the activity is great the large earthquakes may occur at short
-    intervals; but when the activity is small, as in England, shocks of
-    moderate intensity may not be felt more than once or twice per century.
-    A general idea of the relative frequency of the
-    <span class="pagenum" id="Page_246">246</span>large earthquakes in
-    various parts of the world may be easily obtained by an inspection of
-    the table on page 240.</p>
-
-  <p>Between the years 1850 and 1857 Kluge found that in the world there had
-    been 4,620 earthquakes, which is, upon the average, nearly two per day.
-    This estimate of the frequency of earthquakes of sufficient intensity
-    to be recorded without the aid of instruments is, however, much below
-    the truth. In Japan alone there probably occurs, as a daily average, a
-    number at least equal to that which has been just given for the whole
-    world. Boussingault considered that, in the Andes, earthquakes were
-    occurring every instant of time.<a id="FNanchor_97" href="#Footnote_97" class="fnanchor">[97]</a></p>
-
-  <p>To state definitely how many earthquakes are felt in the world on the
-    average every day is, from the data which we have at our command, an
-    impossibility. Perhaps there may be ten, perhaps there may be 100. The
-    question is one which remains to be decided by statistics which have
-    yet to be compiled.</p>
-
-  <p>After a large earthquake, smaller shocks usually occur at short
-    intervals. At first the succession of disturbances are separated from
-    each other by perhaps only a few minutes or hours. Later on, the
-    intensity of these shocks usually decreases, and the intervals between
-    them become greater and greater, until, finally, after perhaps a few
-    months, the seismic activity of the area assumes a quiescent state.</p>
-
-  <p>The great earthquake which overtook Concepcion on February 20, 1835,
-    was followed by a succession of shocks like those just referred to,
-    there being registered, between the date of the destructive shock and
-    March 4, 300 smaller disturbances.</p>
-
-  <p>During the twenty-four hours succeeding the destruction of Lima
-    (October 28, 1746), 200 shocks were <span class="pagenum" id="Page_247">247</span>
-    counted, and up to the 24th of
-    February in the following year 451 shocks were felt.</p>
-
-  <p>At St. Thomas, in 1868, 283 shocks were counted in nine and a quarter
-    hours.</p>
-
-  <p>Similar examples might be taken from the description of almost
-    all destructive earthquakes of which we have records. For a large
-    earthquake to occur, and not to be accompanied by a train of succeeding
-    earthquakes, is exceptional. Sometimes we find that a large number of
-    small earthquakes have occurred without a large one being felt. Seismic
-    storms of this description have happened, even in England—for instance,
-    in the year 1750, which appears to have been a year of earthquakes for
-    many portions of the globe.</p>
-
-  <p>In this year, which is known as the ‘earthquake year,’ shocks were felt
-    in England as follows: On March 14, in Surrey; March 18, in south-west
-    of England; April 2, at Chester; June 7, at Norwich; August 23, in
-    Lincolnshire; September 30, Northamptonshire.</p>
-
-  <p><em>Synchronism of earthquakes.</em>—One of the first writers who drew
-    attention to the fact that two shocks of earthquakes have been felt
-    simultaneously at distant places was David Milne, who published a list
-    of these occurrences.<a id="FNanchor_98" href="#Footnote_98" class="fnanchor">[98]</a></p>
-
-  <p>In two instances, February and March 1750, shocks were simultaneously
-    felt in England and Italy. In September 1833 shocks appear to have been
-    simultaneously felt in England and Peru. These and many other similar
-    examples are discussed by Mallet, who thinks with Milne that these
-    coincidences are in every probability matters of accident. According to
-    Fuchs, Calabria and Sicily appear often to have had earthquakes at the
-    same time, as for instance in 1169, 1535, 1638,
-    <span class="pagenum" id="Page_248">248</span>when the town Euphemia
-    sank, and in the years 1770, 1776, 1780, and 1783.</p>
-
-  <p>A remarkable example of coincidence occurred on November 16, 1827, when
-    a terrible earthquake was felt in Columbia, and at the same time a
-    shock occurred on the Ochotsk plains, nearly antipodal to each other.</p>
-
-  <p>Kluge also gives a large number of instances of simultaneous
-    earthquakes; thus, on January 23, 1855, on the same day that
-    Wellington, New Zealand, so severely suffered, there was a heavy
-    earthquake in the Siebengeberge, and also in North America. To this
-    might be added the fact that the last destructive earthquake in Japan
-    occurred within a few days of this time.</p>
-
-  <p>Sometimes neighbouring countries where earthquakes are common are
-    equally remarkable by their utter want of synchronism. For example,
-    Southern Italy and Syria are said never to be shaken simultaneously.</p>
-
-  <p><em>Secondary earthquakes.</em>—Although it is possible that the simultaneous
-    occurrence of earthquakes in distant regions may sometimes be a
-    matter of chance, it must also be remarked that the shaking produced
-    by one earthquake may be sufficient to cause ground which is in a
-    critical state to give way, and thus the first disturbance becomes the
-    originator of a second earthquake. Admitting that an earthquake, as
-    it radiates from its centre, may act in such a manner, we see that a
-    feeble disturbance might be the ultimate cause in the production of a
-    destructive earthquake, just as the disturbance of a stone upon the
-    face of a scarp might, by its impact upon other stones, cause many tons
-    of material to be dislodged.</p>
-
-  <p>It is also easy to conceive how the seismic activity of two districts
-    may be dependent upon each other. Inasmuch as these secondary shocks
-    are direct effects of<span class="pagenum" id="Page_249">249</span>
-    primary disturbances, they might have been treated in a previous chapter.</p>
-
-  <p>As examples of consequent or secondary earthquakes Fuchs tells us that
-    when small earthquakes take place in Constantinople and Asia Minor,
-    earthquakes are felt in Bukharest, Galazy, and Kronstadt.</p>
-
-  <p>The great Lisbon earthquake also appears to have given rise to several
-    consequent disturbances. One was in Derbyshire, occurring at 11 a.m.
-    It was sufficiently violent to cause plaster to fall from the sides
-    of a room and a chasm to open on the surface of the ground. Some
-    miners working underground were so alarmed that they endeavoured to
-    escape to the surface. During twenty minutes there were three distinct
-    disturbances.</p>
-
-  <p>Another shock was felt at Cork.<a id="FNanchor_99" href="#Footnote_99" class="fnanchor">[99]</a></p>
-
-  <p>Although these disturbances own a consequence of the Lisbon earthquake
-    they might properly perhaps be attributed to the pulsations produced by
-    the shock at Lisbon, which spread through England and other countries
-    without being felt.</p>
-
-  <p>The shocks which men felt in New Zealand and New South Wales in 1868
-    were probably secondary shocks, due to the disturbance at Arequipa and
-    other places on the South American coast.</p>
-
-  <p>These so-called secondary earthquakes, although in many instances
-    they may be due to earth pulsations produced by earthquake, or to the
-    immediate sensible shaking of a large earthquake, may perhaps, in
-    certain instances, be attributed to some widespread disturbance beneath
-    the crust of the earth. The occurrence of periods where all earthquake
-    countries suffer, unusual disturbances indicate the probability of such
-    underground phenomena.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XIV">
-    <span class="pagenum" id="Page_250">250</span>
-    <h2>CHAPTER XIV.<br />
-      <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    The occurrence of earthquakes in relation to the position of the
-    heavenly bodies—Earthquakes and the moon—Earthquakes and the sun;
-    and the seasons; the months—Planets and meteors—Hours at which
-    earthquakes are frequent—Earthquakes and sun spots—Earthquakes
-    and the aurora.</div>
-
-  <p><em>The position of the heavenly bodies and the occurrence of
-      earthquakes.</em>—Since the earliest times, in searching for the cause of
-    various natural phenomena, man has turned his energies towards the
-    heavens. One of the earliest observations was the connection that
-    exists between the season of the year and the motions of the heavenly
-    bodies. Tides were seen to be influenced by the moon. In later times
-    it has been discovered that periods of maximum magnetic disturbances
-    occur every ten or eleven years with the sun spots, and Herr Kreil, of
-    Vienna, tells us our satellite, the moon, has also an influence upon
-    the magnet.</p>
-
-  <p>From day to day we see the bond connecting our planet with the sun, the
-    moon, and other heavenly bodies which are outside us gradually becoming
-    closer.</p>
-
-  <p>Inasmuch as many phenomena, like the motion of the tides, the rise and
-    fall of the barometer, fluctuations in temperature, are all more or
-    less directly connected with the relative position of our planet with
-    regard to the sun<span class="pagenum" id="Page_251">251</span>
-    and moon, any coincidence between the phases of
-    these bodies and the occurrence of earthquakes more or less involves a
-    time relationship with the other phenomena resultant on lunar and solar
-    influences.</p>
-
-  <p><em>Earthquakes and the position of the moon.</em>—Many earthquake
-    investigators have attempted to show the connection between earthquakes
-    and the phases of the moon.</p>
-
-  <p>The first and most successful worker in this branch of seismology was
-    Professor Alexis Perrey, of Dijon, who, after many years of arduous
-    labour in tabulating and examining catalogues of earthquakes, showed
-    that earthquakes were more likely to occur at the following periods
-    than at others.</p>
-
-  <p>1. They are more frequent at new or full moon (syzygies) than at half
-    moon (quadratures).</p>
-
-  <p>2. They are more frequent when the moon is nearest the earth (perigee)
-    than when she is farthest off (apogee).</p>
-
-  <p>3. They are more frequent when the moon is on the meridian than when
-    she is on the horizon.</p>
-
-  <p>These results were obtained by Perrey after analysing his catalogues
-    by three different and independent methods, and they were confirmed
-    by the report of a committee appointed by the Academy of Sciences. It
-    must, however, be remarked that in several instances anomalies occur,
-    and also that the difference between the number of earthquakes at any
-    two periods is not a very large one. Thus, for instance, the annual
-    catalogues compiled by Perrey from 1844 to 1847, the earthquakes in
-    perigee are to those in apogee as 47 : 39. Between the years 1843 and
-    1872 Perrey finds that 3,290 shocks occurred at the moon’s perigee, and
-    3,015 at the apogee.<a id="FNanchor_100" href="#Footnote_100" class="fnanchor">[100]</a></p>
-
-  <p><span class="pagenum" id="Page_252">252</span></p>
-
-  <p>Between 1761 and 1800 earthquakes occurred as follows:—</p>
-
-  <table summary="Earthquakes per distance of moon">
-    <tr>
-      <td class="tdl">In Perigee</td>
-      <td>526</td>
-    </tr>
-    <tr>
-      <td class="tdl">Apogee</td>
-      <td>465</td>
-    </tr>
-  </table>
-
-  <p>The following table shows the results which enabled Perrey to deduce
-    his first law.</p>
-
-  <p>Dividing the period of lunation into quarters, with the time of
-    syzygies and quadratures as the centres of these quarters, he found
-    that the earthquakes were distributed as follows.</p>
-
-  <table class="collapse" summary="Earthquakes per phase of moon">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th class="ball">Totals</th>
-      <th class="ball">Syzygies</th>
-      <th class="ball">Quadratures</th>
-      <th class="ball">Difference<br />in favour of<br />the Syzygies</th>
-    </tr>
-    <tr>
-      <td class="bl br">1843–1847</td>
-      <td class="tdr br"><div>1,604</div></td>
-      <td class="tdr br"><div>850·48</div></td>
-      <td class="tdr br"><div>753·52</div></td>
-      <td class="tdr br"><div>69·96</div></td>
-    </tr>
-    <tr>
-      <td class="bl br">1848–1852</td>
-      <td class="tdr br"><div>2,049</div></td>
-      <td class="tdr br"><div>1,053·53</div></td>
-      <td class="tdr br"><div>995·47</div></td>
-      <td class="tdr br"><div>58·06</div></td>
-    </tr>
-    <tr>
-      <td class="bl br">1853–1857</td>
-      <td class="tdr br"><div>3,018</div></td>
-      <td class="tdr br"><div>1,534·13</div></td>
-      <td class="tdr br"><div>1,483·87</div></td>
-      <td class="tdr br"><div>50·26</div></td>
-    </tr>
-    <tr>
-      <td class="bl br">1858–1862</td>
-      <td class="tdr br"><div>3,140</div></td>
-      <td class="tdr br"><div>1,602·99 </div></td>
-      <td class="tdr br"><div>1,537·41</div></td>
-      <td class="tdr br"><div>65·98</div></td>
-    </tr>
-    <tr>
-      <td class="bl br">1863–1867</td>
-      <td class="tdr br"><div>2,845</div></td>
-      <td class="tdr br"><div>1,463·42</div></td>
-      <td class="tdr br"><div>1,381·58</div></td>
-      <td class="tdr br"><div>81·84</div></td>
-    </tr>
-    <tr>
-      <td class="bl br">1868–1872</td>
-      <td class="tdr br"><div>4,593</div></td>
-      <td class="tdr br"><div>2,333·48</div></td>
-      <td class="tdr br"><div>2,259·52</div></td>
-      <td class="tdr br"><div>73·96</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="bl br">1843–1872</td>
-      <td class="tdr br"><div>17,249</div></td>
-      <td class="tdr br"><div>8,838·03</div></td>
-      <td class="tdr br"><div>8,410·97</div></td>
-      <td class="tdr br"><div>427·06</div></td>
-    </tr>
-  </table>
-
-  <p>The reported earthquakes between 1751 and 1843 are shown to conform
-    with the same rule.<a id="FNanchor_101" href="#Footnote_101" class="fnanchor">[101]</a> Julius Schmidt, astronomer at Athens, found
-    for the earthquakes of Eastern Europe and adjacent countries for the
-    years 1776 to 1873 that there were more earthquakes when the moon was
-    in perigee. Other maxima were at new moon, and two days after the first
-    quarter. There was a diminution at full moon, and a minimum on the day
-    of the last quarter. As one example of results which are antagonistic
-    to the general results obtained by Perrey may be quoted the results of
-    an examination by Professor W. S. Chaplin of the earthquake recorded at
-    the meteorological observatory in Tokio. The list of earthquakes, 143
-    in number, extending over a period of three years, was recorded by one
-    of Palmieri’s instruments. The results were as follows:—</p>
-
-  <p><span class="pagenum" id="Page_253">253</span></p>
-
-  <p>1. There have been maxima of earthquakes when the moon was two and
-    nine hours east and seven hours west. At the upper transit there is a
-    minimum.</p>
-
-  <p>2. Considering the moon’s position with regard to the sun, at
-    conjunction there were 32, at opposition 37, and at quadrature 74. East
-    of the meridian the maximum was at least four hours.</p>
-
-  <p>3. When the moon was north of the equator these were 68, when south 82.</p>
-
-  <p>4. A maximum of earthquakes seven and eleven days after the moon’s
-    perigee. The fact that these results were obtained for the earthquakes
-    of a special small seismic area renders them more interesting.<a id="FNanchor_102" href="#Footnote_102" class="fnanchor">[102]</a></p>
-
-  <p><em>Frequency of earthquakes in relation to the position of the sun.</em>—The
-    question as to whether there is a connection between the frequency of
-    earthquakes and the relative position of the sun is to a great extent
-    identical with the question as to the relative frequency of earthquakes
-    in the various seasons. It is a subject which we find referred to
-    by writers in the earliest ages. Pliny and Aristotle thought that
-    earthquakes occurred chiefly in spring and autumn. In later times it
-    has been a subject which has been most carefully considered by Merian,
-    von Hoff, Perrey, Mallet, Volger, Kluge, and others who have devoted
-    attention to seismology. In a résumé of the earthquakes of Europe, and
-    of the adjacent parts of Asia and Africa, from <span class="smcap">a.d.</span> 306–1843,
-    Mallet gives the following results:—</p>
-
-  <table class="collapse" summary="Earthquakes in relation to sun">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th colspan="4" class="ball">For Nineteenth Century</th>
-      <th colspan="4" class="ball">For the whole period</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Winter Solstice</td>
-      <td class="">177</td>
-      <td rowspan="3" class="pl0 pr0"><span class="x300a">}</span></td>
-      <td colspan="2" class="tdl br">Solstices</td>
-      <td class="">253</td>
-      <td rowspan="3" class="pl0 pr0"><span class="x300a">}</span></td>
-      <td colspan="2" class="tdl br">Solstices</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Spring Equinox</td>
-      <td class="">151</td>
-      <td rowspan="3" class="pl0 pr0 bb"><span class="x300a">}</span></td>
-      <td class="br">306</td>
-      <td class="">170</td>
-      <td rowspan="3" class="pl0 pr0 bb"><span class="x300a">}</span></td>
-      <td class="br">403</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Summer Solstice</td>
-      <td class="">129</td>
-      <td class="br">Equinoxes</td>
-      <td class="">150</td>
-      <td class="br">Equinoxes</td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl br">Autumnal Equinox</td>
-      <td class="">164</td>
-      <td class="">&nbsp;</td>
-      <td class="br">315</td>
-      <td class="">159</td>
-      <td class="">&nbsp;</td>
-      <td class="br">329</td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_254">254</span></p>
-
-  <p>The above periods were called by Perrey <em>critical epochs</em>, because
-    as a general result of his researches he found that at such periods
-    there was a greater frequency of earthquakes. Fuchs, quoting from
-    Kluge’s tables, extending from 1850–1857, tells us that the recorded
-    earthquakes occurred as follows:—</p>
-
-  <p>
-    In the Northern Hemisphere—
-  </p>
-  <table summary="Earthquakes in relation to sun north">
-    <tr>
-      <td class="tdl">Equinoxes</td>
-      <td><b>1324</b></td>
-    </tr>
-    <tr>
-      <td class="tdl">Solstices</td>
-      <td>1202</td>
-    </tr>
-  </table>
-
-  <p>
-    In the Southern Hemisphere—
-  </p>
-  <table summary="Earthquakes in relation to sun south">
-    <tr>
-      <td class="tdl">Equinoxes</td>
-      <td><b>301</b></td>
-    </tr>
-    <tr>
-      <td class="tdl">Solstices</td>
-      <td>261</td>
-    </tr>
-  </table>
-
-  <p>Earthquakes are, therefore, more frequent at the equinoxes, and this
-    especially at the autumnal equinox. In the northern hemisphere, at
-    the solstices, the greater number of shocks occur about the winter
-    solstices, whilst in the southern hemisphere, about the summer
-    solstices.</p>
-
-  <p>Exceptions, however, are found in Central America and the West Indies,
-    in the Caucasus, and the Ægean Sea.</p>
-
-  <p>The idea that earthquakes had a periodicity dependent upon the position
-    of the heavenly bodies is by no means confined to Europe. In a Japanese
-    work called ‘Jishin Setsu’ (an opinion about earthquakes) by a priest
-    called Tensho, it is stated that the relative positions and movements
-    of the twenty-eight constellations with respect to the moon cause
-    earthquakes. This Tensho asserts after careful calculation, and Falb
-    tells us that all future earthquakes can be predicted.</p>
-
-  <p>In the Kuriles and Kamschatka, Sicily, and in parts of South America,
-    it is said that the equinoxes are regarded as dangerous seasons.</p>
-
-  <p><em>Frequency of earthquakes in relation to the seasons and months.</em>—What
-    is here said respecting the relative<span class="pagenum" id="Page_255">255</span>
-    frequency of earthquakes at the
-    different seasons and months is little more than an extension and
-    critical examination of the results which have been given respecting
-    the frequency of earthquakes in regard to the position of the sun.</p>
-
-  <p>That there is a difference between the number of earthquakes which are
-    felt at one season of the year as compared with those felt at another
-    is a fact which, as seismoscopic observations are extended, is becoming
-    more and more recognised.</p>
-
-  <p>Some of the more important results which were arrived at by Mallet from
-    5,879 observations made in the northern hemisphere, and 223 in the
-    southern hemisphere, may be expressed as follows:—</p>
-
-  <table class="collapse" summary="Earthquakes in relation to sun, hemispheres">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th class="ball">Maxima</th>
-      <th class="ball">Minima</th>
-    </tr>
-    <tr>
-      <td class="tdl vatop bl br">Northern Hemisphere</td>
-      <td class="tdl vatop br">January, also a slight rise in August and October</td>
-      <td class="tdl vatop br">May, June, and July</td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl vatop bl br">Southern Hemisphere</td>
-      <td class="tdl vatop br">November, also May and June</td>
-      <td class="tdl vatop br">March, extending over one month, also August</td>
-    </tr>
-  </table>
-
-  <p>Julius Schmidt, of Athens, who so carefully examined the earthquakes of
-    eastern Europe, came to the following conclusions:—</p>
-
-  <p>For the earthquakes between 1200 and 1873, a maximum on September 26
-    and January 17; a minimum on December 3 and June 13.</p>
-
-  <p>For the earthquakes between 1873 and 1874, a maximum on March 1 and
-    October 1; a minimum on July 7 and December 15.</p>
-
-  <p>For all the earthquakes of eastern Europe, a maximum on January 3; a
-    minimum on July 8, or there was a maximum at perihelion and aphelion.</p>
-
-  <p><span class="pagenum" id="Page_256">256</span></p>
-
-  <p>When the months are grouped together according to the seasons, spring,
-    summer, autumn, and winter, we find that in the northern hemisphere the
-    minimum is in summer and the maximum in winter, whilst in the southern
-    hemisphere (giving the proper months corresponding to its seasons) we
-    find two maxima, one at the commencement of winter, and the other at
-    midsummer, whilst the minima are in spring and autumn.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p256.jpg" width="541" height="454" alt="" />
-    <div class="caption"><span class="smcap">Fig. 36.</span>—Curves of Monthly Seismic Intensity
-      (Mallet).</div>
-  </div>
-
-  <p>In the following table the difference in the number of earthquakes felt
-    at different seasons is given more in detail.</p>
-
-  <p>In examining this table, we must remember that for countries like Peru,
-    Chili, and New Zealand, lying in the southern hemisphere, the records
-    given for the months April to September correspond to the winter months
-    of<span class="pagenum" id="Page_257">257</span> those countries.
-    The Roman numerals indicate the centuries between which the records date.</p>
-
-  <table class="collapse" summary="Earthquakes in relation to season">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th colspan="2" class="ball">&nbsp;</th>
-      <th class="ball">October to March</th>
-      <th class="ball">April to September</th>
-    </tr>
-    <tr class="bb">
-      <td rowspan="14" class="bl br">Northern Regions</td>
-      <td class="tdrt"><div>1.</div></td>
-      <td class="tdl br">Scandinavia and Iceland, xii–xix</td>
-      <td class="tdrb br"><div><b>129</b></div></td>
-      <td class="tdrb br"><div>91</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>2.</div></td>
-      <td class="tdl br">British and Northern Isles, xi–xix</td>
-      <td class="tdrb br"><div><b>123</b></div></td>
-      <td class="tdrb br"><div>94</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>3.</div></td>
-      <td class="tdl br">Belgium, France, and Holland, iv–xix</td>
-      <td class="tdrb br"><div><b>395</b></div></td>
-      <td class="tdrb br"><div>272</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>4.</div></td>
-      <td class="tdl br">Rhone Basin, xvi–xix</td>
-      <td class="tdrb br"><div><b>115</b></div></td>
-      <td class="tdrb br"><div>69</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>5.</div></td>
-      <td class="tdl br">Switzerland and Rhine Basin, ix–xix</td>
-      <td class="tdrb br"><div><b>327</b></div></td>
-      <td class="tdrb br"><div>205</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>6.</div></td>
-      <td class="tdl br">Danube Basin, v–xix</td>
-      <td class="tdrb br"><div><b>147</b></div></td>
-      <td class="tdrb br"><div>128</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>7.</div></td>
-      <td class="tdl br">Spanish Peninsula, xi–xiv</td>
-      <td class="tdrb br"><div><b>114</b></div></td>
-      <td class="tdrb br"><div>87</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>8.</div></td>
-      <td class="tdl br">Italy, Sicily, Sardinia, and Malta, iv–xix</td>
-      <td class="tdrb br"><div><b>650</b></div></td>
-      <td class="tdrb br"><div>581</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>9.</div></td>
-      <td class="tdl br">Turco-Hellenic Territory, Syria, Ægean Isles, and Levant, iv–xix</td>
-      <td class="tdrb br"><div>214</div></td>
-      <td class="tdrb br"><div><b>222</b></div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>10.</div></td>
-      <td class="tdl br">Northern Zone of Asia, xviii–xix</td>
-      <td class="tdrb br"><div><b>46</b></div></td>
-      <td class="tdrb br"><div>36</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>11.</div></td>
-      <td class="tdl br">Japan (Tokio area), 1872–1880 (small earthquakes)</td>
-      <td class="tdrb br"><div><b>213</b></div></td>
-      <td class="tdrb br"><div>157</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>12.</div></td>
-      <td class="tdl br">Japan <span class="smcap">b.c.</span> 295-<span class="smcap">a.d.</span> 1872 (large earthquakes)</td>
-      <td class="tdrb br"><div>165</div></td>
-      <td class="tdrb br"><div><b>188</b></div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>13.</div></td>
-      <td class="tdl br">Algeria and Northern Africa</td>
-      <td class="tdrb br"><div><b>26</b></div></td>
-      <td class="tdrb br"><div>20</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>14.</div></td>
-      <td class="tdl br">United States and Canada, xvii–xix</td>
-      <td class="tdrb br"><div><b>86</b></div></td>
-      <td class="tdrb br"><div>48</div></td>
-    </tr>
-    <tr class="bb">
-      <td rowspan="5" class="bl br">Central Regions</td>
-      <td class="tdrt"><div>15.</div></td>
-      <td class="tdl br">Java, Sumatra, and neighbouring Islands, 1873–4–7–8</td>
-      <td class="tdrb br"><div><b>194</b></div></td>
-      <td class="tdrb br"><div>182</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>16.</div></td>
-      <td class="tdl br">Mexico and Central America, xvi–xix</td>
-      <td class="tdrb br"><div>26</div></td>
-      <td class="tdrb br"><div>26</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>17.</div></td>
-      <td class="tdl br">West Indies (Mallet), xvi–xix</td>
-      <td class="tdrb br"><div>108</div></td>
-      <td class="tdrb br"><div><b>114</b></div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>18.</div></td>
-      <td class="tdl br">West Indies, xvi–xix</td>
-      <td class="tdrb br"><div>296</div></td>
-      <td class="tdrb br"><div><b>343</b></div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>19.</div></td>
-      <td class="tdl br">Cuba, xvi–xix</td>
-      <td class="tdrb br"><div><b>28</b></div></td>
-      <td class="tdrb br"><div>23</div></td>
-    </tr>
-    <tr class="bb">
-      <td rowspan="5" class="bl br bb">Southern Regions</td>
-      <td class="tdrt"><div>20.</div></td>
-      <td class="tdl br">Chili, and La Plata Basin, xvi–xix</td>
-      <td class="tdrb br"><div>89</div></td>
-      <td class="tdrb br"><div>89</div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>21.</div></td>
-      <td class="tdl br">Peru, Columbia, Basin of Amazons, xvi–xix</td>
-      <td class="tdrb br"><div>506</div></td>
-      <td class="tdrb br"><div><b>541</b></div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdrt"><div>22.</div></td>
-      <td class="tdl br">New Zealand, 1869–1879</td>
-      <td class="tdrb br"><div>166</div></td>
-      <td class="tdrb br"><div><b>176</b></div></td>
-    </tr>
-  </table>
-
-  <p>Neglecting those records which show as many earthquakes for the winter
-    months as for the summer months, we see at a glance that generally
-    the greater number of shocks have happened during the colder seasons.
