diff options
| -rw-r--r-- | .gitattributes | 4 | ||||
| -rw-r--r-- | LICENSE.txt | 11 | ||||
| -rw-r--r-- | README.md | 2 | ||||
| -rw-r--r-- | old/60007-0.txt | 12841 | ||||
| -rw-r--r-- | old/60007-0.zip | bin | 229000 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h.zip | bin | 2380578 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/60007-h.htm | 17534 | ||||
| -rw-r--r-- | old/60007-h/images/cover.jpg | bin | 102025 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/divider.png | bin | 3041 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p014.jpg | bin | 79734 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p017.png | bin | 5009 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p023.jpg | bin | 48758 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p028a.jpg | bin | 24155 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p028b.jpg | bin | 18563 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p030.jpg | bin | 41467 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p032.jpg | bin | 25868 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p036.jpg | bin | 18885 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p039.jpg | bin | 99620 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p054.jpg | bin | 19664 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p058.jpg | bin | 39647 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p060.jpg | bin | 87689 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p061.jpg | bin | 66796 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p063.jpg | bin | 8883 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p080.jpg | bin | 11420 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p081.jpg | bin | 10409 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p100.jpg | bin | 30809 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p101.jpg | bin | 48708 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p105.jpg | bin | 98448 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p106.jpg | bin | 100101 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p112.jpg | bin | 33128 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p113.jpg | bin | 100686 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p114.jpg | bin | 100929 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p116.jpg | bin | 98524 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p117.jpg | bin | 9736 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p118.jpg | bin | 8580 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p138.jpg | bin | 73848 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p169.jpg | bin | 101888 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p190.jpg | bin | 97433 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p197.jpg | bin | 22268 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p203.jpg | bin | 66418 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p211.jpg | bin | 24449 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p217.jpg | bin | 54216 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p232.jpg | bin | 48504 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p232a.png | bin | 1708 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p232b.png | bin | 2288 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p242.jpg | bin | 83189 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p256.jpg | bin | 98025 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p318.jpg | bin | 34057 -> 0 bytes | |||
| -rw-r--r-- | old/60007-h/images/i_p321.jpg | bin | 86350 -> 0 bytes |
49 files changed, 17 insertions, 30375 deletions
diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..f5348d9 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #60007 (https://www.gutenberg.org/ebooks/60007) diff --git a/old/60007-0.txt b/old/60007-0.txt deleted file mode 100644 index 9765b19..0000000 --- a/old/60007-0.txt +++ /dev/null @@ -1,12841 +0,0 @@ -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) - - - - - - - - - - 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._ | - | | - +--------------------------------------------------------------------+ - - - - - THE INTERNATIONAL SCIENTIFIC SERIES. - - • • • • • - EACH BOOK COMPLETE IN ONE VOLUME, 12MO, AND BOUND IN CLOTH. - • • • • • - - 1. FORMS OF WATER: A Familiar Exposition of the Origin and - Phenomena of Glaciers. By J. TYNDALL, LL. D., F. R. S. With 25 - Illustrations. $1.50. - - 2. PHYSICS AND POLITICS; Or, Thoughts on the Application of the - Principles of “Natural Selection” and “Inheritance” to Political - Society. By WALTER BAGEHOT. $1.50. - - 3. FOODS. By EDWARD SMITH, M. D., LL. B., F. R. S. With numerous - Illustrations. $1.75. - - 4. MIND AND BODY: The Theories of their Relation. By ALEXANDER - BAIN, LL. D. With 4 Illustrations. $1.50. - - 5. THE STUDY OF SOCIOLOGY. By HERBERT SPENCER. $1.50. - - 6. THE NEW CHEMISTRY. By Professor J. P. COOKE, of Harvard - University. With 31 Illustrations. $2.00. - - 7. ON THE CONSERVATION OF ENERGY. By BALFOUR STEWART, M. A., LL. - D., F. R. S. With 14 Illustrations. $1.50. - - 8. ANIMAL LOCOMOTION; or, Walking, Swimming, and Flying. By J. B. - PETTIGREW, M. D., F. R. S., etc. With 130 Illustrations. $1.75. - - 9. RESPONSIBILITY IN MENTAL DISEASE. By HENRY MAUDSLEY, M. D. - $1.50. - - 10. THE SCIENCE OF LAW. By Professor SHELDON AMOS. $1.75. - - 11. ANIMAL MECHANISM: A Treatise on Terrestrial and Aërial - Locomotion. By Professor E. J. MAREY. With 117 Illustrations. - $1.75. - - 12. THE HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE. By - J. W. DRAPER, M. D., LL. D. $1.75. - - 13. THE DOCTRINE OF DESCENT AND DARWINISM. By Professor OSCAR - SCHMIDT (Strasburg University). With 26 Illustrations. $1.50. - - 14. THE CHEMICAL EFFECTS OF LIGHT AND PHOTOGRAPHY. By Dr. HERMANN - VOGEL (Polytechnic Academy of Berlin). Translation thoroughly - revised. With 100 Illustrations. $2.00. - - 15. FUNGI: Their Nature, Influences, Uses, etc. By M. C. COOKE, - M. A., LL. D. Edited by the Rev. M. J. Berkeley, M. A., F. L. S. - With 109 Illustrations. $1.50. - - 16. THE LIFE AND GROWTH OF LANGUAGE. By Professor WILLIAM DWIGHT - WHITNEY, of Yale College. $1.50. - - 17. MONEY AND THE MECHANISM OF EXCHANGE. By W. STANLEY - JEVONS, M. A., F. R. S. $1.75. - - 18. THE NATURE OF LIGHT, with a General Account of Physical Optics. - By Dr. EUGENE LOMMEL. With 188 Illustrations and a Table - of Spectra in Chromo-lithography. $2.00. - - 19. ANIMAL PARASITES AND MESSMATES. By Monsieur VAN - BENEDEN. With 83 Illustrations. $1.50. - - 20. FERMENTATION. By Professor SCHÜTZENBERGER. With 28 - Illustrations. $1.50. - - 21. THE FIVE SENSES OF MAN. By Professor BERNSTEIN. With - 91 Illustrations. $1.75. - - 22. THE THEORY OF SOUND IN ITS RELATION TO MUSIC. By Professor - PIETRO BLASERNA. With numerous Illustrations. $1.50. - - 23. STUDIES IN SPECTRUM ANALYSIS. By J. NORMAN LOCKYER, F. - R. S. With 6 Photographic Illustrations of Spectra, and numerous - Engravings on Wood. $2.50. - - 24. A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By Professor - R. H. THURSTON. With 163 Illustrations. $2.50. - - 25. EDUCATION AS A SCIENCE. By ALEXANDER BAIN, LL. D. $1.75. - - 26. STUDENTS’ TEXT-BOOK OF COLOR; Or, Modern Chromatics. With - Applications to Art and Industry. By Professor OGDEN N. - ROOD, Columbia College. New edition. With 130 Illustrations. - $2.00. - - 27. THE HUMAN SPECIES. By Professor A. DE QUATREFAGES, - Membre de l’Institut. $2.00. - - 28. THE CRAYFISH: An Introduction to the Study of Zoölogy. By - T. H. HUXLEY, F. R. S. With 82 Illustrations. $1.75. - - 29. THE ATOMIC THEORY. By Professor A. WURTZ. Translated - by E. Cleminshaw, F. C. S. $1.50. - - 30. ANIMAL LIFE AS AFFECTED BY THE NATURAL CONDITIONS OF EXISTENCE. - By KARL SEMPER. With 2 Maps and 106 Woodcuts. $2.00. - - 31. SIGHT: An Exposition of the Principles of Monocular and - Binocular Vision. By JOSEPH LE CONTE, LL. D. With 132 - Illustrations. $1.50. - - 32. GENERAL PHYSIOLOGY OF MUSCLES AND NERVES. By Professor J. - ROSENTHAL. With 75 Illustrations. $1.50. - - 33. ILLUSIONS: A Psychological Study. By JAMES SULLY. $1.50. - - 34. THE SUN. By C. A. YOUNG, Professor of Astronomy in the - College of New Jersey. With numerous Illustrations. $2.00. - - 35. VOLCANOES: What they Are and what they Teach. By JOHN W. - JUDD, F. R. S., Professor of Geology in the Royal School of - Mines. With 96 Illustrations. $2.00. - - 36. SUICIDE: An Essay in Comparative Moral Statistics. By HENRY - MORSELLI, M. D., Professor of Psychological Medicine, Royal - University, Turin. $1.75. - - 37. THE FORMATION OF VEGETABLE MOULD, THROUGH THE ACTION OF WORMS. - With Observations on their Habits. By CHARLES DARWIN, LL. - D., F. R. S. With Illustrations. $1.50. - - 38. THE CONCEPTS AND THEORIES OF MODERN PHYSICS. By J. B. STALLO. - $1.75. - - 39. THE BRAIN AND ITS FUNCTIONS. By J. LUYS. $1.50. - - 40. MYTH AND SCIENCE. By TITO VIGNOLI. $1.50. - - 41. DISEASES OF MEMORY: An Essay in the Positive Psychology. By - TH. RIBOT, author of “Heredity.” $1.50. - - 42. ANTS, BEES, AND WASPS. A Record of Observations of the Habits - of the Social Hymenoptera. By Sir JOHN LUBBOCK, Bart., F. - R. S., D. C. L., LL. D., etc. $2.00. - - 43. SCIENCE OF POLITICS. By SHELDON AMOS. $1.75. - - 44. ANIMAL INTELLIGENCE. By GEORGE J. ROMANES. $1.75. - - 45. MAN BEFORE METALS. By N. JOLY, Correspondent of the - Institute. With 148 Illustrations. $1.75. - - 46. THE ORGANS OF SPEECH AND THEIR APPLICATION IN THE FORMATION - OF ARTICULATE SOUNDS. By G. H. VON MEYER, Professor in - Ordinary of Anatomy at the University of Zürich. With 47 - Woodcuts. $1.75. - - 47. FALLACIES: A View of Logic from the Practical Side. By - ALFRED SIDGWICK, B. A., Oxon. $1.75. - - 48. ORIGIN OF CULTIVATED PLANTS. By ALPHONSE DE CANDOLLE. $2.00. - - 49. JELLY-FISH, STAR-FISH, AND SEA-URCHINS. Being a Research on - Primitive Nervous Systems. By GEORGE J. ROMANES. $1.75. - - 50. THE COMMON SENSE OF THE EXACT SCIENCES. By the late WILLIAM - KINGDON CLIFFORD. $1.50. - - 51. PHYSICAL EXPRESSION: Its Modes and Principles. By FRANCIS - WARNER, M. D., Assistant Physician, and Lecturer on Botany to - the London Hospital, etc. With 51 Illustrations. $1.75. - - 52. ANTHROPOID APES. By ROBERT HARTMANN, Professor in the - University of Berlin. With 63 Illustrations. $1.75. - - 53. THE MAMMALIA IN THEIR RELATION TO PRIMEVAL TIMES. By OSCAR - SCHMIDT. $1.50. - - 54. COMPARATIVE LITERATURE. By HUTCHESON MACAULAY POSNETT, - M. A., LL. D., F. L. S., Barrister-at-Law; Professor of Classics - and English Literature, University College, Auckland, New - Zealand, author of “The Historical Method,” etc. $1.75. - - 55. EARTHQUAKES AND OTHER EARTH MOVEMENTS. By JOHN MILNE, - Professor of Mining and Geology in the Imperial College of - Engineering, Tokio, Japan. With 38 Figures. - - - New York: D. APPLETON & CO., 1, 3, & 5 Bond Street. - - - - - VOLCANOES: - - _WHAT THEY ARE AND WHAT THEY TEACH_. - - =By J. W. JUDD=, - Professor of Geology in the Royal School of Mines (London). - • • • • • - With Ninety-six Illustrations. 12mo. Cloth, $2.00. - • • • • • - -“The volume before us is one of the pleasantest science manuals we have -read for some time.”—_Athenæum._ - -“Mr. Judd’s summary is so full and so concise, that it is almost -impossible to give a fair idea in a short review.”—_Pall Mall Gazette._ - -“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.”—_New York Home -Journal._ - -“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.”—_Hartford Times._ - -“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.”—_Boston Gazette._ - -“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.”—_Philadelphia Times._ - -“A succinct and excellent treatise on a very interesting -subject.”—_Philadelphia North American._ - - • • • • • -_For sale by all booksellers; or sent by mail, post paid, on receipt of - price._ - • • • • • - New York: D. APPLETON & CO., 1, 3, & 5 Bond Street. - - - THE CONCEPTS AND THEORIES OF MODERN PHYSICS. - - =By J. B. STALLO=. - • • • • • - 12mo, cloth $1.75. - • • • • • - -“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.”—_New York Sun._ - -“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.”—_The Popular Science Monthly._ - -“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.”—_New York World._ - -“The volume is an important contribution to scientific discussion, -and is marked by closeness of reasoning, and clearness and cogency of -statement.”