-    In the southern hemisphere, so far as the records go, this is not
-    true. In the northern regions, out of fourteen examples there are
-    two exceptions. In the central regions there are two cases where the
-    greatest number of earthquakes have been recorded in the winter months,
-    and two cases<span class="pagenum" id="Page_258">258</span>
-    where the greatest number have been recorded for the summer.</p>
-
-  <p>Altogether, out of twenty-two examples, there are only six exceptions
-    to the rule. These exceptions altogether occur among records many of
-    which are ancient, and are, therefore, more open to error than lists
-    which have been compiled in modern times.</p>
-
-  <p>Because small earthquakes are seldom noticed by persons out in the
-    open air, it might be expected that the number of earthquakes observed
-    in warm countries at one portion of the year would be equal to those
-    observed in any other season. Such an argument, however, would hardly
-    apply to most of the records which are quoted, as they refer to
-    destructive disturbances.</p>
-
-  <p>If, however, we take the records made in tropical countries from the
-    table just given, we see that in such countries there have been almost
-    as many observations of earthquakes at one season as at any other.</p>
-
-  <p>Another fact which might be adduced against the rule that the
-    greater number of earthquakes occur during the winter months would
-    be the comparison of a table of earthquakes recorded previous to the
-    nineteenth century. By doing this we see that for certain countries the
-    winter rule is inverted, and that the greater number of shocks are felt
-    during the summer.</p>
-
-  <p>Notwithstanding these objections to Perrey’s conclusions, the balance
-    of evidence is in favour of his general result, and we may conclude
-    that during the colder portions of the year we may expect more shakings
-    than during the warmer portions. Comparing the number of earthquakes of
-    winter and autumn to those of summer and spring, they are to each other
-    in the proportion of 4 : 3.</p>
-
-  <p>A fairer way to examine this question, and to determine what is
-    probably the present state of seismic activity in our globe, would
-    be only to consider the earthquakes
-    <span class="pagenum" id="Page_259">259</span> which have taken place in
-    comparatively recent times, laying especial stress upon those
-    observations which have been made with the assistance of automatic
-    instruments, or those which have been collected by persons interested
-    in these investigations.</p>
-
-  <p>For this purpose the following table, showing the distribution of
-    earthquakes in different countries during the nineteenth century, has
-    been compiled.</p>
-
-  <p>The arrangement is mensual. Where the number of earthquakes in any
-    month is above the average, the number is printed in large type; where
-    below the average, in small type.</p>
-
-
-  <p class="center"><span class="smcap">Earthquakes of the Nineteenth Century, chiefly from Perrey.</span></p>
-
-  <div class="left hang2">Key:</div>
-
-  <table class="ml5 mb2 lh1" summary="Earthquakes by country and month key">
-    <tr><td>Jan</td> <td>January</td></tr>
-    <tr><td>Feb</td> <td>February</td></tr>
-    <tr><td>Mar</td> <td>March</td></tr>
-    <tr><td>Apr</td> <td>April</td></tr>
-    <tr><td>May</td> <td>May</td></tr>
-    <tr><td>Jun</td> <td>June</td></tr>
-    <tr><td>Jul</td> <td>July</td></tr>
-    <tr><td>Aug</td> <td>August</td></tr>
-    <tr><td>Sep</td> <td>September</td></tr>
-    <tr><td>Oct</td> <td>October</td></tr>
-    <tr><td>Nov</td> <td>November</td></tr>
-    <tr><td>Dec</td> <td>December</td></tr>
-    <tr><td>Ave</td> <td>Average per month</td></tr>
-  </table>
-
-  <table class="collapse" summary="Earthquakes by country and month">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th class="ball">Jan</th>
-      <th class="ball">Feb</th>
-      <th class="ball">Mar</th>
-      <th class="ball">Apr</th>
-      <th class="ball">May</th>
-      <th class="ball">Jun</th>
-      <th class="ball">Jul</th>
-      <th class="ball">Aug</th>
-      <th class="ball">Sep</th>
-      <th class="ball">Oct</th>
-      <th class="ball">Nov</th>
-      <th class="ball">Dec</th>
-      <th class="ball">Ave</th>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Scandinavia and Iceland</td>
-      <td class="tdrb br"><div class="large">17</div></td>
-      <td class="tdrb br"><div class="large">11</div></td>
-      <td class="tdrb br"><div class="large">11</div></td>
-      <td class="tdrb br"><div>7</div></td>
-      <td class="tdrb br"><div>7</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div class="large">10</div></td>
-      <td class="tdrb br"><div class="large">10</div></td>
-      <td class="tdrb br"><div class="large">11</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div>9·3</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">British Isles and Northern Isles</td>
-      <td class="tdrb br"><div class="large">9</div></td>
-      <td class="tdrb br"><div class="large">9</div></td>
-      <td class="tdrb br"><div class="large">10</div></td>
-      <td class="tdrb br"><div>7</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div>5</div></td>
-      <td class="tdrb br"><div class="large">11</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div class="large">11</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div>9<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">France, Belgium, Holland</td>
-      <td class="tdrb br"><div class="large">27</div></td>
-      <td class="tdrb br"><div class="large">17</div></td>
-      <td class="tdrb br"><div class="large">21</div></td>
-      <td class="tdrb br"><div>13</div></td>
-      <td class="tdrb br"><div>13</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div>15</div></td>
-      <td class="tdrb br"><div class="large">17</div></td>
-      <td class="tdrb br"><div>15</div></td>
-      <td class="tdrb br"><div class="large">17</div></td>
-      <td class="tdrb br"><div class="large">21</div></td>
-      <td class="tdrb br"><div class="large">25</div></td>
-      <td class="tdrb br"><div>17<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Basin of the Rhone</td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div class="large">8</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>4</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div class="large">8</div></td>
-      <td class="tdrb br"><div class="large">14</div></td>
-      <td class="tdrb br"><div>6·6</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Basin of the Rhine and Switzerland</td>
-      <td class="tdrb br"><div class="large">15</div></td>
-      <td class="tdrb br"><div class="large">17</div></td>
-      <td class="tdrb br"><div>13</div></td>
-      <td class="tdrb br"><div>12</div></td>
-      <td class="tdrb br"><div>11</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div>12</div></td>
-      <td class="tdrb br"><div>11</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div class="large">17</div></td>
-      <td class="tdrb br"><div class="large">24</div></td>
-      <td class="tdrb br"><div class="large">25</div></td>
-      <td class="tdrb br"><div>14<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Basin of the Danube</td>
-      <td class="tdrb br"><div class="large">14</div></td>
-      <td class="tdrb br"><div class="large">15</div></td>
-      <td class="tdrb br"><div>9</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div class="large">16</div></td>
-      <td class="tdrb br"><div>11</div></td>
-      <td class="tdrb br"><div>11</div></td>
-      <td class="tdrb br"><div class="large">16</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div>11·8</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Spanish Peninsula</td>
-      <td class="tdrb br"><div class="large">10</div></td>
-      <td class="tdrb br"><div>5</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div class="large">7</div></td>
-      <td class="tdrb br"><div>4</div></td>
-      <td class="tdrb br"><div>6</div></td>
-      <td class="tdrb br"><div class="large">10</div></td>
-      <td class="tdrb br"><div>5</div></td>
-      <td class="tdrb br"><div class="large">9</div></td>
-      <td class="tdrb br"><div class="large">11</div></td>
-      <td class="tdrb br"><div class="large">7</div></td>
-      <td class="tdrb br"><div>5</div></td>
-      <td class="tdrb br"><div>7<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Italian Peninsula, Sicily, Sardinia, and Malta</td>
-      <td class="tdrb br"><div class="large">44</div></td>
-      <td class="tdrb br"><div class="large">44</div></td>
-      <td class="tdrb br"><div class="large">48</div></td>
-      <td class="tdrb br"><div class="large">43</div></td>
-      <td class="tdrb br"><div class="large">40</div></td>
-      <td class="tdrb br"><div>34</div></td>
-      <td class="tdrb br"><div class="large">41</div></td>
-      <td class="tdrb br"><div class="large">46</div></td>
-      <td class="tdrb br"><div>27</div></td>
-      <td class="tdrb br"><div class="large">45</div></td>
-      <td class="tdrb br"><div>26</div></td>
-      <td class="tdrb br"><div class="large">39</div></td>
-      <td class="tdrb br"><div>39<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Turco-Hellenic Territory, Syria, Ægean Islands, and Levant</td>
-      <td class="tdrb br"><div class="large">22</div></td>
-      <td class="tdrb br"><div class="large">20</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div>16</div></td>
-      <td class="tdrb br"><div>15</div></td>
-      <td class="tdrb br"><div>14</div></td>
-      <td class="tdrb br"><div class="large">22</div></td>
-      <td class="tdrb br"><div>14</div></td>
-      <td class="tdrb br"><div class="large">17</div></td>
-      <td class="tdrb br"><div>12</div></td>
-      <td class="tdrb br"><div>14</div></td>
-      <td class="tdrb br"><div>16<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Northern Zone of Asia</td>
-      <td class="tdrb br"><div>4</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div>4</div></td>
-      <td class="tdrb br"><div>4</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div class="large">7</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>4</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div>4·7</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">1876–1881, Japan (Tokio area)</td>
-      <td class="tdrb br"><div class="large">39</div></td>
-      <td class="tdrb br"><div class="large">41</div></td>
-      <td class="tdrb br"><div class="large">41</div></td>
-      <td class="tdrb br"><div>30</div></td>
-      <td class="tdrb br"><div class="large">33</div></td>
-      <td class="tdrb br"><div>30</div></td>
-      <td class="tdrb br"><div>27</div></td>
-      <td class="tdrb br"><div>21</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div>28</div></td>
-      <td class="tdrb br"><div class="large">34</div></td>
-      <td class="tdrb br"><div class="large">43</div></td>
-      <td class="tdrb br"><div>31·4</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Japan (large earthquakes)</td>
-      <td class="tdrb br"><div>&mdash;</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div>&mdash;</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div>&mdash;</div></td>
-      <td class="tdrb br"><div>&mdash;</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div>2<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Algeria and North’rn Africa</td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div class="large">7</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div class="large">8</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div>3·8</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">United States and Canada</td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>&mdash;</div></td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div class="large">7</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div>3·8</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Java, Sumatra, &amp;c., 1873–4–5–7 and 9</td>
-      <td class="tdrb br"><div class="large">35</div></td>
-      <td class="tdrb br"><div>30</div></td>
-      <td class="tdrb br"><div class="large">38</div></td>
-      <td class="tdrb br"><div>33</div></td>
-      <td class="tdrb br"><div>22</div></td>
-      <td class="tdrb br"><div class="large">36</div></td>
-      <td class="tdrb br"><div>27</div></td>
-      <td class="tdrb br"><div class="large">40</div></td>
-      <td class="tdrb br"><div>24</div></td>
-      <td class="tdrb br"><div class="large">35</div></td>
-      <td class="tdrb br"><div>30</div></td>
-      <td class="tdrb br"><div>26</div></td>
-      <td class="tdrb br"><div>31<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Mexico and Central America</td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div>2·5</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Antilles</td>
-      <td class="tdrb br"><div>9</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div class="large">19</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div>9</div></td>
-      <td class="tdrb br"><div class="large">16</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div class="large">13</div></td>
-      <td class="tdrb br"><div class="large">12</div></td>
-      <td class="tdrb br"><div>11·8</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Cuba</td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div>3</div></td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div>2</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div class="large">5</div></td>
-      <td class="tdrb br"><div class="large">6</div></td>
-      <td class="tdrb br"><div class="large">4</div></td>
-      <td class="tdrb br"><div>4<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Chili and La Plata</td>
-      <td class="tdrb br"><div class="large">14</div></td>
-      <td class="tdrb br"><div>10</div></td>
-      <td class="tdrb br"><div class="large">14</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div class="large">19</div></td>
-      <td class="tdrb br"><div>11</div></td>
-      <td class="tdrb br"><div class="large">16</div></td>
-      <td class="tdrb br"><div class="large">15</div></td>
-      <td class="tdrb br"><div class="large">16</div></td>
-      <td class="tdrb br"><div>9</div></td>
-      <td class="tdrb br"><div class="large">27</div></td>
-      <td class="tdrb br"><div>8</div></td>
-      <td class="tdrb br"><div>13·9</div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Peru, Columbia, Basins of Amazons, xvi–xix</td>
-      <td class="tdrb br"><div class="large">92</div></td>
-      <td class="tdrb br"><div>83</div></td>
-      <td class="tdrb br"><div class="large">92</div></td>
-      <td class="tdrb br"><div>27</div></td>
-      <td class="tdrb br"><div class="large">106</div></td>
-      <td class="tdrb br"><div>79</div></td>
-      <td class="tdrb br"><div class="large">94</div></td>
-      <td class="tdrb br"><div class="large">93</div></td>
-      <td class="tdrb br"><div class="large">97</div></td>
-      <td class="tdrb br"><div>77</div></td>
-      <td class="tdrb br"><div>72</div></td>
-      <td class="tdrb br"><div class="large">90</div></td>
-      <td class="tdrb br"><div>87<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">New Zealand, 1869–79</td>
-      <td class="tdrb br"><div class="large">31</div></td>
-      <td class="tdrb br"><div>27</div></td>
-      <td class="tdrb br"><div class="large">37</div></td>
-      <td class="tdrb br"><div>23</div></td>
-      <td class="tdrb br"><div>22</div></td>
-      <td class="tdrb br"><div class="large">31</div></td>
-      <td class="tdrb br"><div>27</div></td>
-      <td class="tdrb br"><div class="large">36</div></td>
-      <td class="tdrb br"><div class="large">37</div></td>
-      <td class="tdrb br"><div>21</div></td>
-      <td class="tdrb br"><div>27</div></td>
-      <td class="tdrb br"><div>23</div></td>
-      <td class="tdrb br"><div>28·5</div></td>
-    </tr>
-    <tr>
-      <td class="tdc bl br">Jan. 1850, Dec. 1857</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Northern Hemisphere</td>
-      <td class="tdrb br"><div class="large">153</div></td>
-      <td class="tdrb br"><div class="large">162</div></td>
-      <td class="tdrb br"><div>143</div></td>
-      <td class="tdrb br"><div class="large">161</div></td>
-      <td class="tdrb br"><div>126</div></td>
-      <td class="tdrb br"><div>124</div></td>
-      <td class="tdrb br"><div>141</div></td>
-      <td class="tdrb br"><div class="large">156</div></td>
-      <td class="tdrb br"><div class="large">154</div></td>
-      <td class="tdrb br"><div class="large">171</div></td>
-      <td class="tdrb br"><div class="large">151</div></td>
-      <td class="tdrb br"><div class="large">168</div></td>
-      <td class="tdrb br"><div>150<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Southern Hemisphere</td>
-      <td class="tdrb br"><div class="large">72</div></td>
-      <td class="tdrb br"><div>43</div></td>
-      <td class="tdrb br"><div class="large">61</div></td>
-      <td class="tdrb br"><div class="large">66</div></td>
-      <td class="tdrb br"><div>46</div></td>
-      <td class="tdrb br"><div>42</div></td>
-      <td class="tdrb br"><div class="large">53</div></td>
-      <td class="tdrb br"><div>39</div></td>
-      <td class="tdrb br"><div class="large">54</div></td>
-      <td class="tdrb br"><div class="large">55</div></td>
-      <td class="tdrb br"><div class="large">57</div></td>
-      <td class="tdrb br"><div>46</div></td>
-      <td class="tdrb br"><div>53<span class="s">·0</span></div></td>
-    </tr>
-    <tr>
-      <td class="tdc bl br">1821–1830</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-      <td class="br">&nbsp;</td>
-    </tr>
-    <tr>
-      <td class="tdl bl br">Northern Hemisphere</td>
-      <td class="tdrb br"><div class="large">31</div></td>
-      <td class="tdrb br"><div class="large">36</div></td>
-      <td class="tdrb br"><div class="large">31</div></td>
-      <td class="tdrb br"><div>29</div></td>
-      <td class="tdrb br"><div class="large">33</div></td>
-      <td class="tdrb br"><div class="large">33</div></td>
-      <td class="tdrb br"><div>20</div></td>
-      <td class="tdrb br"><div class="large">31</div></td>
-      <td class="tdrb br"><div>24</div></td>
-      <td class="tdrb br"><div class="large">41</div></td>
-      <td class="tdrb br"><div>26</div></td>
-      <td class="tdrb br"><div class="large">34</div></td>
-      <td class="tdrb br"><div>30<span class="s">·0</span></div></td>
-    </tr>
-    <tr class="bb">
-      <td class="tdl bl br">Southern Hemisphere</td>
-      <td class="tdrb br"><div class="large">2</div></td>
-      <td class="tdrb br"><div>&mdash;</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div class="large">2</div></td>
-      <td class="tdrb br"><div class="large">3</div></td>
-      <td class="tdrb br"><div class="large">2</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div>1</div></td>
-      <td class="tdrb br"><div>1·6</div></td>
-    </tr>
-  </table>
-
-  <p><span class="pagenum" id="Page_260">260</span></p>
-
-  <p>A glance at this table shows that for most countries in the northern
-    hemisphere the rule that there are generally more earthquakes during
-    the winter months—that is, from October to March—holds good. For
-    countries which lie comparatively near to the Equator, and also for
-    those countries in the southern hemisphere, the rule is not so clear.
-    When examining this table it must be remembered that it does not enable
-    us to judge of the relative frequency of earthquakes in different
-    countries, inasmuch as the periods over which the records were taken
-    are different in different cases.</p>
-
-  <p>To the above table might be added the records of P. Merian, who
-    examined the earthquakes felt in Basle up to 1831. As a result he
-    found that during the winter months eighty shocks had been felt,
-    whilst during the summer only forty. Taking the records for the two
-    hemispheres from 1850–1857, compiled by Kluge,<a id="FNanchor_103" href="#Footnote_103" class="fnanchor">[103]</a> in the northern
-    hemisphere we have in the months between October and March 948 shocks
-    against 862 in the remainder of the year. In the same months in the
-    southern hemisphere we have for the corresponding periods the numbers
-    337 and 300, and thus both hemispheres would appear to follow the same
-    rule. If, however, we examine the table we see that the two seasons
-    are not so pronounced for the southern hemisphere as they are for
-    the northern, and that there may be two or three periods of maximum
-    disturbance as has been previously indicated.</p>
-
-  <p><em>Earthquakes and the planets and meteors.</em>—Just as the moon and the sun
-    may exert an attractive influence upon the earth and cause earthquakes
-    to predominate at certain seasons rather than at others, several
-    investigators of seismic phenomena have thought that the planets might
-    act in a similar manner.</p>
-
-  <p><span class="pagenum" id="Page_261">261</span></p>
-
-  <p>M. J. Delauney, from a study of Perrey’s tables of earthquakes from
-    1750–1842, found two groups of maxima each with a period of about
-    twelve years, one commencing in 1759 and the other in 1756. Two
-    other groups with twenty-eight year periods respectively commence in
-    1756 and 1773. These groups coincide with the times when Jupiter and
-    Saturn reach the mean longitudes of 265° and 135°. From this Delauney
-    concludes that earthquakes have a maximum when the planets are in the
-    mean longitudes just mentioned.</p>
-
-  <p>The increased number of earthquakes, especially in November, are
-    attributed to the passage of the earth through swarms of meteors, and
-    in like manner supposes the influence of Jupiter and Saturn to be due
-    to their passing through meteor streams situated in mean longitudes
-    135° and 265°.</p>
-
-  <p>As a consequence of this he predicts an increase of earthquakes in the
-    years 1886, 1891, 1898, 1900, &amp;c.<a id="FNanchor_104" href="#Footnote_104" class="fnanchor">[104]</a></p>
-
-  <p>Dr. E. Naumann, who critically examined the large earthquakes of Japan,
-    showed that there was an approximate coincidence between many of the
-    disturbances and the thirty-three year period of meteoric showers.<a id="FNanchor_105" href="#Footnote_105" class="fnanchor">[105]</a></p>
-
-  <p>Humboldt states that a great shower of meteors was seen at Quito before
-    the great earthquake of Riobamba (Feb. 4, 1797). The earthquakes of
-    1766 and 1799 at Cumana are also said to have been accompanied with
-    meteoric showers. Mallet gives a list of large earthquakes which
-    occurred at the times when meteors were observed.<a id="FNanchor_106" href="#Footnote_106" class="fnanchor">[106]</a></p>
-
-  <p><em>The hours at which earthquakes are most frequent.</em>—From the
-    examination of a catalogue of over 2,000 earthquakes
-    <span class="pagenum" id="Page_262">262</span> which occurred
-    in various parts of the world between the years 1850 and 1857, made by
-    Kluge, it is found that both for the northern and southern hemispheres
-    the observations which were made during the night generally exceed
-    those which were made during the day.</p>
-
-  <table class="collapse" summary="Day vs. night earthquakes">
-    <tr>
-      <th class="ball">&nbsp;</th>
-      <th colspan="2" class="ball">Number of Earthquakes</th>
-    </tr>
-    <tr>
-      <th class="bl br">&nbsp;</th>
-      <th class="br">Day</th>
-      <th class="br">Night</th>
-    </tr>
-    <tr>
-      <td class="bl br">In the Northern Hemisphere</td>
-      <td class="br">938</td>
-      <td class="br">1592</td>
-    </tr>
-    <tr class="bb">
-      <td class="bl br">In the Southern Hemisphere</td>
-      <td class="br">292</td>
-      <td class="br">357</td>
-    </tr>
-  </table>
-
-  <p class="noindent">In the northern hemisphere the greatest number were observed between 10
-    <span class="smcap">p.m.</span> and 12 <span class="smcap">p.m.</span> (360 shocks), and the fewest between
-    12 and 2 <span class="smcap">p.m.</span> (139 shocks). In the southern hemisphere, the
-    greatest number were observed at night between 12 and 1, and the
-    smallest number between 1 and 2 and 4 and 5 in the afternoon.<a id="FNanchor_107" href="#Footnote_107" class="fnanchor">[107]</a>
-    These distinctions, however, are less distinctly marked as we approach
-    the Equator. Schmidt found for the earthquakes of the Orient between
-    1774 and 1873, that shocks had been most frequent about half-past two
-    <span class="smcap">a.m.</span>, and less frequent about 1 <span class="smcap">p.m.</span> With regard
-    to these conclusions, which have been reached with much labour, we
-    might be inclined to think that they are partially to be explained on
-    the supposition that more observations are made during the night than
-    during the day—the personal experience of residents in an earthquake
-    country being, that many earthquakes which occur during the day are
-    passed by unnoticed, whilst those which occur during the night are
-    recorded by thousands of observers. Such a view is certainly confirmed
-    by the instrumental records obtained in Japan. From 1872 to 1880
-    inclusive there were 261 shocks recorded, 132 of which occurred between
-    the hours of 6 <span class="smcap">p.m.</span> and 6 <span class="smcap">a.m.</span>
-    <span class="pagenum" id="Page_263">263</span></p>
-
-  <p><em>Earthquakes and sun spots.</em>—Of late years considerable attention
-    has been drawn to a coincidence between the occurrence of sun spots,
-    magnetic disturbances, rainfall, and other natural phenomena.</p>
-
-  <p>These periods of sun spots occur about every eleven years, and appear
-    to be coincident with the periodical return of the planet Jupiter. In
-    Japan, Dr. E. Naumann sought for a coincidence between these periods of
-    sun spots and earthquakes, but without any marked results.</p>
-
-  <p>Schmidt, who carefully compared his lists of earthquakes with the
-    appearance of sun spots, came to the conclusion that there was no
-    marked coincidence. The occurrence of earthquakes had sometimes
-    synchronised with sun spots, whilst at other times there had been a
-    maximum of sun spots and no earthquakes.</p>
-
-  <p>M. R. Wolf<a id="FNanchor_108" href="#Footnote_108" class="fnanchor">[108]</a> apparently considers that earthquakes, like volcanic
-    eruptions and the appearance of the aurora, are coincident with sun
-    spots.</p>
-
-  <p>Kluge, however, came to the conclusion that when there are few sun
-    spots, earthquakes, like volcanic eruptions and magnetic disturbances,
-    have been at a maximum.</p>
-
-  <p>M. A. Poey, who examined a catalogue of the earthquakes of Mexico
-    and the Antilles, extending from 1634 to 1870, shows by a table that
-    earthquakes have come in groups, first at the maxima and then at the
-    minima period of sun spots. Out of thirty-eight groups, seventeen
-    being at the maximum and seventeen at the minimum, the remaining four
-    are exceptions to the rule, being between the maximum and minimum.
-    Phenomena which are dependent upon heat occur with the minima of sun
-    spots, and those dependent upon cold with the maxima.<a id="FNanchor_109" href="#Footnote_109" class="fnanchor">[109]</a></p>
-
-  <p><span class="pagenum" id="Page_264">264</span></p>
-
-  <p><em>Earthquakes and the aurora.</em>—The possible connection between
-    earthquakes and the aurora is a subject which has attracted some
-    attention. Boué has especially made a careful examination of this
-    subject.<a id="FNanchor_110" href="#Footnote_110" class="fnanchor">[110]</a></p>
-
-  <p>He comes to the conclusion that if we compare the monthly periods of
-    earthquake frequency and the aurora there is an agreement between
-    the two. Comparing Perrey’s tables of earthquakes from the fourth to
-    the nineteenth century, with tables of the aurora, one-third of both
-    phenomena have occurred, not only in the same day, but often at the
-    same hour. Between 1834 and 1847, 457 earthquakes are given and 351
-    notices of the aurora.</p>
-
-  <p class="mb0">Out of these:—</p>
-
-  <div class="left ml5">
-    48 occur on the same day,<br />
-    &nbsp; 5 occur in the same hour,<br />
-    30 approximate to the same time.
-  </div>
-
-  <p>The nearer together that these phenomena have occurred the stronger
-    have they been.</p>
-
-  <p>Professor M. S. di Rossi brings forward many examples where there
-    has been a coincidence between the appearance of the aurora and
-    earthquakes. On 139 nights out of 211 days the aurora was seen in
-    some parts of Italy, and ninety-three times earthquakes were felt. On
-    forty-six occasions earthquakes and aurora took place together.<a id="FNanchor_111" href="#Footnote_111" class="fnanchor">[111]</a> In
-    considering the probability of a connection existing between these two
-    phenomena, we must bear in mind that the aurora is at no great height
-    above the surface of our earth, and, further, that it can be partially
-    imitated. The fact that in earthquake countries, like Japan, the aurora
-    is practically never seen, would indicate that
-    <span class="pagenum" id="Page_265">265</span>we can neither regard
-    this imperfectly understood phenomenon either as an effect or cause
-    of earthquakes. That earthquakes and the appearance of the aurora in
-    certain countries should not sometimes coincide is an impossibility.</p>
-
-  <p>Dr. Stukeley, who, it must be remembered, attempted to correlate
-    the phenomena of earthquakes and electricity, when writing of the
-    disturbances which shook England in 1849 and 1850, says that the
-    weather had been unusually warm, the aurora borealis frequent and of
-    unusually bright colours, whilst the whole year was remarkable for its
-    fire-balls, lightnings, and corruscations.<a id="FNanchor_112" href="#Footnote_112" class="fnanchor">[112]</a></p>
-
-  <p>The aurora was observed before the commencement of the Maestricht
-    earthquakes in 1751<a id="FNanchor_113" href="#Footnote_113" class="fnanchor">[113]</a> whilst at the time of the shock flashes of
-    light like lightning were observed in the sky.</p>
-
-  <p>Glimmering lights were seen in the sky before the New England
-    earthquakes (Nov. 18, 1755), and again, before the disturbances which
-    occurred in the same region in 1727, peculiar flashes of light were
-    seen.</p>
-
-  <p>Preceding the Sicilian earthquake of 1692 strange lights were seen
-    in the sky. Ignis fatui have also been observed with earthquakes. At
-    the time of auroral displays Bertelli has observed microseismical
-    disturbances, and M. S. di Rossi, who has made similar observations,
-    thinks that there is an intimate connection between the aurora and
-    earthquakes; the aurora either occurring in a period of earthquakes, or
-    else taking the place of earthquakes.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XV">
-    <span class="pagenum" id="Page_266">266</span>
-    <h2>CHAPTER XV.<br />
-      <span class="subhead">BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE
-        AND EARTHQUAKES.</span>
-    </h2>
-  </div>
-
-  <p><em>Changes in the barometer and earthquakes.</em>—Mallet, who collected
-    together a number of examples of earthquakes which have occurred with
-    a fall of the barometer, and a number which have happened with a rise,
-    concludes that there are as many instances of the one as of the other.
-    At the great earthquake of Calabria, in 1783, the barometer was very
-    low. The earthquake of the Rhine (February 23, 1828) was preceded by
-    a gradual fall of the barometer, which reached its lowest point upon
-    that day. After the earthquake the barometer again rose. The earthquake
-    of February 22, 1880, in Japan, was accompanied by exactly similar
-    phenomena. Caldcleugh, who observed the heavy shocks in Chili (February
-    20, 1835), noticed that on February 17 and 18 the barometer fell 5/10
-    inches. Similar phenomena were observed before the succeeding smaller
-    shocks. After the shocks the barometer again rose. Principal Dawson,
-    speaking of the earthquakes of Canada, observes that some of the shocks
-    have been accompanied with a low barometer.</p>
-
-  <p>P. Merian, who examined the connection between the Swiss earthquakes
-    and atmospheric pressure, found that out of twenty-two earthquakes
-    observed in Basle between<span class="pagenum" id="Page_267">267</span>
-    1755 and 1836, thirteen of these were
-    local shocks, of which eight were accompanied with sudden changes of
-    pressure. Of the remaining nine, which were only felt slightly in
-    Basle, no change in atmospheric pressure was observed. Of thirty-six
-    earthquakes which, between 1826 and 1836, were felt in Switzerland,
-    thirty were chiefly confined to Switzerland, and ten of these occurred
-    with a low or falling barometer.</p>
-
-  <p>Humboldt is of opinion that earthquakes only occur with changes in
-    barometric pressure in those countries where earthquakes are few; and
-    he gives examples where the regular variations of the barometer have
-    gone on without interruption at the time of earthquakes.</p>
-
-  <p>Frederick Hoffmann, who examined fifty-seven earthquakes which occurred
-    at Palermo between 1788 and 1838, came to the following result:—</p>
-
-  <table summary="Palermo barometer readings">
-    <tr>
-      <td class="tdl">The barometer was sinking</td>
-      <td class="tdl">in 20 cases</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; rising</td>
-      <td class="tdl">in 16&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; at a minimum</td>
-      <td class="tdl">in&nbsp; &nbsp;7&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; maximum</td>
-      <td class="tdl">in&nbsp; &nbsp;3&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; undetermined</td>
-      <td class="tdl">in 11&nbsp; &nbsp; „<a id="FNanchor_114" href="#Footnote_114" class="fnanchor">[114]</a></td>
-    </tr>
-  </table>
-
-  <p>The observations of M. S. di Rossi apparently show that the earthquakes
-    in Italy chiefly occur with a barometrical depression and with sudden
-    jumps in atmospheric pressure.</p>
-
-  <p>Schmidt, who examined the earthquakes of the Orient, which occurred
-    between 1858 and 1873, says that they were rare with a high barometer,
-    but numerous when the barometer was low.</p>
-
-  <p>From an examination of a table of 396 earthquakes (May 8, 1875-Dec.
-    1881) felt in Tokio, furnished to me by Mr. Arai Ikunosuke, the
-    director of the meteorological department, I obtained the following
-    results:—</p>
-
-  <p><span class="pagenum" id="Page_268">268</span></p>
-
-  <table summary="Tokio barometer readings">
-    <tr>
-      <td class="tdl">The barometer was rising</td>
-      <td class="tdl">in 169 cases</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; falling</td>
-      <td class="tdl">in 154&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; steady</td>
-      <td class="tdl">in&nbsp; &nbsp;73&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; below the monthly mean</td>
-      <td class="tdl">in 189&nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; above</td>
-      <td class="tdl">in 192&nbsp; &nbsp; „</td>
-    </tr>
-  </table>
-
-  <p>From this it would appear that in Japan at least the movements of the
-    barometer do not show any marked connection with the occurrence of
-    earthquakes.</p>
-
-  <p>When considering this question we must remember the marked effects
-    which a lowering of the barometer produces upon certain volcanoes and
-    solfataras. The volumes of steam emitted from Stromboli and from some
-    of the solfataras in Tuscany hold a marked connection with atmospheric
-    pressure as the quantity of fire damp given off from coal seams—these
-    being greatest when the barometer is low. At certain changes of
-    the weather it is said that the volcano of Vulture, near Melfi,
-    emits noises. These phenomena at once place volcanic phenomena and
-    barometrical pressure in direct relationship.</p>
-
-  <p><em>Changes in temperature.</em>—If, with an earthquake, it should happen
-    that there is a change in the height of the barometer, we should
-    naturally expect that this might be accompanied with the changes in the
-    temperature, in the wind, and in other atmospheric phenomena which are
-    more or less connected with the height of the barometer.</p>
-
-  <p>Many times it has been observed that after an earthquake there has been
-    a sudden fall in the temperature. Such was the case with the Yokohama
-    earthquakes of 1880.</p>
-
-  <p>Cotte endeavours to show that the earthquakes of Lisbon produced a
-    change upon the temperature of all Europe. In the year which followed
-    this earthquake storms were more common than usual.</p>
-
-  <p>Kluge has collected together a large number of
-    <span class="pagenum" id="Page_269">269</span> examples when there has
-    been a fall of temperature at the time of an earthquake.<a id="FNanchor_115" href="#Footnote_115" class="fnanchor">[115]</a></p>
-
-  <p>At Kiachta, in Siberia, at the time of the earthquake of December
-    27, 1856, the thermometer fell from 12° to 25° R. We must, however,
-    remember that there are many cases known where the thermometer rose.</p>
-
-  <p>M. S. di Rossi remarks that we have the highest records of temperature
-    in the years richest in earthquakes. Thus, in 1873, at the time of the
-    earthquakes in Central and Northern Italy, an abnormal high temperature
-    was remarked. Japanese writers have remarked upon the unusual heat
-    which has shaken their countries. The temperature of subterranean
-    waters have been known to increase before earthquakes.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XVI">
-    <span class="pagenum" id="Page_270">270</span>
-    <h2>CHAPTER XVI.<br />
-      <span class="subhead">RELATION OF SEISMIC TO VOLCANIC PHENOMENA.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Want of synchronism between earthquakes and volcanic
-    eruptions—Synchronism between earthquakes and volcanic
-    eruptions—Conclusion.</div>
-
-  <p><em>Connection between earthquakes and volcanic eruptions.</em>—Insomuch as
-    it is a recognised fact that regions which are characterised by their
-    seismic activity are chiefly those which are also characterised by
-    the number of their volcanoes, it is generally assumed that these
-    two phenomena have an intimate relation. The residents in a volcanic
-    country, when seeking for the origin of an earthquake, invariably turn
-    towards the volcanoes which surround them. If a neighbouring volcano
-    is in a state of activity, it is often regarded as a safeguard against
-    seismic convulsions, in other cases it is looked upon as being the
-    cause of such disturbances. In certain instances both of these views
-    have apparently been corroborated. When we consider that an earthquake
-    and a volcanic eruption may both be the result of some great internal
-    convulsion, and that first one and then the other may take place in the
-    same neighbourhood, it is natural to expect that when these internal
-    forces have expended themselves in the production of one of these
-    phenomena, it is not so likely that they should exhibit themselves in
-    the other. The inhabitants of Sicily and Naples, we are
-    <span class="pagenum" id="Page_271">271</span> told, regard
-    eruptions of Etna and Vesuvius as safeguards against earthquakes. A
-    similar belief is to be found in portions of South America with regard
-    to the volcanoes for which that country is so celebrated.</p>
-
-  <p>From an examination of the records of the large earthquakes and the
-    volcanic eruptions which have taken place in Japan during the last
-    2,000 years, Dr. Naumann found that there was often an approximate
-    coincidence between the times of the occurrence of these phenomena,
-    suggesting the idea that the efforts which had been sufficient to
-    establish the volcano had at the same time been sufficient to shake the
-    ground.</p>
-
-  <p>Of destructive earthquakes which have occurred at the time of volcanic
-    eruptions, and of examples when these phenomena have occurred at widely
-    separated intervals, the records are extremely numerous.</p>
-
-  <p><em>Want of synchronism between earthquakes and volcanic eruptions.</em>—Many
-    of the great earthquakes of South America do not appear to have been
-    connected with volcanic eruptions.</p>
-
-  <p>The great earthquakes of the world, like those of Calabria and Lisbon,
-    which took place in regions which are not volcanic, have not, Fuchs
-    tells us, taken place in conjunction with volcanic outbursts.</p>
-
-  <p>In Japan, as in the Sandwich Islands and in many other parts of the
-    globe, the small earthquakes which occur almost daily do not appear to
-    show any marked connection with volcanic disturbances.</p>
-
-  <p>In 1881, during the eruption of Natustake, a volcano lying about
-    a hundred miles north of Tokio, there was neither an increase nor
-    a decrease in the earthquakes which were felt in Tokio. Similar
-    remarks apply to the state of seismic activity of 1876–77, when
-    Oshima, a volcanic island about seventy miles to the south of Tokio,
-    was in eruption.<span class="pagenum" id="Page_272">272</span>
-    In the Sandwich Islands Mauna Loa seems to have
-    its eruptions independently of the disturbances which shake these
-    islands.<a id="FNanchor_116" href="#Footnote_116" class="fnanchor">[116]</a></p>
-
-  <p><em>Synchronism of earthquakes and volcanic eruptions.</em>—Although many
-    examples like the above may be quoted, which apparently show an utter
-    want of connection between earthquakes and volcanoes, we must not
-    overlook that class of earthquakes which almost invariably accompany
-    all great volcanic disturbances. In fact the sudden explosions which
-    take place at volcanic foci, as, for instance, at the commencement
-    of an eruption, are enumerated as one of the causes which produce
-    earthquakes. Earthquakes like these usually continue until the pressure
-    of the steam and lava have found for themselves an opening. As compared
-    with the total number of earthquakes which are recorded, they form but
-    an insignificant portion.</p>
-
-  <p>The direct connection which exists between these phenomena has, no
-    doubt, done very much to spread the popular belief that all earthquakes
-    may be connected with volcanic eruptions. As examples where this
-    connection has existed we might quote from almost all the volcanic
-    countries in the world.</p>
-
-  <p>Thus, Fuchs tells us that on October 6, 1737, almost the whole of
-    Kamschatka and the Kurile Islands were disturbed by movements which
-    were simultaneous with the outbreak of the great volcano Klutschenskja
-    of North Kamschatka.</p>
-
-  <p>One of the earliest records of a severe earthquake and a volcanic
-    eruption occurring simultaneously is found in the accounts of the
-    destruction of Herculaneum and Pompeii. The throwing up of Monte Nuovo
-    in the neighbourhood of Pozzuoli was accompanied with a dreadful
-    earthquake.<a id="FNanchor_117" href="#Footnote_117" class="fnanchor">[117]</a></p>
-
-  <p><span class="pagenum" id="Page_273">273</span></p>
-
-  <p>In 1868 the earthquake of Arequipa was accompanied by the opening of
-    the volcano Misti, on its north side. The distance to the volcano is
-    about fourteen miles.</p>
-
-  <p>At the time of the eruptions of Kilauea in 1789 the ground shook and
-    rocked so that persons could not stand.</p>
-
-  <p>The first eruption of the volcano Irasu, in Costa Rica (1783), was
-    accompanied by violent earthquakes.<a id="FNanchor_118" href="#Footnote_118" class="fnanchor">[118]</a> The smoke and flames which
-    are said to have issued from the side of Mount Fojo at the time of the
-    Lisbon earthquake are regarded by some as having been volcanic. Others
-    thought that the phenomena, rather than being on the side of Fojo,
-    which showed no traces of volcanic action, had taken place in the ocean.</p>
-
-  <p>At the time of the great earthquake at Concepcion (1835), whilst the
-    waves were coming in, two great submarine eruptions were observed. One,
-    behind the Isle of Quiriquina, appeared like a column of smoke. The
-    other, in the bay of San Vicente, appeared to form a whirlpool. The
-    sea-water became black, and had a sulphurous smell, there being a vast
-    eruption of gas in bubbles. Many fish were killed.<a id="FNanchor_119" href="#Footnote_119" class="fnanchor">[119]</a></p>
-
-  <p>With this same earthquake, near to Juan Fernandez, about one mile from
-    the shore, the sea appeared to boil, and a high column of smoke was
-    thrown into the air. At night flames were seen.</p>
-
-  <p>In 1861, when Mendoza was destroyed and 10,000 inhabitants killed, a
-    volcano at the foot of which Mendoza is situated burst into eruption.</p>
-
-  <p>The earthquake of 1822 at Valdivia was accompanied by eruptions of the
-    neighbouring mountains, which only lasted a few minutes.</p>
-
-  <p>At the time of the Leghorn shocks (January 16–27, 1742) some fishermen
-    observed a part of the sea to rage
-    <span class="pagenum" id="Page_274">274</span>violently, to raise itself to a
-    great height, and then rush landwards.<a id="FNanchor_120" href="#Footnote_120" class="fnanchor">[120]</a></p>
-
-  <p>In 1797, when Riobamba was destroyed, the neighbouring volcanoes were
-    not affected, but Mount Pasto, 120 miles distant, suddenly ceased to
-    throw out its usual column of water.</p>
-
-  <p>On the night of December 10, 1874, a strong shock was felt in New
-    England, whilst at 4.45 <span class="smcap">a.m.</span> on December 11 a shock was felt
-    in the Pic du Midi, in the Pyrenees. In the middle of December there
-    were volcanic outbursts in Iceland.<a id="FNanchor_121" href="#Footnote_121" class="fnanchor">[121]</a></p>
-
-  <p>It is possible that these occurrences might be the results of some
-    widespread disturbance beneath the crust of the earth, or perhaps even
-    of widely extended earth pulsations. The probability, however, is that
-    these coincidences are accidental. When we remember that in a small
-    area like the northern half of Japan alone there are periods when there
-    are at least two shocks per day on the average, it is impossible for
-    these coincidences not to exist. Less frequently coincidences between
-    the larger disturbances must occur. Over and above these accidental
-    coincidences, it would appear that in the world’s history periods
-    have occurred when earthquakes were unusually frequent, and at such
-    times distant countries have suffered simultaneously. This approximate
-    coincidence in period, which has been referred to when speaking of the
-    distribution of destructive earthquakes in historical time, does not
-    imply an exact synchronism in the single shocks.</p>
-
-  <p>Small earthquakes, or, more properly speaking, local tremblings, are a
-    necessary accompaniment of almost all volcanic eruptions. Tremors of
-    this description are seldom, however, felt beyond the crater, or at the
-    most upon the flanks of the mountain where the eruption is going on.</p>
-
-  <p><span class="pagenum" id="Page_275">275</span></p>
-
-  <p>They are due to the explosive action of steam bursting through the
-    molten lava.</p>
-
-  <p><em>Volcanic eruption succeeding earthquakes.</em>—Sometimes it has happened
-    that an earthquake, or a series of earthquakes, have terminated with
-    the formation of volcanic vents.</p>
-
-  <p>As an example of a volcanic outburst terminating a seismic disturbance,
-    may be mentioned the appearance of a new volcano in the centre of
-    Lake Ilopango, as a sequel to the shocks which had disturbed that
-    neighbourhood in 1879.<a id="FNanchor_122" href="#Footnote_122" class="fnanchor">[122]</a></p>
-
-  <p>In 1750 there were continuous shakings lasting over three months at
-    Manilla. These terminated with an eruption of a small island in the
-    middle of a neighbouring lake. Three days after the commencement of
-    this eruption, four other small islands rose in the same lake.<a id="FNanchor_123" href="#Footnote_123" class="fnanchor">[123]</a></p>
-
-  <p>Antonio d’Ulloa, when speaking of the Andes, remarks that after a
-    volcanic eruption the shocks cease.<a id="FNanchor_124" href="#Footnote_124" class="fnanchor">[124]</a></p>
-
-  <p><em>Conclusion.</em>—Looking at this question generally, insomuch as the
-    greatest number of volcanic eruptions appear, according to Fuchs,
-    to have taken place in summer, whilst the greatest number of its
-    earthquakes have apparently taken place in winter, it would seem that
-    the two phenomena are without any direct connection, unless it be that
-    both are different effects of a common cause.</p>
-
-  <p>Regarded in this manner, an earthquake may be looked upon as an
-    uncompleted effort to establish a volcano. To use the words of Mallet,
-    ‘The forces of explosion and impulse are the same in both; they differ
-    only in degree <span class="pagenum" id="Page_276">276</span>
-    of energy, or on the varying sorts and degrees of
-    resistance opposed to them.’<a id="FNanchor_125" href="#Footnote_125" class="fnanchor">[125]</a></p>
-
-  <p>Although we have many examples of earthquakes having occurred without
-    volcanic eruptions, and, on the other hand, of volcanic eruptions
-    without earthquakes, volcanoes may still be regarded as ‘safety-valves
-    of the earth’s crust,’ which, by giving relief to internal stresses,
-    guard us against the effects of earthquakes.</p>
-
-  <p>That many earthquakes are felt at Copiapo is attributed to the fact
-    that in the neighbouring mountains there are no volcanic vents.</p>
-
-  <p>We must not, however, overrate the protective influence of volcanoes.