—_Boston Journal._ - - • • • • • -_For sale by all booksellers; or sent by mail, post paid, on receipt of - price._ - • • • • • - New York: D. APPLETON & CO., 1, 3, & 5 Bond St. - - - • • • • • - -Transcriber’s Notes: - - Text enclosed by underscores is in italics (_italics_). - - Text enclosed by equals is in bold (=bold=). - - Blank pages have been removed. - - Silently corrected typographical errors. - - Spelling and hyphenation variations made consistent. - - Front publication list moved to the back. - - Tables pages 77, 89: removed unneeded right braces. - - Tables pages 240, 257: changed to use cell borders instead of right braces. - - Table page 259: Northern Hemisphere average 15·0 corrected to 150. - - - - - -End of the Project Gutenberg EBook of Earthquakes and Other Earth Movements, by -John Milne - -*** END OF THIS PROJECT GUTENBERG EBOOK EARTHQUAKES, OTHER EARTH MOVEMENTS *** - -***** This file should be named 60007-0.txt or 60007-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/0/0/0/60007/ - -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) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. Special rules, set forth in the General Terms of Use part -of this license, apply to copying and distributing Project -Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm -concept and trademark. Project Gutenberg is a registered trademark, -and may not be used if you charge for the eBooks, unless you receive -specific permission. If you do not charge anything for copies of this -eBook, complying with the rules is very easy. You may use this eBook -for nearly any purpose such as creation of derivative works, reports, -performances and research. They may be modified and printed and given -away--you may do practically ANYTHING in the United States with eBooks -not protected by U.S. copyright law. Redistribution is subject to the -trademark license, especially commercial redistribution. - -START: FULL LICENSE - -THE FULL PROJECT GUTENBERG LICENSE -PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK - -To protect the Project Gutenberg-tm mission of promoting the free -distribution of electronic works, by using or distributing this work -(or any other work associated in any way with the phrase "Project -Gutenberg"), you agree to comply with all the terms of the Full -Project Gutenberg-tm License available with this file or online at -www.gutenberg.org/license. - -Section 1. General Terms of Use and Redistributing Project -Gutenberg-tm electronic works - -1.A. By reading or using any part of this Project Gutenberg-tm -electronic work, you indicate that you have read, understand, agree to -and accept all the terms of this license and intellectual property -(trademark/copyright) agreement. If you do not agree to abide by all -the terms of this agreement, you must cease using and return or -destroy all copies of Project Gutenberg-tm electronic works in your -possession. If you paid a fee for obtaining a copy of or access to a -Project Gutenberg-tm electronic work and you do not agree to be bound -by the terms of this agreement, you may obtain a refund from the -person or entity to whom you paid the fee as set forth in paragraph -1.E.8. - -1.B. "Project Gutenberg" is a registered trademark. It may only be -used on or associated in any way with an electronic work by people who -agree to be bound by the terms of this agreement. There are a few -things that you can do with most Project Gutenberg-tm electronic works -even without complying with the full terms of this agreement. See -paragraph 1.C below. There are a lot of things you can do with Project -Gutenberg-tm electronic works if you follow the terms of this -agreement and help preserve free future access to Project Gutenberg-tm -electronic works. See paragraph 1.E below. - -1.C. The Project Gutenberg Literary Archive Foundation ("the -Foundation" or PGLAF), owns a compilation copyright in the collection -of Project Gutenberg-tm electronic works. Nearly all the individual -works in the collection are in the public domain in the United -States. If an individual work is unprotected by copyright law in the -United States and you are located in the United States, we do not -claim a right to prevent you from copying, distributing, performing, -displaying or creating derivative works based on the work as long as -all references to Project Gutenberg are removed. Of course, we hope -that you will support the Project Gutenberg-tm mission of promoting -free access to electronic works by freely sharing Project Gutenberg-tm -works in compliance with the terms of this agreement for keeping the -Project Gutenberg-tm name associated with the work. You can easily -comply with the terms of this agreement by keeping this work in the -same format with its attached full Project Gutenberg-tm License when -you share it without charge with others. - -1.D. The copyright laws of the place where you are located also govern -what you can do with this work. Copyright laws in most countries are -in a constant state of change. If you are outside the United States, -check the laws of your country in addition to the terms of this -agreement before downloading, copying, displaying, performing, -distributing or creating derivative works based on this work or any -other Project Gutenberg-tm work. The Foundation makes no -representations concerning the copyright status of any work in any -country outside the United States. - -1.E. Unless you have removed all references to Project Gutenberg: - -1.E.1. The following sentence, with active links to, or other -immediate access to, the full Project Gutenberg-tm License must appear -prominently whenever any copy of a Project Gutenberg-tm work (any work -on which the phrase "Project Gutenberg" appears, or with which the -phrase "Project Gutenberg" is associated) is accessed, displayed, -performed, viewed, copied or distributed: - - 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. - -1.E.2. If an individual Project Gutenberg-tm electronic work is -derived from texts not protected by U.S. copyright law (does not -contain a notice indicating that it is posted with permission of the -copyright holder), the work can be copied and distributed to anyone in -the United States without paying any fees or charges. If you are -redistributing or providing access to a work with the phrase "Project -Gutenberg" associated with or appearing on the work, you must comply -either with the requirements of paragraphs 1.E.1 through 1.E.7 or -obtain permission for the use of the work and the Project Gutenberg-tm -trademark as set forth in paragraphs 1.E.8 or 1.E.9. - -1.E.3. If an individual Project Gutenberg-tm electronic work is posted -with the permission of the copyright holder, your use and distribution -must comply with both paragraphs 1.E.1 through 1.E.7 and any -additional terms imposed by the copyright holder. Additional terms -will be linked to the Project Gutenberg-tm License for all works -posted with the permission of the copyright holder found at the -beginning of this work. - -1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm -License terms from this work, or any files containing a part of this -work or any other work associated with Project Gutenberg-tm. - -1.E.5. Do not copy, display, perform, distribute or redistribute this -electronic work, or any part of this electronic work, without -prominently displaying the sentence set forth in paragraph 1.E.1 with -active links or immediate access to the full terms of the Project -Gutenberg-tm License. - -1.E.6. You may convert to and distribute this work in any binary, -compressed, marked up, nonproprietary or proprietary form, including -any word processing or hypertext form. However, if you provide access -to or distribute copies of a Project Gutenberg-tm work in a format -other than "Plain Vanilla ASCII" or other format used in the official -version posted on the official Project Gutenberg-tm web site -(www.gutenberg.org), you must, at no additional cost, fee or expense -to the user, provide a copy, a means of exporting a copy, or a means -of obtaining a copy upon request, of the work in its original "Plain -Vanilla ASCII" or other form. Any alternate format must include the -full Project Gutenberg-tm License as specified in paragraph 1.E.1. - -1.E.7. Do not charge a fee for access to, viewing, displaying, -performing, copying or distributing any Project Gutenberg-tm works -unless you comply with paragraph 1.E.8 or 1.E.9. - -1.E.8. You may charge a reasonable fee for copies of or providing -access to or distributing Project Gutenberg-tm electronic works -provided that - -* You pay a royalty fee of 20% of the gross profits you derive from - the use of Project Gutenberg-tm works calculated using the method - you already use to calculate your applicable taxes. The fee is owed - to the owner of the Project Gutenberg-tm trademark, but he has - agreed to donate royalties under this paragraph to the Project - Gutenberg Literary Archive Foundation. Royalty payments must be paid - within 60 days following each date on which you prepare (or are - legally required to prepare) your periodic tax returns. Royalty - payments should be clearly marked as such and sent to the Project - Gutenberg Literary Archive Foundation at the address specified in - Section 4, "Information about donations to the Project Gutenberg - Literary Archive Foundation." - -* You provide a full refund of any money paid by a user who notifies - you in writing (or by e-mail) within 30 days of receipt that s/he - does not agree to the terms of the full Project Gutenberg-tm - License. You must require such a user to return or destroy all - copies of the works possessed in a physical medium and discontinue - all use of and all access to other copies of Project Gutenberg-tm - works. - -* You provide, in accordance with paragraph 1.F.3, a full refund of - any money paid for a work or a replacement copy, if a defect in the - electronic work is discovered and reported to you within 90 days of - receipt of the work. - -* You comply with all other terms of this agreement for free - distribution of Project Gutenberg-tm works. - -1.E.9. If you wish to charge a fee or distribute a Project -Gutenberg-tm electronic work or group of works on different terms than -are set forth in this agreement, you must obtain permission in writing -from both the Project Gutenberg Literary Archive Foundation and The -Project Gutenberg Trademark LLC, the owner of the Project Gutenberg-tm -trademark. Contact the Foundation as set forth in Section 3 below. - -1.F. - -1.F.1. Project Gutenberg volunteers and employees expend considerable -effort to identify, do copyright research on, transcribe and proofread -works not protected by U.S. copyright law in creating the Project -Gutenberg-tm collection. Despite these efforts, Project Gutenberg-tm -electronic works, and the medium on which they may be stored, may -contain "Defects," such as, but not limited to, incomplete, inaccurate -or corrupt data, transcription errors, a copyright or other -intellectual property infringement, a defective or damaged disk or -other medium, a computer virus, or computer codes that damage or -cannot be read by your equipment. - -1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right -of Replacement or Refund" described in paragraph 1.F.3, the Project -Gutenberg Literary Archive Foundation, the owner of the Project -Gutenberg-tm trademark, and any other party distributing a Project -Gutenberg-tm electronic work under this agreement, disclaim all -liability to you for damages, costs and expenses, including legal -fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT -LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE -PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE -TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE -LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR -INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH -DAMAGE. - -1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a -defect in this electronic work within 90 days of receiving it, you can -receive a refund of the money (if any) you paid for it by sending a -written explanation to the person you received the work from. If you -received the work on a physical medium, you must return the medium -with your written explanation. The person or entity that provided you -with the defective work may elect to provide a replacement