-    In the Sandwich Islands we see the columns of liquid lava in
-    neighbouring mountains standing at different heights, indicating a
-    want of subterranean connection between these vents. In consequence
-    of this it would seem that enormous pressures might be generated in
-    the neighbourhood of one of these mountains without finding relief at
-    the other. When we have conditions like these, it would seem that the
-    eruption of a volcano may have little or no influence in protecting
-    neighbouring districts.</p>
-
-  <p>This may possibly be the explanation of the fact that in 1835
-    Concepcion was destroyed, notwithstanding there being an unusual
-    activity in the volcanic vents of the neighbouring mountains.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XVII">
-    <span class="pagenum" id="Page_277">277</span>
-    <h2>CHAPTER XVII.<br />
-      <span class="subhead">THE CAUSE OF EARTHQUAKES.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Modern views respecting the cause of earthquakes—Earthquakes due
-    to faulting—To explosions of steam—To volcanic evisceration—To
-    chemical degradation—Attractive influence of the heavenly
-    bodies—The effect of oceanic tides—Variation in atmospheric
-    pressure—Fluctuation in temperature—Winds and earthquakes—Rain
-    and earthquakes—Conclusion.</div>
-
-  <p>As the results of modern inquiries respecting the cause of earthquakes,
-    we see many investigators chiefly attributing these phenomena to
-    special causes. A few attribute them to several causes. It seems to us
-    that they might be attributed to very many causes which often act in
-    a complex manner. The primary causes are telluric heats, solar heat,
-    and variations in gravitating influences. These may be the principal,
-    and sometimes the immediate, cause of an earthquake. The secondary
-    causes are those dependent upon the primary causes, such as expansions
-    and contractions of the earth’s crust, variations in temperature,
-    barometrical pressure, rain, wind, the attractive influences of the
-    sun and moon in producing tides in the ocean or the earth’s crust,
-    variations in the distribution of stress upon the earth’s surface
-    caused by processes of degradation, the alterations in the position of
-    isogeothermal surfaces, &amp;c.</p>
-
-  <p>The part which may be played by these various causes
-    <span class="pagenum" id="Page_278">278</span> in the production
-    of oscillations, pulsations, and tremors will be referred to.</p>
-
-  <p><em>Earthquakes consequent on faulting.</em>—In the chapter on Earth
-    Oscillations, the causes producing the phenomena of elevation and
-    depression are briefly indicated.</p>
-
-  <p>By the variations in stress accompanying elevations and depressions,
-    cracks are produced. Inasmuch as compression would crush the rocks
-    constituting the earth’s crust, we must conclude with Captain Dutton
-    that these cracks are formed by tension. By elevation, the upper rigid
-    crust of the earth is stretched, and fissures are produced. The sudden
-    formation of these fissures or faults gives rise to earthquakes, and
-    perhaps also to volcanic vents. That earthquake and volcanic regions
-    are situated on areas where there is evidence of rapid elevation is
-    strikingly illustrated round the shores of the Pacific.</p>
-
-  <p>Lasaulx considered that the earthquake of Herzogenrath was more
-    or less intimately connected with the great mountain fissure—the
-    <em>Feldbiss</em>—which crosses the coal region of the Wurm.<a id="FNanchor_126" href="#Footnote_126" class="fnanchor">[126]</a> The sudden
-    elevation or sinking of large areas at the time of an earthquake may be
-    a consequence of these dislocations.</p>
-
-  <p>It has already been pointed out that the earthquake region of Japan
-    is the one where we have evidence of recent and rapid elevation. That
-    certain earthquakes of this region may possibly be the result of
-    faulting we have the evidence of our senses and of our instruments. The
-    sudden blows and jolts which are sometimes felt are indicative of the
-    sliding of one mass of rock across another.</p>
-
-  <p>Should the ground be simply torn asunder, this tearing would give rise
-    to a series of waves of distortion, vibrating in directions parallel
-    to the plane of the fissure. Supposing this motion to be propagated
-    to a number of <span class="pagenum" id="Page_279">279</span>
-    surrounding stations, it would be recorded at each of
-    these as having the same direction. To those situated on a line forming
-    a continuation of the strike of the fissure, the vibrations would
-    advance so to speak <em>end on</em>, whilst to those stations lying in a line
-    perpendicular to the strike of the fissure, the motion would advance
-    <em>broadside on</em>.</p>
-
-  <p>Motions like these latter have been recorded in Tokio, where
-    earthquakes which from time observations were known to have come from
-    the faulted and rising region to the south have been registered as a
-    series of east and west motions, or vibrations transverse to this line
-    of propagation.</p>
-
-  <p>It must, however, be here mentioned that the registration of only
-    transverse motion may possibly be due to the extinction of normal
-    motion, although this is not generally regarded as probable.</p>
-
-  <p>It would therefore appear that certain earthquakes and faults are
-    closely related phenomena, the former being an immediate effect of
-    the latter. Faults are due to earth oscillations, and to a variety of
-    causes producing disturbances in the equilibrium of the earth’s crust;
-    the principal cause of all these phenomena being alterations in the
-    distribution of heat, and the attractive force of gravity.</p>
-
-  <p><em>Earthquakes consequent on the explosion of steam.</em>—Humboldt regarded
-    volcanoes and earthquakes as the results of a common cause, which he
-    formulated as ‘the reaction of the fiery interior of the earth upon
-    its rigid crust.’ Certain investigators, who have endeavoured to
-    reduce Humboldt’s explanation to definite limits, have suggested that
-    earthquakes may be due to sudden outbursts of steam beneath the crust
-    of the earth, and its final escape through cracks and fissures.</p>
-
-  <p>Admitting that steam may accumulate by separating
-    <span class="pagenum" id="Page_280">280</span> out from the cooling
-    interior of our globe, its sudden explosion might be brought about by
-    its own expansive force, or by the movements in the bubbling mass from
-    which it originated.</p>
-
-  <p>Others, however, rather than regard the steam as being a primeval
-    constituent of the earth’s interior, imagine it arises from the gradual
-    percolation of water from the surface of the earth down to volcanic
-    foci, into which it is admitted against opposing pressures, by virtue
-    of capillary action.</p>
-
-  <p>Mallet, in his account of the Neapolitan earthquake, shows that the
-    whole of the observed phenomena can be accounted for by the admission
-    of steam into a fissure, which by the expansive force exerted on its
-    walls was rent open. Just as at the Geysers we hear the thud and
-    feel the trembling produced by the sudden evolution and condensation
-    of steam, so may steam by its sudden evolution and condensation in
-    the ground beneath us give rise to a series of shocks of varying
-    intensity, accompanied by intermediate vibratory motions—that is to
-    say, a motion which, as judged of by our feelings, is not unlike many
-    earthquakes. Often it may happen that the result of the explosion may
-    be the production of a fault, or at least a fissure; and thus in the
-    resulting movements we may have a variety of vibrations, some being
-    those of compression and distortion, produced by the blow of the
-    explosion, and others being those of distortion alone, produced by the
-    shearing action which may have taken place by the opening of the fault.
-    Sometimes one set of these vibrations may be prominent, and sometimes
-    the other. Thus, when we say that an earthquake has shown evidence by
-    the nature of its vibrations that it was produced by a fault, this
-    by no means precludes the possibility that an explosion of steam may
-    also have been connected<span class="pagenum" id="Page_281">281</span>
-    with the production of the disturbance.
-    Mallet threw out the suggestion that the opening of fissures beneath
-    the ocean might admit water to volcanic foci. During the time that the
-    water was in the spheroidal state, the preliminary tremors, so common
-    to many earthquakes, would be produced. These would be followed by the
-    explosion, or series of explosions, constituting the shock or shocks of
-    the earthquakes.</p>
-
-  <p>The chief reasons for believing that the earthquakes of North-Eastern
-    Japan are partly due to explosive efforts are:—</p>
-
-  <p>1. That the greater number of disturbances, perhaps ninety per cent.,
-    originate beneath the sea, where we may imagine that the ground,
-    under the superincumbent hydrostatic pressure, is continuously being
-    saturated with moisture.</p>
-
-  <p>2. Many of the diagrams show that the prominent vibrations, of which
-    there are usually from one to three, in a given disturbance have the
-    same character as those produced by an explosive like dynamite, the
-    greatest and probably the most rapid motions being inwards towards the
-    origin.</p>
-
-  <p>It may here be remarked that a very large proportion of the destructive
-    earthquakes of the world have originated beneath the sea, as has often
-    been testified by the succeeding sea waves. Also, it must be observed,
-    that earthquake countries, like volcanic countries, are chiefly those
-    which have a coast line sloping at a steep angle beneath the sea—that
-    is to say, earthquakes are frequent along coasts bordered by deep water.</p>
-
-  <p>The earthquakes which occur at volcanic foci constitute another class
-    of disturbances which may be accredited to the explosive efforts of
-    steam.</p>
-
-  <p><em>Earthquakes due to volcanic evisceration.</em>—By the
-    <span class="pagenum" id="Page_282">282</span> ejection of ashes
-    and lava from volcanic vents, there is an extensive evisceration of
-    the neighbouring ground. When we look at a volcano like Fujiyama,
-    13,000 feet in height, and at least fifty miles in circumference, and
-    remember that the mass of cinders and slag of which it is composed
-    came from beneath the area on which it rests, the point to be wondered
-    at is, that earthquakes, consequent on the collapse of subterranean
-    hollows, are not more frequent than they are. At the time of a single
-    eruption of a volcano, the quantity of lava ejected amounts to many
-    thousand millions of cubic feet. In 1783 the quantity of lava ejected
-    from Skaptas Joknee, in Iceland, was estimated as surpassing ‘in
-    magnitude the bulk of Mont Blanc.’<a id="FNanchor_127" href="#Footnote_127" class="fnanchor">[127]</a> Admitting that hollow spaces
-    are the results of these eruptions, and that in consequence of this
-    evisceration the ground is rendered unstable, the instability being
-    increased by the additional load placed above the eviscerated area, it
-    would seem that from time to time earthquakes are inevitable.</p>
-
-  <p>Facts, however, teach us that volcanoes act as safety valves, and
-    that, as a rule, at or shortly after an eruption, earthquakes cease.
-    The relationship of earthquakes to volcanic eruptions would therefore
-    indicate, notwithstanding the arguments put forward to show that an
-    area loaded by a volcano has in consequence of the evisceration and the
-    load a quaquaversal dip, that evisceration does not take place beneath
-    volcanoes as is usually supposed, and we may conclude that it is but
-    few earthquakes which have an origin due to these causes.</p>
-
-  <p><em>Earthquakes and evisceration by chemical degradation.</em>—A powerful
-    agent, which tends to the formation of subterranean hollows, is
-    chemical degradation. The effects of this have been often measured by
-    quantitative <span class="pagenum" id="Page_283">283</span>
-    analysis of the solid materials which are daily carried
-    away by many of our springs. In limestone districts this is very great.
-    Prof. Ramsay estimates that the mineral matter discharged annually by
-    the hot springs of Bath is equivalent in bulk to a column 140 feet in
-    height and 9 feet in diameter. At San Filippo, in Tuscany, the solid
-    matter discharged from the springs has formed a hill a mile and a
-    quarter long, a third of a mile broad, and 250 feet in thickness.<a id="FNanchor_128" href="#Footnote_128" class="fnanchor">[128]</a>
-    Many other examples of subterranean chemical degradation will be found
-    in text-books of geology.</p>
-
-  <p>By this chemical action large cavernous hollows are produced. Beneath
-    a volcano it is probable that liquid material immediately takes the
-    place of that which is ejected, and that hollows are not formed as in
-    the case of chemical degradation. If a cavern becomes too large, it
-    eventually collapses.</p>
-
-  <p>Of the falling in of large excavations we have examples in large mines.
-    As a consequence, not only is a trembling produced, but also a noise,
-    which is so like that produced by certain earthquakes that the South
-    American miners have but one word, ‘bramido,’ to express both.<a id="FNanchor_129" href="#Footnote_129" class="fnanchor">[129]</a></p>
-
-  <p>Boussingault, who was an advocate for the theory that many earthquakes
-    are produced by the sinking of the ground, calls attention to the fact
-    that we have evidences of the subsidence of great mountains, like
-    the Andes, the districts around which are so well known for their
-    earthquakes. Capac Urcu is one of these mountains which legends tell us
-    has decreased in height.</p>
-
-  <p>The variation in the height of mountains is a subject which deserves
-    attention. That mountains may possibly be hollow, we have the
-    remarkable results attained by <span class="pagenum" id="Page_284">284</span>
-    Captain Herschel, who found that the
-    attractive force of gravity in the neighbourhood of the Himalayas was
-    not so great as it ought to have been had these mountains been solid.
-    The Rev. O. Fisher gives another explanation of this phenomenon.
-    Palmieri considers that the terrible earthquake which devastated
-    Casamicciola (1881) was due to the hot springs having gradually eaten
-    out cavernous spaces beneath the town. The extremely local character of
-    this shock was certainly favourable to such a view.</p>
-
-  <p>The earthquake which, in 1840, caused Mount Cernans, in the Jura, to
-    fall, is also attributed to the solvent action of waters in undermining
-    its foundations. This undermining action was in great measure probably
-    due to a large spring, which, twenty-five years previously, had
-    disappeared, and which subsequently may possibly have been slowly
-    disintegrating the foundations of the mountain. Earthquakes of this
-    order would be principally confined to districts where there are rocks
-    which are more or less soluble, as, for instance, rock salt, gypsum,
-    and limestone.</p>
-
-  <p><em>Earthquakes and the attractive influences of the heavenly bodies.</em>—The
-    most important attractions exercised upon our planet are those due to
-    the sun and moon. To these influences we owe the tides in our ocean,
-    and possibly elastic tides in the earth’s crust. Some theorists would
-    also insist upon liquid tides in the fluid interior of our earth. The
-    nature of the earth’s interior is, however, a question on which there
-    is a diversity of opinion.</p>
-
-  <p>One doctrine, which, until recent years, received much support, was
-    that the interior of the earth was a reservoir of molten matter
-    contained within a thin crust. Hopkins showed that the least
-    possible thickness of such a crust must be from 800 or 1,000 miles,
-    otherwise the<span class="pagenum" id="Page_285">285</span>
-    motions of precession and nutation would be subject to interference.</p>
-
-  <p>M. Delauney objected to the views of Hopkins, on the supposition that
-    the fluid interior of the earth had a certain viscosity.</p>
-
-  <p>Sir William Thomson arrives at the conclusion that the earth on the
-    whole must be more rigid than a continuous solid globe of glass. Mr.
-    George H. Darwin’s investigations on the bodily tides of viscous or
-    semi-elastic spheroids tend to strengthen the arguments of Sir William
-    Thomson.</p>
-
-  <p>Some philosophers hold the view that the central portion of the earth,
-    although intensely hot, is solid by pressure, whilst the outer crust is
-    solid by cooling. Between the two there is a shell of liquid or viscous
-    molten matter.</p>
-
-  <p>Another argument is, that although the interior of the globe may be
-    solid, it is only retained in that condition by an immense pressure, on
-    the relief of which it is liquefied—it is potentially liquid.</p>
-
-  <p>As these views, and the arguments for and against them, are to be
-    found in all modern text-books of geology, we will at once proceed
-    to consider the effect of solar and lunar attractive influences in
-    producing earthquakes upon a globe which is either solid, partially
-    solid, or which has an interior wholly liquid.</p>
-
-  <p><em>Effect of the attractive influences of the sun and moon.</em>—In 1854
-    M. F. Zantedeschi put forward the view—that it is probable there is
-    a continual tendency of the earth to protuberance in the direction
-    of the radii vectores of the two luminaries which attract it. In
-    consequence of these protuberances, pendulums ought at one time to
-    swing more slowly than at others. Zantedeschi remarks that the periods
-    of earthquakes appear to confirm
-    <span class="pagenum" id="Page_286">286</span> such a view, insomuch as they occur
-    more often at the syzygies, or epoch of the spring tides, than at neap
-    tides—an observation found in the works of Georges Baglivi (1703) and
-    Joseph Toaldo (1770).<a id="FNanchor_130" href="#Footnote_130" class="fnanchor">[130]</a></p>
-
-  <p>Prof. Perrey, of Dijon, who did so much for seismology, held the view
-    that the preponderance in the number of earthquakes felt at particular
-    seasons was possibly due to the attractive influence of the sun and
-    moon producing a tide in the fluid interior of the earth, which, acting
-    on the solid crust, produced fractures.</p>
-
-  <p>Rudolf Falb, whose writings have of late years attracted considerable
-    attention, brings forward views which may be regarded as amplifications
-    of those suggested by Perrey.</p>
-
-  <p>According to Falb, the inner portion of the earth must be regarded as
-    fluid. In the crust above this fluid reservoir are cracks and channels,
-    into which, by the attraction of the moon and sun, the fluid is drawn.
-    On entering these cracks cooling takes place, together with explosions
-    of gas and subterranean volcanic disturbances. The attractions
-    producing the internal tides required by Falb are chiefly dependent
-    upon the following factors:—</p>
-
-  <p>1. The nearness and distance of the sun from the earth (January 1 and
-    July 1).</p>
-
-  <p>2. The position of the moon with regard to the earth, which in every
-    twenty-seven days is once near and once distant.</p>
-
-  <p>3. The phases of the moon—whether full or new moon (syzygies), or
-    whether first or last quarter (quadratures).</p>
-
-  <p>4. The equinoxes, the position of the sun in the equator, and the
-    relative position of the earth.</p>
-
-  <p>5. The position of the moon relative to the equator.</p>
-
-  <p><span class="pagenum" id="Page_287">287</span></p>
-
-  <p>6. The concurrence of the ‘centrifugal force’ of the earth with the
-    last quarter of the moon.</p>
-
-  <p>7. The entrance of the moon on the ecliptic—the so-called nodes.</p>
-
-  <p>Assuming that earthquakes are wholly consequent on these attractions,
-    it at once becomes possible to predict their occurrence. This Falb
-    does, and when his predictions have been fulfilled he has certainly
-    gained notoriety.</p>
-
-  <p>He commenced by the predictions of great storms. In 1873 he predicted
-    the destructive earthquake of Belluno, which earned for himself a
-    eulogistic poem, which he has republished in his ‘Gedanken und Studien
-    über Vulkanismus.’ After this, in 1874, he predicted the eruption of
-    Etna. He also explained why, in <span class="smcap">b.c.</span> 4000, there should have
-    been a great flood, and for <span class="smcap">a.d.</span> 6400 he predicts a repetition
-    of such an occurrence.</p>
-
-  <p>When we approach the question of the extent to which the attraction
-    of the sun and moon may influence the production of earthquakes, a
-    question which we have to answer is, whether it is likely that the
-    attractive power of the moon is so great that it could draw up the
-    crust the earth beyond its elastic limits. We know what it can do with
-    water. It can lift up a hemispherical shell 8,000 miles in diameter
-    about two or three feet higher at its crown than it lifts the earth.
-    Even supposing the solid crust to be lifted 100 times the apparent
-    rise of the tide, is it likely that a hemispherical arch 8,000 miles
-    in diameter when it is raised 200 feet at its crown could by any
-    possibility suffer fracture? If an arch is 12,000 miles in length, all
-    that we here ask is, whether the materials which compose the arch are
-    sufficiently elastic to allow themselves to be so far stretched that
-    the crown may be raised 200 feet. The result which we should arrive
-    at is apparently so obvious that actual calculation seems hardly
-    <span class="pagenum" id="Page_288">288</span>
-    necessary. If we regard the earth as being solid, the question resolves
-    itself into the inquiry as to whether a column of rock, which is equal
-    in length to the diameter of the earth, or about 8,000 miles, can be
-    elongated 200 feet without a fracture. This is equivalent to asking
-    whether a piece of rock one yard in length can be stretched one seventy
-    thousandth of a foot. Considering that this is a quantity which is
-    scarcely appreciable under the most powerful of our microscopes, we
-    must also regard this as a question which it is hardly necessary to
-    enter into calculations about before giving it an answer. To vary
-    the method of treating such a question, may we not ask what is the
-    utmost limit to which it would be possible to raise up or stretch the
-    crust of the earth without danger of a fracture? Thus, for instance,
-    to what extent might a column of rock be elongated without danger
-    of its being broken? From what we know of the tenacity of materials
-    like brick and their moduli of elasticity, it would seem possible to
-    stretch a bar of rock 8,000 miles in length for approximately half
-    a mile before expecting it to break. As to whether there is a wave,
-    the height of which is equal to half this quantity, running round our
-    earth as successive portions of its surface pass beneath the attracting
-    influences of the sun and moon, is a phenomenon which, if it exists,
-    would probably long ago have met with a practical demonstration.</p>
-
-  <p>The deformation which a solid globe or spherical shell would
-    experience under the attractive influences of the sun and moon has
-    been investigated by Lamé, Thomson, Darwin, and other physicists and
-    mathematicians.</p>
-
-  <p>A conclusion that we are led to as one result of these valuable
-    investigations is, that if the interior of the earth be fluid, and
-    covered with a thin shell, then enormous
-    <span class="pagenum" id="Page_289">289</span> elastic tides must be
-    produced. A consequent phenomenon, dependent on the existence of these
-    tides, would be a marked regularity in the occurrence of earthquakes.
-    As this marked regularity does not exist, we must conclude that
-    earthquakes are not due to the attractive influences of the sun and
-    moon acting upon the thin crust of the earth covering a fluid interior.
-    The periodicity of earthquakes corroborates the conclusions of Sir
-    William Thomson, who remarks that if the earth were not extremely rigid
-    the enormous elastic tides which must result would be sufficient to
-    lift the waters of the ocean up and down so that the oceanic tide would
-    be obliterated.</p>
-
-  <p>Assuming that the earth has the rigidity assigned to it by mathematical
-    and physical investigators, we nevertheless have travelling round our
-    earth, following the attractions of the moon and sun, a tidal stress.
-    This stress, imposed upon an area in a critical state, may cause it to
-    give way, and thus be the origin of an earthquake. Earthquakes ought
-    therefore to be more numerous when these stresses are the greatest.</p>
-
-  <p>The periods of maximum stress or greatest pull exerted by the moon and
-    sun will occur when these bodies are nearest to our planet—that is, in
-    perigee and perihelion, and again when they are acting in conjunction
-    or at the syzygies. That earthquakes are <em>slightly</em> more numerous
-    at these particular periods than at others is a strong reason for
-    believing that the attractions of the moon and sun enter into the list
-    of causes producing these phenomena.</p>
-
-  <p>Had there been a strongly marked distinction in the number of
-    earthquakes occurring at these particular seasons as compared with
-    others, we might have attributed earthquakes to the existence of
-    elastic tides of a sensible<span class="pagenum" id="Page_290">290</span>
-    magnitude. As the facts stand, it appears
-    that the maximum pulls exerted by the moon and sun are only sufficient
-    to cause a slight preponderance in the number of earthquakes felt at
-    particular seasons, and therefore that these pulls only result in
-    earthquakes when the distorting effort has been exerted on an area
-    which, by volcanic evisceration, the pressure of included gases, and
-    other causes, is on the verge of yielding.</p>
-
-  <p><em>Earthquakes and the tides.</em>—If we assume that earthquakes are in many
-    cases due to the overloading of an area and its consequent fracture,
-    such loading may occur by the rising of the tide. A belief that the
-    earthquakes of Japan were attributable to the tides may be found in the
-    diary of Richard Cocks under the date November 7, 1618, who remarks:—</p>
-
-  <p>‘And, as we retorned, about ten aclock, hapned a greate earthquake,
-    which caused many people to run out of their howses. And about the lyke
-    hower the night following hapned an other, this countrey being much
-    subject to them. And that which is comunely markd, they allwais hapen
-    at a hie water (or full sea); so it is thought it chauseth per reason
-    is much wind blowen into hollow caves under ground at a loe water, and
-    the sea flowing in after, and stoping the passage out, causeth these
-    earthquakes, to fynd passage or vent for the wind shut up.’<a id="FNanchor_131" href="#Footnote_131" class="fnanchor">[131]</a></p>
-
-  <p>Although we may not acquiesce in Cock’s views respecting the imprisoned
-    wind, it would seem that a comparison of the occurrence of earthquakes
-    and the state of the tide would be a legitimate research. Inasmuch
-    as the stresses which are brought to bear upon an area by the rising
-    of the tide are so very much greater than those due to barometrical
-    changes, it is not unlikely <span class="pagenum" id="Page_291">291</span>
-    that a marked connection would be found.
-    But it must be remembered that because researches, so far as they have
-    gone, tend to show that earth movements are more frequent when an area
-    is relieved of a load, it is not unlikely that the greatest number of
-    earthquakes may be found to occur at low water. Prof. W. S. Chaplin
-    attempted to make this investigation in Japan, but not being able to
-    obtain the necessary information respecting the tides, was compelled to
-    relinquish this interesting work.</p>
-
-  <p>Every foot of rise in a tide is equivalent to a load being placed on
-    the area over which the tide takes place of sixty-two pounds to the
-    square foot. This load is not evenly distributed, but stops abruptly at
-    a coast line. Lastly, it may be observed that many coast lines are not
-    simultaneously subjected to stresses consequent upon this load. Japan,
-    for instance, may be regarded as an arch placed horizontally. The
-    area near the crown of this arch is loaded by the tidal wave crossing
-    the Pacific before the areas near the abutment, and farther there is
-    a horizontal pressure at the crown which, if Japan were like a raft,
-    would tend, as the tide advanced, to straighten its bow-like form, but
-    as the wave passed its abutments to increase its curvature.</p>
-
-  <p>Prof. G. Darwin has calculated the amount of rise and fall of a shore
-    line due to tidal loads (see <a href="#Page_336">p. 336</a>, ‘Earth Pulsations’). The result
-    of these calculations apparently indicates that these loads may have a
-    considerable influence upon the stability of an area in a more or less
-    critical condition.</p>
-
-  <p>Mr. J. Carruthers suggests that tidal action may hold a general but
-    indirect relationship to volcanic and seismic action by the retardation
-    it causes on the earth’s rotation. By this retardation the polar axis
-    tends to lengthen, and<span class="pagenum" id="Page_292">292</span>
-    tensile stresses are induced, resulting in
-    fracture. The fluid interior of the earth, being no longer restrained,
-    would move polewards, and, leaving equatorial portions unsupported,
-    this would gradually collapse. The primary fractures would be north and
-    south, while the secondary fractures would be east and west.<a id="FNanchor_132" href="#Footnote_132" class="fnanchor">[132]</a></p>
-
-  <p>That the rise of the tide is accompanied by a greater percolation
-    of water to volcanic foci, which, in consequence, assume a greater
-    state of activity, is a theory which was advanced many years ago. To
-    determine how far tides may directly be connected with earthquakes, the
-    necessary records have yet to be examined.</p>
-
-  <p><em>Variations in atmospheric pressure.</em>—When we consider the immense
-    load which, by a sudden rise of the barometer, is placed upon the area
-    over which this rise takes place, it is not difficult to imagine that
-    this rise may occasionally be the final cause which makes the crust of
-    the earth to give way. A barometric rise of an inch is equivalent to a
-    load of about seventy-two pounds being put upon every square foot of
-    area over which this rise takes place. On the other hand, a fall in the
-    barometric column indicates that a load has been removed, and whatever
-    elastic effort may be exerted by subterranean forces in endeavouring to
-    escape, being met by less resistance, they may burst these bonds, and
-    an earthquake will result. For reasons such as these the final cause
-    of earthquakes has often been attributed to variations in atmospheric
-    pressure. In Japan there are practically as many earthquakes with a
-    high barometer as with a low one.</p>
-
-  <p>The extent to which barometric fluctuations have acted as final causes
-    in the production of earthquakes may be judged of by a comparison of
-    the times of barometric<span class="pagenum" id="Page_293">293</span>
-    variation and the times at which earthquakes have occurred.</p>
-
-  <p>Three important laws of barometric variation are the following:—</p>
-
-  <p>1. In the world generally the average barometric pressure is highest in
-    winter. (Exceptions occur near Iceland and in the North Pacific.)</p>
-
-  <p>2. The summer and winter monthly mean barometer differs least near the
-    equator and over the great oceans. They differ most over the great
-    continents and generally with increasing latitude.</p>
-
-  <p>3. The greatest number of barometrical fluctuations usually take place
-    in winter.</p>
-
-  <p>Inasmuch as there are generally more earthquakes in winter than in
-    summer, the first of these laws would indicate that this might be due
-    to the greater load which acts upon the crust of the earth at that
-    season. The second law would indicate that the distinction between
-    the winter and summer earthquakes ought to be most marked in high
-    latitudes, which, if we refer to the table on p. 257, we observe to be
-    borne out by the results of observation. The countries where there are
-    as many earthquakes in winter as in summer are chiefly those in low
-    latitudes. The number of these countries from which we have records
-    are, however, few.</p>
-
-  <p>Facts opposed to the idea that earthquakes may be caused by an increase
-    of barometric pressure are the results of observations like those of
-    Schmidt and Rossi, which show that earthquakes chiefly occur with a low
-    barometer.</p>
-
-  <p>Assuming that these latter observations will be found by future
-    investigators to be generally true, we must conclude that the relief
-    of atmospheric pressure has an influence upon the occurrence of
-    earthquakes. Such a<span class="pagenum" id="Page_294">294</span>
-    conclusion would partially accord with the third
-    barometrical law, or the fact that there are more occasions on which we
-    get a low barometer during the winter months.</p>
-
-  <p>Other writers who have examined this question are Volger, Kluge,
-    Andrès, and Poly. The latter investigator sought a connection between
-    earthquakes and revolving storms, in the centres of which there is
-    usually an abnormal decrease of atmospheric pressure. If an area over
-    which such a sudden change in pressure took place was in a critical
-    state, it is not difficult to see that storms such as Poly refers to
-    might sometimes be accompanied by earthquakes.</p>
-
-  <p><em>Fluctuations in temperature.</em>—Inasmuch as fluctuations in temperature
-    are governed by the sun, it may at once be said that there is a
-    connection between earthquakes and readings of the thermometer.
-    Certainly earthquakes occur mostly during the cold months or in
-    winter. Similarly, as changes in temperature are so closely connected
-    with barometric fluctuations, and these are said to have a direct
-    influence upon the yielding of the earth’s crust, seismic phenomena are
-    indirectly linked to fluctuations in temperature. A rise in temperature
-    is usually accompanied by a fall in the barometer, and this in turn may
-    be a condition favourable for the occurrence of an earthquake.</p>
-
-  <p>If we regard solar heat as an agent causing expansions or contractions
-    in the earth’s crust, then fluctuations in temperature become an
-    immediate cause of earthquakes. The probability, however, is that
-    solar heat has little or no connection with the final cause producing
-    earthquakes, although at the same time coincidences between the
-    occurrence of earthquakes and unusual fluctuations in temperature may
-    from time to time be observed.</p>
-
-  <p><em>Winds and earthquakes.</em>—Although it may be
-    <span class="pagenum" id="Page_295">295</span> admitted that high winds
-    exert enormous pressures upon mountain ranges, and might occasionally
-    give rise to stresses causing rocky masses in unstable equilibrium to
-    give way, the coincidences which have been established between the
-    occurrence of storms and earthquakes can usually only be regarded as
-    occurrences which have synchronised by chance.</p>
-
-  <p>Storms are usually accompanied with a barometric depression, and the
-    relation of diminutions in atmospheric pressure to earthquakes has been
-    discussed.</p>
-
-  <p><em>Rain and earthquakes.</em>—It has already been shown that earthquakes
-    have occasionally been found to coincide with rain and rainy seasons.
-    Whether the saturation of the ground with moisture or the percolation
-    of the same to volcanic foci may be a direct effect producing
-    earthquakes it is difficult to say. The probability, however, is
-    that, rain being dependent on phenomena like changes in temperature,
-    barometric fluctuations, and winds, we must regard it and the
-    earthquakes which happen to coincide with these precipitations of
-    moisture as congruent effects of more general causes.</p>
-
-  <p><em>Conclusion.</em>—Although it would be an easy matter to discuss the
-    relationship of earthquakes and other phenomena, we must conclude
-    that the primary cause of earthquakes is endogenous to our earth, and
-    that exogenous phenomena, like the attraction of the sun and moon and
-    barometric fluctuations, play but a small part in the actual production
-    of these phenomena, their greatest effect being to cause a slight
-    preponderance in the number of earthquakes at particular seasons. They
-    may, therefore, sometimes be regarded as final causes. The majority of
-    earthquakes are due to explosive efforts at volcanic foci. The greater
-    number of these explosions take place beneath the sea, and are probably
-    due to the<span class="pagenum" id="Page_296">296</span>
-    admission of water through fissures to the heated rocks
-    beneath. A smaller number of earthquakes originate at actual volcanoes.
-    Some earthquakes are produced by the sudden fracture of rocky strata or
-    the production of faults. This may be attributable to stresses brought
-    about by elevatory pressure. Lastly, we have earthquakes due to the
-    collapse of underground excavations.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XVIII">
-    <span class="pagenum" id="Page_297">297</span>
-    <h2>CHAPTER XVIII.<br />
-      <span class="subhead">PREDICTION OF EARTHQUAKES.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    General nature of predictions—Prediction by the observation of
-    unusual phenomena (alteration in the appearance and taste of
-    springs; underground noises; preliminary tremors; earthquake
-    prophets—warnings furnished by animals, &amp;c.)—Earthquake warning.</div>
-
-  <p><em>General nature of predictions.</em>—Ever since seismology has been
-    studied, one of the chief aims of its students has been to discover
-    some means which would enable them to foretell the coming of an
-    earthquake, and the attempts which have been made by workers in various
-    countries to correlate these occurrences with other well-marked
-    phenomena may be regarded as attempts in this direction.</p>
-
-  <p>Ability to herald the approach of these calamities would unquestionably
-    be an inestimable boon to all who dwell in earthquake-shaken countries,
-    and the attempts which have been made both here and in other places are
-    extremely praiseworthy. In almost all countries where earthquakes are
-    of common occurrence these movements of the earth have been more or
-    less connected with certain phenomena which, in the popular mind, are
-    supposed to be associated with an approach of an earthquake.</p>
-
-  <p>If predictions were given in general terms, and they only referred
-    to time, inasmuch as on the average there are in the world several
-    shakings per day, we should<span class="pagenum" id="Page_298">298</span>
-    always find that predictions were
-    verified. We might even go further and predict that on certain days
-    earthquakes would occur in certain countries, and still find that in
-    many instances our supposed power of foresight had not deceived us.
-    Thus, for instance, in Japan, where on the average there are probably
-    one or two shakings every day, if prognostications were never correct
-    there would be a violation of the laws of chance.</p>
-
-  <p>What is required from those who undertake to forewarn us of an
-    earthquake is an indication not only of the time at which the
-    disturbance will happen, but also an indication of the area in which
-    it is to occur. Those who dwell in an area where there are certain
-    well-defined periods during which seismic activity is at a maximum—if
-    ten or fourteen days should have passed without a shock—might, in many
-    instances, find that a prophecy that there would be an earthquake
-    within the next few days would prove itself correct. Also, if a severe
-    shock had taken place, a prophecy that there would be a second or
-    third smaller disturbance within a short period would also meet with
-    verification.</p>
-
-  <p>Certain persons with whom I am intimate appear to have persuaded
-    themselves that they can foretell the coming of an earthquake by the
-    sultry state of the atmosphere or a certain oppressiveness they feel,
-    and an instinctive feeling arises that an earthquake is at hand.</p>
-
-  <p>It is said that a few minutes before many of the shocks which shook
-    New England between 1827 and 1847 people could foretell the coming
-    disturbance by an alteration in their stomach.<a id="FNanchor_133" href="#Footnote_133" class="fnanchor">[133]</a> No doubt many who
-    dwell in earthquake countries, and have been alarmed by earthquakes,
-    are at times subject to nervous expectancy.</p>
-
-  <p>The author has had such sensations himself, due, perhaps,
-    <span class="pagenum" id="Page_299">299</span> to a
-    knowledge that it was the earthquake season, that there had been no
-    disturbance for some weeks, and a consequent increasing state of
-    nervous presentments. In consequence of this, not only has he carefully
-    prepared his instruments for the coming shock, but he has written and
-    telegraphed to friends to do the same.</p>
-
-  <p>Sometimes these guesses have proved correct. One remarkable instance
-    was a few hours prior to the severe shock of February 22, 1880, when
-    he communicated with his friends in Yokohama and asked them to see
-    that their instruments were in good order. Oftener, however, his
-    prognostications have been incorrect. The point in connection with
-    this subject which he wishes to be remarked is, that the instances
-    where earthquakes occurred shortly after the receipt of his letters
-    are carefully remembered, and often mentioned, but the instances in
-    which earthquakes did not occur appear to be entirely forgotten. He is
-    led to mention these facts because they appear to be an experimental
-    proof of what has taken place in bygone times, and what still takes
-    place, especially amongst savages—namely, that the record of that which
-    is remarkable survives, whilst that which is of every-day occurrence
-    quickly dies. Had the records of all prognostications been preserved,
-    the probability is that we should find that they had, in the majority
-    of cases, been incorrect, whilst it would have been but in very few
-    instances they had been fulfilled.</p>
-
-  <p><em>Prediction by the observation of natural phenomena.</em>—The above remarks
-    may perhaps help us to understand the prognostications of the ancient
-    philosophers about which Professor Antonio Favaro, of Padua, has
-    written.<a id="FNanchor_134" href="#Footnote_134" class="fnanchor">[134]</a> Cicero in the ‘De Divinatione,’ speaking on this subject,
-    says that ‘God has not predicted so much
-    <span class="pagenum" id="Page_300">300</span>as the divine intelligence of
-    man.’—‘Non Deus prævidet tantum, sed et divini in genii viri.’ Favaro
-    regards these predictions, however, as the result of observations of
-    nature which show it is possible that indications of coming earthquakes
-    had been announced by variations in the gas given out from subterranean
-    sources, the change in colour, taste, level, temperature of the water
-    in springs, &amp;c.</p>
-
-  <p>In 1843 a bishop of Ischia forewarned his people of a conning
-    earthquake, and thus was instrumental in the saving of many lives.
-    Naturally, in an age of superstition, the bishop would be regarded as
-    a prophet, but Favaro considers that the prognostication was probably
-    due to a knowledge of premonitory signs as exhibited in changes in the
-    characters of mineral waters.</p>
-
-  <p>The shock of 1851, at Melfi, was in this way predicted by the Capuchin
-    fathers, who observed that a lake near their door became frothy and
-    turbulent.</p>
-
-  <p>Underground noises have led persons to the belief that an earthquake
-    was at hand. It was in this way that Viduari, a prisoner at Lima,
-    predicted the destruction of that city.</p>
-
-  <p>Before the earthquake of 1868, so severely felt at Iquique, the
-    inhabitants were terrified by loud subterranean noises.</p>
-
-  <p>That underground noises have preceded earthquakes by considerable
-    intervals appears to be a fact, but, at the same time, it must
-    be remembered that similar noises have often occurred without an
-    earthquake having taken place.</p>
-
-  <p>Farmers predicted the earthquake of St. Remo, in 1831, by underground
-    noises.</p>
-
-  <p>On the day before the earthquake which, in 1873, shook Mount Baldo,
-    the inhabitants of Puos, a village north of Lake Santa Croce, heard
-    underground noises.</p>
-
-  <p><span class="pagenum" id="Page_301">301</span></p>
-
-  <p>Before the earthquakes which, in 1783, shook Calabria and Sicily, fish
-    are said to have appeared in great numbers on the coast of Sicily, and
-    the whirlpool of Charybdis assumed an unusual excited state.</p>
-
-  <p>It is said that Pherecydes predicted the earthquakes of Lacedemon and
-    Helm out, by the taste of the water in the very deep well at the castle
-    of Lovain.<a id="FNanchor_135" href="#Footnote_135" class="fnanchor">[135]</a></p>
-
-  <p>The writer of an article on the Lisbon earthquake says that ‘after the
-    24th I felt apprehensive, as I observed the same prognostics as on the
-    afternoon of October 31, that is, the weather was severe, the wind
-    northerly, a fog came from the sea, the water in a fountain ran of a
-    yellow clay colour, and’ he adds, ‘from midnight to the morning of the
-    25th I felt five shocks.’<a id="FNanchor_136" href="#Footnote_136" class="fnanchor">[136]</a></p>
-
-  <p>At the present time Rudolf Falb, following a theory based upon the
-    attractive influences of the sun and moon, tells us the time at which
-    we are to expect earthquakes.</p>
-
-  <p>That occasionally there are signs attendant on earthquakes, although
-    we cannot give them a physical explanation, we cannot doubt. Also we
-    know that in certain areas earthquakes are more likely to occur at
-    one season than at another. Should earthquakes be foretold with the
-    assistance of knowledge of this description, the predictions at once
-    become the result of the application of certain natural laws, and are
-    not to be regarded as predictions in the popularly accepted sense of
-    that term, any more than the arrival of a friend is predicted by the
-    previous receipt of a telegram announcing his coming.</p>
-
-  <p>Rather than accredit the ancients and those of more modern times who,
-    in consequence of their feelings, have recorded the coming of an
-    earthquake, with a knowledge of premonitory signs, we might in many
-    instances regard <span class="pagenum" id="Page_302">302</span>
-    the records of those prognostications as the survival
-    of accidental guesses, and, as such, examples of the survival of the
-    useless.</p>
-
-  <p>The effect of accidental occurrences of this description upon an
-    uneducated mind, in engendering superstition, is a subject which has
-    often been dwelt upon, and the difficulty of eradicating the same—as
-    may be judged of by the following accident which came under the
-    observation of Mr. T. B. Lloyd and the author, in 1873, when travelling
-    in Newfoundland—will be easily appreciated.</p>
-
-  <p>At the time to which I refer, my companion was bringing a canoe down
-    the rapids below the Grand Pond in a country which is practically
-    uninhabited, and where an Indian trapper would perhaps be the only
-    person met with, and this not more than once a year. Whilst shooting
-    the rapids one of the Indians, Reuben Soulian, shot at a deer passing
-    up one bank of the river. That the deer had been hit was testified by a
-    trail of blood which bespattered the rocks. Subsequently several more
-    shots were fired, and it was believed by all that the deer was killed.
-    Soulian quickly followed the animal to the spot where it was supposed
-    to have fallen. Some time after he returned, having failed to find any
-    trace of the animal. He was greatly agitated, but eventually became
-    melancholy, saying that the sudden disappearance of the animal was a
-    sure sign that some of his relations had suddenly died. About two hours
-    afterwards Mr. Lloyd’s party met with a party of Indians coming up
-    the river, the first they had seen for four weeks, who told them that
-    Soulian’s sister had just died on the coast.</p>
-
-  <p>In the northern part of South America certain shocks are anticipated by
-    preliminary vibrations which cause a little bell attached to a T-shaped
-    frame (cruz sonante) to ring. There are, however, persons (trembloron)
-    who are<span class="pagenum" id="Page_303">303</span>
-    supposed to be endowed with seismic foresight, whose verdicts
-    are much relied upon.</p>
-
-  <p>In Caraccas it is said that nearly every street in the river suburb has
-    an earthquake Cassandra or two. Some of these go so far not only as
-    to predict the coming seisms, but also the vicissitudes of particular
-    streets.<a id="FNanchor_137" href="#Footnote_137" class="fnanchor">[137]</a> Earthquake prophets are, however, by no means confined to
-    the new world, and many examples of them may be found in the histories
-    of countries where earthquakes have been felt.</p>
-
-  <p>The story of the crazy lifeguardsman who prophesied an earthquake to
-    take place in London on April 4, 1691, is an example. The Rev. Sig.
-    Pasquel E. Perdini, writing on the earthquakes at Leghorn in 1742,
-    says that ‘a Milanese astrologer predicted this earthquake for January
-    27, by which “misfortune” the faith and credit given to the astrologer
-    gained him more reverence and honour than the prophets and holy
-    gospel.’ Before the time at which he predicted a second shock, people
-    removed away from Leghorn.</p>
-
-  <p><em>Warnings furnished by animals.</em>—A study of the warnings furnished by
-    animals is also interesting. Several of the natives in Caraccas possess
-    oracular quadrupeds, such as dogs, cats, and jerboas, which anticipate
-    coming dangers by their restlessness.</p>
-
-  <p>Before the catastrophe of 1812, at Caraccas, a Spanish stallion broke
-    out from its stable and escaped to the highlands, which was regarded
-    as the result of the prescience of a coming calamity. Before the
-    disturbances of 1822 and 1835, which shook Chili, immense flocks of sea
-    birds flew inland, as if they had been alarmed by the commencement of
-    some suboceanic disturbance. Before this
-    <span class="pagenum" id="Page_304">304</span>last shock it is also related
-    that all the dogs escaped from the city of Talcahuano.</p>
-
-  <p><em>Earthquake warning.</em>—What has here been said respecting the prediction
-    of earthquakes is necessarily imperfect—many of the signs which are
-    popularly supposed to enable persons to foretell the coming of an
-    earthquake having already been mentioned in previous chapters. That
-    we shall yet be able to prepare ourselves against the coming of
-    earthquakes, by the applications of laws governing these disturbances,
-    is not an unreasonable hope.</p>
-
-  <p>With an electric circuit which is closed by a movement of the ground,
-    we are already in a position to warn the dwellers in surrounding
-    districts that a movement is approaching.</p>
-
-  <p>An earthquake which travelled at the rate of four seconds to the
-    mile might, if it were allowed to close a circuit which fired a gun
-    at a station fifteen miles distant, give the inhabitants at that
-    place a minute’s warning to leave their houses. The inhabitants of
-    Australia and the western shores of the Pacific might, by telegraphic
-    communication, receive eighteen to twenty-five hours’ warning of the
-    coming of destructive sea waves resulting from earthquakes in South
-    America.</p>
-
-  <p>Although warnings like these might have their value, that which is
-    chiefly required is to warn the dwellers at and near an earthquake
-    centre of coming disturbances.</p>
-
-  <p>What the results of the observations on earth tremors will lead to is
-    problematical.</p>
-
-  <p>Should microseismic observation enable us to say when and where the
-    minute movements of the soil will reach a head, a valuable contribution
-    to the insurance of human safety in earthquake regions will have been
-    attained.</p>
-
-  <p>As to whether the movements of tromometers are destined to become
-    barometric-like warnings of increased
-    <span class="pagenum" id="Page_305">305</span> activity beneath the earth
-    crust, or whether they are only due to vibrations of the earth
-    crust produced by variations in atmospheric pressure, has yet to be
-    investigated.</p>
-
-  <p>Other phenomena which may probably forewarn us of the coming of an
-    earthquake are phenomena resultant on the stresses brought to bear upon
-    the rocky crust previous to its fracture, or phenomena due to changes
-    in the position and condition of heated materials beneath the earth’s
-    surface. Amongst these may be mentioned electrical disturbances, which
-    appear to be so closely related to seismic phenomena.</p>
-
-  <p>At the time of earthquakes telegraph lines have been disturbed, but
-    as to what may happen before an earthquake we have as yet but little
-    information. The subject of earthquake warning is of importance to many
-    countries, and is deserving of attention.</p>
-
-  <p>As our knowledge of earth movements, and their attendant phenomena,
-    increases, there is but little doubt that laws will gradually be
-    formulated, and in the future, as telluric disturbances increase, a
-    large black ball gradually ascending a staff may warn the inhabitants
-    on the land of a coming earthquake, with as much certainty as the ball
-    upon a pole at many seaports warns the mariner of coming storms.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XIX">
-    <span class="pagenum" id="Page_306">306</span>
-    <h2>CHAPTER XIX.<br />
-      <span class="subhead">EARTH TREMORS.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Artificially produced tremors—Observations of Kater, Denman,
-    Airy, Palmer, Paul—Natural tremors—Observations of Zöllner,
-    M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With
-    seismoscopes, microphones, pendulums—Work in Italy—Bertelli,
-    Count Malvasia, M. S. di Rossi—Instruments employed in
-    Italy—Tromometers, microseismographs, microphones—Results
-    obtained in Italy—in Japan—Cause of microseismic motion.</div>
-
-  <p class="noindent"><span class="smcap">During</span> the past few years considerable attention has been drawn towards
-    the study of small vibratory motions of the ground, which to the
-    unaided senses are usually passed by without recognition. These motions
-    are called <em>earth tremors</em>. Their discovery appears to have been due
-    to accident, and not to the results of inductive reasoning. No sooner
-    had philosophers contrived astronomical and other instruments for the
-    purpose of making refined measurements and observations than they at
-    once discovered that they had an enemy to contend against in the form
-    of microscopic earthquakes.</p>
-
-  <p><em>Artificially produced tremors.</em>—Artificial disturbances of this
-    description exist in all our towns, and near a railway line they are
-    perceptible with every passing train. Those who have used microscopes
-    of high power will readily appreciate how small a disturbance of
-    the ground is visible in the apparent movement of the object under
-    examination.</p>
-
-  <p><span class="pagenum" id="Page_307">307</span></p>
-
-  <p>Captain Kater found that he could not perform his pendulum experiments
-    in London on account of the vibrations produced by the rolling of
-    carriages. Captain Denman, who made some observations on artificially
-    produced tremors, found that a goods train produced an effect 1,100
-    feet distant in marshy ground over sandstone. Vertically, however,
-    above a tunnel through the sandstone, the effects only extended 100
-    feet.</p>
-
-  <p>A remarkable example of the trouble which artificially produced earth
-    vibrations have occasioned those who make astronomical observations
-    occurred some twenty years ago at the Greenwich Observatory. When
-    determining the collimation error of the transit circle by means of the
-    reflexion of a star in a tray of mercury, it was found that on certain
-    nights the surface of the mercury was in such a state of trembling
-    that the observers were unable to complete their observations until
-    long after midnight. After obtaining a series of dates on which these
-    disturbances occurred, it was found that they coincided with public and
-    bank holidays, on which days crowds of the poorer classes of London
-    flocked to Greenwich Park, and there amused themselves with running and
-    rolling down the hill on which the observatory is situated. On these
-    occasions it was found that the disturbances in the mercury were such
-    that observations could not be made until two or three hours after the
-    crowds had been turned out of the neighbouring park.<a id="FNanchor_138" href="#Footnote_138" class="fnanchor">[138]</a></p>
-
-  <p>To obviate this difficulty Sir George Airy suspended his dish of
-    mercury in a system of india-rubber bands, and in this way succeeded in
-    eating the intruders up.</p>
-
-  <p>Lieutenant-Colonel H. S. Palmer, R.E., when engaged with the transit
-    of Venus expedition in New Zealand, in 1874, was troubled with
-    vibrations produced from a <span class="pagenum" id="Page_308">308</span>
-    neighbouring railway. To escape the enemy
-    he intrenched his instruments by placing them in pits. With pits
-    3½ feet deep he found himself sufficiently protected. The distance
-    from the line was about 400 yards, and the soil through which the
-    disturbances were propagated was a coarse pebbly gravel.<a id="FNanchor_139" href="#Footnote_139" class="fnanchor">[139]</a></p>
-
-  <p>Before the United States Naval Observatory was established at
-    Washington, Professor H. M. Paul was deputed to make a tremor survey
-    to discover stable ground. The results of these experiments were
-    exceedingly interesting. By watching the reflected image of a star in
-    a dish of mercury a passing train would be noticed at the distance of
-    a mile. Its approach could be detected by the trembling of the image
-    before its coming could be heard. At one point of observation the
-    disturbance appeared to be cut off by a ravine. The strata was gravel
-    and clay.<a id="FNanchor_140" href="#Footnote_140" class="fnanchor">[140]</a></p>
-
-  <p>These few examples of artificially produced tremors, to which many
-    more might be added, have been given because they teach us something
-    respecting their nature. Hitherto earth tremors have only been regarded
-    as intruders, which it was necessary to escape from or destroy. From
-    what has been said they appear to be a superficial disturbance which
-    is propagated to an enormous distance. This distance appears to
-    depend upon the propagating medium, upon the intensity of the initial
-    disturbance, and upon its duration. In the observation of these
-    artificial disturbances, which are accessible to every one, and which
-    hitherto have been so neglected, we have undoubtedly a fruitful source
-    of study.</p>
-
-  <p><em>Natural tremors.</em>—Next let us turn to those microscopical disturbances
-    of our soil which are due to natural
-    <span class="pagenum" id="Page_309">309</span>causes. Thus far they seem to
-    have been recorded wherever instruments suitable for their detection
-    have been erected, and it is not improbable that they are common to the
-    surface of the whole globe.</p>
-
-  <p>Some of the more definite observations which have been made upon earth
-    tremors were those made in connection with experiments on the deviation
-    of the vertical due to the attractive influence of the moon and sun.</p>
-
-  <p>Professor Zöllner, who invented the horizontal pendulum which he used
-    in the attempt to measure the change in level due to lunar and solar
-    attraction, found his instruments so sensitive that the readings were
-    always changing.</p>
-
-  <p>The most interesting observations which were made upon small
-    disturbances of the soil were those of M. d’Abbadie, who carried on his
-    experiments at Abbadia, in Subernoa, near Hendaye, 400 mètres distant
-    from the Atlantic, and 62 mètres above sea level. The soil was a loamy
-    rock. Here M. d’Abbadie constructed a concrete cone 8 mètres in height,
-    which was pierced down the centre by a vertical hole or well, which was
-    continued two mètres below the cone into the solid rock. At the bottom
-    of this hole or well a pool of mercury was formed which reflected
-    the image of cross wires placed at the top of the hole. These cross
-    wires and their reflection were observed by means of a microscope.
-    The observations consisted in noting the displacement and azimuth of
-    the reflected image relatively to the real image of the wires. After
-    allowing this structure five years to settle, M. d’Abbadie commenced
-    his observations. To find the surface of the mercury tranquil was
-    a rare occurrence. Sometimes the mercury appeared to be in violent
-    motion, although both the air and neighbouring sea were perfectly calm.
-    At times the reflected image would disappear as if the mercury had been
-    disturbed by a microscopic earthquake.</p>
-
-  <p><span class="pagenum" id="Page_310">310</span></p>
-
-  <p>The relative positions of the images were in part governed by the state
-    of the tide. Altogether the movements were so strange that M. d’Abbadie
-    did not venture any speculations as to their cause, but he remarks
-    that the cause of the changes he observed were sometimes neither
-    astronomical nor thermometrical. These observations, the principal
-    object of which was to determine changes in level rather than earth
-    vibrations, were carried on between the years 1868 and 1872.<a id="FNanchor_141" href="#Footnote_141" class="fnanchor">[141]</a></p>
-
-  <p><em>Observations at Cambridge.</em>—Another instructive set of observations
-    were those which were made in the years 1880–1882, by George and Horace
-    Darwin, in the Cavendish Laboratory, at Cambridge. The main object
-    in these experiments was to determine the disturbing influence of
-    gravity produced by lunar attraction. The result which was obtained,
-    however, showed that the soil at Cambridge was in such an incessant
-    state of vibration that whatever pull the moon may have exerted upon
-    the instrument which was employed was masked by the magnitude of the
-    effects produced by the earth tremors, and the experiments had, in
-    consequence, to be abandoned.</p>
-
-  <p>The principle of this instrument was similar to one devised by Sir
-    William Thomson, and put up by him in his laboratory at Glasgow.
-    As erected by the brothers Darwin, at Cambridge, it was briefly as
-    follows: A pendulum, which was a massive cylinder of pure copper,
-    was hung by a copper wire, about four feet long, inside a hollow
-    cylindrical tube rising from a stone support. A small mirror was then
-    hung by two silk fibres, one of which was fastened to the bob and the
-    other to the stone basement. A ray of light sent from a lamp on to
-    the mirror was reflected to a scale seven feet distant, and by this
-    magnification any motion of the bob relatively to
-    <span class="pagenum" id="Page_311">311</span>the stone support
-    was magnified 50,000 times. In several ways the apparatus was insulated
-    from all accidental disturbances. The spot of light was observed from
-    another room by means of a telescope. This instrument was so delicate
-    that even at the distance of sixteen feet the shifting of your weight
-    from one foot to the other caused the spot of light to run along the
-    scale. So sensitive was the instrument that, notwithstanding its being
-    cut off from the surrounding soil by a trench filled with water and
-    the whole instrument being immersed in water to damp out the small
-    vibrations, it would seem that the ground was in a constant state of
-    tremor; in fact, so persistent and irregular were these movements that
-    it seemed impossible to separate them from the perturbations due to the
-    attraction of the moon.<a id="FNanchor_142" href="#Footnote_142" class="fnanchor">[142]</a></p>
-
-  <p>As a result of observations like these, the world had gradually
-    forced upon it the fact that the ground on which we live is probably
-    everywhere in what is practically an incessant state of vibration.</p>
-
-  <p>This led those who were interested in the study of earth movements to
-    establish special apparatus for the purpose of recording these motions
-    with the hope of eventually discovering the laws by which they were
-    governed.</p>
-
-  <p><em>Experiments in Japan.</em>—The simpler forms of apparatus which have been
-    used in Japan may be described as delicate forms of seismoscopes,
-    which, in addition to recording earth tremors, also record the
-    occurrence of small earthquakes.</p>
-
-  <p>A simple contrivance which may be used for the purpose of recording
-    small earthquakes can be made with a small compass needle.</p>
-
-  <p>If a light, small sensitive compass needle be placed on
-    <span class="pagenum" id="Page_312">312</span>a table, it
-    will be found that a small piece of iron like a nail may be pushed so
-    near to it that the needle assumes a position of extremely unstable
-    equilibrium. If the table now receives the slightest tap or shake this
-    condition is overcome, and the needle flies to the iron and there
-    remains. By making the support of the needle and the iron the poles of
-    an electric circuit it is possible to register the time at which motion
-    took place with considerable accuracy.</p>
-
-  <p>With crude apparatus like this a large number of small earth
-    disturbances have been recorded.</p>
-
-  <p>Another form of apparatus, employed in Japan, has been a delicately
-    constructed <em>circuit closer</em>. The motions of this instrument were
-    recorded by causing an electro-magnet to deflect a pencil which was
-    tracing a circle on a revolving dial. The revolving dial was a disc of
-    wood covered with paper fixed to the hour-hand axle of a common clock.</p>
-
-  <p>A third form of apparatus used in Japan consisted of a small piece of
-    sheet lead about the size of a threepenny piece suspended by a short
-    loop from a rigid support. Projecting from the lead a fine wire, about
-    two inches in length, passed freely through a hole in a metallic plate.
-    By the slightest motion of the support the small pendulum of lead was
-    set into a state of tremor and caused its pointer to come in contact
-    with one or other side of the hole in the metal plate and thus to close
-    an electric circuit.</p>
-
-  <p>A more refined kind of apparatus which has been employed in Japan was
-    similar to that used by the Darwins at Cambridge. This was so arranged
-    that any deflection of the mirror was permanent until the instrument
-    was reset, and in this way the maximum disturbance which had taken
-    place between each observation was recorded.</p>
-
-  <p><span class="pagenum" id="Page_313">313</span></p>
-
-  <p>In addition to these and other contrivances, experiments were made with
-    microphones.</p>
-
-  <p>The microphones used were small doubly pointed pencils of carbon
-    about three centimetres long, saturated with mercury, and supported
-    vertically in pivot holes bored in other pieces of carbon, which were
-    the terminals of an electric circuit. These microphones were screwed
-    down on the top of stakes driven deeply into the ground. They were
-    covered with a glass shade thickly greased at its base. The stakes were
-    in the ground at the bottom of a small pit—about two feet square and
-    two feet deep—which was lined with a box. The box was covered with a
-    lid, and earth to the depth of nine inches or one foot. One of these
-    pits was in the middle of a lawn in the front of my house, and the
-    other was at the foot of a hill at the back of the house. The wires
-    from the microphone passed through the side of the box into a bamboo
-    tube and thence up to my dining-room and bed-room. In one of the
-    circuits there were three Daniell’s cells, a telephone, and a small
-    galvanometer. I used the galvanometer because I found that when there
-    was sufficient motion in the microphone to produce a sound in the
-    telephone a motion in the needle of a galvanometer was produced. If in
-    any case motion took place in the magnetic needle during my absence, it
-    was held deflected by a small piece of iron with which it was brought
-    into contact by the movement.</p>
-
-  <p>The sensitiveness of the arrangement may be judged of from the fact
-    that if a person walked on the grass within six feet of the microphone,
-    each step caused a creak in the telephone, and the needle of the
-    galvanometer was caused to swing and come in contact with the iron.
-    Dogs running on the grass had no effect. A small stone one or two
-    inches in diameter thrown from the
-    <span class="pagenum" id="Page_314">314</span> house so that it fell near to the
-    microphone pit caused a sharp creak in the telephone and a movement in
-    the needle.</p>
-
-  <p>The nature of the records I received from this contrivance may be
-    judged of from the following extract from my papers.</p>
-
-  <table summary="Microphone contrivance readings">
-    <tr>
-      <th>&nbsp;</th>
-      <th class="tdl">&nbsp;h. m.</th>
-    </tr>
-    <tr>
-      <td class="tdl">1879. Nov. 12th</td>
-      <td class="tdl">7&nbsp; &nbsp;0 <span class="smcap">p.m.</span> contact of needle</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;</td>
-      <td class="tdl">7&nbsp; &nbsp;2&nbsp; „&nbsp; difficult to set the needle</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;</td>
-      <td class="tdl">7&nbsp; &nbsp;3&nbsp; „&nbsp; needle swings and telephone creaks</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;</td>
-      <td class="tdl">7&nbsp; &nbsp;4&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;</td>
-      <td class="tdl">7&nbsp; &nbsp;5&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;</td>
-      <td class="tdl">7&nbsp; &nbsp;6&nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;</td>
-      <td class="tdl">7 10&nbsp; „&nbsp; 3 more swings</td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;</td>
-      <td class="tdl">7 11&nbsp; „&nbsp; again&nbsp; &nbsp; &nbsp; &nbsp; „</td>
-    </tr>
-  </table>
-
-  <p class="noindent">Here I went out, took away the covering, and examined the microphone.
-    Nothing wrong was to be observed. All that I saw was one small ant. I
-    do not think that this could have caused the disturbance, because it
-    could not get near the instrument.</p>
-
-  <p>On the succeeding nights I experienced similar disturbances, and it
-    seemed as if they might possibly have been the prelude to several small
-    shocks which occurred about this time (November 15, 16, and 17). On
-    November 17, at 8 <span class="smcap">a.m.</span>, the needle was found in contact, and
-    again at 5 <span class="smcap">p.m.</span>, and at 6 <span class="smcap">p.m.</span> the shock of a small
-    earthquake was felt <em>which caused a rattling sound in the telephone for
-      about one minute after the motion had appeared to cease</em>. The needle
-    swung considerably, but did not come in contact.</p>
-
-  <p>The great objection to these observations is that it is possible
-    that the movements and sounds which I have recorded might, with the
-    exception of one case when the shaking was actually felt, possibly have
-    been produced by causes other than that of the movement of the ground.
-    To determine this I subsequently put up two distinct sets
-    <span class="pagenum" id="Page_315">315</span> of apparatus
-    to determine whether the motions of each were synchronous. So far as
-    I went this appeared only to be sometimes the case:—but this is a
-    question difficult to determine, unless a recorder of time be added to
-    the apparatus.</p>
-
-  <p>The greatest objection to observations of this sort is that the
-    sensibility of the instrument is not constant. After a current has been
-    running for several days it is no longer sensible to slight shocks, it
-    appears as if its resistance had been increased. To overcome this it is
-    necessary to resharpen the carbon points and bore out the pivot holes
-    every three or four days. Farther, the battery varies. This might to
-    some extent be overcome by using a battery with large plates. These
-    two causes tend to reduce the sensitiveness of the galvanometer-like
-    recorder—the deflection of the needle gradually becoming less and less,
-    and therefore day by day needing a greater swing to bring it into
-    contact with the iron. For reasons such as these this instrument, to be
-    used successfully, appears to require considerable attention.</p>
-
-  <p>Another form of microphone employed by the author consisted of an
-    aluminium wire standing vertically on a metallic plate, its upper end
-    passing loosely through a hole in an aluminium wire standard.</p>
-
-  <p>The upper end of the vertical wire was loaded with lead. This
-    contrivance possesses all the sensitiveness of an ordinary microphone,
-    whilst, if it receives a sudden impulse, there is a sudden break in the
-    current, and the vertical wire is thrown from one side to the other of
-    the hole in the standard.</p>
-
-  <p>After many months of tiresome observation with instruments of this
-    description, and after eliminating all motions which might have been
-    produced by accidental causes, the general result obtained showed that
-    in Tokio<span class="pagenum" id="Page_316">316</span>
-    there were movements of the soil to be detected every day,
-    and sometimes many times per day, which to ordinary persons were passed
-    by unnoticed.</p>
-
-  <p><em>Work in Italy.</em>—The most satisfactory observations which have been
-    made upon microseismic disturbances are those which have been made
-    during the last ten years in Italy. The father of systematical
-    microseismical research appears to have been Father Timoteo Bertelli,
-    of Florence.</p>
-
-  <p>In 1870 Father Bertelli suspended a pendulum in a cellar, and observed
-    it with a microscope. As the result of his observations it was
-    announced that he had perceived the earthquakes which shook Romagna,
-    although to the ordinary observer in Florence these shakings had not
-    been perceptible.</p>
-
-  <p>In 1873 Bertelli, by means of microscopes fixed in several azimuths,
-    made 5,500 observations on free pendulums. He also made observations on
-    reflections from the surface of mercury.<a id="FNanchor_143" href="#Footnote_143" class="fnanchor">[143]</a></p>
-
-  <p>One result of these observations was to show that microseismic motions
-    increased with a fall of the barometer. Similar observations were made
-    at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near
-    Rome. On January 14, 15, and February 25 these three observers at their
-    respective stations simultaneously observed great disturbances.</p>
-
-  <p>Similar investigations were made at Nice by M. le Baron Prost.</p>
-
-  <p>Although doubt was cast upon Bertelli’s observations they appear to
-    have been the origin of a series of microseismical observations, a
-    distinguished leader in which is Professor Rossi, who, in 1874, found
-    that large earthquakes were almost always preceded or accompanied with
-    microseismical<span class="pagenum" id="Page_317">317</span>
-    storms. In 1878 Professor Rossi worked upon these small
-    disturbances with the assistance of the microphone and telephone, and
-    his first results were published by Professor Palmieri.</p>
-
-  <p>Many of Professor Rossi’s observations were made in the grotto of Roca
-    de Papa, 700 mètres high and eighteen mètres under the soil. Here over
-    6,000 observations were made by means of microscopes, on pendulums of
-    different lengths, suspended in tubes cut in the solid rock.</p>
-
-  <p><em>Instruments employed in Italy.</em>—It is impossible to describe in detail
-    the various forms of apparatus which have been used by the Italian
-    investigators. A description of one or two of the more important
-    instruments may not, however, be out of place, inasmuch as they will
-    assist the reader to understand the manner in which the various results
-    respecting the laws governing microseismic movements have been arrived
-    at.</p>
-
-  <p>The most important of these instruments is the <em>Normal Tromometer</em> of
-    Bertelli and Rossi.</p>
-
-  <p>This consists of a pendulum 1½ metres long, carrying, by means of
-    a very fine wire, a weight of 100 grammes. To the base of the bob
-    a vertical stile is attached, and the whole is enclosed in a tube
-    terminated, at its base, by a glass prism of such a form that when
-    looked through horizontally the motion of the stile can be seen in
-    all azimuths. In front of this prism a microscope is placed. Inside
-    the microscope there is a micromatic scale, so arranged that it can
-    be turned to coincide with the apparent direction of oscillation of
-    the point of the stile. In this way not only can the amplitude of the
-    motion of the stile be measured, but also its azimuth. The extent of
-    vertical motion is measured by the up and down motion of the stile due
-    to the elasticity of the supporting<span class="pagenum" id="Page_318">318</span>
-    wire. This instrument is shown in the accompanying drawing.</p>
-
-  <div class="figleft">
-    <img src="images/i_p318.jpg" width="303" height="545" alt="" />
-    <div class="caption">
-      <span class="smcap">Fig. 37.</span>—Normal Tromometer.<br />
-      <span class="small">
-        <span class="smcap">b</span>, bob of pendulum; <span class="smcap">p</span>, prism;
-        <span class="smcap">m</span>, microscope; <span class="smcap">s</span>, rim of scale.