copy in -lieu of a refund. If you received the work electronically, the person -or entity providing it to you may choose to give you a second -opportunity to receive the work electronically in lieu of a refund. If -the second copy is also defective, you may demand a refund in writing -without further opportunities to fix the problem. - -1.F.4. Except for the limited right of replacement or refund set forth -in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO -OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT -LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. - -1.F.5. Some states do not allow disclaimers of certain implied -warranties or the exclusion or limitation of certain types of -damages. If any disclaimer or limitation set forth in this agreement -violates the law of the state applicable to this agreement, the -agreement shall be interpreted to make the maximum disclaimer or -limitation permitted by the applicable state law. The invalidity or -unenforceability of any provision of this agreement shall not void the -remaining provisions. - -1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the -trademark owner, any agent or employee of the Foundation, anyone -providing copies of Project Gutenberg-tm electronic works in -accordance with this agreement, and any volunteers associated with the -production, promotion and distribution of Project Gutenberg-tm -electronic works, harmless from all liability, costs and expenses, -including legal fees, that arise directly or indirectly from any of -the following which you do or cause to occur: (a) distribution of this -or any Project Gutenberg-tm work, (b) alteration, modification, or -additions or deletions to any Project Gutenberg-tm work, and (c) any -Defect you cause. - -Section 2. Information about the Mission of Project Gutenberg-tm - -Project Gutenberg-tm is synonymous with the free distribution of -electronic works in formats readable by the widest variety of -computers including obsolete, old, middle-aged and new computers. It -exists because of the efforts of hundreds of volunteers and donations -from people in all walks of life. - -Volunteers and financial support to provide volunteers with the -assistance they need are critical to reaching Project Gutenberg-tm's -goals and ensuring that the Project Gutenberg-tm collection will -remain freely available for generations to come. In 2001, the Project -Gutenberg Literary Archive Foundation was created to provide a secure -and permanent future for Project Gutenberg-tm and future -generations. To learn more about the Project Gutenberg Literary -Archive Foundation and how your efforts and donations can help, see -Sections 3 and 4 and the Foundation information page at -www.gutenberg.org - - - -Section 3. Information about the Project Gutenberg Literary Archive Foundation - -The Project Gutenberg Literary Archive Foundation is a non profit -501(c)(3) educational corporation organized under the laws of the -state of Mississippi and granted tax exempt status by the Internal -Revenue Service. The Foundation's EIN or federal tax identification -number is 64-6221541. Contributions to the Project Gutenberg Literary -Archive Foundation are tax deductible to the full extent permitted by -U.S. federal laws and your state's laws. - -The Foundation's principal office is in Fairbanks, Alaska, with the -mailing address: PO Box 750175, Fairbanks, AK 99775, but its -volunteers and employees are scattered throughout numerous -locations. Its business office is located at 809 North 1500 West, Salt -Lake City, UT 84116, (801) 596-1887. Email contact links and up to -date contact information can be found at the Foundation's web site and -official page at www.gutenberg.org/contact - -For additional contact information: - - Dr. Gregory B. Newby - Chief Executive and Director - gbnewby@pglaf.org - -Section 4. Information about Donations to the Project Gutenberg -Literary Archive Foundation - -Project Gutenberg-tm depends upon and cannot survive without wide -spread public support and donations to carry out its mission of -increasing the number of public domain and licensed works that can be -freely distributed in machine readable form accessible by the widest -array of equipment including outdated equipment. Many small donations -($1 to $5,000) are particularly important to maintaining tax exempt -status with the IRS. - -The Foundation is committed to complying with the laws regulating -charities and charitable donations in all 50 states of the United -States. Compliance requirements are not uniform and it takes a -considerable effort, much paperwork and many fees to meet and keep up -with these requirements. We do not solicit donations in locations -where we have not received written confirmation of compliance. To SEND -DONATIONS or determine the status of compliance for any particular -state visit www.gutenberg.org/donate - -While we cannot and do not solicit contributions from states where we -have not met the solicitation requirements, we know of no prohibition -against accepting unsolicited donations from donors in such states who -approach us with offers to donate. - -International donations are gratefully accepted, but we cannot make -any statements concerning tax treatment of donations received from -outside the United States. U.S. laws alone swamp our small staff. - -Please check the Project Gutenberg Web pages for current donation -methods and addresses. Donations are accepted in a number of other -ways including checks, online payments and credit card donations. To -donate, please visit: www.gutenberg.org/donate - -Section 5. General Information About Project Gutenberg-tm electronic works. - -Professor Michael S. Hart was the originator of the Project -Gutenberg-tm concept of a library of electronic works that could be -freely shared with anyone. For forty years, he produced and -distributed Project Gutenberg-tm eBooks with only a loose network of -volunteer support. - -Project Gutenberg-tm eBooks are often created from several printed -editions, all of which are confirmed as not protected by copyright in -the U.S. unless a copyright notice is included. Thus, we do not -necessarily keep eBooks in compliance with any particular paper -edition. - -Most people start at our Web site which has the main PG search -facility: www.gutenberg.org - -This Web site includes information about Project Gutenberg-tm, -including how to make donations to the Project Gutenberg Literary -Archive Foundation, how to help produce our new eBooks, and how to -subscribe to our email newsletter to hear about new eBooks. - diff --git a/old/60007-0.zip b/old/60007-0.zip Binary files differdeleted file mode 100644 index f726cfc..0000000 --- a/old/60007-0.zip +++ /dev/null diff --git a/old/60007-h.zip b/old/60007-h.zip Binary files differdeleted file mode 100644 index f6272a5..0000000 --- a/old/60007-h.zip +++ /dev/null diff --git a/old/60007-h/60007-h.htm b/old/60007-h/60007-h.htm deleted file mode 100644 index 4b34c84..0000000 --- a/old/60007-h/60007-h.htm +++ /dev/null @@ -1,17534 +0,0 @@ -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> -<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> - -<head> - <meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> - <meta http-equiv="Content-Style-Type" content="text/css" /> - <title> - Earthquakes and Other Earth Movements, by John Milne–A Project Gutenberg eBook - </title> - - <link rel="coverpage" href="images/cover.jpg" /> - - <style type="text/css"> - body { - margin-left: 10%; - margin-right: 10%; - } - - h1, h2, h3, h4 { - text-align: center; - clear: both; - } - - h1 { - font-size: 250%; - font-weight: normal; - margin-top: 8%; - margin-bottom: 2%; - page-break-before: avoid; - } - - h2 { - margin-top: 5%; - margin-bottom: 2%; - font-size: 130%; - page-break-before: avoid; - } - - h3 { - font-size: medium; - font-weight: normal; - margin-top: 5%; - } - - h4 { - font-size: small; - font-weight: normal; - margin-top: 3%; - margin-bottom: 1%; - } - - .chapter { - clear: both; - margin-top: 10%; - page-break-before: always; - } - - p { - margin-top: .5em; - text-align: justify; - margin-bottom: .5em; - text-indent: 1em; - } - - .hang1 { - padding-left: 3.0em; - text-indent: -2.0em; - } - - .hang2 { - padding-left: 3.0em; - text-indent: -1.0em; - } - - hr { - width: 35%; - margin: 1% 32.5%; - clear: both; - } - - hr.page { - width: 65%; - margin: 5% 17.5%; - } - - hr.tb { - width: 45%; - margin: 3% 27.5%; - } - - hr.r15 { - width: 15%; - margin: auto 42.5%; - } - - sub { font-size: x-small; } - sup { font-size: x-small; } - - /* === fonts === */ - .large { font-size: large; } - .xlarge { font-size: x-large; } - .xxlarge { font-size: xx-large; } - .ssmall { font-size: 90%; } - .small { font-size: small; } - .xsmall { font-size: x-small; } - .smcap { font-variant: small-caps; } - .gespertt { letter-spacing: 0.1em; } - .s { visibility: hidden; } - - /* === Alignment === */ - .mt0 { margin-top: 0; } - .mt1 { margin-top: 1%; } - .mt2 { margin-top: 2%; } - .mt3 { margin-top: 3%; } - .mt10 { margin-top: 10%; } - .mb0 { margin-bottom: 0; } - .mb1 { margin-bottom: 1.5%; } - .mb2 { margin-bottom: 2%; } - .ml5 { margin-left: 10%; } - .mr1 { margin-right: 1%; } - .pt2 { padding-top: 2%; } - .pl0 { padding-left: 0; } - .pr0 { padding-right: 0; } - .pr1 { padding-right: 4%; } - .lh1 { line-height: 1em; } - .center { text-align: center; } - .icenter { text-align: center; display:inline-block; } - .left { text-align: left; } - .right { - float: right; - vertical-align: middle; - } - .vatop { vertical-align: top} - .vm { vertical-align: middle; } - .nowrap { display: inline-block; } - - .noindent { - text-indent: 0; - margin-left: 0%; - } - - .rightinset { - text-align: right; - margin-right: 2%; - } - - .subhead { - font-size: small; - margin-top: 3%; - display: inline-block; - } - - .summary { - font-size: small; - font-weight: normal; - margin-bottom: 2%; - display: inline-block; - text-align: justify; - padding-left: 1.4em; - text-indent: -1.4em; - } - - /* === Page #s === */ - .pagenum { /* uncomment the next line for invisible page numbers */ - /* visibility: hidden; */ - right: 1%; - font-size: x-small; - background-color: inherit; - color: silver; - text-indent: 0; - text-align: right; - position: absolute; - border: thin solid silver; - padding: .1em .2em; - font-style: normal; - font-variant: normal; - font-weight: normal; - text-decoration: none; - } - - ul { - list-style-type: none; - text-align: left; - display: inline-block; - } - - ol { - text-align: left; - text-indent: 0.6em; - display: inline-block; - } - - li.biblio { - text-align: justify; - text-indent: -1em; - padding-left: 1.2em; - margin-top: 0.4em; - margin-bottom: 0.4em; - } - - .list-container { text-align: center; } - - /* === Tables === */ - table { margin: auto; } - - th { - font-size: small; - text-align: center; - padding-right: 0.4em; - padding-left: 0.4em; - } - - td { - vertical-align: middle; - padding-right: 0.4em; - padding-left: 0.4em; - padding-top: 0; - padding-bottom: 0; - } - - .tdpage { - font-size: x-small; - line-height: 50%; - } - - .tdpage div { text-align: right; } - - .tdl { - text-align: left; - padding-left: 1.4em; - text-indent: -1.0em; - } - - .tdl0 { - text-align: left; - padding-left: 0.8em; - text-indent: -1.4em; - } - - .tdr div { - text-align: right; - padding-right: 0.4em; - } - .tdc div { text-align: center; } - - .tdrt { vertical-align: top; } - .tdrt div { text-align: right; } - - .tdrx div { - text-align: right; - padding-right: 1.5em; - } - - .tdrb { - padding-left: 0; - vertical-align: bottom; - } - .tdrb div { text-align: right; } - - .tdr0 div { - text-align: right; - padding-left: 0; - } - - .tdr1 { - text-align: right; - padding-right: 0; - } - - .tdhang { - text-align: left; - padding-left: 1.2em; - text-indent: -1.2em; - } - - .tdc { - text-align: center; - margin-top: 5%; - margin-bottom: 2%; - } - - .tdh { - text-align: center; - margin-top: 5%; - margin-bottom: 2%; - font-size: large; - } - - .tdr2 { - text-align: right; - padding-right: 15%; - } - - table { border-collapse: collapse; } - - .collapse { border-collapse: collapse; } - - table#AbbotsResults { - border-collapse: collapse; - } - table#AbbotsResults th { - border: 1px solid black; - } - table#AbbotsResults td { - border: 1px solid black; - } - - table#amplitude2 { - border-collapse: collapse; - } - table#amplitude2 th { - border: 1px solid