-      </span>
-    </div>
-  </div>
-
-  <p>Another apparatus is the <em>Microseismograph</em> of Professor Rossi. Here we
-    have an arrangement which gives automatic records of slight motions.
-    It consists of four pendulums, each about three feet long, suspended
-    so that they form the corners of a square platform. In the centre
-    of this platform a fifth, but rather longer, pendulum is suspended.
-    The four pendulums are each connected just above their bobs to the
-    central pendulum with loose silk threads. Fixed to the centre of each
-    of these threads, and held vertically by a light spring, is a needle,
-    so adjusted that each thread is depressed to form an obtuse angle of
-    about 155°. These needles form the terminals of an electric circuit,
-    the other termination of which is a small cup of mercury placed just
-    below the lower end of the needle. By a horizontal swing of one of the
-    pendulums this arrangement causes the needle to move vertically, but
-    with a slightly multiplied amplitude. By this motion the needle comes
-    in contact with the mercury, and an electro-magnet with a lever and
-    pencil is caused to make a mark on a band of paper moved by clockwork.
-    The<span class="pagenum" id="Page_319">319</span>
-    five pendulums being of different lengths allows the apparatus to
-    respond ‘to seismic waves of different
-    velocities.’<a id="FNanchor_144" href="#Footnote_144" class="fnanchor">[144]</a></p>
-
-  <p>Lastly, we have Professor Rossi’s <em>Microphone</em>. This consists of a
-    metallic swing arranged like the beam of a balance. By means of a
-    movable weight at one end of the beam this is so adjusted that it
-    falls down until it comes in contact with a metallic stop. This can be
-    so adjusted that a slight tap will cause the beam to slightly jump from
-    the stop. The beam and the stop form two poles of an electric circuit,
-    in which there is a telephone. The slightest motion in a <em>vertical</em>
-    direction causes a fluctuation in the current passing between the stop
-    and the beam, and a consequent noise is heard in the telephone.</p>
-
-  <p>With instruments analogous to these, observations have been made by
-    various observers in all portions of Italy, extending over a period
-    of ten years. Every precaution appears to have been taken to avoid
-    accidental disturbances, and the experiments have been repeated in a
-    variety of forms.</p>
-
-  <p><em>Results obtained in Italy.</em>—The results which from time to time have
-    been announced are of the greatest interest to those who study the
-    physics of the earth’s crust, and they appear to be leading to the
-    establishment of laws of scientific value.</p>
-
-  <p>It would seem that the soil of Italy is in incessant movement, there
-    being periods of excessive activity usually lasting about ten days.
-    Such periods are called seismic storms. These storms are separated by
-    periods of relative calm. These storms have their greater regularity
-    in winter, and sharp maximums are to be observed in spring and autumn.
-    In the midst of such a period or
-    <span class="pagenum" id="Page_320">320</span>at its end there is usually an
-    earthquake. Usually these storms are closely related to barometric
-    depressions. To distinguish these movements from those which occur
-    under high pressure, the latter are called baro seismic movements, and
-    the former <em>vulcano seismic</em> movements. The relation of these storms
-    to barometric fluctuation has been observed to have been very marked
-    during the time of a volcanic eruption.</p>
-
-  <p>At the commencement of a storm the motions are usually small, and one
-    storm, lasting two or three days, may be joined to another storm.
-    In such a case the action may be a local one. It has been observed
-    that a barometrical depression tended to bring a storm to a maximum,
-    whilst an increase of pressure would cause it to disappear. Sometimes
-    these actions are purely local, but at other times they may affect a
-    considerable tract of land.</p>
-
-  <p>If a number of pendulums of different length are observed at the same
-    place, there is a general similarity in their movements, but it is also
-    evident that the free period of the pendulum more or less disturbs
-    the character of the record. The greatest amplitude of motion in a
-    set of pendulums is not reached simultaneously by all the pendulums,
-    and at every disturbance the movement of one will predominate. From
-    this Rossi argues that the character of the microseismical motions is
-    not constant. Bertelli observed that the direction of oscillation of
-    the pendulums is different at different places, but each place will
-    have its particular direction dependent upon the direction of valleys
-    and chains of mountains in the neighbourhood. Rossi shows that the
-    directions of movement are perpendicular to the direction of lines of
-    faults, the lips of these fractures rising and falling, and producing
-    two sets of waves, one set parallel to the line of fracture,
-    <span class="pagenum" id="Page_321">321</span> and the
-    other perpendicular to such a direction. These movements, according
-    to Bertelli, have no connection with the wind, rain, change of
-    temperature, and atmospheric electricity.</p>
-
-  <div class="figcenter">
-    <img src="images/i_p321.jpg" width="542" height="444" alt="" />
-    <div class="caption"><span class="smcap">Fig. 38.</span></div>
-  </div>
-
-  <p>The disturbances, as recorded at different towns, are not always
-    strictly synchronous, but succeed each other at short intervals. If,
-    however, we take monthly curves of the disturbances as recorded at
-    different towns in Italy, we see that these are similar in character.
-    The maximum of disturbance occurs about the winter solstice, and the
-    minimum about the summer solstice, and in this respect they exhibit
-    a perfect accordance with the curves drawn by Mallet to show the
-    periodicity of earthquakes. The accompanying curves taken from one
-    of Bertelli’s original memoirs not only show this general result but
-    also show<span class="pagenum" id="Page_322">322</span>
-    the close accord there is between the results obtained at
-    different towns during successive months.</p>
-
-  <p>At Florence, before a period of earthquakes there is an increase in the
-    amplitude and frequency of vertical movements. These vertical movements
-    do not appear to coincide with the barometrical disturbances, but they
-    appear to be connected with the seismic disturbances.</p>
-
-  <p>They are usually accompanied with noises in the telephone, but as the
-    microphone is so constructed as to be more sensitive to vertical motion
-    than to horizontal motion, this is to be expected. This vertical motion
-    would appear to be a local action, inasmuch as the accompanying motions
-    of an earthquake which originates at a distance are horizontal.</p>
-
-  <p>Storms of microseismical motions appear to travel from point to point.</p>
-
-  <p>Sometimes a local earthquake is not noticed in the tromometer,
-    whilst one which occurred at a distance, although it may be small,
-    is distinctly observed. To explain this, Bertelli suggests the
-    existence of nodes. Similar conclusions were arrived at by Rossi when
-    experimenting on different portions of the sides of Vesuvius. Galli
-    noticed an augmentation in microseismic activity when the sun and moon
-    are near the meridian. Grablovitz found from Bertelli’s observations
-    a maximum two or three days before the syzygies, and a minimum
-    three days after these periods. He also found that the principal
-    large disturbances occurred in the middle of periods separating
-    the quadrature from the syzygies, the apogee from perigee, and the
-    lunistigi period from the nodes, whilst the smallest disturbances
-    happened in the middle of periods opposed to these.</p>
-
-  <p>P. C. Melzi says that the curves of microseismical motions,
-    earthquakes, lunar and solar motions, show a concordance with each
-    other.</p>
-
-  <p><span class="pagenum" id="Page_323">323</span></p>
-
-  <p>With the microphone Rossi hears sounds which he describes as roarings,
-    explosions, occurring isolated or in volleys, metallic and bell-like
-    sounds, ticking, &amp;c., which, he says, revealed natural telluric
-    phenomena. Sometimes these have been intolerably loud. At Vesuvius the
-    vertical shocks corresponded with a sound like volleys of musketry,
-    whilst the undulatory shocks gave the roaring. Some of these sounds
-    could be imitated artificially by rubbing together the conducting wires
-    in the same manner in which the rocks must rub against each other in
-    an earthquake. Other sounds were imitated by placing the microphone
-    on a vessel of boiling water, or by putting it on a marble slab and
-    scratching and tapping the under side of it.</p>
-
-  <p>These, then, are some of the more important results which have been
-    arrived at by the study of microseismic motions. One point which seems
-    worthy of attention is that they appear to be more law-abiding than
-    their more violent relations, the earthquakes, and as phenomena in
-    which natural laws are to be traced they are certainly deserving our
-    attention. As to whether they will ever become the means of forewarning
-    ourselves against earthquakes is yet problematical. Their systematic
-    study, however, will enable us to trace the progress of a microseismic
-    storm from point to point, and it is not impossible that we may yet
-    be enabled to foretell where the storm may reach its climax as an
-    earthquake. These, I believe, are the views of Professor di Rossi, who
-    is at the present time engaged in the establishment of a system of
-    microseismic observations throughout Italy.</p>
-
-  <p>Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer
-    was in a state of agitation, and similar disturbances were observed at
-    Livorno, Florence, and Bologna.</p>
-
-  <p><span class="pagenum" id="Page_324">324</span></p>
-
-  <p>Since February 1883 I have observed a tromometer in Japan, and such
-    results as have been obtained accord with results obtained in Italy.
-    The increase in microseismical activity with a fall of the barometer
-    is very marked. Other peculiarities in the behaviour of the instrument
-    will be referred to under ‘Earth Pulsations.’</p>
-
-  <p><em>Cause of microseismic movements.</em>—As to the cause of tromometric
-    movements, we have a field for speculation. Possibly they may be
-    due to slight vibratory motions produced in the soil by the bending
-    and crackling of rocks produced by their rise upon the relief of
-    atmospheric pressure. If this were so we should expect similar
-    movements to be produced at the time of an increase of pressure. Rossi
-    suggests that they may be the result of an increased escape of vapour
-    from the molten materials beneath the crust of the earth, consequent
-    upon a relief of pressure. The similarity of some of the sounds which
-    are heard with the microphone to those produced by boiling water are
-    suggestive of this, and Rossi quotes instances when underground noises
-    like those which we should expect to hear from a boiling fluid have
-    been heard before earthquakes without the aid of microphones. One
-    instance was that of Viduari, a prisoner in Lima, who, two days before
-    the shock of 1824, repeatedly predicted the same in consequence of the
-    noises he heard.</p>
-
-  <p>A possible cause of disturbances of this order may be small but
-    sudden fluctuations in barometric pressure, which are visible during
-    a storm. During a small typhoon on September 15, 1881, when in the
-    Kurile Islands, I observed that the needle of an aneroid worked back
-    and forth with a period of from one to three seconds. This continued
-    for several hours. With every gust of wind the needle suddenly rose
-    and then immediately fell. At times it trembled. These movements were
-    observed in the open<span class="pagenum" id="Page_325">325</span>
-    air. The extent of these sudden variations
-    was approximately from ·03 to ·05 inches. Beckoning an increase of
-    barometrical pressure of one hundredth of an inch as equivalent to a
-    load of twenty million pounds on the square mile, during this storm
-    there must have been the equivalent of loads of from 60 to 100 million
-    pounds to the square mile continually placed on and removed from a
-    considerable tract of the earth’s surface. If the period of application
-    of these stresses approximately coincide with the natural vibrational
-    period of the area affected, it would surely seem, especially when
-    we reflect upon the effect of an ordinary carriage, that tremors of
-    considerable magnitude ought to be produced.</p>
-
-  <p>An inspection of the following few observations taken from my note-book
-    for the same typhoon will suggest that even the large and slower
-    variations are capable of producing tremulous motions.</p>
-
-  <table summary="Typhoon barometer readings">
-    <tr>
-      <th>Time<br />h.&nbsp; m.</th>
-      <th>Barometer<br />reading</th>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; 12&nbsp; &nbsp;5 <span class="smcap">p.m.</span></td>
-      <td class="tdc">29·02</td>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; 12 10&nbsp; &nbsp;„</td>
-      <td class="tdc">29·05</td>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; 12 12&nbsp; &nbsp;„</td>
-      <td class="tdc">29·07</td>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; 12 13&nbsp; &nbsp;„</td>
-      <td class="tdc">29·05</td>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; 12 25&nbsp; &nbsp;„</td>
-      <td class="tdc">29·10</td>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; 12 50&nbsp; &nbsp;„</td>
-      <td class="tdc">29·00</td>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; &nbsp; 1 10&nbsp; &nbsp;„</td>
-      <td class="tdc">29·00</td>
-    </tr>
-    <tr>
-      <td>&nbsp; &nbsp; &nbsp; 1 20&nbsp; &nbsp;„</td>
-      <td class="tdc">29·07</td>
-    </tr>
-  </table>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XX">
-    <span class="pagenum" id="Page_326">326</span>
-    <h2>CHAPTER XX.<br />
-      <span class="subhead">EARTH PULSATIONS.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Definition of an earth pulsation—Indications of
-    pendulums—Indications of levels—Other phenomena indicating
-    the existence of earth pulsations—Disturbances in lakes and
-    oceans—Phenomena resultant on earth pulsations—Cause of earth
-    pulsations.</div>
-
-  <p class="noindent"><span class="smcap">The</span> object of the present chapter is to show that from time to time it
-    is very probable that slow but large wave-like undulations travel over
-    or disturb the surface of the globe.</p>
-
-  <p>These movements, which have escaped our attention on account of their
-    slowness in period, for want of another term I call earth pulsations.</p>
-
-  <p>The existence of movements such as these may be indicated to us by
-    changes in the level of bodies of water like seas and lakes, by the
-    movements of delicate levels, by the displacement of the bob of a
-    pendulum relatively to some point on the earth above which it hangs,
-    and by other phenomena which will be enumerated.</p>
-
-  <p><em>Indication of pendulums.</em>—Pendulums which have been suspended for the
-    purposes of seismometrical observations have, both by observers in
-    Italy and Japan, been seen to have moved a short distance out from, and
-    then back to, their normal position.</p>
-
-  <p>This motion has simply taken place on one side of their central
-    position, and is not due to a swing. The
-    <span class="pagenum" id="Page_327">327</span> character of these records
-    is such that we might imagine the soil on which the support of the
-    pendulum had rested to have been slowly tilted, and slowly lowered.
-    They are the most marked on those pendulums provided with an index
-    writing a record of its motions on a smoked glass plate, which index is
-    so arranged that it gives a multiplied representation of the relative
-    motion between it and the earth. As motions of this sort might be
-    possibly due to the action of moisture in the soil tilting the support
-    of the pendulum, and to a variety of other accidental causes, we cannot
-    insist on them as being certain indications that there are slow tips in
-    the soil, but for the present allow them to remain as possible proof of
-    such phenomena.</p>
-
-  <p>Evidence of displacement of the vertical, which are more definite than
-    the above, are those made by Bertelli, Rossi, Count Malvasia, and other
-    Italian observers, who, whilst recording earth tremors, have spent so
-    much time in watching the vibrations of stiles of delicate pendulums
-    by means of microscopes. As a result of these observations we are
-    told that the point about which the stile of a pendulum oscillates
-    is variable. These displacements take place in various azimuths, and
-    they appear to be connected with changes of the barometer. I have made
-    similar observations in Japan.</p>
-
-  <p>From this, and from the fact that it is found that a number of
-    different pendulums differently situated on the same area give similar
-    evidence of these movements, it would hardly seem that these phenomena
-    could be attributed to causes like changes in temperature and moisture.
-    M. S. di Rossi lays stress on this point, especially in connection with
-    his microseismograph, where there are a number of pendulums of unequal
-    length which give indications of a like character. The direction in
-    which<span class="pagenum" id="Page_328">328</span>
-    these tips of the soil take place—which phenomena are noticeable
-    in seismic as well as microseismic motions—Rossi states are related to
-    the direction of certain lines of faulting.</p>
-
-  <p><em>Indications of levels.</em>—Bubbles of delicate levels can be easily seen
-    to change their position with meteorological variations; but Rossi also
-    tells us that they change their position, sometimes not to return for
-    a long time, during a microseismic storm. Here again we have another
-    phenomenon pointing to the fact that microseismic disturbances are the
-    companions of slow alterations in level.</p>
-
-  <p>One of the most patient observers of levels has been M. Plantamour,
-    who commenced his observations in 1878, at Sécheron, on the Lake of
-    Geneva. He used two levels, one placed north and south, and the other
-    east and west. During the summer of 1878 the east end rose, but at the
-    end of September a depression set in. The diurnal movements had their
-    maximum and minimum at 6 and 7.45 <span class="smcap">a.m.</span> and <span class="smcap">p.m.</span> The
-    total amplitude was 4·89″. The variations of the east and west level
-    appeared to be due to the temperature, but the movements of the north
-    and south level were dependent upon an unknown cause.</p>
-
-  <p>Between October 1, 1879, and September 30, 1880, the east end fell
-    rapidly, from the middle of November up to December 26, amounting to
-    88·71″. It then rose 6·55″ to January 5, and then fell again. On
-    January 28 it reached 89·95″, after which it rose.</p>
-
-  <p>Between October 4, 1879, and January 28, 1880, the movement was 95·8″,
-    against 28·08″ of the previous year.</p>
-
-  <p>These movements were not due alone to temperature. The north and south
-    level, which was not influenced by<span class="pagenum" id="Page_329">329</span>
-    the cold of the winter, moved
-    4·56″. In the previous year 4·89″.<a id="FNanchor_145" href="#Footnote_145" class="fnanchor">[145]</a></p>
-
-  <p>From February 17 to June 5, 1883, the author observed in Tokio the
-    bubbles of two delicate levels, one placed north and south, and the
-    other east and west. They were placed under glass cases on the head of
-    a stone column. The column, which is inside a brick building, rests
-    on a concrete foundation, and is about ten years old. It is in no way
-    connected with the building. The temperature of the room has a daily
-    variation of about 1° Fahr.</p>
-
-  <p>In both these levels diurnal changes are very marked. Occasionally they
-    are enormously great. Thus, on March 25, the readings of the south end
-    of the north south level were as follows:—</p>
-
-  <table summary="Tokio level readings">
-    <tr>
-      <th>&nbsp;</th>
-      <th class="left">Time<br />h.&nbsp; m.</th>
-      <th>Readings.</th>
-    </tr>
-    <tr>
-      <td>25th.</td>
-      <td class="left">4 00 <span class="smcap">p.m.</span></td>
-      <td class="left">104·5</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 &nbsp; 5 &nbsp;„</td>
-      <td class="left">103</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 10 &nbsp;„</td>
-      <td class="left">102</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 25 &nbsp;„</td>
-      <td class="left">101</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 30 &nbsp;„</td>
-      <td class="left">100</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 40 &nbsp;„</td>
-      <td class="left">&nbsp; 98</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 42 &nbsp;„</td>
-      <td class="left">&nbsp; 99·5</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 45 &nbsp;„</td>
-      <td class="left">100</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 50 &nbsp;„</td>
-      <td class="left">101</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">4 55 &nbsp;„</td>
-      <td class="left">101</td>
-    </tr>
-    <tr>
-      <td>&nbsp;</td>
-      <td class="left">5 00 &nbsp;„</td>
-      <td class="left">100</td>
-    </tr>
-    <tr>
-      <td>26th.</td>
-      <td class="left">7 00 <span class="smcap">a.m.</span></td>
-      <td class="left">105</td>
-    </tr>
-  </table>
-
-  <p>Usually this level moves through about three divisions per day.</p>
-
-  <p>From March 25 to May 4 it travelled from 98 to 127. Since then, to June
-    5, it has descended to 116. During this period the east west level
-    has been <em>comparatively</em> quiet. One division of the north south level
-    equals about 2″ of arc.</p>
-
-  <p><span class="pagenum" id="Page_330">330</span></p>
-
-  <p>Many of these changes may be due to changes in temperature, variations
-    in moisture, and other local actions. Some of them, however, are hardly
-    explicable on such assumptions. The fact that the general direction in
-    change of the vertical, as indicated by a tromometer standing on the
-    same column with the levels, showed that the change which was taking
-    place was rather in the column than in the instruments.</p>
-
-  <p>The fact also that at the time of a barometrical depression a
-    <em>pulse-like surge</em> can be seen in the levels, having a period averaging
-    about three seconds and sometimes amounting to about one second of
-    arc, is a phenomenon hardly to be attributed to sudden fluctuations in
-    moisture or temperature, but indicates real changes in level.<a id="FNanchor_146" href="#Footnote_146" class="fnanchor">[146]</a></p>
-
-  <p>In addition to variation in the bubbles of levels which come on more or
-    less gradually, we have many recorded instances of <em>sudden</em> alterations
-    taking place in these instruments.</p>
-
-  <p>Examples of what may have been a slow oscillating motion of the earth’s
-    crust are referred to by Mr. George Darwin in a Report to the British
-    Association in 1882.</p>
-
-  <p>One of them was made by M. Magnus Nyrén, at Pulkova, who, when engaged
-    in levelling the axis of a telescope, observed spontaneous oscillation
-    in the bulb of the level.</p>
-
-  <p>This was on May 10 (April 28), 1877. The complete period was about
-    20 seconds, the amplitude being 1·5″ and 2″. One hour and fourteen
-    minutes before this he observes that there had been a severe earthquake
-    at Iquique, the distance to which in a straight line was 10,600
-    kilomètres, and on an arc of a great circle, 12,500 kilomètres.
-    On September 20 (8), in 1867, Mr. Wagner
-    <span class="pagenum" id="Page_331">331</span>had observed at Pulkova
-    oscillations of 3″, seven minutes before which there had been an
-    earthquake at Malta. On April 4 (March 23), 1868, an agitation of the
-    level had been observed by Mr. Gromadzki, five minutes before which
-    there had been an earthquake in Turkestan. Similar observations had
-    been made twice before. These, however, had not been connected with any
-    earthquakes—at least, Mr. Darwin remarks—with certainty.</p>
-
-  <p><em>Phenomena analogous to the pendulum and level observations.</em>—As
-    examples of phenomena which are analogous to those made on pendulums
-    and levels, the following may be noticed. On March 20, 1881, at 9
-    <span class="smcap">p.m.</span> a watchmaker in Buenos Ayres observed that all his
-    clocks oscillating north and south suddenly began to increase their
-    amplitude, until some of them became twice as great as before. Similar
-    observations were made in all the other shops. No motion of the earth
-    was detected. Subsequently it was learnt that this corresponded with an
-    earthquake in Santiago and Mendoza.<a id="FNanchor_147" href="#Footnote_147" class="fnanchor">[147]</a></p>
-
-  <p>Another remarkable example illustrating the like phenomena is furnished
-    by the observations which were made on December 21, 1860, by means of a
-    barometer in San Francisco, which oscillated, with periods of rest, for
-    half an hour. No shock was felt, nor is it likely that it was a local
-    accident, as it could not be produced artificially. On the following
-    day, however, a violent earthquake was experienced at Santiago.<a id="FNanchor_148" href="#Footnote_148" class="fnanchor">[148]</a></p>
-
-  <p>At the time or shortly after the great Lisbon earthquake, curious
-    phenomena were observed in distant countries, which only appear to be
-    explicable on the assumption of the existence of earth pulsations.</p>
-
-  <p>Thus at Amsterdam and other towns, chandeliers in churches were
-    observed to swing. At Haarlem water was
-    <span class="pagenum" id="Page_332">332</span>thrown over the sides of tubs,
-    and it is expressly mentioned that no motion was perceived in the
-    ground.</p>
-
-  <p>At the Hague a tallow chandler was surprised at the clashing noise
-    made by his candles, and this the more so because no motion was felt
-    underfoot.</p>
-
-  <p><em>Unusual disturbances in bodies of water.</em>—At the time of large
-    earthquakes it would appear that earth pulsations are produced, which
-    exhibit themselves in countries where the actual shaking of the
-    earthquake is not felt, by disturbances in bodies of waters like lakes
-    and seas.</p>
-
-  <p>Some remarkable examples of these disturbances are to be found in the
-    records of the great Lisbon earthquake. This earthquake, as a violent
-    movement of the ground, was chiefly felt in Spain, Portugal, northern
-    Italy, the south of France and Germany, northern Africa, Madeira, and
-    other Atlantic islands. In other countries further distant, as, for
-    instance, Great Britain, Holland, Scandinavia, and North America,
-    although the records are numerous, the only phenomena which were
-    particularly observed was the slow oscillations of the waters in lakes,
-    ponds, canals, &amp;c. In some instances the observers especially remark
-    that there was no motion in the soil.</p>
-
-  <p>Pebley Dam, in Derbyshire, which is a large body of water covering
-    some thirty acres, commenced to oscillate from the south. A canal near
-    Godalming flowed eight feet over the walk on the north side.</p>
-
-  <p>Coniston Water, in Cumberland, which is about five miles long,
-    oscillated for about five minutes, rising a yard up its shores. Near
-    Durham a pond, forty yards long and ten broad, rose and fell about one
-    foot for six or seven minutes. There were four or five ebbs and flows
-    per minute.</p>
-
-  <p><span class="pagenum" id="Page_333">333</span></p>
-
-  <p>Loch Lomond rose and fell through about two and a half feet every
-    five minutes, and all the other lochs in Scotland seem to have been
-    similarly agitated.</p>
-
-  <p>At Shirbrun Castle, in Oxfordshire, where the water in some moats and
-    ponds was very carefully observed, it was noticed that the floods
-    began gently, the velocity then increased, till at last with great
-    impetuosity they reached their full height. Here the water remained for
-    a little while, until the ebb commenced, at first gently, but finally
-    with great rapidity. At two extremities of a moat about 100 yards long,
-    it was found that the sinkings and risings were almost simultaneous.
-    The motions in a pond a short distance from the moat were also
-    observed, and it was found that the risings and sinkings of the two did
-    not agree.</p>
-
-  <p>During these motions there were several maxima.</p>
-
-  <p>These few examples of the motions of waters, without any record of the
-    motions of the ground, at the time of the Lisbon earthquake, must be
-    taken as examples of a very large number of similar observations of
-    which we have detailed accounts.</p>
-
-  <p>Like agitations, it must also be remembered, were perceived in North
-    America and in Scandinavia, and if the lakes of other distant countries
-    had been provided with sufficiently delicate apparatus, it is not
-    unlikely that similar disturbances would have been recorded.</p>
-
-  <p>Besides these movements in the waters of seas and lakes, at or about
-    the time of great earthquakes, we have records of like movements, which
-    take place as independent phenomena.</p>
-
-  <p>Thus we read that on October 22, 1755, the waters of Lake Ontario rose
-    and fell five and a half feet several times in the course of half an
-    hour.<a id="FNanchor_149" href="#Footnote_149" class="fnanchor">[149]</a> On March 31, 1761,
-    <span class="pagenum" id="Page_334">334</span>Loch Ness rose suddenly for the period
-    of three-quarters of an hour.<a id="FNanchor_150" href="#Footnote_150" class="fnanchor">[150]</a></p>
-
-  <p>As another example of the disturbance of water at the time of a great
-    earthquake in districts where the earthquake was not felt, may be
-    mentioned the swelling of the waters of the Marañon, in 1746, on the
-    night when Callao was overwhelmed.</p>
-
-  <p>Sudden variations in the level of the water have been many times
-    observed in the North American lakes. The changes in level which
-    sometimes take place in the Genfer and Boden lakes are supposed to have
-    some relation to the condition of the atmosphere. A rising and falling
-    of especial note took place on April 18, 1855.</p>
-
-  <p>In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’</p>
-
-  <p>From the observations and calculations of Prof. Forel it would seem
-    that the period of the ‘seiches’ depends upon the dimensions of the
-    lakes; the calculated periods dependent on the depths of the lakes
-    being approximately equal to the observed periods.<a id="FNanchor_151" href="#Footnote_151" class="fnanchor">[151]</a></p>
-
-  <p>W. T. Bingham, writing on the volcanoes of the Hawaiian Islands,
-    remarks that it is not unusual for the sea to be agitated by great and
-    unusual tides, and that such sea waves have not been attended with
-    volcanic eruptions or seismic disturbances. Thus in May 1819 the tide
-    rose and fell thirteen times. On November 7, 1837, there was an ebb and
-    flow of eight feet every twenty-eight minutes. Again, on May 17, 1841,
-    like phenomena, unaccompanied by any other unusual occurrences, were
-    recorded.<a id="FNanchor_152" href="#Footnote_152" class="fnanchor">[152]</a></p>
-
-  <p>Phenomena which may possibly hold a relationship to earth pulsations
-    are the periodical swellings of the ocean on the coast of Peru. Dr. C.
-    F. Winslow, who made a long <span class="pagenum" id="Page_335">335</span>
-    period observation upon the coast of Peru,
-    found ‘the highest tides to prevail at Callao and Paita in December
-    and January,’ and ‘also a series of enormous waves or sea-swells to
-    be thrown from time to time upon the coast, varying from twenty-four
-    to twenty-seven hours in continuance, accompanied by unusual height
-    of the tide during the same period.’ During June and July the ocean
-    was unusually tranquil. These phenomena do not appear to be connected
-    with great atmospheric storms, nor do they hold any relation to the
-    prevailing wind. They increase with and accompany the swelling of the
-    tides, and occur generally, but not always, about full moon.</p>
-
-  <p>Sometimes they break suddenly upon the coast. ‘<em>They are annual and
-    constant in their periodicity.</em>’</p>
-
-  <p>The periodical swellings are most noticeable between Tumbez 3° S.L. and
-    the Chincha Islands 14° S.L.</p>
-
-  <p>These oceanic phenomena synchronise with the periodic intensity of
-    earthquake phenomena in that part of the globe, and these with tidal
-    movements.<a id="FNanchor_153" href="#Footnote_153" class="fnanchor">[153]</a></p>
-
-  <p><em>Other phenomena possibly attributable to earth pulsations.</em>—If we
-    assume that earth pulsations have an existence, these many phenomena
-    which are otherwise difficult to understand meet with an explanation.
-    The curious effects which were produced in the springs at Toplitz at
-    the time of the Lisbon earthquake may have been due to a pulse-like
-    wave. The flow of the principal spring was greatly increased. Before
-    the increase it became turbid and at one time stopped. Subsequently it
-    became clear and flowed as usual, but the water was hotter and more
-    strongly mineralised. Sudden changes in the flow of underground waters
-    which from time to time are observed may be attributed to like causes.
-    Secondary earthquakes such as occurred after the Lisbon
-    <span class="pagenum" id="Page_336">336</span>earthquake,
-    as for instance in Derbyshire, may have been produced by pulsations
-    disturbing the equilibrium of ground in a critical state.</p>
-
-  <p>The falling in of subterranean excavations is also possibly connected
-    with these phenomena.</p>
-
-  <p><em>Possible causes of earth pulsations.</em>—Mr. George Darwin, in a report
-    to the British Association (1882), has shown that movements of
-    considerable magnitude may occur in the earth’s crust in consequence of
-    fluctuations in barometrical pressure. (A rise of the barometer over an
-    area is equivalent to loading that area with a weight, in consequence
-    of which it is depressed. When the barometer falls, the load is removed
-    from the area, which, in virtue of its elasticity, rises to its
-    original position. This fall and rise of the ground completes a single
-    pulsation.)</p>
-
-  <p>On the assumption that the earth has a rigidity like steel, Mr. Darwin
-    calculates that if the barometer rises an inch over an area like
-    Australia, the load is sufficient to sink that continent two or three
-    inches.</p>
-
-  <p>The tides which twice a day load our shores cause the land to rise and
-    fall in a similar manner. On the shores of the Atlantic, Mr. Darwin
-    has calculated that this rise and fall of the land may be as much as 5
-    inches. By these risings and fallings of the land the inclination of
-    the surface is so altered that the stile of a plummet suspended from a
-    rigid support ought not always to hang over the same spot. There would
-    be a deflection of the vertical.</p>
-
-  <p>In short, calculations respecting the effects of loads of various
-    descriptions, which we know are by natural operations continually
-    being placed upon and removed from the surface of various areas of the
-    earth’s surface, indicate that slow pulsatory movements of the earth’s
-    <span class="pagenum" id="Page_337">337</span>
-    surface must be taking place, causing variations in inclination of one
-    portion of the earth’s crust relatively to another.</p>
-
-  <p>Although it is possible that phenomena like the surging of levels may
-    be attributable to causes like these, we can hardly attribute the other
-    phenomena to such agencies.</p>
-
-  <p>Rather than seek an explanation from agencies exogenous to our earth,
-    we might perhaps with advantage appeal to the endogenous phenomena of
-    our planet. When the barometer falls, which we have shown corresponds
-    to an upward motion of the earth’s crust, we know, from the results of
-    experiments, that microseismic motions are particularly noticeable.</p>
-
-  <p>As a pictorial illustration of what this really means, we may imagine
-    ourselves to be residing on the loosely fitting lid of a large
-    cauldron, the relief of the external pressure over which increases the
-    activity of its internal ebullition—the jars attendant on which are
-    gradually propagated from their endogenous source to the exterior of
-    our planet. This travelling outwards would take place much in the same
-    way that the vibrations consequent to the rattle and jar of a large
-    factory slowly spread themselves farther and farther from the point
-    where they were produced.</p>
-
-  <p>Admitting an action of this description to take place, it would then
-    follow that this extra liberation of gaseous material beneath the
-    earth’s crust would result in an increased upward pressure from within,
-    and a tendency on the part of the earth’s crust to elevation. If we
-    accept this as an explanation of the increased activity of a tremor
-    indicator, then such an instrument may be regarded as a barometer,
-    measuring by its motions the variations in the internal pressure of our
-    planet.</p>
-
-  <p><span class="pagenum" id="Page_338">338</span></p>
-
-  <p>The relief of external pressure and the increase of internal pressure,
-    it will be observed, both tend in the same direction—namely, to an
-    elevation of the earth’s crust.</p>
-
-  <p>This explanation of the increased activity of earth tremors, which
-    has also been suggested by M. S. di Rossi, is here only advanced as a
-    speculation, more probable perhaps than many others.</p>
-
-  <p>We know how a mass of sulphur which has been fused in the presence of
-    water in a closed boiler gives up in the form of steam the occluded
-    moisture upon the relief of pressure. In a similar manner we see steam
-    escaping from volcanic vents and cooling streams of lava. We also
-    know how gas escapes from the pores and cavities in a seam of coal
-    on the fall of the barometrical column. We also know that certain
-    wells increase the height of their column under like conditions. The
-    latter of these phenomena, resulting in an increase in the rate of
-    drainage of an area by its tendency to render such an area of less
-    weight, facilitates its rise. If we follow the views of Mr. Mallet in
-    considering that the pressures exerted on the crust of our earth may in
-    volcanic regions be roughly estimated by the height of a column of lava
-    in the volcanoes of such districts, we see that in the neighbourhood of
-    a volcano like Cotopaxi the upward pressures must be enormously great.