black; - } - - table#propogation td { - vertical-align: top; - } - - .br { border-right: 1px solid black; } - .bb { border-bottom: 1px solid black; } - .bl { border-left: 1px solid black; } - .ball { border: 1px solid black; } - - /* === Footnotes === */ - .footheader { - font-size: 150%; - text-align: center; - text-decoration: underline; - margin-bottom: 2%; - } - .footnotes { - border: dashed 1px; - margin-top: 10%; - text-align: justify - } - .footnote { - margin-left: 10%; - margin-right: 10%; - margin-bottom: 1%; - font-size: 0.9em; - } - .footnote .label { - position: absolute; - right: 84%; - text-align: right; - } - - .fnanchor { - vertical-align: super; - font-size: .8em; - text-decoration: none; - } - - /* === Images === */ - .figcenter { - margin: 3% auto 2% auto; - text-align: center; - page-break-inside: avoid; - } - - .caption { - margin-bottom: 1%; - font-size: 90%; - text-align: center; - } - - img { - max-width: 100%; - height: auto; - } - - .iglyph-a { - width: auto; - vertical-align: -0.6em; - } - - .iglyph-b { - width: auto; - vertical-align: -2.0em; - } - - .x200 { - font-size: 220%; - padding-bottom: 1%; - padding-right: 0; - text-align: right; - line-height: 0.8em; - } - - .x200a { - font-size: 220%; - vertical-align: -0.4em; - } - - .x200b { - font-size: 220%; - vertical-align: -0.2em; - } - - .x300 { - font-size: 350%; - padding-bottom: 30%; - text-align: right; - line-height: 0.8em; - } - - .x300a { - font-size: 350%; - padding-right: 0; - padding-left: 0; - line-height: 80%; - vertical-align: 0.2em; - } - - .x400 { - font-size: 480%; - padding-bottom: 2%; - padding-right: 0; - padding-left: 0; - text-align: right; - line-height: 0.8em; - } - - /* === Math === */ - span.frac { - display: inline-block; - position: relative; - vertical-align: middle; - font-size: 120%; - text-align: center; - text-indent: 0; - } - - span.frac > sup { - display: block; - font-size: medium; - } - - span.frac > span { - display: none; - } - - span.frac > sub { - display: block; - border-top: thin solid black; - font-size: medium; - } - - span.sqrt { - padding-top: 2%; - border-top: thin solid black; - } - - span.sqrt2 { - padding-top: 0.3em; - padding-left: 0.2em; - padding-right: 0.2em; - border-top: medium solid black; - } - - span.sqrt3 { - border-top: thin solid black; - } - - @media screen, print { - .figright { - float: right; - margin: 1% 0 1% 2%; - page-break-inside: avoid; - } - - .figleft { - float: left; - margin: 1% 2% 1% 0; - page-break-inside: avoid; - } - } - - @media handheld { - body { margin: 0; } - - /* page break instead of rule for chapter heading */ - hr.page { display: none; } - - h2 { page-break-before: always; } - - /* repeating these here seems to override epubmaker 'float' treatment */ - .figright { - float: right; - margin: 1% 0 1% 2%; - page-break-inside: avoid; - } - - .figleft { - float: left; - margin: 1% 2% 1% 0; - page-break-inside: avoid; - } - } - - /* === Transcriber's notes === */ - .transnote { - background-color: #E6E6FA; - color: black; - font-size: smaller; - padding: 0.5em; - margin-bottom: 5em; - font-family: sans-serif, serif; - } - - </style> -</head> - -<body> - - -<pre> - -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) - - - - - - -</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, &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> III. <span class="smcap">Earth Pulsations</span>.</div></td> - </tr> - <tr> - <td colspan="3" class="tdc"><div> 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, &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, &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, &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.</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> &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> = 2π <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> = 2π <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> = 2π <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 /> - „ - „ in line of lamination, 5,415 - „ „ - </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> - = <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> - > <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> - = <i>f</i> <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> < 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>, &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>, &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>, &c., for - the sake of argument, vary inversely as <i>r</i>, <i>r</i><sub>1</sub>, <i>r</i><sub>2</sub>, &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> </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 ± </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">„ „ „ </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">„ „ „ </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">„ „ „ </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"> 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 „ „ </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"> „ „ „ </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 „ „ </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"> „ „ „ </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"> 70 „ „ </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"> „ „ „ </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> </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 „ „ </td> - <td class="tdc"><div>8,730</div></td> - </tr> - <tr> - <td class="left"> 70 „ 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> </th> - <th>Feet per second</th> - </tr> - <tr> - <td class="left"> 200 lbs. of dynamite gave for 1 mile</td> - <td class="tdc"><div>8,730</div></td> - </tr> - <tr> - <td class="left"> „ „ „ „ 5 miles</td> - <td class="tdc"><div>5,250</div></td> - </tr> - <tr> - <td class="left">50,000 „ „ „ 8 „</td> - <td class="tdc"><div>8,300</div></td> - </tr> - <tr> - <td class="left"> „ „ „ „ 13½ „</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">—</td> - <td class="tdc br">211</td> - <td class="tdc br">—</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">—</td> - <td class="tdc br">—</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">—</td> - <td class="tdc br">—</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">—</td> - <td class="tdc br bb">—</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> ·0 to 0·15 mm.</td> - </tr> - <tr> - <td class="tdr2"><div>7</div></td> - <td> ·15 „ 0·5 „</td> - </tr> - <tr> - <td class="tdr2"><div>8</div></td> - <td> ·5 „ 2·5 „</td> - </tr> - <tr> - <td class="tdr2"><div>2</div></td> - <td>2·5 „ more „</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"> 3·5</td> - <td class="br bb"> 4·0</td> - <td class="br bb"> 4·5</td> - <td class="br bb"> 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,’ - &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>) = - <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> = <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> = 2<i>g</i> × - <span class="frac"><sup><span class="smcap">k</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span> × - <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> = - <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> = 2<i>gl</i> × <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 - <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, &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> = <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> = - <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> = - <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> = - <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> = - <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"> </td> - <td class="br"> </td> - <td class="br"> </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"> </td> - <td class="br"> </td> - <td class="br"> </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"> </td> - <td class="br"> </td> - <td class="br"> </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:—</td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </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"> </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"> </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">—</td> - <td class="tdc br">—</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"> </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"> </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">—</td> - <td class="tdc br">—</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"> „ „ 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"> „ „ 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"> „ „ 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"> „ „ 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"> „ „ „</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"> „ Spoleto „ „ </td> - <td class="tdr"><div>4,101</div></td> - <td class="tdl"> „ „</td> - </tr> - <tr> - <td class="tdl"> „ Perugia to Orvieto „ </td> - <td class="tdr"><div>601</div></td> - <td class="tdl"> „ „</td> - </tr> - <tr> - <td class="tdl"> „ „ „ Ancona</td> - <td class="tdr"><div>1,640</div></td> - <td class="tdl"> „ „</td> - </tr> - <tr> - <td class="tdl"> „ „ „ 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"> „ „</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 36 seconds</td> - <td>1882 October 25th 21 seconds</td> - </tr> - <tr> - <td class="tdl br">1881 January 7th 14–31 „</td> - <td class="tdl">1883 February 6th 23 „</td> - </tr> - <tr> - <td class="tdl br"> „ March 8th 60 „</td> - <td class="tdl"> „ March 1 53 „</td> - </tr> - <tr> - <td class="tdl br"> „ „ 17th. 66 „</td> - <td class="tdl"> „ „ „ 63 „</td> - </tr> - <tr> - <td class="tdl br"> „ November 15th 31 „</td> - <td class="tdl"> „ „ 8th 27 „</td> - </tr> - <tr> - <td class="tdl br">1882 February 16th 22 „</td> - <td class="tdl"> „ „ 11th 26 „</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. 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. 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. 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, &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, &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. 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, &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"> </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"> </th> - <th class="bl"> </th> - <th class="bl">h.</th> - <th class="br">m.</th> - <th class="br"> </th> - <th class="br"> </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"> </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"> </th> - <th class="bl"> </th> - <th class="tdr bl"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="br"> </th> - <th class="tdr br"> </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"> </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"> </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"> </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"> </th> - <th class="bl"> </th> - <th class="tdr bl"><div>h.</div></th> - <th class="tdr"><div>m.