-    Further, the phenomena of earthquakes and volcanoes indicate that these
-    pressures are variable. Before a volcano bursts forth we should expect
-    that there would be in its vicinity an upward bulging of the crust, and
-    after its formation a fall. Further, it is not difficult to conjecture
-    other possible means by which such pressures may obtain relief.</p>
-
-  <p>Should these pressures then find relief without rupturing the
-    surface, it is not difficult to imagine them as the
-    <span class="pagenum" id="Page_339">339</span> originators of
-    vast pulsations which may be recorded on the surface of the earth as
-    wave-like motions of slow period.</p>
-
-  <p>As an explanation of the strange movements observed on seas and lakes,
-    Kluge brings forward the following strange and remarkable theory. The
-    oxygen of the air is magnetic, whilst water is diamagnetic and the
-    earth magnetic: we have, therefore, in our seas and lakes a diamagnetic
-    body lying between and being, consequently, repelled from two magnetic
-    bodies. By variation in temperature, the balance of repulsions
-    exerted by the air and the earth is destroyed. Thus, by an elevation
-    of temperature the air expands and flows away from the heated area,
-    where, in consequence, there is less oxygen. The result of this is,
-    that the repulsion of the air upon the waters is less than that of the
-    earth upon the waters, and the waters are in consequence raised up. By
-    a falling of temperature the waters may be depressed, and by either
-    of these actions waves may be produced without the intervention of
-    earthquakes or earth pulsations.</p>
-
-  <p>The more definite kinds of information which we have to bring forward,
-    tending to prove the existence of earth pulsations, too slow in period
-    to be experienced by ordinary observers, are those which appear to be
-    resultant phenomena of great earthquakes.</p>
-
-  <p>The phenomena that we are certain of in connection with earth
-    vibrations, whether these vibrations are produced artificially by
-    explosions of dynamite in bore-holes, or whether they are produced
-    naturally by earthquakes, are, first, that a disturbance as it dies out
-    at a given point often shows in the diagrams obtained by seismographs a
-    decrease in period; and, secondly, a similar decrease in the period of
-    the disturbance takes place as the disturbance spreads.</p>
-
-  <p><span class="pagenum" id="Page_340">340</span></p>
-
-  <p>As examples of these actions I will quote the following.</p>
-
-  <p>The diagram of the disturbance of March 1, 1882, taken at Yokohama,
-    shows that the vibrations at the commencement of the disturbance had a
-    period of about three per second, near the middle of the disturbance
-    the period is about 1·1, whilst near the end the period has decreased
-    to ·46. That is to say, the backward and forward motion of the ground
-    at the commencement of the earthquake was six times as great as it was
-    near the end, when to make one complete oscillation it took between two
-    and three seconds. Probably the period became still less, but was not
-    recorded owing to the insensibility of the instruments to such slow
-    motions.<a id="FNanchor_154" href="#Footnote_154" class="fnanchor">[154]</a></p>
-
-  <p>We have not yet the means of comparing together diagrams of two or more
-    earthquakes, one having been taken near to the origin, and the other
-    at a distance. The only comparisons which I have been enabled to make
-    have been those of diagrams taken of the same earthquake, one in Tokio
-    and the other in Yokohama. As this base is only sixteen miles, and the
-    earthquake may have originated at a distance of several hundreds of
-    miles, comparisons like these can be of but little value.</p>
-
-  <p>Other diagrams illustrating the same point are those obtained at three
-    stations in a straight line, but at different distances from the
-    origin of a disturbance produced by exploding a charge of dynamite in
-    a bore-hole. A simple inspection of these diagrams shows that at the
-    near station the disturbance consisted of backward and forward motions,
-    which, as compared with the same disturbance as recorded at a more
-    distant station, were very rapid. Further, by examining the diagram
-    of the motions, <span class="pagenum" id="Page_341">341</span>
-    say, at the near station, it is clearly evident that
-    the period of the backward and forward motion rapidly decreased as the
-    motion died out.</p>
-
-  <p>These illustrations are given as examples out of a large series of
-    other records, all showing like results.</p>
-
-  <p>An observation which confirms the records obtained from seismographs
-    respecting the increase in period of an earthquake as it dies out
-    I have had opportunities of twice making with my levels. After all
-    perceptible motion of the ground subsequent upon a moderately severe
-    shock had died away, I have distinctly seen the bubble in one of these
-    levels slowly pulsating with an irregular period of from one to five
-    seconds.</p>
-
-  <p>Although we must draw a distinction between earth waves and water
-    waves, we yet see that in these points they present a striking
-    likeness. Let us take, for example, any of the large earthquake waves
-    which have originated off the coast of South America, and then radiated
-    outwards, until they spread across the Pacific, to be recorded in Japan
-    and other countries perhaps twenty-five hours afterwards, at a distance
-    of nearly 9,000 miles from their origin. Near this origin they appeared
-    as walls of water which were seen rapidly advancing towards the coast.
-    These have been from twenty to two hundred feet in height, and they
-    succeeded each other at rapid intervals, until finally they died out as
-    a series of gentle waves. By the time these walls of water traversed
-    the Pacific, to, let us say, Japan, they broadened out to a swell so
-    flat that it could not be detected on the smoothest water excepting
-    along shore lines where the water rose and fell like the tide. Instead
-    of a wall of water sixty feet in height, we had long flat undulations
-    perhaps eight feet in height, but with a distance from crest to crest
-    of from one to two hundred miles.</p>
-
-  <p><span class="pagenum" id="Page_342">342</span></p>
-
-  <p>If we turn to the effects of large earthquakes as exhibited on the
-    land, I think that we shall find records of phenomena which are only
-    to be explained on the assumption of an action having taken place
-    analogous to that which takes place so often in the ocean, or an action
-    similar to that exhibited by small earthquakes, and artificially
-    produced disturbances, if greatly exaggerated.</p>
-
-  <p>The only explanation for the phenomena accompanying the Lisbon
-    earthquake appears to be that the short quick vibrations which had
-    ruined so many cities in Portugal had, by the time that they had
-    radiated to distant countries, gradually become changed into long flat
-    waves having a period of perhaps several minutes. In countries like
-    England these pulse-like movements were too gentle to be perceived,
-    except in the effects produced by tipping up the beds of lakes and
-    ponds.</p>
-
-  <p>The phenomenon was not unlike that of a swell produced by a distant
-    storm. It would seem possible that in some cases pulsations producing
-    phenomena like the ‘seiches’ of Switzerland might have their origin
-    beneath the ocean, or deep down beneath the earth’s crust. Perhaps,
-    instead of commencing with the ‘snap and jar’ of an earthquake, they
-    may commence as a heaving or sinking of a considerable area, which
-    may be regarded as an uncompleted effort in the establishment of an
-    earthquake or a volcano.</p>
-
-  <p>From what has now been said it would seem that earth pulsations
-    are phenomena with a real existence, and that some of these are
-    attributable to earthquakes. On the other hand, certain earthquakes are
-    attributable to earth pulsations. Some of the phenomena which have been
-    brought forward have only a possible connection with these movements,
-    and they yet require investigation.
-    <span class="pagenum" id="Page_343">343</span> Elastic tides in the earth’s
-    crust have for long been realities in the minds of physicists. These,
-    however, are due to lunar and solar influences, and are regular in
-    their action. The tidal-like movements called pulsations are of greater
-    magnitude, and their goings and comings are irregular.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="CHAPTER_XXI">
-    <span class="pagenum" id="Page_344">344</span>
-    <h2>CHAPTER XXI.<br />
-      <span class="subhead">EARTH OSCILLATIONS.</span>
-    </h2>
-  </div>
-
-  <div class="summary">
-    Evidences of oscillation—Examples of oscillation—Temple of Jupiter
-    Serapis—Observations of Darwin—Causes of oscillation.</div>
-
-  <p><em>Evidences of oscillation.</em>—By earth oscillations are meant those slow
-    and quiet changes in the relative level of the sea and land which
-    geologists speak of as elevations or subsidences. These movements are
-    especially characteristic of volcanic and earthquake-shaken countries.</p>
-
-  <p>As evidences of elevations we appeal to phenomena like raised beaches,
-    sea-worn caves, raised coral reefs, and the remains of other dead
-    organisms like barnacles, and the borings of lithodomous shells in and
-    on the rocks of many coasts high above the level of the highest tides.
-    As a proof that subsidence has taken place, there is the evidence
-    afforded by submerged forests, the prolongation of certain valleys
-    beneath the bed of the ocean, the formation of coral islands, the
-    peculiar distribution of the plants and animals which we find in many
-    countries, and the submergence of works of human construction. Inasmuch
-    as these phenomena are discussed so fully in many treatises on physical
-    geology, the references to them here will be made as brief as possible.
-    Elevations and depressions which have taken place at the time of large
-    earthquakes<span class="pagenum" id="Page_345">345</span>
-    in a paroxysmal manner have already been mentioned. The
-    movements referred to in this chapter, although generally taking place
-    with extreme slowness, in certain instances, by an increase in their
-    rapidity, have approached in character to earth pulsations. In most
-    instances it would appear that the upward movement of the ground, which
-    may be likened to a process of tumefaction, goes on so gently that it
-    only becomes appreciable after the lapse of many generations.</p>
-
-  <p><em>Examples of movements.</em>—Lyell estimated that the average rate of rise
-    in Scandinavia has been about two and a half feet per century. At the
-    North Cape the rise may have been as much as five or six feet per
-    century. Observations made at the temple of Jupiter Serapis, between
-    October 1822 and July 1838, showed that the ground was sinking at the
-    rate of about one inch in four years. Since the Roman period, when this
-    temple was built, the ground has sunk twenty feet below the waves. Now
-    the floor of the temple is on the level of the sea. Lyell remarks that
-    if we reflect on the dates of the principal oscillations at this place
-    there appears to be connection between the movements of upheaval and a
-    local development of volcanic heat, whilst periods of depression are
-    concurrent with periods of volcanic quiescence.<a id="FNanchor_155" href="#Footnote_155" class="fnanchor">[155]</a></p>
-
-  <p>As examples of movements even more rapid than those at the Temple of
-    Jupiter Serapis we refer to an account of the earthquakes in Vallais
-    (November 1755), when the ground about a mountain at a small distance
-    from Brigue sank about a thumb’s-breadth every twenty-four hours. This
-    took place between December 9 and February 26.<a id="FNanchor_156" href="#Footnote_156" class="fnanchor">[156]</a></p>
-
-  <p>Another remarkable example of earth movement is
-    <span class="pagenum" id="Page_346">346</span>given in the account
-    of the earthquake at Scarborough, on December 29, 1737, when the head
-    of the spa water well was forced up in the air about ten yards high. At
-    this time the sands on the shore are said to have risen so slowly that
-    people came out to watch them.<a id="FNanchor_157" href="#Footnote_157" class="fnanchor">[157]</a></p>
-
-  <p>Two other examples of rapid earth movement are taken from Professor
-    Rossi’s ‘Meteorologia Endogena.’ Professor D. Seghetti, writing to
-    Professor Rossi, says that a few lustres ago (one lustre = twenty
-    years) Mount S. Giovanni hid the towns Jenne and Subiaco from each
-    other. From Subiaco the church at Jenne is now visible, which a few
-    years ago was invisible. The people at Jenne also can see more than
-    formerly. The supposition is that the side of Mount S. Giovanni is
-    lowered. This fact corresponds to a fact stated by Professor Carina,
-    who says that forty or fifty years ago from Granaiola you could not see
-    either the church of S. Maria Assunta di Citrone or the church of S.
-    Pietro di Corsena. Now you can see both.<a id="FNanchor_158" href="#Footnote_158" class="fnanchor">[158]</a></p>
-
-  <p>For a remarkable example illustrating the connection between seismic
-    activity and elevation we are indebted to the patient labours of
-    Darwin, who carefully investigated the evidences of elevation which
-    are visible upon the western coasts of South America. These evidences,
-    consisting of marks of erosion, caves, ancient beaches, sand dunes,
-    terraces of gravel, &amp;c., were traced between latitudes 45° 35′ to 12°
-    5′, a distance north and south of 2,075 geographical miles, and there
-    is but little doubt that they extend much farther. As deduced from
-    observations upon upraised shells alone, a summary of Mr. Darwin’s
-    observations are contained in the following table:—</p>
-
-  <p><span class="pagenum" id="Page_347">347</span></p>
-
-  <table summary="Darwin's observations">
-    <tr>
-      <th>&nbsp;</th>
-      <th class="tdr"><div>Feet</div></th>
-    </tr>
-    <tr>
-      <td class="tdl">At Chiloe the recent elevation has been</td>
-      <td class="tdr"><div>350</div></td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;„&nbsp; Concepcion&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdr"><div>625 to 1,000</div></td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;„&nbsp; Valparaiso&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdr"><div>1,300</div></td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;„&nbsp; Coquimbo&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdr"><div>252</div></td>
-    </tr>
-    <tr>
-      <td class="tdl">&nbsp;„&nbsp; Lima&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; „</td>
-      <td class="tdr"><div>85</div></td>
-    </tr>
-  </table>
-
-  <p>Shells, similar to those clinging to uplifted rocks, which are
-    evidences of these elevations, still exist in the neighbouring seas,
-    and in the same proportionate numbers as they are found in the upraised
-    beds. In addition to this, Mr. Darwin shows us that at Lima, during the
-    Indo-human period, the elevation has been at least eighty-five feet.
-    At Valparaiso, during the last 220 years, the rise was about nineteen
-    feet, and in the seventeen years subsequent to 1817 the rise has been
-    ten or eleven feet, a portion only of which can be attributed to
-    earthquakes. In 1834 the rise there was apparently still in progress.</p>
-
-  <p>At Chiloe there has been a gradual elevation of about four feet in four
-    years. These, together with numerous other examples, testify to the
-    gradual but, as compared with other parts of the globe, exceedingly
-    rapid rise of the ground upon the western shores of South America.<a id="FNanchor_159" href="#Footnote_159" class="fnanchor">[159]</a>
-    The most important point to be noticed is that this district of rapid
-    elevation is one of the most earthquake-shaken regions of the world.
-    And further, judging from Darwin’s remarks, in those portions of it
-    where the movements have been the most extensive, and at the same time
-    probably the most rapid, the seismic disturbances appear to have been
-    the most noticeable.</p>
-
-  <p>Similar remarks may be applied to Japan, it being in those districts
-    where evidences of recent elevation are abundant that earthquakes
-    are numerous. Thus, in the bay of Yedo, where we have borings of
-    lithodomi in the tufaceous cliffs ten feet above high-water mark,
-    which, <span class="pagenum" id="Page_348">348</span>
-    inasmuch as the rock in which they are found is soft and easily
-    weathered, indicate an exceedingly rapid elevation, earthquakes are of
-    common occurrence.</p>
-
-  <p>From the evidences of elevation which we have upon the South American
-    coast, Japan, and in other countries, it appears that these movements
-    are intermittent, there being periods of rest, when sea cliffs are
-    denuded, and perhaps even periods of subsidence. There is also evidence
-    to show that, although these movements have been gradual from time to
-    time, they have been aided by starts occasioned by earthquakes.</p>
-
-  <p>As to whether earthquakes are more numerous during periods of
-    elevation, or of subsidence, or during the intermediate periods of
-    rest, we have no evidence.</p>
-
-  <p>Sudden displacements which occasionally accompany earthquakes might, it
-    was said, sometimes be regarded as the <em>cause</em> of an earthquake, and
-    sometimes as the <em>effect</em>.</p>
-
-  <p>The slow elevations here referred to may be looked upon as being one of
-    the more important factors in the production of earthquakes. By various
-    causes the rocky coast is bent until, having reached the limit of its
-    elasticity, it snaps, and, in flying back like a broken spring, causes
-    the jars and tremors of an earthquake.</p>
-
-  <p>If this is the case, then the number of earthquakes felt in a district
-    which is being elevated may possibly be a function of the rate of
-    elevation.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="APPENDIX">
-    <span class="pagenum" id="Page_349">349</span>
-    <h2>APPENDIX.</h2>
-  </div>
-
-  <div class="figcenter">
-    <img src="images/divider.png" width="80" height="9" alt="" />
-  </div>
-
-  <h3><span class="smcap">List of the Principal Books, Papers, Periodicals, which are
-    referred to in the Preceding Pages.</span></h3>
-
-  <hr class="r15" />
-  <p class="hang1"><i>For a more complete bibliography of earthquakes refer to Mallet’s
-    catalogue of works given in his report to the British Association
-    in 1858.</i></p>
-
-  <hr class="r15" />
-  <p class="hang1">A True and Particular Relation of the Dreadful Earthquake which
-    happened at Lima, &amp;c. (1746). 1768.</p>
-
-  <p class="hang1">Abbot, Gen. H. L. On the Velocity of Transmission of Earth Waves.
-    <cite>Am. Jour. Sci.</cite> XV., March 1878.</p>
-
-  <p class="hang1">— Shock of the Explosion at Hallet’s Point, Nov. 14, 1876.
-    <i>Battalion Press.</i></p>
-
-  <p class="hang1">Alexander, Prof. T. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">American Journal of Science.</p>
-
-  <p class="hang1">Annali del reale osservatorio meteorologico Vesuviano.</p>
-
-  <p class="hang1">Annual Register, The.</p>
-
-  <p class="hang1">Anonymous, A Chronological and Historical Account of the most
-    Memorable Earthquakes in the World, &amp;c. 1750.</p>
-
-  <p class="hang1">— A Vindication of the Bishop of London’s Letter occasioned by the
-    Late Earthquake. 1750.</p>
-
-  <p class="hang1">— Phenomena of the Great Earthquake of Nov. 1, 1755.</p>
-
-  <p class="hang1">— Serious Thoughts occasioned by the Late Earthquake at Lisbon.
-    1755.</p>
-
-  <p class="hang1">Asiatic Society of Japan, Transactions of.</p>
-
-  <p class="hang1">Ayrton, Prof. W. E. <i>See</i> Perry, J.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Bárceno, M. Estudio del Terremoto (May 17, 1879) Mexico. 1879.</p>
-
-  <p class="hang1">Beke, Dr. C. T. Mount Sinai a Volcano.</p>
-
-  <p><span class="pagenum" id="Page_350">350</span></p>
-
-  <p class="hang1">Bissett, Rev. J. A Sermon (on account of the Earthquake at Lisbon,
-    Nov. 1, 1755). 1757.</p>
-
-  <p class="hang1">Bittner, A. Beiträge zur Kenntniss des Erdbebens von Belluno vom
-    29. Juni 1873.</p>
-
-  <p class="hang1">— Sitzungsb. der K. Akad. d. Wissensch., lxix. II. Abth., 1874.</p>
-
-  <p class="hang1">Bollettino del Vulcanismo Italiano.</p>
-
-  <p class="hang1">Boué, Dr. A. Ueber das Erdbeben welches Mittel-Albanien im
-    October d. J. so schrecklich getroffen hat. <cite>Die K. Akad. d.
-    Wissenschaften</cite>, Nov. 1851.</p>
-
-  <p class="hang1">— Parallele der Erdbeben, des Nordlichtes und des Erdmagnetismus.</p>
-
-  <p class="hang1">— Ueber die Nothwendigkeit die Erdbeben und vulkanischen
-    Erscheinungen genauer als bis jetzt beobachten zu lassen. <cite>Die K.
-    Akad. d. Wissenschaften</cite>, 1851 and 1857.</p>
-
-  <p class="hang1">Bouguer, M. Of the Volcanoes and Earthquakes in Peru.</p>
-
-  <p class="hang1">British Association, Reports of.</p>
-
-  <p class="hang1">Brunton, R. H. Constructive Art in Japan. <cite>Trans. Asiatic Soc. of
-    Japan</cite>, II. and III., Pt. 2.</p>
-
-  <p class="hang1">Bryce, J. Report to British Association, 1841.</p>
-
-  <p class="hang1">Buffour, M. The Natural History of Earthquakes and Volcanoes.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">C. H. A Physical Discussion of Earthquakes, &amp;c. 1693.</p>
-
-  <p class="hang1">Canterbury, Thomas, Lord Archbishop of, The Theory and History of
-    Earthquakes.</p>
-
-  <p class="hang1">Casariego, E. A. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Cawley, G. Some Remarks on Construction in Brick and Wood, &amp;c.
-    <cite>Trans. Asiatic Soc. of Japan</cite>, VI. Plate ii.</p>
-
-  <p class="hang1">Chaplin, Prof. W. S. An Examination of the Earthquakes recorded
-    at the Meteorological Observatory, Tokio. <cite>Trans. Asiatic Soc. of
-    Japan</cite>, VI. Part ii.</p>
-
-  <p class="hang1">Comptes Rendus.</p>
-
-  <p class="hang1">Credner, H. Das Dippoldiswalder Erdbeben vom Oktober 1877.</p>
-
-  <p class="hang1">— Zeitschr. f. d. Naturwiss. f. Sachsen u. Thüringen.</p>
-
-  <p class="hang1">— Das Vogtländisch-erzgebirgische Erdbeben, 23. Nov. 1875.</p>
-
-  <p class="hang1">— Zeitschr. f. d. gesammt. Naturwissenschaften, xlviii., Oktober.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Dan, T. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Darwin, Charles. Researches on Geology and Natural History.</p>
-
-  <p class="hang1">— Geological observations.</p>
-
-  <p class="hang1">Darwin, G. H. Reports on Lunar Disturbance of Gravity to British
-    Association, 1881. 1882.</p>
-
-  <p><span class="pagenum" id="Page_351">351</span></p>
-
-  <p class="hang1">Diffenbach, F. Plutonismus und Vulkanismus in der Periode von
-    1868–1872, und ihre Beziehungen zu den Erdbeben im Rheingebiet.</p>
-
-  <p class="hang1">Doelter, C. von. Ueber die Eruptivgebilde von Fleims, nebst einigen
-    Bemerkungen über den Bau älterer Vulcane.</p>
-
-  <p class="hang1">— lxxiv. Band d. Sitzungsb. d. K. Akad. d. Wissensch., I. Abth.,
-    Dec. Heft, Jahrg. 1876.</p>
-
-  <p class="hang1">Doolittle, Rev. T. Earthquakes Explained and Practically Improved,
-    &amp;c. 1693.</p>
-
-  <p class="hang1">Doyle, P. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Emerson, Prof., B.A. Review of Von Seebachs’ Earthquake of March 6,
-    1872. <cite>Am. Jour. Sci.</cite>, Series III.</p>
-
-  <p class="hang1">Ewing, Prof. J. A. Earthquake Measurement. A memoir published by
-    the Tokio University. 1883.</p>
-
-  <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Falb, R. Gedanken und Studien über den Vulkanismus, &amp;c. 1875.</p>
-
-  <p class="hang1">— Grundzüge zu einer Theorie der Erdbeben und Vulkanausbrüche.</p>
-
-  <p class="hang1">— Das Erdbeben von Belluno. ‘Sirius,’ Bd. VI., Heft ii.</p>
-
-  <p class="hang1">Flamstead, J. A Letter concerning Earthquakes. 1693.</p>
-
-  <p class="hang1">Forel, F. A. Les Tremblements de Terre (Suisse). <cite>Arch. des
-    Sciences Physiques et Naturelles</cite>, VI. p. 461.</p>
-
-  <p class="hang1">— Tremblement de Terre du 30 Décembre 1879.</p>
-
-  <p class="hang1">Fuchs, Karl. Vulkane und Erdbeben.</p>
-
-  <p class="hang1">— <cite>Die Vulkanischen Erscheinungen der Erde.</cite></p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Garcia, J. C. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Geinitz, Dr. E. Das Erdbeben von Iquique am 9. Mai 1877, &amp;c. <cite>Die
-    K. Leop.-Carol.-Deutschen Akademie der Naturforscher</cite>, Band xl., Nr. 9.</p>
-
-  <p class="hang1">Gentleman’s Magazine, The.</p>
-
-  <p class="hang1">Geographical Society, Proceedings of.</p>
-
-  <p class="hang1">Geological Society, Proceedings of.</p>
-
-  <p class="hang1">Girard, Dr. H. Ueber Erdbeben und Vulkane. 1845.</p>
-
-  <p class="hang1">Gray, T. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">— On Instruments for Measuring and Recording Earthquake Motions.
-    Phil. Mag. Sept. 1881.</p>
-
-  <p class="hang1">— On Recent Earthquake Investigation. <cite>The Chrysanthemum</cite>, 1881.
-    Guiscardi, Prof. G. Notizie del Vesuvio. 1857.</p>
-
-  <p class="hang1">— Il terremoto di Casamicciola del 4 Marzo. 1881.</p>
-
-  <p>&nbsp;</p>
-
-  <p><span class="pagenum" id="Page_352">352</span></p>
-
-  <p class="hang1">Hales, S., D.D., F.R.S. Some Considerations on the Causes of
-    Earthquakes. 1750.</p>
-
-  <p class="hang1">Hamilton, Sir W. Observations on Mount Vesuvius, Mount Etna, &amp;c.
-    1774.</p>
-
-  <p class="hang1">Hattori, I. Destructive Earthquakes in Japan. <cite>Trans. Asiatic Soc.
-    of Japan</cite>, V. Plate i.</p>
-
-  <p class="hang1">Heim, Prof. A. Les Tremblements de Terre et leur Etude
-    Scientifique. 1880.</p>
-
-  <p class="hang1">— Prof. A. Die Schweizerischen Erdbeben in 1881–1882.</p>
-
-  <p class="hang1">Hoeffer, Prof. H. Die Erdbeben Kärntens und deren Stosslinien. <cite>Die
-    Kais. Akademie d. Wissenschaften</cite>, Band xlii.</p>
-
-  <p class="hang1">Höfer, Prof. H. Das Erdbeben von Belluno, am 29. Juni 1873.
-    <cite>Sitzungsb. der K. Akad. d. Wissensch.</cite>, I. Abth., Band lxxiv.</p>
-
-  <p class="hang1">Hoff, K. E. A. von. Geschichte der durch Ueberlieferung
-    nachgewiesenen natürlichen Veränderungen der Erdoberfläche. 1822.</p>
-
-  <p class="hang1">Hooke, R., M.D., F.R.S. Discourses concerning Earthquakes.</p>
-
-  <p class="hang1">Hopkins, William. Report to the British Association on the
-    Geological Theories of Elevation and Earthquakes. 1847.</p>
-
-  <p class="hang1">Horton, Rev. Mr. An Account of the Earthquake which happened at
-    Leghorn in Italy (Jan. 1742). 1750.</p>
-
-  <p class="hang1">Humboldt, Alexander von. Cosmos.</p>
-
-  <p class="hang1">— Travels.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Jeitteles, L. A. Bericht über das Erdbeben am 15. Januar 1858.</p>
-
-  <p class="hang1">— Sitzungsberichte der mathem.-naturw. Classe d. K. Akad. d.
-    Wissensch., xxxv. S. 511.</p>
-
-  <p class="hang1">Judd, J. W., Prof. Volcanoes, What they Are and What they Teach.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Knipping, E. Verzeichniss von Erdbeben wahrgenommen in Tokio,
-    &amp;c. <cite>Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde
-    Ostasiens</cite>, Heft 14.</p>
-
-  <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Lasaulx, A. von. Das Erdbeben von Herzogenrath am 22. October 1873.</p>
-
-  <p class="hang1">Lemery, M. A Physico-Chemical Explanation of Subterranean Fires,
-    Earthquakes, &amp;c.</p>
-
-  <p class="hang1">Lescasse, M. J. Etude sur les Constructions Japonaises, &amp;c.
-    <cite>Mémoires de la Société des Ingénieurs Civils</cite>.</p>
-
-  <p class="hang1">Lister, M., M.D., F.R.S. Of the Nature of Earthquakes.</p>
-
-  <p class="hang1">Little, Rev. J. Conjectures on the Physical Causes of Earthquakes
-    and Volcanoes. 1820.</p>
-
-  <p>&nbsp;</p>
-
-  <p><span class="pagenum" id="Page_353">353</span></p>
-
-  <p class="hang1">Mallet, R. The Neapolitan Earthquake, Vol. II. <cite>Reports to the
-    British Association</cite>, 1850, 1851, 1852, 1854, 1858, 1861.</p>
-
-  <p class="hang1">— Secondary Effects of the Earthquake of Cachar. <cite>Proc. Geolog.
-    Soc.</cite>, 1872.</p>
-
-  <p class="hang1">— Dynamics of Earthquakes. <cite>Trans. Royal Irish Acad.</cite> 1846.</p>
-
-  <p class="hang1">Milne, David. Reports to British Association, 1841, 1843, 1844.</p>
-
-  <p class="hang1">Milne, John. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">— On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) <cite>Trans.
-    Royal Soc.</cite> 1882.</p>
-
-  <p class="hang1">— On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) <cite>Proc.
-    Royal Soc.</cite> No. 217, 1881.</p>
-
-  <p class="hang1">— Earthquake Observations and Experiments in Japan (with T. Gray,
-    B.Sc., F.R.S.E.) <cite>Phil. Mag.</cite>, Nov. 1881.</p>
-
-  <p class="hang1">— On the Elasticity and Strength Constants of certain Rocks (with
-    T. Gray, B.Sc., F.R.S.E.) <cite>Jour. Geolog. Soc.</cite>, 1882.</p>
-
-  <p class="hang1">— A Visit to the Volcano of Oshima. <cite>Geolog. Mag.</cite>, Dec. 2, Vol.
-    IV., pp. 193–197, 255.</p>
-
-  <p class="hang1">— On the Form of Volcanoes. <cite>Geolog. Mag.</cite>, Dec. 2, Vol. V., and
-    Dec. 2, Vol. VI.</p>
-
-  <p class="hang1">— Note upon the Cooling of the Earth, &amp;c. <cite>Geolog. Mag.</cite>, Dec. 2.,
-    Vol. VII., p. 99.</p>
-
-  <p class="hang1">— Investigation of the Earthquake Phenomena of Japan. <cite>Rep. Brit.
-    Assoc.</cite>, 1881 and 1882.</p>
-
-  <p class="hang1">— A Large Crater. <cite>Popular Science Review.</cite></p>
-
-  <p class="hang1">— The Volcanoes of Japan (a series of Articles). <cite>Japan Gazette.</cite></p>
-
-  <p class="hang1">— Earthquake Literature of Japan (a series of Articles). <cite>Japan Gazette.</cite></p>
-
-  <p class="hang1">— The Earthquake of Dec. 23, 1880. <cite>The Crysanthemum</cite>, 1881.</p>
-
-  <p class="hang1">— Earthquake Motion. <cite>The Crysanthemum</cite>, 1882.</p>
-
-  <p class="hang1">— Seismology in Japan. <cite>Nature</cite>, Oct. 1882.</p>
-
-  <p class="hang1">— Earth Movements. <cite>The Times</cite>, Oct. 12, 1882.</p>
-
-  <p class="hang1">Mitchell, Rev. J. Conjectures Concerning the Cause and Observations
-    upon the Phenomena of Earthquakes. 1760.</p>
-
-  <p class="hang1">Mohr, Dr. F. Geschichte der Erde. 1875.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Naturkundig Tijdschrift voor Nederlandsch Indie. 1875–1880.</p>
-
-  <p class="hang1">Naumann, Dr. E. Ueber Erdbeben und Vulkanausbrüche in Japan. <cite>Mitt.
-    d. Deutsch. Gesellsch. für Natur- und Völkerkunde Ostasiens.</cite> Heft 15.</p>
-
-  <p><span class="pagenum" id="Page_354">354</span></p>
-
-  <p class="hang1">Noggerath, Dr. J. Die Erdbeben vom 29. Juli 1846 im Rheingebiet, &amp;c.</p>
-
-  <p class="hang1">— Die Erdbeben im Vispthale (1855).</p>
-
-  <p class="hang1">— Die Erdbeben im Rheingebiet in den Jahren 1868, 1869, 1870.</p>
-
-  <p class="hang1">— Jahrgänge d. Verbandlungen d. Natur. Vereins für 1870. <cite>Rheinland
-    u. Westphalen</cite>, xxvii.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Oldham, Dr. Secondary Effects of the Earthquake of Cachas. <cite>Proc.
-    Geolog. Soc.</cite> 1872.</p>
-
-  <p class="hang1">— Thermal Springs of India. <cite>Memoirs of Geolog. Survey of India</cite>,
-    XIX. Plate 2.</p>
-
-  <p class="hang1">— A Catalogue of Indian Earthquakes. <cite>Memoirs of Geolog. Survey of
-    India</cite>, XIX. Plate 3.</p>
-
-  <p class="hang1">— The Cachas Earthquake. <cite>Memoirs of Geolog. Survey of India</cite>, XIX.
-    Plate 1.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Palmer, Col. H. S. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Palmieri, Prof. L., e Scacchi, A. Della Regione Volcanica del Monte
-    Vulture, e del Tremuoto ivi avvenuto nel dì 14 Agosto 1851, 1852.</p>
-
-  <p class="hang1">— Annali del reale Osservatorio Meteorologico Vesuviano.</p>
-
-  <p class="hang1">— Il Vesuvio, il Terremoto d’ Isernia e l’eruzione sottomarina di
-    Santorino. <cite>R. Accad. d. Sci. Fis. e Mat. di Napoli</cite>, iv. 1866.</p>
-
-  <p class="hang1">— Sul recente Terremoto di Corleone. <cite>R. Accad. d. Sci. Fis. e
-      Mat.</cite>, v. 1876.</p>
-
-  <p class="hang1">— Il Terremoto di Scio del dì 4 Aprile. <cite>R. Accad. d. Sci. Fis. e
-      Mat. di Napoli</cite>, v. 1881.</p>
-
-  <p class="hang1">— Sul Terremoto di Casamicciola del 4 Marzo 1881. <cite>R. Accad. d.
-      Sci. Fis. e. Mat. di Napoli</cite>. 1881.</p>
-
-  <p class="hang1">Paul, Prof. H. M. See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Perrey, Prof. A. Earthquake Catalogue and Memoirs. (For list see
-    Mallet’s Report to British Association. 1858.)</p>
-
-  <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Perry, J., and W. E. Ayrton. On a Neglected Principle that may be
-    Employed in Earthquake Measurement.</p>
-
-  <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Philosophical Magazine.</p>
-
-  <p class="hang1">Pickering, Rev. R. An Address to those who have either retired
-    or intend to leave Town under the Imaginary Apprehension of the
-    Approaching Shock of another Earthquake. 1750.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Ray, J., F.R.S. A Summary of the Causes of the Alterations which
-    have happened to the Face of the Earth.</p>
-
-  <p><span class="pagenum" id="Page_355">355</span></p>
-
-  <p class="hang1">Rockstroh, E. Informe de la Comision Científica del Instituto
-    Nacional de Guatemala, nombrada por el Sr. Ministro de Instruccion
-    Pública para el Estudio de los Fenómenos Volcánicos en el Lago de
-    Ilopango. 1880.</p>
-
-  <p class="hang1">Rockwood, Prof. C. G. Notes on Earthquakes. Annually in the <cite>Am.
-      Jour. Sci.</cite></p>
-
-  <p class="hang1">— Japanese Seismology. <cite>Am. Jour. Sci.</cite>, XXII. Dec. 1881.</p>
-
-  <p class="hang1">Romaine, W. A Discourse occasioned by the Late Earthquake. 1755.</p>
-
-  <p class="hang1">Rossi, Prof. M. S. di. Intorno all’ odierna fase dei Terremoti in
-    Italia, e segnatamente sul Terremoto in Casamicciola del 4 Marzo
-    1881. <cite>Società Geografica Italiana.</cite> 1881.</p>
-
-  <p class="hang1">— La Meteorologia Endogena, 2 vols.</p>
-
-  <p class="hang1">Royal Society, Transactions of.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Scacchi, A. <i>See</i> Palmieri.</p>
-
-  <p class="hang1">Schmidt, Dr. J. F. Untersuchungen über das Erdbeben am 15. Januar
-    1858.</p>
-
-  <p class="hang1">— Studien über Erdbeben. 1879.</p>
-
-  <p class="hang1">— Die Eruption des Vesuv (1855). 1856.</p>
-
-  <p class="hang1">Scrope, G. P. Volcanoes.</p>
-
-  <p class="hang1">Seebach. Das mittle Deutsche Erdbeben (1872). <cite>Mitt. der K.K.
-      geograph. Gesellsch.</cite>, II. Jahrg., 2. Heft, 1873.</p>
-
-  <p class="hang1">Serpieri, Prof. A. C. S. Nuove Osservazioni sul Terremoto avvenuto
-    in Italia il 12 Marzo 1873. <cite>Istituto Lombardo.</cite> 1873.</p>
-
-  <p class="hang1">— Il Terremoto di Rimini della notte 17–18 Marzo 1875.</p>
-
-  <p class="hang1">— Documenti nuove e Riflessioni sul Terremoto della notte 17–18
-    Marzo 1875. <cite>Meteorologia Italiana</cite>, iv. 1875.</p>
-
-  <p class="hang1">— Determinazione delle fasi e delle leggi del grande Terremoto
-    avvenuto in Italia nella notte 17–18 Marzo 1875. <cite>Istituto
-      Lombardo.</cite> 1875.</p>
-
-  <p class="hang1">— Dell’ influenza del Lume Solare sui Terremoti. <cite>Istituto
-      Lombardo.</cite> 1882.</p>
-
-  <p class="hang1">Sherlock, T., D.D. (Lord Bishop of London). A Letter on the
-    occasion of the late Earthquakes. 1750.</p>
-
-  <p class="hang1">Shower, Rev. J., D.D. Practical Reflections on the Earthquakes that
-    have happened in Europe and America, &amp;c. 1750.</p>
-
-  <p class="hang1">Stübel, A. (see Reiss, W.)</p>
-
-  <p class="hang1">Stukeley, Rev. W., M.D., F.R.S. The Philosophy of Earthquakes,
-    Natural and Religious, &amp;c. Plates 1, 2, and 3. 1756.</p>
-
-  <p class="hang1">Sturmius, J. C. A Methodical Account of Earthquakes.</p>
-
-  <p><span class="pagenum" id="Page_356">356</span></p>
-
-  <p class="hang1">Suess, E. Die Erdbeben Niederösterreiches. <cite>Die Kais. Akademie der
-      Wissenschaften</cite>, Bd. xxxiii.</p>
-
-  <p class="hang1">— Die Erdbeben des südlichen Italiens. <cite>Die Kais. Akademie der
-      Wissenschaften</cite>, Bd. xxxiv.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Volger, Dr. G. H. Untersuchungen über das Phänomen der Erdbeben.
-    1857.</p>
-
-  <p>&nbsp;</p>
-
-  <p class="hang1">Wagener, Dr. G. Bemerkungen über Erdbebenmesser und Vorschläge zu
-    einem neuen Instrumente dieser Art. <cite>Mitt. d. Deutsch. Gesellsch.
-      für Natur- und Völkerkunde Ostasiens</cite>, Heft 15.</p>
-
-  <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p>
-
-  <p class="hang1">Winchilsea, The Earl of. A True and Exact Relation of the late
-    Prodigious Earthquake and Eruption of Mount Etna. 1669.</p>
-
-  <p class="hang1">Woodward, J., M.D., F.R.S. Earthquake caused by some Accidental
-    Obstruction of a Continual Subterranean Heat.</p>
-
-
-  <h3><span class="smcap">Seismological Society Of Japan.</span></h3>
-
-  <p class="center">The following are a list of the papers published by this Society:—</p>
-
-  <h4>VOL. I.</h4>
-
-  <p class="hang1">Milne, J. Seismic Science in Japan. 35 pages.</p>
-
-  <p class="hang1">Ewing, J. A. New Form of Pendulum Seismograph. 6 pages, 3 plates.</p>
-
-  <p class="hang1">Gray, T. Seismometer and Torsion Pendulum Seismograph. 8 pages, 2
-    plates.</p>
-
-  <p class="hang1">Mendenhall, T. C. Acceleration of Gravity at Tokio (abstract). 2
-    pages.</p>
-
-  <p class="hang1">Wagener, G., and E. Knipping. New Seismometer and Observations with
-    same. 18 pages, 1 plate.</p>
-
-  <p class="hang1">Milne, J. Earthquake in Japan of Feb. 22, 1880. 116 pages, 5
-    plates, 8 woodcuts.</p>
-
-
-  <h4>VOL. II.</h4>
-
-  <p class="hang1">Milne, J. Recent Earthquakes of Yeddo, Effects on Buildings, &amp;c. 38
-    pages, 2 plates, and many tables.</p>
-
-  <p class="hang1">Mendenhall, T. C. Gravity on Summit of Fujiyama (abstract). 2 pages.</p>
-
-  <p class="hang1">Paul, H. M. Earth Vibrations from Railroad Trains (abstract). 4
-    pages.</p>
-
-  <p class="hang1">Ewing, J. A. Astatic Horizontal Lever Seismograph (abstract). 5
-    pages, 1 plate.</p>
-
-  <p><span class="pagenum" id="Page_357">357</span></p>
-
-  <p class="hang1">Milne, J. Peruvian Earthquake of May, 9, 1877. 47 pages, 2 plates,
-    tables. Constitution, Rules, Officers and Members of the Society,
-    Dec., 1881.</p>
-
-  <h4>VOL. III.</h4>
-
-  <p class="hang1">Gray, T. Steady Points for Earthquake Measurements. 11 pages, 3
-    plates.</p>
-
-  <p class="hang1">Milne, J. Experiments in Observational Seismology. 53 pages, 1
-    plate, tables.</p>
-
-  <p class="hang1">— The Great Earthquakes of Japan. 38 pages, 1 plate, many tables.</p>
-
-  <p class="hang1">Perry, J. Theory of a Rocking Column. 4 pages.</p>
-
-  <p class="hang1">Knipping, E. Earthquake of July 25, 1880, with Dr. Wagener’s
-    Seismometer. 4 pages.</p>
-
-  <p class="hang1">Ewing, J. A. Earthquake Observation at three or more Stations, &amp;c.
-    4 pages.</p>
-
-  <p class="hang1">— Records of three recent Earthquakes. 6 pages, 3 plates.</p>
-
-  <p class="hang1">— Earthquake of March 8, 1881. 8 pages, 1 plate.</p>
-
-  <p class="hang1">Milne, J. Horizontal and Vertical Motion in Earthquake of March 8,
-    1881. 8 pages, 3 plates.</p>
-
-  <p class="hang1">Gray, T. Seismograph for Registering Vertical Motion. 3 pages, 1
-    plate.</p>
-
-  <p class="hang1">Ewing, J. A. Seismometer for Vertical Motion. 3 pages, 1 plate.</p>
-
-  <p class="hang1">Gray, T. Seismograph for Large Motions. 2 pages.</p>
-
-  <p class="hang1">— Compensating a Pendulum to make it Astatic. 3 pages.</p>
-
-  <p class="hang1">Palmer, H. S. Note on Earth Vibrations. 3 pages.</p>
-
-  <p class="hang1">Kuwabara, M. The Hot Springs of Atami. 2 pages.</p>
-
-  <h4>VOL. IV.</h4>
-
-  <p class="hang1">Milne, J. Distribution of Seismic Activity in Japan. 30 pages, 1
-    plate.</p>
-
-  <p class="hang1">Wada, T. Notes on Fujiyama. 7 pages.</p>
-
-  <p class="hang1">Casariego, E. Abella y. Earthquakes of Nueva Vizcaya in 1881. 23
-    pages, 2 maps.</p>
-
-  <p class="hang1">Milne, J. Utilisation of Earth’s Internal Heat. 12 pages.</p>
-
-  <p class="hang1">Ewing, J. A. Earthquake of March 11, 1882. 5 pages.</p>
-
-  <p class="hang1">Doyle, P. Note on an Indian Earthquake. 6 pages.</p>
-
-  <p class="hang1">Milne, J. Systematic Observation of Earthquakes. 31 pages, 5 plates.</p>
-
-  <h4>VOL. V.</h4>
-
-  <p class="hang1">Naumann, Dr. E. Notes on Secular Changes of Magnetic Declination in
-    Japan, p. 1–18.</p>
-
-  <p class="hang1">Casariego, Don E. Abella y. Monografía Geológica del Volcan de
-    Albay ó El Máyon. p. 19–43.</p>
-
-  <p class="hang1"><span class="pagenum" id="Page_358">358</span></p>
-
-  <p class="hang1">Garcia, Don J. Centeno y. Abstract of a Memoir on the Earthquakes
-    on the Island of Luzon in 1880. p. 43–89.</p>
-
-  <p class="hang1">Ewing, Prof. J. A. Seismological Notes.</p>
-
-  <p class="hang1">— A Duplex Pendulum Seismometer.</p>
-
-  <p class="hang1">— The Suspension of a Horizontal Pendulum.</p>
-
-  <p class="hang1">— A Speed Governor for Seismograph Clocks, p. 89–95.</p>
-
-  <p class="hang1">Dan, T., S.B. Notes on the Earthquake at Atami, in the Province of
-    Idzu, on September 29, 1882. p. 95–105.</p>
-
-  <h4>VOL. VI.</h4>
-
-  <p class="hang1">Alexander, Prof. T. The Development of the Record given by a
-    Bracket Machine.</p>
-
-  <p class="hang1">Milne, J. Earth Pulsations.</p>
-
-  <p class="hang1">Ewing, J. A. Note on a Duplex Pendulum with a Single Bob.</p>
-
-  <p class="hang1">Gergens, F. Note on an Iron Casting, Supposed to have been
-    Disturbed whilst Cooling by an Earthquake.</p>
-
-  <p class="hang1">West, C. D. On a Parallel Motion Seismograph.</p>
-
-  <p class="hang1">Ewing, J. A. Certain Methods of Astatic Suspension.</p>
-
-  <p class="hang1">Alexander, T. Ball and Cup Seismometer.</p>
-
-  <p class="hang1">Knipping, Messrs. Paul and. Report on a System for Earthquake
-    Observation.</p>
-
-  <p class="hang1">Catalogues of Earthquakes.</p>
-
-  <hr class="page" />
-  <div class="chapter" id="INDEX">
-    <span class="pagenum" id="Page_359">359</span>
-    <h2>INDEX.</h2>
-  </div>
-
-  <div class="figcenter">
-    <img src="images/divider.png" width="80" height="9" alt="" />
-  </div>
-
-  <ul>
-    <li>Abbadie, M. d’, on earth tremors, <a href="#Page_309">309</a></li>
-    <li>Abbot, General H. L., on the transmission of vibrations, <a href="#Page_62">62</a></li>
-    <li>Abella, M., on the earthquake in the Philippines in 1881, <a href="#Page_77">77</a></li>
-    <li>Activity, on seismic, <a href="#Page_6">6</a></li>
-    <li>Aristotle, on the classification of earthquakes, <a href="#Page_41">41</a></li>
-    <li>Artificial earthquakes, experiments on, <a href="#Page_57">57</a></li>
-    <li>— — intensity of, <a href="#Page_61">61</a></li>
-    <li>Aurora, on the occurrence of, with earthquakes, <a href="#Page_264">264</a></li>
-    <li>Ayrton and Perry, on the effect of soft foundations, <a href="#Page_130">130</a></li>
-    <li>— — on the period of vibration of buildings, <a href="#Page_115">115</a></li>
-    <li>— — on the principle of, <a href="#Page_31">31</a></li>
-    <li>&nbsp;</li>
-    <li>Barometer, effect of changes of, on earthquakes, <a href="#Page_266">266</a></li>
-    <li>Bertelli, on aurora and earthquakes, <a href="#Page_265">265</a></li>
-    <li>— on earth tremors, <a href="#Page_316">316</a>, <a href="#Page_320">320</a></li>
-    <li>— on the normal tromometer of, <a href="#Page_317">317</a></li>
-    <li>Bittner, A., on the buildings of Belluno, <a href="#Page_100">100</a></li>
-    <li>Bridges, on earthquake, <a href="#Page_140">140</a></li>
-    <li>Brunton, R. H., on buildings in earthquake countries, <a href="#Page_123">123</a></li>
-    <li>Buckle, on the history of civilisation, <a href="#Page_1">1</a></li>
-    <li>Builders, interest of the study of earthquakes to, <a href="#Page_3">3</a></li>
-    <li>Buildings, on cracks in, <a href="#Page_98">98</a>, <a href="#Page_108">108</a></li>
-    <li>— the effect of earthquakes on, <a href="#Page_96">96</a></li>
-    <li>— on the irregular destruction of, <a href="#Page_96">96</a></li>
-    <li>— — effect on the end house in a row, <a href="#Page_112">112</a></li>
-    <li>— — church of St. Augustin at Manilla, <a href="#Page_113">113</a></li>
-    <li>— — relation of destruction of, to earthquake motion, <a href="#Page_103">103</a></li>
-    <li>— — protection of, <a href="#Page_143">143</a></li>
-    <li>— — pitch of the roof of, <a href="#Page_110">110</a></li>
-    <li>— — position of openings in the walls of, <a href="#Page_111">111</a></li>
-    <li>— — swing of, <a href="#Page_115">115</a></li>
-    <li>— — period of vibration of, <a href="#Page_115">115</a></li>
-    <li>— — principle of relative periods in, <a href="#Page_116">116</a></li>
-    <li>— — types of, for earthquake countries, <a href="#Page_121">121</a></li>
-    <li>— — effect of underlying rocks on, <a href="#Page_130">130</a></li>
-    <li>— general conclusions regarding, <a href="#Page_144">144</a></li>
-    <li>&nbsp;</li>
-    <li>Cacciatore, definition of the, <a href="#Page_18">18</a></li>
-    <li>Caldcleugh, A., on earthquake frequency, <a href="#Page_245">245</a></li>
-    <li>— on barometric height and earthquakes, <a href="#Page_266">266</a></li>
-    <li>Carruthers, J., on earthquakes and tides, <a href="#Page_291">291</a></li>
-    <li>Centrum, definition of, <a href="#Page_9">9</a></li>
-    <li>— on the depth of, <a href="#Page_213">213</a></li>
-    <li><span class="pagenum" id="Page_360">360</span>— on the maximum depth of, <a href="#Page_218">218</a></li>
-    <li>Centrum, determination of position of, <i>see</i> origins</li>
-    <li>Chaplin, W. S., on the bracket seismometer of, <a href="#Page_27">27</a></li>
-    <li>— on earthquakes and the position of the moon, <a href="#Page_252">252</a></li>
-    <li>Coast line, on the movement of, <a href="#Page_160">160</a></li>
-    <li>Cocks, R., on earthquakes and tides, <a href="#Page_290">290</a></li>
-    <li>Coseismic lines, definition of, <a href="#Page_10">10</a></li>
-    <li>Curves, on microseismic, <a href="#Page_321">321</a></li>
-    <li>&nbsp;</li>
-    <li>Darwin, Charles, on the movement of coast lines, <a href="#Page_160">160</a></li>
-    <li>— George H., on earth tides, <a href="#Page_285">285</a></li>
-    <li>— on tidal loads, <a href="#Page_291">291</a></li>
-    <li>— on earth pulsation, <a href="#Page_330">330</a></li>
-    <li>— on the effect of fluctuations of barometric pressure, <a href="#Page_336">336</a></li>
-    <li>— experiments at Cambridge, <a href="#Page_310">310</a></li>
-    <li>Delauney, M. J., on the influence of the planets on earthquakes, <a href="#Page_261">261</a></li>
-    <li>Diagonic, definition of, <a href="#Page_11">11</a></li>
-    <li>Diastrophic, definition of, <a href="#Page_11">11</a></li>
-    <li>Direction of motion, from instrumental records, <a href="#Page_198">198</a></li>
-    <li>Distribution, on earthquake, <a href="#Page_226">226</a></li>
-    <li>— examples of, <a href="#Page_231">231</a></li>
-    <li>Disturbance, on the propagation of, <a href="#Page_50">50</a></li>
-    <li>Douglas, J., on South American houses, <a href="#Page_126">126</a></li>
-    <li>&nbsp;</li>
-    <li>Earth particle, on the velocity and acceleration of, <a href="#Page_79">79</a></li>
-    <li>Earthquake motion, nature of, as deduced from the feelings, <a href="#Page_67">67</a></li>
-    <li>— direction of, derived from instrumental records, <a href="#Page_69">69</a></li>
-    <li>— duration of, <a href="#Page_71">71</a></li>
-    <li>— period of vibration in, <a href="#Page_74">74</a></li>
-    <li>— examples of extent of, <a href="#Page_75">75</a>–<a href="#Page_77">77</a></li>
-    <li>— absolute intensity of force in, <a href="#Page_83">83</a></li>
-    <li>— radiation of, <a href="#Page_85">85</a></li>
-    <li>— velocity of propagation of, <a href="#Page_87">87</a></li>
-    <li>Earthquake at Lisbon, velocity of propagation of, <a href="#Page_88">88</a></li>
-    <li>Earthquakes, general examples of effects of, <a href="#Page_142">142</a></li>
-    <li>— geological changes produced by, <a href="#Page_161">161</a></li>
-    <li>— hunting, <a href="#Page_187">187</a></li>
-    <li>— distribution of, <a href="#Page_226">226</a></li>
-    <li>— maps, <a href="#Page_189">189</a></li>
-    <li>— secondary, <a href="#Page_248">248</a></li>
-    <li>— table of, for nineteenth century, <a href="#Page_259">259</a></li>
-    <li>— on the course of, <a href="#Page_277">277</a>–<a href="#Page_281">281</a></li>
-    <li>— and tides, <a href="#Page_290">290</a></li>
-    <li>— prediction of, <a href="#Page_297">297</a>–<a href="#Page_304">304</a></li>
-    <li>Elastic waves, nature of, <a href="#Page_44">44</a></li>
-    <li>Emergence, angle of, <a href="#Page_9">9</a></li>
-    <li>Energy, dissipation of, in earthquakes, <a href="#Page_52">52</a></li>
-    <li>— seismic, in relation to geological time, <a href="#Page_234">234</a></li>
-    <li>— — table of, <a href="#Page_240">240</a></li>
-    <li>Epicentrum, definition of, <a href="#Page_9">9</a></li>
-    <li>Euthutropic, definition of, <a href="#Page_11">11</a></li>
-    <li>Ewing, J. A., pendulum seismograph of, <a href="#Page_25">25</a></li>
-    <li>— astatic pendulum of, <a href="#Page_26">26</a></li>
-    <li>— bracket seismograph of, <a href="#Page_26">26</a></li>
-    <li>&nbsp;</li>
-    <li>Falb, R., on the influence of the sun and moon on earthquakes, <a href="#Page_286">286</a></li>
-    <li>Fissures, on the material discharged from, <a href="#Page_148">148</a></li>
-    <li>— on the explanation of, <a href="#Page_151">151</a></li>
-    <li>Focal cavity, definition of, <a href="#Page_9">9</a></li>
-    <li>— on the form of, <a href="#Page_221">221</a></li>
-    <li>Forbes, D., on an earthquake in Mendoza, <a href="#Page_151">151</a></li>
-    <li>Frequency of earthquakes, <a href="#Page_243">243</a></li>
-    <li>Frere, Sir H. Bartle, on geological changes produced by earthquakes, <a href="#Page_161">161</a></li>
-    <li>Fuchs, on sea waves, <a href="#Page_176">176</a></li>
-    <li>— on the movement of the seismic centre, <a href="#Page_233">233</a></li>
-    <li>— on earthquakes and volcanic outbursts, <a href="#Page_271">271</a></li>
-    <li><span class="pagenum" id="Page_361">361</span>— on hot springs, <a href="#Page_157">157</a></li>
-    <li>Fumaroles, the effect of earthquakes on, <a href="#Page_156">156</a></li>
-    <li>&nbsp;</li>
-    <li>Geinitz, Dr., on sea waves, <a href="#Page_182">182</a></li>
-    <li>Geologists, on the interest of seismology to, <a href="#Page_2">2</a></li>
-    <li>Gray, T., astatic pendulum of, <a href="#Page_26">26</a></li>
-    <li>— bracket seismometer of, <a href="#Page_27">27</a></li>
-    <li>— conical pendulum of, <a href="#Page_29">29</a></li>
-    <li>— dead heat pendulum of, <a href="#Page_22">22</a></li>
-    <li>— on the rotation of bodies, <a href="#Page_196">196</a></li>
-    <li>— rolling spheres and cylinders of, <a href="#Page_29">29</a></li>
-    <li>— torsion pendulum seismometer of, <a href="#Page_25">25</a></li>
-    <li>— vertical motion seismometers of, <a href="#Page_32">32</a>, <a href="#Page_33">33</a></li>
-    <li>— and Milne, seismograph of, <a href="#Page_38">38</a></li>
-    <li>&nbsp;</li>
-    <li>Hattori, I., on the large earthquakes of Japan, <a href="#Page_244">244</a></li>
-    <li>Haughton, Prof., list of active volcanoes of, <a href="#Page_227">227</a></li>
-    <li>— method of finding earthquake origins of, <a href="#Page_209">209</a></li>
-    <li>Hills, on the want of support on the face of, <a href="#Page_136">136</a></li>
-    <li>Höfer, on an earthquake at Belluno, <a href="#Page_225">225</a></li>
-    <li>Hoffmann, F., on the barometer and earthquakes, <a href="#Page_267">267</a></li>
-    <li>Hooke, on earthquake motion, <a href="#Page_42">42</a></li>
-    <li>Hopkins, on the thickness of the earth’s crust, <a href="#Page_284">284</a></li>
-    <li>Humboldt, on meteors and earthquakes, <a href="#Page_261">261</a></li>
-    <li>— on the barometer and earthquakes, <a href="#Page_267">267</a></li>
-    <li>— on volcanoes and earthquakes, <a href="#Page_279">279</a></li>
-    <li>&nbsp;</li>
-    <li>Imagination, effect of earthquakes on the, <a href="#Page_2">2</a></li>
-    <li>Instruments, direction of motion derived from, <a href="#Page_198">198</a></li>
-    <li>Intensity, on earthquake, <a href="#Page_51">51</a>, <a href="#Page_71">71</a></li>
-    <li>— seismic curve of, for Kioto, <a href="#Page_242">242</a></li>
-    <li>Isoseismic circles, definition of, <a href="#Page_10">10</a></li>
-    <li>— areas, definition of, <a href="#Page_10">10</a></li>
-    <li>&nbsp;</li>
-    <li>Kluge, on sea waves, <a href="#Page_175">175</a></li>
-    <li>— on earthquake frequency, <a href="#Page_246">246</a></li>
-    <li>— on simultaneous earthquakes, <a href="#Page_248">248</a></li>
-    <li>— on earthquakes and sun spots, <a href="#Page_263">263</a></li>
-    <li>— on earth pulsations, <a href="#Page_339">339</a></li>
-    <li>Kreil, pendulum seismometer of, <a href="#Page_25">25</a></li>
-    <li>&nbsp;</li>
-    <li>Lakes, on disturbances in, <a href="#Page_154">154</a></li>
-    <li>Land, effect of earthquakes on, <a href="#Page_146">146</a>–<a href="#Page_162">162</a></li>
-    <li>— on the reason of movements of, <a href="#Page_162">162</a></li>
-    <li>— on cracks and fissures formed in, <a href="#Page_146">146</a></li>
-    <li>Level, on the use of for earth pulsations, <a href="#Page_328">328</a></li>
-    <li>Literature, on seismic, <a href="#Page_6">6</a></li>
-    <li>— on Japanese earthquake, <a href="#Page_7">7</a></li>
-    <li>&nbsp;</li>
-    <li>Mallet, R., on area of disturbance as a test of seismic energy, <a href="#Page_78">78</a></li>
-    <li>— on clock stopping, <a href="#Page_36">36</a></li>
-    <li>— list of works on earthquakes of, <a href="#Page_5">5</a></li>
-    <li>— curve of seismic energy of, <a href="#Page_238">238</a></li>
-    <li>— definition of earthquake of, <a href="#Page_43">43</a></li>
-    <li>— on earthquake frequency, <a href="#Page_243">243</a></li>
-    <li>— on the influence of the heavenly bodies on earthquakes, <a href="#Page_253">253</a></li>
-    <li>— on maximum depth of origin, <a href="#Page_218">218</a></li>
-    <li>— on pendulum seismometers, <a href="#Page_20">20</a></li>
-    <li>— projection seismometer of, <a href="#Page_17">17</a></li>
-    <li>— on the Neapolitan earthquake, <a href="#Page_69">69</a>, <a href="#Page_77">77</a>, <a href="#Page_83">83</a>, <a href="#Page_97">97</a>, <a href="#Page_103">103</a>, <a href="#Page_132">132</a>, <a href="#Page_142">142</a>, <a href="#Page_218">218</a>, <a href="#Page_280">280</a></li>
-    <li>— on sea waves, <a href="#Page_170">170</a></li>
-    <li>— on the swing of mountains, <a href="#Page_135">135</a></li>
-    <li><span class="pagenum" id="Page_362">362</span>— on propagation from a fissure, <a href="#Page_217">217</a></li>
-    <li>Mallet on the temperature of focal cavity, <a href="#Page_84">84</a></li>
-    <li>Malvasia, M. le Conte, on earth tremors, <a href="#Page_316">316</a></li>
-    <li>Martin, D. S., on the New England earthquake of 1874, <a href="#Page_142">142</a></li>
-    <li>Meizoseismic area, definition of, <a href="#Page_10">10</a></li>
-    <li>Melzi, on curves of microseismic motion, <a href="#Page_322">322</a></li>
-    <li>Meteors, on earthquakes and, <a href="#Page_260">260</a></li>
-    <li>Microseismic movements, on cause of, <a href="#Page_324">324</a></li>
-    <li>Milne, D., on the Lisbon earthquake, <a href="#Page_87">87</a></li>
-    <li>— on earthquake synchronism, <a href="#Page_247">247</a></li>
-    <li>Mitchell, on earthquake motion, <a href="#Page_42">42</a></li>
-    <li>Moon, effect of, on earthquakes, <a href="#Page_251">251</a>, <a href="#Page_285">285</a></li>
-    <li>Mountains, on the swing of, <a href="#Page_135">135</a></li>
-    <li>&nbsp;</li>
-    <li>Naumann, E., on meteors and earthquakes, <a href="#Page_261">261</a></li>
-    <li>— on sun spots and earthquakes, <a href="#Page_263">263</a></li>
-    <li>&nbsp;</li>
-    <li>Ocean, on disturbances in, <a href="#Page_163">163</a>–<a href="#Page_186">186</a></li>
-    <li>Origin, definition of, <a href="#Page_9">9</a></li>
-    <li>— on the determination of, <a href="#Page_187">187</a></li>
-    <li>— position of, deduced from direction of motion, <a href="#Page_192">192</a></li>
-    <li>— — from destruction of buildings, <a href="#Page_194">194</a></li>
-    <li>— — from rotation of bodies, <a href="#Page_195">195</a></li>
-    <li>— — from time of occurrence, <a href="#Page_199">199</a></li>
-    <li>— — examples of methods of calculating, <a href="#Page_200">200</a>–<a href="#Page_212">212</a></li>
-    <li>Oscillations, on earth, <a href="#Page_344">344</a></li>
-    <li>Overturning moment, on the area of greatest, <a href="#Page_53">53</a></li>
-    <li>&nbsp;</li>
-    <li>Palmer, Col. H. S., on earth tremors, <a href="#Page_307">307</a></li>
-    <li>Palmieri, on clock stopping, <a href="#Page_36">36</a>, <a href="#Page_62">62</a></li>
-    <li>Paul, H. M., on earth tremors, <a href="#Page_308">308</a></li>
-    <li>Perrey, A., on the influence of the moon on earthquakes, <a href="#Page_251">251</a></li>
-    <li>Perrey on the periodicity of earthquakes, <a href="#Page_8">8</a></li>
-    <li>Perry, J., on position of openings in walls, <a href="#Page_111">111</a></li>
-    <li>Physicists, on the interest of earthquakes to, <a href="#Page_2">2</a></li>
-    <li>Planets, influence of, on earthquakes, <a href="#Page_260">260</a></li>
-    <li>Plantamour, M., on earth pulsations, <a href="#Page_328">328</a></li>
-    <li>Pleistoseists, definition of, <a href="#Page_10">10</a></li>
-    <li>Poly, M. A., on earthquakes and sun spots, <a href="#Page_263">263</a></li>
-    <li>— on earthquakes and revolving storms, <a href="#Page_294">294</a></li>
-    <li>Prost, M. le Baron, on earth tremors, <a href="#Page_316">316</a></li>
-    <li>Pulsation, on earth, <a href="#Page_4">4</a>, <a href="#Page_326">326</a>–<a href="#Page_343">343</a></li>
-    <li>&nbsp;</li>
-    <li>Records, on receivers of, <a href="#Page_33">33</a></li>
-    <li>Rivers, on disturbances in, <a href="#Page_154">154</a></li>
-    <li>Rockwood, Prof., on American earthquakes, <a href="#Page_6">6</a></li>
-    <li>Ronaldson, T., on San Francisco houses, <a href="#Page_129">129</a></li>
-    <li>Rossi, M. S. di, on an eruption of gas in the Tiber, <a href="#Page_153">153</a></li>
-    <li>— aurora and earthquakes, <a href="#Page_264">264</a></li>
-    <li>— earth tremors, <a href="#Page_317">317</a>, <a href="#Page_320">320</a></li>
-    <li>— earth oscillations, <a href="#Page_346">346</a></li>
-    <li>— earth pulsations, <a href="#Page_327">327</a></li>
-    <li>— microseismograph of, <a href="#Page_318">318</a></li>
-    <li>— microphonic observations of, <a href="#Page_319">319</a>, <a href="#Page_323">323</a></li>
-    <li>— normal tromometer of, <a href="#Page_317">317</a></li>
-    <li>&nbsp;</li>
-    <li>Schmidt, on the influence of barometric pressure on earthquakes, <a href="#Page_267">267</a></li>
-    <li>Sea waves, on nature of, <a href="#Page_165">165</a></li>
-    <li>— on cause of, <a href="#Page_171">171</a></li>
-    <li>— seldom produced by earthquakes which originate inland, <a href="#Page_175">175</a></li>
-    <li>— on velocity of propagation of, <a href="#Page_177">177</a></li>
-    <li>— examples of, <a href="#Page_179">179</a></li>
-    <li><span class="pagenum" id="Page_363">363</span>Seasons, frequency of earthquakes at different, <a href="#Page_254">254</a></li>
-    <li>Seebach, on the determination of origins, <a href="#Page_211">211</a></li>
-    <li>— on the focal cavity, <a href="#Page_224">224</a></li>
-    <li>Seismic vertical, definition of, <a href="#Page_9">9</a></li>
-    <li>Seismic and volcanic phenomena, relation of, <a href="#Page_270">270</a></li>
-    <li>— — conclusions regarding, <a href="#Page_275">275</a>, <a href="#Page_295">295</a></li>
-    <li>Seismology, definition of, <a href="#Page_9">9</a></li>
-    <li>Seismometers, on various forms of, <a href="#Page_17">17</a>–<a href="#Page_40">40</a></li>
-    <li>Seismoscopes, on various forms of, <a href="#Page_13">13</a>–<a href="#Page_20">20</a></li>
-    <li>Serpieri, P. A., on distribution of seismic movement, <a href="#Page_231">231</a></li>
-    <li>Shadows, on earthquake, <a href="#Page_137">137</a></li>
-    <li>Spring, on frequency of earthquakes during, <a href="#Page_156">156</a></li>
-    <li>Stukeley, on earthquake motion, <a href="#Page_42">42</a></li>
-    <li>— earthquakes and aurora, <a href="#Page_265">265</a></li>
-    <li>Succussatore, definition of, <a href="#Page_10">10</a></li>
-    <li>Sun, on the effect of, on earthquakes, <a href="#Page_253">253</a>, <a href="#Page_285">285</a></li>
-    <li>&nbsp;</li>
-    <li>Temperature, effect of changes of, on earthquakes, <a href="#Page_268">268</a>, <a href="#Page_294">294</a></li>
-    <li>Terremoto, definition of, <a href="#Page_10">10</a></li>
-    <li>Thomson, Sir W., on the rigidity of the earth, <a href="#Page_285">285</a></li>
-    <li>Time, on recording apparatus for, <a href="#Page_35">35</a></li>
-    <li>Travagini, F., on earthquake motion, <a href="#Page_42">42</a></li>
-    <li>Trembelores, definition of, <a href="#Page_10">10</a></li>
-    <li>Tremors, on earth, <a href="#Page_3">3</a>, <a href="#Page_306">306</a>–<a href="#Page_325">325</a></li>
-    <li>&nbsp;</li>
-    <li>Understanding, effects of earthquakes on, <a href="#Page_2">2</a></li>
-    <li>&nbsp;</li>
-    <li>Verbeck, on the ball and plate seismometer of, <a href="#Page_31">31</a></li>
-    <li>Vibration, on the nature of earthquake, <a href="#Page_12">12</a></li>
-    <li>Vorticose motion, on, <a href="#Page_70">70</a></li>
-    <li>— definition of, <a href="#Page_10">10</a></li>
-    <li>&nbsp;</li>
-    <li>Wagener, on the pendulum seismometer of, <a href="#Page_25">25</a></li>
-    <li>— vertical motion seismometer of, <a href="#Page_33">33</a></li>
-    <li>— list of earthquakes, <a href="#Page_76">76</a></li>
-    <li>Wave paths, definition of, <a href="#Page_9">9</a></li>
-    <li>Waves, on the nature of earthquake, <a href="#Page_55">55</a></li>
-    <li>— on the interference of, <a href="#Page_138">138</a></li>
-    <li>Wells, on the effect of earthquakes on, <a href="#Page_156">156</a></li>
-    <li>Wenthrop, on the New Zealand earthquake of 1855, <a href="#Page_79">79</a></li>
-    <li>West, on the parallel motion seismometer of, <a href="#Page_28">28</a></li>
-    <li>Winslow, on pulsations of the ocean, <a href="#Page_334">334</a></li>
-    <li>Wolf, R., on earthquakes and sun spots, <a href="#Page_263">263</a></li>
-    <li>Woodward, on earthquake motion, <a href="#Page_42">42</a></li>
-    <li>&nbsp;</li>
-    <li>Young, Dr. T., on earthquake motion, <a href="#Page_43">43</a></li>
-    <li>&nbsp;</li>
-    <li>Zantedeschi, M. F., on the influence of the sun and moon on earthquakes, <a href="#Page_285">285</a></li>
-    <li>Zöllner, on the bracket seismometer of, <a href="#Page_27">27</a></li>
-    <li>— on earth tremors, <a href="#Page_309">309</a></li>
-  </ul>
-
-  <hr class="page" />
-
-  <div class="footnotes">
-    <div class="footheader"><b>Footnotes:</b></div>
-    <div class="footnote">
-      <a id="Footnote_1" href="#FNanchor_1"><span class="label">[1]</span></a>
-      <i>Mémoires de l’Académie Imp. de Dijon</i>, vols. xiv. and xv., 2nd Series, 1855–56.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_2" href="#FNanchor_2"><span class="label">[2]</span></a>
-      <i>Trans. Seis. Soc. of Japan</i>, vol. iii. p. 65.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_3" href="#FNanchor_3"><span class="label">[3]</span></a>
-      <i>Gentleman’s Magazine</i>, 1753.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_4" href="#FNanchor_4"><span class="label">[4]</span></a>
-      1 Kings xix. 11, 12.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_5" href="#FNanchor_5"><span class="label">[5]</span></a>
-      ‘Notes on the Great Earthquake of Japan.’ J. Milne, <i>Trans. Seis. Soc. of Japan</i>, vol. iii.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_6" href="#FNanchor_6"><span class="label">[6]</span></a>
-      See Mallet’s List of Works on Earthquakes, <i>Report of the British Association</i>, 1858, p. 107.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_7" href="#FNanchor_7"><span class="label">[7]</span></a>
-      <i>Quarterly Review</i>, vol. lxiii. p. 61.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_8" href="#FNanchor_8"><span class="label">[8]</span></a>
-      <i>De Mundo</i>, c. iv.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_9" href="#FNanchor_9"><span class="label">[9]</span></a>
-      See <i>Phil. Trans. R. S.</i>, Part III. 1882.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_10" href="#FNanchor_10"><span class="label">[10]</span></a>
-      <i>Report of the British Association</i>, 1851.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_11" href="#FNanchor_11"><span class="label">[11]</span></a>
-      ‘On the Velocity of Transmission of Earth Waves,’ by General H. L, Abbot, <i>American, Journal of Science
-      and Arts</i>, vol. xv. March 1878; ‘Shock of the Explosion at Hallet’s Point,’ by Bvt. Brig.-Gen. Henry
-      L. Abbot, read before the Essayons Club of the Corps of Engineers, Nov. 1876.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_12" href="#FNanchor_12"><span class="label">[12]</span></a>
-      <i>West. Rev.</i>, July 1849.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_13" href="#FNanchor_13"><span class="label">[13]</span></a>
-      <i>Phil. Trans.</i>, L., 1755.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_14" href="#FNanchor_14"><span class="label">[14]</span></a>
-      The solution is taken from Mallet’s <i>Account of the Neapolitan Earthquake</i>, vol. i. p. 155.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_15" href="#FNanchor_15"><span class="label">[15]</span></a>
-      <i>Neapolitan Earthquake</i>, ii. p. 300.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_16" href="#FNanchor_16"><span class="label">[16]</span></a>
-      See <i>Edinburgh Phil. Trans.</i>, vol. xxxi.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_17" href="#FNanchor_17"><span class="label">[17]</span></a>
-      See <i>Report of British Association</i>, 1858, p. 10.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_18" href="#FNanchor_18"><span class="label">[18]</span></a>
-      <i>Meteorologia Endogena</i>, i. p. 306.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_19" href="#FNanchor_19"><span class="label">[19]</span></a>
-      See remarks on the Earthquake ‘Push,’ p. 162.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_20" href="#FNanchor_20"><span class="label">[20]</span></a>
-      See <i>Researches in Geology and Natural History</i>, p. 374.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_21" href="#FNanchor_21"><span class="label">[21]</span></a>
-      ‘The City of Earthquakes,’ H. D. Warner, <i>Atlantic Monthly</i>, March, 1883.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_22" href="#FNanchor_22"><span class="label">[22]</span></a>
-      Mallet, <i>Dynamics of Earthquakes</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_23" href="#FNanchor_23"><span class="label">[23]</span></a>
-      Stud Mill at Haywards.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_24" href="#FNanchor_24"><span class="label">[24]</span></a>
-      See ‘Constructive Art in Japan,’ by R. H. Brunton, C.E., F.R.G.S., F.G.S.,
-      <i>Transactions of Asiatic Society of Japan</i>, December 22, 1873, and January 13, 1875.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_25" href="#FNanchor_25"><span class="label">[25]</span></a>
-      <i>Journal of the American Geographical Society</i>, vol. x.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_26" href="#FNanchor_26"><span class="label">[26]</span></a>
-      <i>Phil. Trans.</i>, li. 1760.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_27" href="#FNanchor_27"><span class="label">[27]</span></a>
-      <i>Ibid.</i>, xviii.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_28" href="#FNanchor_28"><span class="label">[28]</span></a>
-      ‘The City of Earthquakes,’ H. D. Warner, <i>Atlantic Monthly</i>, March 1883.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_29" href="#FNanchor_29"><span class="label">[29]</span></a>
-      T. Ronaldson, <i>A Treatise on Earthquake Dangers &amp;c.</i>
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_30" href="#FNanchor_30"><span class="label">[30]</span></a>
-      <i>Principles of Geology</i>, Lyell, vol. ii. p. 106.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_31" href="#FNanchor_31"><span class="label">[31]</span></a>
-      <i>The Neapolitan Earthquake of 1857</i>, R. Mallet, vol. ii. p. 359.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_32" href="#FNanchor_32"><span class="label">[32]</span></a>
-      <i>Am. J. Sci.</i> x. 191.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_33" href="#FNanchor_33"><span class="label">[33]</span></a>
-      <i>Am. J. Sci.</i> x. 191.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_34" href="#FNanchor_34"><span class="label">[34]</span></a>
-      <i>Reports of British Association</i>, 1858, p. 106.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_35" href="#FNanchor_35"><span class="label">[35]</span></a>
-      See chapter ‘Causes of Earthquakes’ for details of this myth.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_36" href="#FNanchor_36"><span class="label">[36]</span></a>
-      <i>Am. Jour. Sci.</i> vol. x. p. 191.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_37" href="#FNanchor_37"><span class="label">[37]</span></a>
-      <i>The Earth</i>, p. 599.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_38" href="#FNanchor_38"><span class="label">[38]</span></a>
-      Lyell, <i>Principles of Geology</i>, vol. ii. chap. xxix.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_39" href="#FNanchor_39"><span class="label">[39]</span></a>
-      <i>Gent. Mag.</i> vol. xx. p. 212.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_40" href="#FNanchor_40"><span class="label">[40]</span></a>
-      <i>Trans. Seis. Soc.</i> vol. v. p. 67–68.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_41" href="#FNanchor_41"><span class="label">[41]</span></a>
-      <i>Am. Jour. Sci.</i> vol. iv.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_42" href="#FNanchor_42"><span class="label">[42]</span></a>
-      <i>Phil. Trans.</i> vol. xviii.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_43" href="#FNanchor_43"><span class="label">[43]</span></a>
-      Oldham and Mallet, ‘Cachar Earthquake,’ <i>Proc. Geolog. Soc.</i> 1872.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_44" href="#FNanchor_44"><span class="label">[44]</span></a>
-      <i>Phil. Trans.</i> vols. li. and xviii.; <i>Gent. Mag.</i> vol.