</div></th> - <th class="br"> </th> - <th class="tdr"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="br"> </th> - <th class="br"> </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 „</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 „</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 „</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 „</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 „</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 „</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"> </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"> </th> - <th class="tdr bl"><div>°</div></th> - <th class="tdr"><div>′</div></th> - <th class="tdr br"><div> </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"> </th> - <th class="br"> </th> - <th class="br"> </th> - <th class="br"> </th> - <th class="br"> </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> </div></td> - <td class="tdr br"><div> </div></td> - <td class="br"><div> </div></td> - <td class="br"> </td> - <td class="br"><div> </div></td> - <td class="br"> </td> - <td class="br"> </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> </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"> </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> </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"> </td> - <td class="br"> </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> </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> </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"> </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> </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> </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"> </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> </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"> </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> </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"> </td> - <td class="br"> </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> </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"> </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> </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> </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> </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"> </td> - <td class="br"> </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"> </td> - <td class="br"> </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> </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> </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"> </td> - <td class="br"> </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> </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"> </td> - <td class="br"> </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> </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> </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>„ 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 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"> „ New Zealand (Wellington)</td> - <td class="tdl">18 „ 23 „</td> - </tr> - <tr> - <td class="tdl"> „ Samoa</td> - <td class="tdl">14 „ 58 „</td> - </tr> - <tr> - <td class="tdl"> „ Sandwich Islands (Honolulu)</td> - <td class="tdl">14 „ 53 „</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. 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, &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>, &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>, &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>, - &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>, &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>, - &c., successively afterwards.</p> - - <p> - Let <i>t</i><sub>1</sub> - <i>t</i><sub>0</sub> = <i>a</i><br /> - <i>t</i><sub>2</sub> - <i>t</i><sub>0</sub> = <i>b</i><br /> - <i>t</i><sub>3</sub> - <i>t</i><sub>0</sub> = <i>c</i>, &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>, &c. as centres, - describe circles with radii proportional to the known qualities <i>a</i>, - <i>b</i>, <i>c</i>, &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>, &c. may be - represented by the quantities <i>ax</i>, <i>bx</i>, <i>cx</i>, &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"> </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"> </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 /> - &c. &c. &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"> </th> - <th colspan="2" class="ball">Co-ordinates</th> - <th colspan="3" class="ball">Time of arrival</th> - </tr> - <tr> - <th class="bl br"> </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"> </th> - </tr> - <tr> - <td class="tdl bl br">Mejillones</td> - <td class="tdr br"><div><i>a</i> or 0</div></td> - <td class="tdr br"><div><i>b</i> or 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 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 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>, - <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>, &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>, &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>, &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"> </th> - <th colspan="3" class="ball">In feet</th> - </tr> - <tr> - <th class="bl br"> </th> - <th class="br"> </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> </div></td> - <td class="tdr br"><div>127,309</div></td> - <td class="tdr br"><div> </div></td> - </tr> - <tr> - <td class="tdl bl br">Sillien</td> - <td class="tdl br">1858 (Schmidt)</td> - <td class="tdr br"><div> </div></td> - <td class="tdr br"><div>86,173</div></td> - <td class="tdr br"><div> </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. - <img class="iglyph-a" src="images/i_p232a.png" alt="" width="40" height="40" /> - </div> - <div> - Subsequent radial disturbance - <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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>II.</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>IV.</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>23</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>19</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>27</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>21</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>19</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>7</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>2</div></td> - <td class="tdr br"><div>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</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>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</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>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</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>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</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>—</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>—</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>—</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"> </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"> </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=""> </td> - <td class="br">315</td> - <td class="">159</td> - <td class=""> </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"> </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"> </th> - <th colspan="2" class="ball"> </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"> </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>—</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>—</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>—</div></td> - <td class="tdrb br"><div>—</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>—</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, &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"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </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"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </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>—</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, &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"> </th> - <th colspan="2" class="ball">Number of Earthquakes</th> - </tr> - <tr> - <th class="bl br"> </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 /> - 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"> „ „ rising</td> - <td class="tdl">in 16 „</td> - </tr> - <tr> - <td class="tdl"> „ „ at a minimum</td> - <td class="tdl">in 7 „</td> - </tr> - <tr> - <td class="tdl"> „ „ maximum</td> - <td class="tdl">in 3 „</td> - </tr> - <tr> - <td class="tdl"> „ „ undetermined</td> - <td class="tdl">in 11 „<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"> „ „ falling</td> - <td class="tdl">in 154 „</td> - </tr> - <tr> - <td class="tdl"> „ „ steady</td> - <td class="tdl">in 73 „</td> - </tr> - <tr> - <td class="tdl"> „ „ below the monthly mean</td> - <td class="tdl">in 189 „</td> - </tr> - <tr> - <td class="tdl"> „ „ above</td> - <td class="tdl">in 192 „</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, &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, &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, &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> </th> - <th class="tdl"> h. m.</th> - </tr> - <tr> - <td class="tdl">1879. Nov. 12th</td> - <td class="tdl">7 0 <span class="smcap">p.m.</span> contact of needle</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 2 „ difficult to set the needle</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 3 „ needle swings and telephone creaks</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 4 „ „ „ „</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 5 „ „ „ „</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 6 „ „ „ „</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 10 „ 3 more swings</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 11 „ again „</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, &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. m.</th> - <th>Barometer<br />reading</th> - </tr> - <tr> - <td> 12 5 <span class="smcap">p.m.</span></td> - <td class="tdc">29·02</td> - </tr> - <tr> - <td> 12 10 „</td> - <td class="tdc">29·05</td> - </tr> - <tr> - <td> 12 12 „</td> - <td class="tdc">29·07</td> - </tr> - <tr> - <td> 12 13 „</td> - <td class="tdc">29·05</td> - </tr> - <tr> - <td> 12 25 „</td> - <td class="tdc">29·10</td> - </tr> - <tr> - <td> 12 50 „</td> - <td class="tdc">29·00</td> - </tr> - <tr> - <td> 1 10 „</td> - <td class="tdc">29·00</td> - </tr> - <tr> - <td> 1 20 „</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> </th> - <th class="left">Time<br />h. 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> </td> - <td class="left">4 5 „</td> - <td class="left">103</td> - </tr> - <tr> - <td> </td> - <td class="left">4 10 „</td> - <td class="left">102</td> - </tr> - <tr> - <td> </td> - <td class="left">4 25 „</td> - <td class="left">101</td> - </tr> - <tr> - <td> </td> - <td class="left">4 30 „</td> - <td class="left">100</td> - </tr> - <tr> - <td> </td> - <td class="left">4 40 „</td> - <td class="left"> 98</td> - </tr> - <tr> - <td> </td> - <td class="left">4 42 „</td> - <td class="left"> 99·5</td> - </tr> - <tr> - <td> </td> - <td class="left">4 45 „</td> - <td class="left">100</td> - </tr> - <tr> - <td> </td> - <td class="left">4 50 „</td> - <td class="left">101</td> - </tr> - <tr> - <td> </td> - <td class="left">4 55 „</td> - <td class="left">101</td> - </tr> - <tr> - <td> </td> - <td class="left">5 00 „</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, &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, &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> </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"> „ Concepcion „ „</td> - <td class="tdr"><div>625 to 1,000</div></td> - </tr> - <tr> - <td class="tdl"> „ Valparaiso „ „</td> - <td class="tdr"><div>1,300</div></td> - </tr> - <tr> - <td class="tdl"> „ Coquimbo „ „</td> - <td class="tdr"><div>252</div></td> - </tr> - <tr> - <td class="tdl"> „ Lima „ „</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, &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, &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> </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> </p> - - <p class="hang1">C. H. A Physical Discussion of Earthquakes, &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, &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> </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, - &c. 1693.</p> - - <p class="hang1">Doyle, P. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p> </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> </p> - - <p class="hang1">Falb, R. Gedanken und Studien über den Vulkanismus, &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> </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, &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> </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, &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> </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> </p> - - <p class="hang1">Knipping, E. Verzeichniss von Erdbeben wahrgenommen in Tokio, - &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> </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, &c.</p> - - <p class="hang1">Lescasse, M. J. Etude sur les Constructions Japonaises, &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> </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, &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> </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, &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> </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> </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> </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> </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, &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, &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> </p> - - <p class="hang1">Volger, Dr. G. H. Untersuchungen über das Phänomen der Erdbeben. - 1857.