-        xx. 212.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_45" href="#FNanchor_45"><span class="label">[45]</span></a>
-      <i>Trans. Royal Geog. Soc.</i> vol. vi.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_46" href="#FNanchor_46"><span class="label">[46]</span></a>
-      <i>Phil. Trans.</i> vols. xxxvi. and xxxix.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_47" href="#FNanchor_47"><span class="label">[47]</span></a>
-      <i>Am. Jour. of Sci.</i> 1865, vol. xl. p. 365.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_48" href="#FNanchor_48"><span class="label">[48]</span></a>
-      <i>Proc. Geolog. Soc. Ap.</i> 1875, p. 270.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_49" href="#FNanchor_49"><span class="label">[49]</span></a>
-      <i>Gent. Mag.</i> vol. xxi. p. 569.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_50" href="#FNanchor_50"><span class="label">[50]</span></a>
-      <i>Jahrb. f. Min.</i> 1840, p. 173.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_51" href="#FNanchor_51"><span class="label">[51]</span></a>
-      Oldham and Mallet, ‘Cachar Earthquake,’ <i>Trans. Geolog.
-          Soc. Ap.</i> 1872.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_52" href="#FNanchor_52"><span class="label">[52]</span></a>
-      O. Volger, <i>Unters üb. d. Phän. d. Erdb.</i> vol. iii. p.
-        414.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_53" href="#FNanchor_53"><span class="label">[53]</span></a>
-      <i>Meteorologia Endogena</i>, vol. i. p. 166.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_54" href="#FNanchor_54"><span class="label">[54]</span></a>
-      <i>Gent. Mag.</i> vol. xxvi. p. 91.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_55" href="#FNanchor_55"><span class="label">[55]</span></a>
-      <i>Compte Rendu</i>, 1873, p. 66.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_56" href="#FNanchor_56"><span class="label">[56]</span></a>
-      <i>An Historical Account of Earthquakes</i>, p. 46.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_57" href="#FNanchor_57"><span class="label">[57]</span></a>
-      <i>Phil. Trans.</i> vol. xlix. p. 436.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_58" href="#FNanchor_58"><span class="label">[58]</span></a>
-      <i>Am. Jour. Sci.</i> vol. xlv. p. 129.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_59" href="#FNanchor_59"><span class="label">[59]</span></a>
-      <i>Phil. Trans.</i> vol. xlix. p. 547.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_60" href="#FNanchor_60"><span class="label">[60]</span></a>
-      <i>Ibid.</i> vols. xlii. and xxxix.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_61" href="#FNanchor_61"><span class="label">[61]</span></a>
-      <i>Phil. Trans.</i> vol. xlix, part i.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_62" href="#FNanchor_62"><span class="label">[62]</span></a>
-      <i>Compte Rendu</i>, 1873, part ii. p. 66.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_63" href="#FNanchor_63"><span class="label">[63]</span></a>
-      <i>Die Vulcan. Ers. d. Erde</i>, C. W. C. Fuchs.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_64" href="#FNanchor_64"><span class="label">[64]</span></a>
-      <i>Comte Rendu</i>, 1875, p. 693.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_65" href="#FNanchor_65"><span class="label">[65]</span></a>
-      <i>Gent. Mag.</i> vol. xix. p. 190.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_66" href="#FNanchor_66"><span class="label">[66]</span></a>
-      <i>Phil. Trans.</i> vol. xlix. p. 115.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_67" href="#FNanchor_67"><span class="label">[67]</span></a>
-      <i>Gent. Mag.</i> vol. xxi. 1751.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_68" href="#FNanchor_68"><span class="label">[68]</span></a>
-      <i>Jour. Royal Geo. Soc.</i> vol. vi. p. 319.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_69" href="#FNanchor_69"><span class="label">[69]</span></a>
-      Darwin, <i>Geolog. Observations</i>, p. 232.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_70" href="#FNanchor_70"><span class="label">[70]</span></a>
-      <i>Ibid.</i> p. 245.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_71" href="#FNanchor_71"><span class="label">[71]</span></a>
-      Lyell, <i>Principles of Geology</i>, vol. ii. pp. 107–8.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_72" href="#FNanchor_72"><span class="label">[72]</span></a>
-      <i>Gent. Mag.</i> 1733, vol. iii. p. 217.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_73" href="#FNanchor_73"><span class="label">[73]</span></a>
-      ‘Earthquakes of Cutch,’ <i>Jour. Royal Geo. Soc.</i> vol. xl.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_74" href="#FNanchor_74"><span class="label">[74]</span></a>
-      M. Daussy, ‘Sur l’existence probable d’un volcan sousmarin situé ar environ 0° 20′ de lat. S.,
-      et 22° 0′ de long, ouest,’ <i>Comptes Rendus</i>, vol. vi. p. 512.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_75" href="#FNanchor_75"><span class="label">[75]</span></a>
-      <i>Am. Jour. Sci.</i> vol. xlv. p. 133.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_76" href="#FNanchor_76"><span class="label">[76]</span></a>
-      <i>Am. Jour. Sci.</i> vol. xiv. p. 209.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_77" href="#FNanchor_77"><span class="label">[77]</span></a>
-      D. C. F. Winslow, ‘Tides at Tahiti,’ <i>Am. Jour. Sci.</i> 1865, p. 45; also Mallet’s <i>Catalogue of Earthquakes</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_78" href="#FNanchor_78"><span class="label">[78]</span></a>
-      <i>Am. Jour. Sci.</i> vol. i. p. 469.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_79" href="#FNanchor_79"><span class="label">[79]</span></a>
-      Darwin, <i>Researches in Geology, &amp;c.</i>, p. 378.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_80" href="#FNanchor_80"><span class="label">[80]</span></a>
-      Kluge, <i>Jahrb. f. Min.</i> 1861, p. 977.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_81" href="#FNanchor_81"><span class="label">[81]</span></a>
-      Darwin, <i>Voyage of a Naturalist</i>, p. 309.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_82" href="#FNanchor_82"><span class="label">[82]</span></a>
-      Prof. A. D. Bache, <i>United States Coast Survey Report</i>, 1855, p. 342.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_83" href="#FNanchor_83"><span class="label">[83]</span></a>
-      <i>United States Coast Surrey Report</i>, or <i>Am. Jour. Sci.</i> vi. p. 77.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_84" href="#FNanchor_84"><span class="label">[84]</span></a>
-      <i>Petermann’s Mittheilungen</i>, 1877, Heft xii. S. 454, and <i>Nova Acta der
-      Ksl. Leop. Carol. Deutschen Acad. d Naturforscher</i>, Band xl. No. 9.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_85" href="#FNanchor_85"><span class="label">[85]</span></a>
-      J. Milne: ‘Peruvian Earthquake of May 9, 1877.’ See <i>Trans. Seis. Soc. of Japan</i>, vol. ii.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_86" href="#FNanchor_86"><span class="label">[86]</span></a>
-      <i>Report of British Association</i>, 1847, p. 84.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_87" href="#FNanchor_87"><span class="label">[87]</span></a>
-      <i>Das Erdbeben von Herzogenrath, &amp;c.</i>, p. 134.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_88" href="#FNanchor_88"><span class="label">[88]</span></a>
-      <i>Phil. Trans.</i> vol. li.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_89" href="#FNanchor_89"><span class="label">[89]</span></a>
-      See <i>Am. Jour. Sci.</i> 1872.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_90" href="#FNanchor_90"><span class="label">[90]</span></a>
-      David Milne says that ‘out of 110 shocks recorded in
-      England, thirty-one originated in Wales, thirty-one along the south
-      coast of England, fourteen on the borders of Yorkshire and Derbyshire,
-      and five or six in Cumberland.’
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_91" href="#FNanchor_91"><span class="label">[91]</span></a>
-      E. Suess, <i>Die Erdbeben Niederösterreiches</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_92" href="#FNanchor_92"><span class="label">[92]</span></a>
-      H. Hoeffer, <i>Die Erdbeben Kärntens</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_93" href="#FNanchor_93"><span class="label">[93]</span></a>
-      <i>Six Lectures on Physical Geography</i>, by Rev. S. Haughton, F.R.S., chap. i.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_94" href="#FNanchor_94"><span class="label">[94]</span></a>
-      Ramsay, ‘Geological History of Mountain Chains,’ <i>Mining Journal</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_95" href="#FNanchor_95"><span class="label">[95]</span></a>
-      A notable example of a rapid diminution in the number of
-      earthquakes felt at a place is that of Comrie in Scotland. In 1839–40,
-      no less than sixty shocks were felt in eleven months. In 1842–43, about
-      thirty shocks were felt, and in the following year thirty-seven. Since
-      this time the number of shocks has decreased until they are almost of
-      as rare occurrence at Comrie as in other portions of the British Isles.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_96" href="#FNanchor_96"><span class="label">[96]</span></a>
-      <i>Phil. Trans.</i> vol. i. 1836.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_97" href="#FNanchor_97"><span class="label">[97]</span></a>
-      <i>Am. Jour. of Sci.</i> vol. xxxvii. p. 1.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_98" href="#FNanchor_98"><span class="label">[98]</span></a>
-      Milne, ‘British Earthquakes,’ <i>Edin. Phil. Jour.</i> vol.
-        xxxi.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_99" href="#FNanchor_99"><span class="label">[99]</span></a>
-      <i>Phil. Trans.</i> vol. xlix. pt. i.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_100" href="#FNanchor_100"><span class="label">[100]</span></a>
-      <i>Compte Rendus</i>, 1875, p. 690.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_101" href="#FNanchor_101"><span class="label">[101]</span></a>
-      <i>Am. Jour. Sci.</i> vol. xi. p. 233.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_102" href="#FNanchor_102"><span class="label">[102]</span></a>
-      <i>Transactions of the Asiatic Society of Japan</i>, vol. vi. pt. i. p. 353.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_103" href="#FNanchor_103"><span class="label">[103]</span></a>
-      Kluge, <i>Ueber die Ursachen</i>, &amp;c., p. 74.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_104" href="#FNanchor_104"><span class="label">[104]</span></a>
-      <i>Am. Jour. Sci.</i> vol. xix. p. 162.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_105" href="#FNanchor_105"><span class="label">[105]</span></a>
-      <i>Mitt. d. Deutsch.</i> Ges., Aug. 1878.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_106" href="#FNanchor_106"><span class="label">[106]</span></a>
-      <i>Report to British Association</i>, 1850, p. 74.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_107" href="#FNanchor_107"><span class="label">[107]</span></a>
-      Fuchs, <i>Die Vulkanischen Erscheinungen der Erde</i>, p. 424.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_108" href="#FNanchor_108"><span class="label">[108]</span></a>
-      <i>Bern. Naturf. Gesellschaft</i>, 1852.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_109" href="#FNanchor_109"><span class="label">[109]</span></a>
-      <i>Comptes Rendus</i>, 1874, Jan. to June, p. 51.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_110" href="#FNanchor_110"><span class="label">[110]</span></a>
-      Boué, <i>Parallele der Erdbeben, Nordlichter und Erdmagnetismus, in Sitz. der K. A. d. Wissensch</i>. 1856, vol. iv. p. 395.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_111" href="#FNanchor_111"><span class="label">[111]</span></a>
-      <i>Meteorologia Endogena</i>, vol. i. p. 107, &amp;c.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_112" href="#FNanchor_112"><span class="label">[112]</span></a>
-      <i>Phil. Trans.</i> vol. lxviii. p. 221.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_113" href="#FNanchor_113"><span class="label">[113]</span></a>
-      <i>Gent. Mag.</i> vol. xxvii. p. 508.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_114" href="#FNanchor_114"><span class="label">[114]</span></a>
-      <i>Die Vulkanischen Erscheinungen der Erde</i>, p. 419.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_115" href="#FNanchor_115"><span class="label">[115]</span></a>
-      Petermann’s <i>Geogr. Mitth.</i> 1858, sec. 246.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_116" href="#FNanchor_116"><span class="label">[116]</span></a>
-      <i>Notes on volcanoes of the Hawaiian Islands</i>, W. T. Brigham, Mem. Boston Soc. of Nat. Hist., 1868.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_117" href="#FNanchor_117"><span class="label">[117]</span></a>
-      <i>Gent. Mag.</i> vol. xxiii., 1753.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_118" href="#FNanchor_118"><span class="label">[118]</span></a>
-      <i>Jour. Royal Geog. Soc.</i> vol. vi.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_119" href="#FNanchor_119"><span class="label">[119]</span></a>
-      <i>Ibid.</i> vol. vi.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_120" href="#FNanchor_120"><span class="label">[120]</span></a>
-      <i>Phil. Trans.</i> vol. xlii.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_121" href="#FNanchor_121"><span class="label">[121]</span></a>
-      <i>Am. Jour. Sci.</i> vol. x. p. 191.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_122" href="#FNanchor_122"><span class="label">[122]</span></a>
-      ‘Earthquakes of San Salvador, December 21–30, 1879.’ <i>Am. Jour. Sci.</i> vol. xix. p. 415.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_123" href="#FNanchor_123"><span class="label">[123]</span></a>
-      <i>Gent. Mag.</i> 1757, p. 323.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_124" href="#FNanchor_124"><span class="label">[124]</span></a>
-      <i>Phil. Trans.</i> vol. li., 1760.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_125" href="#FNanchor_125"><span class="label">[125]</span></a>
-      Mallet, <i>Report to Brit. Ass.</i>, 1858, p. 67.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_126" href="#FNanchor_126"><span class="label">[126]</span></a>
-      Von Lasaulx, <i>Earthquakes of Herzogenrath</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_127" href="#FNanchor_127"><span class="label">[127]</span></a>
-      Lyell, <i>Principles</i>, vol. ii. p. 51.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_128" href="#FNanchor_128"><span class="label">[128]</span></a>
-      Lyell, <i>Principles</i>, vol. i. p. 402.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_129" href="#FNanchor_129"><span class="label">[129]</span></a>
-      Fuchs, p. 464.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_130" href="#FNanchor_130"><span class="label">[130]</span></a>
-      <i>Comptes Rendus</i>, August 1854.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_131" href="#FNanchor_131"><span class="label">[131]</span></a>
-      <i>Nature</i>, April 26, 1883.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_132" href="#FNanchor_132"><span class="label">[132]</span></a>
-      <i>Phil. Soc.</i>, Wellington, New Zealand, 1875.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_133" href="#FNanchor_133"><span class="label">[133]</span></a>
-      <i>Phil. Trans.</i>, vol. xlii.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_134" href="#FNanchor_134"><span class="label">[134]</span></a>
-      M. S. di Rossi, <i>Earthquakes of Casamicciola</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_135" href="#FNanchor_135"><span class="label">[135]</span></a>
-      <i>Phil. Trans.</i>, vol. xviii. 1683–5.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_136" href="#FNanchor_136"><span class="label">[136]</span></a>
-      <i>Ibid.</i> vol. xlix.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_137" href="#FNanchor_137"><span class="label">[137]</span></a>
-      H. D. Warner, ‘The City of Earthquakes,’ <i>Atlantic Monthly</i>, March 1833.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_138" href="#FNanchor_138"><span class="label">[138]</span></a>
-      Palmer, <i>Trans. Seis. Soc. of Japan</i>, vol. iii. p. 148.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_139" href="#FNanchor_139"><span class="label">[139]</span></a>
-      Palmer, <i>Trans. Seis. Soc. of Japan</i>, vol. iii. p. 148.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_140" href="#FNanchor_140"><span class="label">[140]</span></a>
-      Paul, <i>Trans. Seis. Soc. of Japan</i>, vol. ii. p. 41.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_141" href="#FNanchor_141"><span class="label">[141]</span></a>
-      G. H. and H. Darwin, <i>Reports of British Association</i>, 1881.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_142" href="#FNanchor_142"><span class="label">[142]</span></a>
-      <i>Reports of British Association</i>, 1881.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_143" href="#FNanchor_143"><span class="label">[143]</span></a>
-      <i>Comptes Rendus</i>, 1875, January to June, p. 685.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_144" href="#FNanchor_144"><span class="label">[144]</span></a>
-      <i>Tel. Jour.</i>, November 15, 1881.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_145" href="#FNanchor_145"><span class="label">[145]</span></a>
-      <i>Minutes and proceedings of the Institute of Civil Engineers</i>, vol. lx. p. 412, and vol. lxiv. p. 343.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_146" href="#FNanchor_146"><span class="label">[146]</span></a>
-      <i>See</i> ‘Earth Tremors,’ p. 309, experiments of M. d’Abbadie, &amp;c.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_147" href="#FNanchor_147"><span class="label">[147]</span></a> <i>Meteorologia Endogena.</i>
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_148" href="#FNanchor_148"><span class="label">[148]</span></a> <i>Ibid.</i>
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_149" href="#FNanchor_149"><span class="label">[149]</span></a>
-      <i>Phil. Trans.</i> vol. xlix. p. 544.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_150" href="#FNanchor_150"><span class="label">[150]</span></a>
-      <i>Annual Register</i>, vol. iv. 1761, p. 92.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_151" href="#FNanchor_151"><span class="label">[151]</span></a>
-      <i>Phil. Mag.</i>, May 1876, p. 447.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_152" href="#FNanchor_152"><span class="label">[152]</span></a>
-      <i>Boston Soc. Nat. Hist.</i>, 1868.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_153" href="#FNanchor_153"><span class="label">[153]</span></a>
-      ‘Notes on Tides at Tahiti,’ &amp;c., <i>Am. Jour. Sci.</i> 1866, vol. xlii. p. 45.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_154" href="#FNanchor_154"><span class="label">[154]</span></a>
-      <i>Trans. Seis. Soc. of Japan</i>, vol. iv. Milne, <i>Systematic Observation of Earthquakes</i>.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_155" href="#FNanchor_155"><span class="label">[155]</span></a>
-      <i>Principles of Geology</i>, vol. ii. 177.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_156" href="#FNanchor_156"><span class="label">[156]</span></a>
-      <i>Gent. Mag.</i>, vol. xxvii. p. 448.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_157" href="#FNanchor_157"><span class="label">[157]</span></a>
-      <i>Phil. Trans.</i>, vol. xli. p. 805.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_158" href="#FNanchor_158"><span class="label">[158]</span></a>
-      <i>Meteorologia Endogena</i>, vol. i. pp. 186, 187.
-    </div>
-
-    <div class="footnote">
-      <a id="Footnote_159" href="#FNanchor_159"><span class="label">[159]</span></a>
-      Darwin, <i>Geological Observations</i>, p. 275 <i>et seq.</i>
-    </div>
-  </div>
-
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-      $1.50.</li>
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-      College of New Jersey. With numerous Illustrations. $2.00.</li>
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-      Primitive Nervous Systems. By <span class="smcap">George J. Romanes</span>. $1.75.</li>
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-        Kingdon Clifford</span>. $1.50.</li>
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-        Warner</span>, M. D., Assistant Physician, and Lecturer on Botany to
-      the London Hospital, etc. With 51 Illustrations. $1.75.</li>
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-      University of Berlin. With 63 Illustrations. $1.75.</li>
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-        Schmidt</span>. $1.50.</li>
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-      author of “The Historical Method,” etc. $1.75.</li>
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-    <li class="biblio">EARTHQUAKES AND OTHER EARTH MOVEMENTS. By <span class="smcap">John Milne</span>,
-      Professor of Mining and Geology in the Imperial College of
-      Engineering, Tokio, Japan. With 38 Figures.</li>
-  </ol>
-
-  <hr class="r15" />
-
-  <p class="center">New York: D. APPLETON &amp; CO., 1, 3, &amp; 5 Bond Street.</p>
-
-  <hr class="page" />
-
-  <div class="xxlarge gespertt center">VOLCANOES:</div>
-
-  <div class="center gespertt"><i>WHAT THEY ARE AND WHAT THEY TEACH</i>.</div>
-
-  <p class="center"><b>By J. W. JUDD</b>,<br />
-    <span class="small">Professor of Geology in the Royal School of Mines (London).</span></p>
-
-  <hr class="r15" />
-  <p class="center">With Ninety-six Illustrations. 12mo. Cloth, $2.00.</p>
-  <hr class="r15" />
-
-  <p>“The volume before us is one of the pleasantest science manuals we have
-    read for some time.”—<i>Athenæum.</i></p>
-
-  <p>“Mr. Judd’s summary is so full and so concise, that it is almost
-    impossible to give a fair idea in a short review.”—<i>Pall Mall Gazette.</i></p>
-
-  <p>“Professor Judd discusses the nature of volcanic action, the internal
-    structure of volcanic mountains, the distribution of volcanoes upon
-    the surface of the globe, their activity in different periods of the
-    earth’s existence, the use of volcanoes in the economy of nature, the
-    various theories that have been made to explain volcanic action. He has
-    abbreviated in this volume a vast amount of information, which has a
-    fascinating interest for many minds by reason of its relation to the
-    past history and future destiny of this little bubble of earth upon
-    which we sail through the infinite spaces of ether.”—<i>New York Home
-      Journal.</i></p>
-
-  <p>“The book gives an exhaustive statement of the phenomena of volcanoes,
-    and of the facts in the formation of mountain-chains, and relates a
-    mass of interesting observations and facts, the results of patient and
-    extensive personal investigation and study, mostly in different places
-    in Southern Europe, but not neglecting the world’s larger volcanoes in
-    other regions.”—<i>Hartford Times.</i></p>
-
-  <p>“A fascinating example of patient observation, sound judgment, and
-    acute reasoning. Under Professor Judd’s skillful treatment the volcano
-    is forced not only to tell its own history, but also to solve a number
-    of earth problems seemingly disconnected with it; and the story is
-    told in strong, nervous language, and with an earnestness and subdued
-    enthusiasm that are delightfully stimulating.”—<i>Boston Gazette.</i></p>
-
-  <p>“Professor Judd first points out the errors in the old definition of
-    a volcano. The volcanic hole is very often not on the summit, but
-    on the side, sometimes at the base of the mountain or hill, and it
-    sends forth steam rather than smoke, and the supposed raging flames
-    are nothing more than the glowing light of a mass of molten material
-    reflected from those vapor-clouds. So our old ignorance vanishes, and
-    in this admirable work the internal structure of volcanic mountains,
-    the nature and products of volcanic action, and the distribution of the
-    materials rejected from volcanic vents, the succession of operations
-    taking place at volcanic centers, are all very ably and clearly
-    discussed.”—<i>Philadelphia Times.</i></p>
-
-  <p>“A succinct and excellent treatise on a very interesting
-    subject.”—<i>Philadelphia North American.</i></p>
-
-  <hr class="r15" />
-
-  <p class="center"><i>For sale by all booksellers; or sent by mail, post paid, on receipt of
-    price.</i></p>
-
-  <hr class="r15" />
-  <p class="center">New York: D. APPLETON &amp; CO., 1, 3, &amp; 5 Bond Street.</p>
-
-  <hr class="page" />
-  <p class="xxlarge center"><span class="smcap">The Concepts and Theories of Modern Physics.</span></p>
-
-  <p class="center"><b>By J. B. STALLO.</b></p>
-
-  <hr class="r15" />
-  <p class="center">12mo, cloth&nbsp; &nbsp; &nbsp; $1.75.</p>
-
-  <hr class="r15" />
-
-  <p>“Judge Stallo’s work is an inquiry into the validity of those
-    mechanical conceptions of the universe which are now held as
-    fundamental in physical science. He takes up the leading modern
-    doctrines which are based upon this mechanical conception, such as
-    the atomic constitution of matter, the kinetic theory of gases, the
-    conservation of energy, the nebular hypothesis, and other views, to
-    find how much stands upon solid empirical ground, and how much rests
-    upon metaphysical speculation. Since the appearance of Dr. Draper’s
-    ‘Religion and Science,’ no book has been published in the country
-    calculated to make so deep an impression on thoughtful and educated
-    readers as this volume.... The range and minuteness of the author’s
-    learning, the acuteness of his reasoning, and the singular precision
-    and clearness of his style, are qualities which very seldom have been
-    jointly exhibited in a scientific treatise.”—<i>New York Sun.</i></p>
-
-  <p>“Judge J. B. Stallo, of Cincinnati, is a German by birth, and came to
-    this country at about the age of seventeen. He was early familiar with
-    science, and he lectured for some years in an Eastern college; but
-    at length he adopted the profession of law. He is also remembered by
-    many as an author, having a number of years ago written a metaphysical
-    treatise of marked ability for one of his youthful years. His present
-    book must be read deliberately, must be studied to be appreciated; but
-    the students of science, as well as those of metaphysics, are certain
-    to be deeply interested in its logical developments. It is a timely and
-    telling contribution to the philosophy of science, imperatively called
-    for by the present exigencies in the progress of knowledge. It is to be
-    commended equally for the solid value of its contents and the scholarly
-    finish of its execution.”—<i>The Popular Science Monthly.</i></p>
-
-  <p>“The book is of vital interest to a much larger class than
-    specialists—to all, in fact, who value clear thinking or are interested
-    in the accuracy more than the progress of scientific thought. It deals
-    with the results and theories of physical science, and in no sense
-    with the processes of the laboratory. It is written with a clearness
-    that is uncommon in philosophic works and with a desire to find truth,
-    conscious of the fact that a prime prerequisite of finding it is to
-    clear the way of accumulated and fast-settling untruths. It is a
-    scientific rebuke, as severe as it is lucid, of the scientists who
-    leave their apparatus and go star-gazing: here is the pit into which
-    they have fallen.”—<i>New York World.</i></p>
-
-  <p>“The volume is an important contribution to scientific discussion,
-    and is marked by closeness of reasoning, and clearness and cogency of
-    statement.”—<i>Boston Journal.</i></p>
-
-  <hr class="r15" />
-  <p class="center"><i>For sale by all booksellers; or sent by mail, post paid, on receipt of
-      price.</i></p>
-
-  <hr class="r15" />
-  <p class="center">New York: D. APPLETON &amp; CO., 1, 3, &amp; 5 Bond Street.</p>
-
-  <div class="transnote mt10">
-    <p class="large"><b>Transcriber’s Notes:</b></p>
-    <p>Blank pages have been removed.</p>
-    <p>Silently corrected typographical errors.</p>
-    <p>Spelling and hyphenation variations made consistent.</p>
-    <p>Front publication list moved to the back.</p>
-    <p>Tables pages 77, 89: removed unneeded right braces.</p>
-    <p>Tables pages 240, 257: changed to use cell borders instead of large right braces.</p>
-    <p>Table page 259: Northern Hemisphere average 15·0 corrected to 150.</p>
-  </div>
-
-
-
-
-
-
-
-
-<pre>
-
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