</p> - - <p> </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, &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, &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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </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> </li> - <li>Understanding, effects of earthquakes on, <a href="#Page_2">2</a></li> - <li> </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> </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> </li> - <li>Young, Dr. T., on earthquake motion, <a href="#Page_43">43</a></li> - <li> </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 &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, &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, &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>, &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, &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, &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,’ &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> - - <hr class="page" /> - - <div class="center mb2">THE<br /><span class="xlarge">INTERNATIONAL SCIENTIFIC SERIES.</span></div> - <hr class="r15" /> - <p class="center"><span class="smcap">Each book complete in One Volume, 12mo, and bound in Cloth.</span></p> - <hr class="r15" /> - - <ol> - <li class="biblio">FORMS OF WATER: A Familiar Exposition of the Origin and - Phenomena of Glaciers. By <span class="smcap">J. Tyndall</span>, LL. D., F. R. S. - With 25 Illustrations. $1.50.</li> - - <li class="biblio">PHYSICS AND POLITICS; Or, Thoughts on the Application of the - Principles of “Natural Selection” and “Inheritance” to Political - Society. By <span class="smcap">Walter Bagehot</span>. $1.50.</li> - - <li class="biblio">FOODS. By <span class="smcap">Edward Smith</span>, M. D., LL. B., F. R. S. With - numerous Illustrations. $1.75.</li> - - <li class="biblio">MIND AND BODY: The Theories of their Relation. By <span class="smcap">Alexander - Bain</span>, LL. D. With 4 Illustrations. $1.50.</li> - - <li class="biblio">THE STUDY OF SOCIOLOGY. By <span class="smcap">Herbert Spencer</span>. $1.50.</li> - - <li class="biblio">THE NEW CHEMISTRY. By Professor <span class="smcap">J. P. Cooke</span>, of Harvard - University. With 31 Illustrations. $2.00.</li> - - <li class="biblio">ON THE CONSERVATION OF ENERGY. By <span class="smcap">Balfour Stewart</span>, M. - A., LL. D., F. R. S. With 14 Illustrations. $1.50.</li> - - <li class="biblio">ANIMAL LOCOMOTION; or, Walking, Swimming, and Flying. By <span class="smcap">J. - B. Pettigrew</span>, M. D., F. R. S., etc. With 130 Illustrations. - $1.75.</li> - - <li class="biblio">RESPONSIBILITY IN MENTAL DISEASE. By <span class="smcap">Henry Maudsley</span>, M. - D. $1.50.</li> - - <li class="biblio">THE SCIENCE OF LAW. By Professor <span class="smcap">Sheldon Amos</span>. $1.75.</li> - - <li class="biblio">ANIMAL MECHANISM: A Treatise on Terrestrial and Aërial - Locomotion. By Professor <span class="smcap">E. J. Marey</span>. With 117 - Illustrations. $1.75.</li> - - <li class="biblio">THE HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE. By - <span class="smcap">J. W. Draper</span>, M. D., LL. D. $1.75.</li> - - <li class="biblio">THE DOCTRINE OF DESCENT AND DARWINISM. By Professor <span class="smcap">Oscar - Schmidt</span> (Strasburg University). With 26 Illustrations. $1.50.</li> - - <li class="biblio">THE CHEMICAL EFFECTS OF LIGHT AND PHOTOGRAPHY. By Dr. - <span class="smcap">Hermann Vogel</span> (Polytechnic Academy of Berlin). Translation - thoroughly revised. With 100 Illustrations. $2.00.</li> - - <li class="biblio">FUNGI: Their Nature, Influences, Uses, etc. By <span class="smcap">M. C. - Cooke</span>, M. A., LL. D. Edited by the Rev. M. J. Berkeley, M. A., - F. L. S. With 109 Illustrations. $1.50.</li> - - <li class="biblio">THE LIFE AND GROWTH OF LANGUAGE. By Professor <span class="smcap">William - Dwight Whitney</span>, of Yale College. $1.50.</li> - - <li class="biblio">MONEY AND THE MECHANISM OF EXCHANGE. By <span class="smcap">W. Stanley - Jevons</span>, M. A., F. R. S. $1.75.</li> - - <li class="biblio">THE NATURE OF LIGHT, with a General Account of Physical Optics. - By Dr. <span class="smcap">Eugene Lommel</span>. With 188 Illustrations and a Table - of Spectra in Chromo-lithography. $2.00.</li> - - <li class="biblio">ANIMAL PARASITES AND MESSMATES. By Monsieur <span class="smcap">Van - Beneden</span>. With 83 Illustrations. $1.50.</li> - - <li class="biblio">FERMENTATION. By Professor <span class="smcap">Schützenberger</span>. With 28 - Illustrations. $1.50.</li> - - <li class="biblio">THE FIVE SENSES OF MAN. By Professor <span class="smcap">Bernstein</span>. With - 91 Illustrations. $1.75.</li> - - <li class="biblio">THE THEORY OF SOUND IN ITS RELATION TO MUSIC. By Professor - <span class="smcap">Pietro Blaserna</span>. With numerous Illustrations. $1.50.</li> - - <li class="biblio">STUDIES IN SPECTRUM ANALYSIS. By <span class="smcap">J. Norman Lockyer</span>, F. - R. S. With 6 Photographic Illustrations of Spectra, and numerous - Engravings on Wood. $2.50.</li> - - <li class="biblio">A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By Professor - <span class="smcap">R. H. Thurston</span>. With 163 Illustrations. $2.50.</li> - - <li class="biblio">EDUCATION AS A SCIENCE. By <span class="smcap">Alexander Bain</span>, LL. D. - $1.75.</li> - - <li class="biblio">STUDENTS’ TEXT-BOOK OF COLOR; Or, Modern Chromatics. With - Applications to Art and Industry. By Professor <span class="smcap">Ogden N. - Rood</span>, Columbia College. New edition. With 130 Illustrations. - $2.00.</li> - - <li class="biblio">THE HUMAN SPECIES. By Professor <span class="smcap">A. de Quatrefages</span>, - Membre de l’Institut. $2.00.</li> - - <li class="biblio">THE CRAYFISH: An Introduction to the Study of Zoölogy. By - <span class="smcap">T. H. Huxley</span>, F. R. S. With 82 Illustrations. $1.75.</li> - - <li class="biblio">THE ATOMIC THEORY. By Professor <span class="smcap">A. Wurtz</span>. Translated - by E. Cleminshaw, F. C. S. $1.50.</li> - - <li class="biblio">ANIMAL LIFE AS AFFECTED BY THE NATURAL CONDITIONS OF EXISTENCE. - By <span class="smcap">Karl Semper</span>. With 2 Maps and 106 Woodcuts. $2.00.</li> - - <li class="biblio">SIGHT: An Exposition of the Principles of Monocular and - Binocular Vision. By <span class="smcap">Joseph Le Conte</span>, LL. D. With 132 - Illustrations. $1.50.</li> - - <li class="biblio">GENERAL PHYSIOLOGY OF MUSCLES AND NERVES. By Professor <span class="smcap">J. - Rosenthal</span>. With 75 Illustrations. $1.50.</li> - - <li class="biblio">ILLUSIONS: A Psychological Study. By <span class="smcap">James Sully</span>. - $1.50.</li> - - <li class="biblio">THE SUN. By <span class="smcap">C. A. Young</span>, Professor of Astronomy in the - College of New Jersey. With numerous Illustrations. $2.00.</li> - - <li class="biblio">VOLCANOES: What they Are and what they Teach. By <span class="smcap">John W. - Judd</span>, F. R. S., Professor of Geology in the Royal School of - Mines. With 96 Illustrations. $2.00.</li> - - <li class="biblio">SUICIDE: An Essay in Comparative Moral Statistics. By <span class="smcap">Henry - Morselli</span>, M. D., Professor of Psychological Medicine, Royal - University, Turin. $1.75.</li> - - <li class="biblio">THE FORMATION OF VEGETABLE MOULD, THROUGH THE ACTION OF WORMS. - With Observations on their Habits. By <span class="smcap">Charles Darwin</span>, LL. - D., F. R. S. With Illustrations. $1.50.</li> - - <li class="biblio">THE CONCEPTS AND THEORIES OF MODERN PHYSICS. By <span class="smcap">J. B. - Stallo</span>. $1.75.</li> - - <li class="biblio">THE BRAIN AND ITS FUNCTIONS. By <span class="smcap">J. Luys</span>. $1.50.</li> - - <li class="biblio">MYTH AND SCIENCE. By <span class="smcap">Tito Vignoli</span>. $1.50.</li> - - <li class="biblio">DISEASES OF MEMORY: An Essay in the Positive Psychology. By - <span class="smcap">Th. Ribot</span>, author of “Heredity.” $1.50.</li> - - <li class="biblio">ANTS, BEES, AND WASPS. A Record of Observations of the Habits - of the Social Hymenoptera. By Sir <span class="smcap">John Lubbock</span>, Bart., F. - R. S., D. C. L., LL. D., etc. $2.00.</li> - - <li class="biblio">SCIENCE OF POLITICS. By <span class="smcap">Sheldon Amos</span>. $1.75.</li> - - <li class="biblio">ANIMAL INTELLIGENCE. By <span class="smcap">George J. Romanes</span>. $1.75.</li> - - <li class="biblio">MAN BEFORE METALS. By <span class="smcap">N. Joly</span>, Correspondent of the - Institute. With 148 Illustrations. $1.75.</li> - - <li class="biblio">THE ORGANS OF SPEECH AND THEIR APPLICATION IN THE FORMATION - OF ARTICULATE SOUNDS. By <span class="smcap">G. H. von Meyer</span>, Professor in - Ordinary of Anatomy at the University of Zürich. With 47 Woodcuts. - $1.75.</li> - - <li class="biblio">FALLACIES: A View of Logic from the Practical Side. By - <span class="smcap">Alfred Sidgwick</span>, B. A., Oxon. $1.75.</li> - - <li class="biblio">ORIGIN OF CULTIVATED PLANTS. By <span class="smcap">Alphonse de Candolle</span>. - $2.00.</li> - - <li class="biblio">JELLY-FISH, STAR-FISH, AND SEA-URCHINS. Being a Research on - Primitive Nervous Systems. By <span class="smcap">George J. Romanes</span>. $1.75.</li> - - <li class="biblio">THE COMMON SENSE OF THE EXACT SCIENCES. By the late <span class="smcap">William - Kingdon Clifford</span>. $1.50.</li> - - <li class="biblio">PHYSICAL EXPRESSION: Its Modes and Principles. By <span class="smcap">Francis - Warner</span>, M. D., Assistant Physician, and Lecturer on Botany to - the London Hospital, etc. With 51 Illustrations. $1.75.</li> - - <li class="biblio">ANTHROPOID APES. By <span class="smcap">Robert Hartmann</span>, Professor in the - University of Berlin. With 63 Illustrations. $1.75.</li> - - <li class="biblio">THE MAMMALIA IN THEIR RELATION TO PRIMEVAL TIMES. By <span class="smcap">Oscar - Schmidt</span>. $1.50.</li> - - <li class="biblio">COMPARATIVE LITERATURE. By <span class="smcap">Hutcheson Macaulay Posnett</span>, - M. A., LL. D., F. L. S., Barrister-at-Law; Professor of Classics - and English Literature, University College, Auckland, New Zealand, - author of “The Historical Method,” etc. $1.75.</li> - - <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 & CO., 1, 3, & 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 & CO., 1, 3, & 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 $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 & CO., 1, 3, & 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> - - - - - -End of the Project Gutenberg EBook of Earthquakes and Other Earth Movements, by -John Milne - -*** END OF THIS PROJECT GUTENBERG EBOOK EARTHQUAKES, OTHER EARTH MOVEMENTS *** - -***** This file should be named 60007-h.htm or 60007-h.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/0/0/0/60007/ - -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) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. Special rules, set forth in the General Terms of Use part -of this license, apply to copying and distributing Project -Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm -concept and trademark. Project Gutenberg is a registered trademark, -and may not be used if you charge for the eBooks, unless you receive -specific permission. If you do not charge anything for copies of this -eBook, complying with the rules is very easy. You may use this eBook -for nearly any purpose such as creation of derivative works, reports, -performances and research. They may be modified and printed and given -away--you may do practically ANYTHING in the United States with eBooks -not protected by U.S. copyright law. Redistribution is subject to the -trademark license, especially commercial redistribution. - -START: FULL LICENSE - -THE FULL PROJECT GUTENBERG LICENSE -PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK - -To protect the Project Gutenberg-tm mission of promoting the free -distribution of electronic works, by using or distributing this work -(or any other work associated in any way with the phrase "Project -Gutenberg"), you agree to comply with all the terms of the Full -Project Gutenberg-tm License available with this file or online at -www.gutenberg.org/license. - -Section 1. General Terms of Use and Redistributing Project -Gutenberg-tm electronic works - -1.A. By reading or using any part of this Project Gutenberg-tm -electronic work, you indicate that you have read, understand, agree to -and accept all the terms of this license and intellectual property -(trademark/copyright) agreement. If you do not agree to abide by all -the terms of this agreement, you must cease using and return or -destroy all copies of Project Gutenberg-tm electronic works in your -possession. If you paid a fee for obtaining a copy of or access to a -Project Gutenberg-tm electronic work and you do not agree to be bound -by the terms of this agreement, you may obtain a refund from the -person or entity to whom you paid the fee as set forth in paragraph -1.E.8. - -1.B. "Project Gutenberg" is a registered trademark. It may only be -used on or associated in any way with an electronic work by people who -agree to be bound by the terms of this agreement. There are a few -things that you can do with most Project Gutenberg-tm electronic works -even without complying with the full terms of this agreement. See -paragraph 1.C below. There are a lot of things you can do with Project -Gutenberg-tm electronic works if you follow the terms of this -agreement and help preserve free future access to Project Gutenberg-tm -electronic works. See paragraph 1.E below. - -1.C. The Project Gutenberg Literary Archive Foundation ("the -Foundation" or PGLAF), owns a compilation copyright in the collection -of Project Gutenberg-tm electronic works. Nearly all the individual -works in the collection are in the public domain in the United -States. If an individual work is unprotected by copyright law in the -United States and you are located in the United States, we do not -claim a right to prevent you from copying, distributing, performing, -displaying or creating derivative works based on the work as long as -all references to Project Gutenberg are removed. Of course, we hope -that you will support the Project Gutenberg-tm mission of promoting -free access to electronic works by freely sharing Project Gutenberg-tm -works in compliance with the terms of this agreement for keeping the -Project Gutenberg-tm name associated with the work. You can easily -comply with the terms of this agreement by keeping this work in the -same format with its attached full Project Gutenberg-tm License when -you share it without charge with others. - -1.D. The copyright laws of the place where you are located also govern -what you can do with this work. Copyright laws in most countries are -in a constant state of change. If you are outside the United States, -check the laws of your country in addition to the terms of this -agreement before downloading, copying, displaying, performing, -distributing or creating derivative works based on this work or any -other Project Gutenberg-tm work. The Foundation makes no -representations concerning the copyright status of any work in any -country outside the United States. - -1.E. Unless you have removed all references to Project Gutenberg: - -1.E.1. The following sentence, with active links to, or other -immediate access to, the full Project Gutenberg-tm License must appear -prominently whenever any copy of a Project Gutenberg-tm work (any work -on which the phrase "Project Gutenberg" appears, or with which the -phrase "Project Gutenberg" is associated) is accessed, displayed, -performed, viewed, copied or distributed: - - 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. - -1.E.2. If an individual Project Gutenberg-tm electronic work is -derived from texts not protected by U.S. copyright law (does not -contain a notice indicating that it is posted with permission of the -copyright holder), the work can be copied and distributed to anyone in -the United States without paying any fees or charges. If you are -redistributing or providing access to a work with the phrase "Project -Gutenberg" associated with or appearing on the work, you must comply -either with the requirements of paragraphs 1.E.1 through 1.E.7 or -obtain permission for the use of the work and the Project Gutenberg-tm -trademark as set forth in paragraphs 1.E.8 or 1.E.9. - -1.E.3. If an individual Project Gutenberg-tm electronic work is posted -with the permission of the copyright holder, your use and distribution -must comply with both paragraphs 1.E.1 through 1.E.7 and any -additional terms imposed by the copyright holder. Additional terms -will be linked to the Project Gutenberg-tm License for all works -posted with the permission of the copyright holder found at the -beginning of this work. - -1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm -License terms from this work, or any files containing a part of this -work or any other work associated with Project Gutenberg-tm. - -1.E.5. Do not copy, display, perform, distribute or redistribute this -electronic work, or any part of this electronic work, without -prominently displaying the sentence set forth in paragraph 1.E.1 with -active links or immediate access to the full terms of the Project -Gutenberg-tm License. - -1.E.6. You may convert to and distribute this work in any binary, -compressed, marked up, nonproprietary or proprietary form, including -any word processing or hypertext form. However, if you provide access -to or distribute copies of a Project Gutenberg-tm work in a format -other than "Plain Vanilla ASCII" or other format used in the official -version posted on the official Project Gutenberg-tm web site -(www.gutenberg.org), you must, at no additional cost, fee or expense -to the user, provide a copy, a means of exporting a copy, or a means -of obtaining a copy upon request, of the work in its original "Plain -Vanilla ASCII" or other form. Any alternate format must include the -full Project Gutenberg-tm License as specified in paragraph 1.E.1. - -1.E.7. Do not charge a fee for access to, viewing, displaying, -performing, copying or distributing any Project Gutenberg-tm works -unless you comply with paragraph 1.E.8 or 1.E.9. - -1.E.8. You may charge a reasonable fee for copies of or providing -access to or distributing Project Gutenberg-tm electronic works -provided that - -* You pay a royalty fee of 20% of the gross profits you derive from - the use of Project Gutenberg-tm works calculated using the method - you already use to calculate your applicable taxes. The fee is owed - to the owner of the Project Gutenberg-tm trademark, but he has - agreed to donate royalties under this paragraph to the Project - Gutenberg Literary Archive Foundation. Royalty payments must be paid - within 60 days following each date on which you prepare (or are - legally required to prepare) your periodic tax returns. Royalty - payments should be clearly marked as such and sent to the Project - Gutenberg Literary Archive Foundation at the address specified in - Section 4, "Information about donations to the Project Gutenberg - Literary Archive Foundation." - -* You provide a full refund of any money paid by a user who notifies - you in writing (or by e-mail) within 30 days of receipt that s/he - does not agree to the terms of the full Project Gutenberg-tm - License. You must require such a user to return or destroy all - copies of the works possessed in a physical medium and discontinue - all use of and all access to other copies of Project Gutenberg-tm - works. - -* You provide, in accordance with paragraph 1.F.3, a full refund of - any money paid for a work or a replacement copy, if a defect in the - electronic work is discovered and reported to you within 90 days of - receipt of the work. - -* You comply with all other terms of this agreement for free - distribution of Project Gutenberg-tm works. - -1.E.9. If you wish to charge a fee or distribute a Project -Gutenberg-tm electronic work or group of works on different terms than -are set forth in this agreement, you must obtain permission in writing -from both the Project Gutenberg Literary Archive Foundation and The -Project Gutenberg Trademark LLC, the owner of the Project Gutenberg-tm -trademark. Contact the Foundation as set forth in Section 3 below. - -1.F. - -1.F.1. Project Gutenberg volunteers and employees expend considerable -effort to identify, do copyright research on, transcribe and proofread -works not protected by U.S. copyright law in creating the Project -Gutenberg-tm collection. Despite these efforts, Project Gutenberg-tm -electronic works, and the medium on which they may be stored, may -contain "Defects," such as, but not limited to, incomplete, inaccurate -or corrupt data, transcription errors, a copyright or other -intellectual property infringement, a defective or damaged disk or -other medium, a computer virus, or computer codes that damage or -cannot be read by your equipment. - -1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right -of Replacement or Refund" described in paragraph 1.F.3, the Project -Gutenberg Literary Archive Foundation, the owner of the Project -Gutenberg-tm trademark, and any other party distributing a Project -Gutenberg-tm electronic work under this agreement, disclaim all -liability to you for damages, costs and expenses, including legal -fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT -LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE -PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE -TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE -LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR -INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH -DAMAGE. - -1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a -defect in this electronic work within 90 days of receiving it, you can -receive a refund of the money (if any) you paid for it by sending a -written explanation to the person you received the work from. If you -received the work on a physical medium, you must return the medium -with your written explanation. The person or entity that provided you -with the defective work may elect to provide a replacement copy in -lieu of a refund. If you received the work electronically, the person -or entity providing it to you may choose to give you a second -opportunity to receive the work electronically in lieu of a refund. If -the second copy is also defective, you may demand a refund in writing -without further opportunities to fix the problem. - -1.F.4. Except for the limited right of replacement or refund set forth -in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO -OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT -LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. - -1.F.5. Some states do not allow disclaimers of certain implied -warranties or the exclusion or limitation of certain types of -damages. If any disclaimer or limitation set forth in this agreement -violates the law of the state applicable to this agreement, the -agreement shall be interpreted to make the maximum disclaimer or -limitation permitted by the applicable state law. The invalidity or -unenforceability of any provision of this agreement shall not void the -remaining provisions. - -1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the -trademark owner, any agent or employee of the Foundation, anyone -providing copies of Project Gutenberg-tm electronic works in -accordance with this agreement, and any volunteers associated with the -production, promotion and distribution of Project Gutenberg-tm -electronic works, harmless from all liability, costs and expenses, -including legal fees, that arise directly or indirectly from any of -the following which you do or cause to occur: (a) distribution of this -or any Project Gutenberg-tm work, (b) alteration, modification, or -additions or deletions to any Project Gutenberg-tm work, and (c) any -Defect you cause. - -Section 2. Information about the Mission of Project Gutenberg-tm - -Project Gutenberg-tm is synonymous with the free distribution of -electronic works in formats readable by the widest variety of -computers including obsolete, old, middle-aged and new computers. It -exists because of the efforts of hundreds of volunteers and donations -from people in all walks of life. - -Volunteers and financial support to provide volunteers with the -assistance they need are critical to reaching Project Gutenberg-tm's -goals and ensuring that the Project Gutenberg-tm collection will -remain freely available for generations to come. In 2001, the Project -Gutenberg Literary Archive Foundation was created to provide a secure -and permanent future for Project Gutenberg-tm and future -generations. To learn more about the Project Gutenberg Literary -Archive Foundation and how your efforts and donations can help, see -Sections 3 and 4 and the Foundation information page at -www.gutenberg.org - - - -Section 3. Information about the Project Gutenberg Literary Archive Foundation - -The Project Gutenberg Literary Archive Foundation is a non profit -501(c)(3) educational corporation organized under the laws of the -state of Mississippi and granted tax exempt status by the Internal -Revenue Service. The Foundation's EIN or federal tax identification -number is 64-6221541. Contributions to the Project Gutenberg Literary -Archive Foundation are tax deductible to the full extent permitted by -U.S. federal laws and your state's laws. - -The Foundation's principal office is in Fairbanks, Alaska, with the -mailing address: PO Box 750175, Fairbanks, AK 99775, but its -volunteers and employees are scattered throughout numerous -locations. Its business office is located at 809 North 1500 West, Salt -Lake City, UT 84116, (801) 596-1887. Email contact links and up to -date contact information can be found at the Foundation's web site and -official page at www.gutenberg.org/contact - -For additional contact information: - - Dr. Gregory B. Newby - Chief Executive and Director - gbnewby@pglaf.org - -Section 4. Information about Donations to the Project Gutenberg -Literary Archive Foundation - -Project Gutenberg-tm depends upon and cannot survive without wide -spread public support and donations to carry out its mission of -increasing the number of public domain and licensed works that can be -freely distributed in machine readable form accessible by the widest -array of equipment including outdated equipment. Many small donations -($1 to $5,000) are particularly important to maintaining tax exempt -status with the IRS. - -The Foundation is committed to complying with the laws regulating -charities and charitable donations in all 50 states of the United -States. Compliance requirements are not uniform and it takes a -considerable effort, much paperwork and many fees to meet and keep up -with these requirements. We do not solicit donations in locations -where we have not received written confirmation of compliance. To SEND -DONATIONS or determine the status of compliance for any particular -state visit www.gutenberg.org/donate - -While we cannot and do not solicit contributions from states where we -have not met the solicitation requirements, we know of no prohibition -against accepting unsolicited donations from donors in such states who -approach us with offers to donate. - -International donations are gratefully accepted, but we cannot make -any statements concerning tax treatment of donations received from -outside the United States. U.S. laws alone swamp our small staff. - -Please check the Project Gutenberg Web pages for current donation -methods and addresses. Donations are accepted in a number of other -ways including checks, online payments and credit card donations. To -donate, please visit: www.gutenberg.org/donate - -Section 5. General Information About Project Gutenberg-tm electronic works. - -Professor Michael S. Hart was the originator of the Project -Gutenberg-tm concept of a library of electronic works that could be -freely shared with anyone. For forty years, he produced and -distributed Project Gutenberg-tm eBooks with only a loose network of -volunteer support. - -Project Gutenberg-tm eBooks are often created from several printed -editions, all of which are confirmed as not protected by copyright in -the U.S. unless a copyright notice is included. Thus, we do not -necessarily keep eBooks in compliance with any particular paper -edition. - -Most people start at our Web site which has the main PG search -facility: www.gutenberg.org - -This Web site includes information about Project Gutenberg-tm, -including how to make donations to the Project Gutenberg Literary -Archive Foundation, how to help produce our new eBooks, and how to -subscribe to our email newsletter to hear about new eBooks. - - - -</pre> - -</body> - -</html> diff --git a/old/60007-h/images/cover.jpg b/old/60007-h/images/cover.jpg Binary files differdeleted file mode 100644 index c7881aa..0000000 --- a/old/60007-h/images/cover.jpg +++ /dev/null diff --git a/old/60007-h/images/divider.png b/old/60007-h/images/divider.png Binary files differdeleted file mode 100644 index 02bfed4..0000000 --- a/old/60007-h/images/divider.png +++ /dev/null diff --git a/old/60007-h/images/i_p014.jpg b/old/60007-h/images/i_p014.jpg Binary files differdeleted file mode 100644 index e47a61e..0000000 --- a/old/60007-h/images/i_p014.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p017.png b/old/60007-h/images/i_p017.png Binary files differdeleted file mode 100644 index ce74ab4..0000000 --- a/old/60007-h/images/i_p017.png +++ /dev/null diff --git a/old/60007-h/images/i_p023.jpg b/old/60007-h/images/i_p023.jpg Binary files differdeleted file mode 100644 index b50cc22..0000000 --- a/old/60007-h/images/i_p023.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p028a.jpg b/old/60007-h/images/i_p028a.jpg Binary files differdeleted file mode 100644 index 28cf4af..0000000 --- a/old/60007-h/images/i_p028a.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p028b.jpg b/old/60007-h/images/i_p028b.jpg Binary files differdeleted file mode 100644 index d92db89..0000000 --- a/old/60007-h/images/i_p028b.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p030.jpg b/old/60007-h/images/i_p030.jpg Binary files differdeleted file mode 100644 index e254ccb..0000000 --- a/old/60007-h/images/i_p030.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p032.jpg b/old/60007-h/images/i_p032.jpg Binary files differdeleted file mode 100644 index f5dd014..0000000 --- a/old/60007-h/images/i_p032.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p036.jpg b/old/60007-h/images/i_p036.jpg Binary files differdeleted file mode 100644 index 61df854..0000000 --- a/old/60007-h/images/i_p036.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p039.jpg b/old/60007-h/images/i_p039.jpg Binary files differdeleted file mode 100644 index c722d8e..0000000 --- a/old/60007-h/images/i_p039.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p054.jpg b/old/60007-h/images/i_p054.jpg Binary files differdeleted file mode 100644 index 6b2a043..0000000 --- a/old/60007-h/images/i_p054.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p058.jpg b/old/60007-h/images/i_p058.jpg Binary files differdeleted file mode 100644 index 8972007..0000000 --- a/old/60007-h/images/i_p058.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p060.jpg b/old/60007-h/images/i_p060.jpg Binary files differdeleted file mode 100644 index 006a578..0000000 --- a/old/60007-h/images/i_p060.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p061.jpg b/old/60007-h/images/i_p061.jpg Binary files differdeleted file mode 100644 index 67b5572..0000000 --- a/old/60007-h/images/i_p061.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p063.jpg b/old/60007-h/images/i_p063.jpg Binary files differdeleted file mode 100644 index 8823211..0000000 --- a/old/60007-h/images/i_p063.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p080.jpg b/old/60007-h/images/i_p080.jpg Binary files differdeleted file mode 100644 index a2095a8..0000000 --- a/old/60007-h/images/i_p080.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p081.jpg b/old/60007-h/images/i_p081.jpg Binary files differdeleted file mode 100644 index 0647461..0000000 --- a/old/60007-h/images/i_p081.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p100.jpg b/old/60007-h/images/i_p100.jpg Binary files differdeleted file mode 100644 index 3b36ae7..0000000 --- a/old/60007-h/images/i_p100.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p101.jpg b/old/60007-h/images/i_p101.jpg Binary files differdeleted file mode 100644 index fb45112..0000000 --- a/old/60007-h/images/i_p101.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p105.jpg b/old/60007-h/images/i_p105.jpg Binary files differdeleted file mode 100644 index 15742fe..0000000 --- a/old/60007-h/images/i_p105.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p106.jpg b/old/60007-h/images/i_p106.jpg Binary files differdeleted file mode 100644 index 37bc7d1..0000000 --- a/old/60007-h/images/i_p106.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p112.jpg b/old/60007-h/images/i_p112.jpg Binary files differdeleted file mode 100644 index 029fb0e..0000000 --- a/old/60007-h/images/i_p112.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p113.jpg b/old/60007-h/images/i_p113.jpg Binary files differdeleted file mode 100644 index 16424f0..0000000 --- a/old/60007-h/images/i_p113.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p114.jpg b/old/60007-h/images/i_p114.jpg Binary files differdeleted file mode 100644 index c9880d6..0000000 --- a/old/60007-h/images/i_p114.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p116.jpg b/old/60007-h/images/i_p116.jpg Binary files differdeleted file mode 100644 index d3c35d2..0000000 --- a/old/60007-h/images/i_p116.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p117.jpg b/old/60007-h/images/i_p117.jpg Binary files differdeleted file mode 100644 index 7770e1b..0000000 --- a/old/60007-h/images/i_p117.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p118.jpg b/old/60007-h/images/i_p118.jpg Binary files differdeleted file mode 100644 index 3e47954..0000000 --- a/old/60007-h/images/i_p118.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p138.jpg b/old/60007-h/images/i_p138.jpg Binary files differdeleted file mode 100644 index 23510f6..0000000 --- a/old/60007-h/images/i_p138.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p169.jpg b/old/60007-h/images/i_p169.jpg Binary files differdeleted file mode 100644 index 3657dea..0000000 --- a/old/60007-h/images/i_p169.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p190.jpg b/old/60007-h/images/i_p190.jpg Binary files differdeleted file mode 100644 index 1cb6aec..0000000 --- a/old/60007-h/images/i_p190.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p197.jpg b/old/60007-h/images/i_p197.jpg Binary files differdeleted file mode 100644 index 0328041..0000000 --- a/old/60007-h/images/i_p197.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p203.jpg b/old/60007-h/images/i_p203.jpg Binary files differdeleted file mode 100644 index bc289b8..0000000 --- a/old/60007-h/images/i_p203.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p211.jpg b/old/60007-h/images/i_p211.jpg Binary files differdeleted file mode 100644 index dea70c6..0000000 --- a/old/60007-h/images/i_p211.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p217.jpg b/old/60007-h/images/i_p217.jpg Binary files differdeleted file mode 100644 index 3248cf5..0000000 --- a/old/60007-h/images/i_p217.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p232.jpg b/old/60007-h/images/i_p232.jpg Binary files differdeleted file mode 100644 index 3a7e905..0000000 --- a/old/60007-h/images/i_p232.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p232a.png b/old/60007-h/images/i_p232a.png Binary files differdeleted file mode 100644 index e5f4c73..0000000 --- a/old/60007-h/images/i_p232a.png +++ /dev/null diff --git a/old/60007-h/images/i_p232b.png b/old/60007-h/images/i_p232b.png Binary files differdeleted file mode 100644 index 3e8e63e..0000000 --- a/old/60007-h/images/i_p232b.png +++ /dev/null diff --git a/old/60007-h/images/i_p242.jpg b/old/60007-h/images/i_p242.jpg Binary files differdeleted file mode 100644 index a2e2301..0000000 --- a/old/60007-h/images/i_p242.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p256.jpg b/old/60007-h/images/i_p256.jpg Binary files differdeleted file mode 100644 index 48b13e8..0000000 --- a/old/60007-h/images/i_p256.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p318.jpg b/old/60007-h/images/i_p318.jpg Binary files differdeleted file mode 100644 index 1da2ff1..0000000 --- a/old/60007-h/images/i_p318.jpg +++ /dev/null diff --git a/old/60007-h/images/i_p321.jpg b/old/60007-h/images/i_p321.jpg Binary files differdeleted file mode 100644 index 10d4cf9..0000000 --- a/old/60007-h/images/i_p321.jpg +++ /dev/null |
