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+The Project Gutenberg EBook of A History of Science, Volume 3(of 5), by 
+Henry Smith Williams
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: A History of Science, Volume 3(of 5)
+
+Author: Henry Smith Williams
+
+Release Date: April, 1999  [Etext #1707]
+Posting Date: November 18, 2009
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HISTORY OF SCIENCE, V3 ***
+
+
+
+
+Produced by Charles Keller
+
+
+
+
+
+A HISTORY OF SCIENCE
+
+
+MODERN DEVELOPMENT OF THE PHYSICAL SCIENCES
+
+
+
+By Henry Smith Williams, M.D., Ll.D.
+
+Assisted By Edward H. Williams, M.D.
+
+In Five Volumes
+
+Volume III.
+
+
+
+CONTENTS
+
+  BOOK III
+
+  CHAPTER I. THE SUCCESSORS OF NEWTON IN ASTRONOMY
+
+  The work of Johannes Hevelius--Halley and Hevelius--Halley's observation
+  of the transit of Mercury, and his method of determining the parallax of
+  the planets--Halley's observation of meteors--His inability to
+  explain these bodies--The important work of James Bradley--Lacaille's
+  measurement of the arc of the meridian--The determination of the
+  question as to the exact shape of the earth--D'Alembert and his
+  influence upon science--Delambre's History of Astronomy--The
+  astronomical work of Euler.
+
+  CHAPTER II. THE PROGRESS OF MODERN ASTRONOMY
+
+  The work of William Herschel--His discovery of Uranus--His discovery
+  that the stars are suns--His conception of the universe--His deduction
+  that gravitation has caused the grouping of the heavenly bodies--The
+  nebula, hypothesis,--Immanuel Kant's conception of the formation of the
+  world--Defects in Kant's conception--Laplace's final solution of the
+  problem--His explanation in detail--Change in the mental attitude of the
+  world since Bruno--Asteroids and satellites--Discoveries of Olbersl--The
+  mathematical calculations of Adams and Leverrier--The discovery of the
+  inner ring of Saturn--Clerk Maxwell's paper on the stability of Saturn's
+  rings--Helmholtz's conception of the action of tidal friction--Professor
+  G. H. Darwin's estimate of the consequences of tidal action--Comets
+  and meteors--Bredichin's cometary theory--The final solution of the
+  structure of comets--Newcomb's estimate of the amount of cometary dust
+  swept up daily by the earth--The fixed stars--John Herschel's studies
+  of double stars--Fraunhofer's perfection of the refracting
+  telescope--Bessel's measurement of the parallax of a star,--Henderson's
+  measurements--Kirchhoff and Bunsen's perfection of the
+  spectroscope--Wonderful revelations of the spectroscope--Lord Kelvin's
+  estimate of the time that will be required for the earth to become
+  completely cooled--Alvan Clark's discovery of the companion star of
+  Sirius--The advent of the photographic film in astronomy--Dr. Huggins's
+  studies of nebulae--Sir Norman Lockyer's "cosmogonic guess,"--Croll's
+  pre-nebular theory.
+
+  CHAPTER III. THE NEW SCIENCE OF PALEONTOLOGY
+
+  William Smith and fossil shells--His discovery that fossil rocks are
+  arranged in regular systems--Smith's inquiries taken up by Cuvier--His
+  Ossements Fossiles containing the first description of hairy
+  elephant--His contention that fossils represent extinct species
+  only--Dr. Buckland's studies of English fossil-beds--Charles Lyell
+  combats catastrophism,--Elaboration of his ideas with reference to
+  the rotation of species--The establishment of the doctrine of
+  uniformitarianism,--Darwin's Origin of Species--Fossil man--Dr.
+  Falconer's visit to the fossil-beds in the valley of the
+  Somme--Investigations of Prestwich and Sir John Evans--Discovery of the
+  Neanderthal skull,--Cuvier's rejection of human fossils--The finding
+  of prehistoric carving on ivory--The fossil-beds of America--Professor
+  Marsh's paper on the fossil horses in America--The Warren mastodon,--The
+  Java fossil, Pithecanthropus Erectus.
+
+  CHAPTER IV. THE ORIGIN AND DEVELOPMENT OF MODERN GEOLOGY
+
+  James Hutton and the study of the rocks--His theory of the earth--His
+  belief in volcanic cataclysms in raising and forming the continents--His
+  famous paper before the Royal Society of Edinburgh, 1781---His
+  conclusions that all strata of the earth have their origin at the bottom
+  of the sea---His deduction that heated and expanded matter caused the
+  elevation of land above the sea-level--Indifference at first shown this
+  remarkable paper--Neptunists versus Plutonists--Scrope's classical work
+  on volcanoes--Final acceptance of Hutton's explanation of the origin
+  of granites--Lyell and uniformitarianism--Observations on the gradual
+  elevation of the coast-lines of Sweden and Patagonia--Observations on
+  the enormous amount of land erosion constantly taking place,--Agassiz
+  and the glacial theory--Perraudin the chamois-hunter, and his
+  explanation of perched bowlders--De Charpentier's acceptance of
+  Perraudin's explanation--Agassiz's paper on his Alpine studies--His
+  conclusion that the Alps were once covered with an ice-sheet--Final
+  acceptance of the glacial theory--The geological ages--The work of
+  Murchison and Sedgwick--Formation of the American continents--Past,
+  present, and future.
+
+  CHAPTER V. THE NEW SCIENCE OF METEOROLOGY
+
+  Biot's investigations of meteors--The observations of Brandes and
+  Benzenberg on the velocity of falling stars--Professor Olmstead's
+  observations on the meteoric shower of 1833--Confirmation of Chladni's
+  hypothesis of 1794--The aurora borealis--Franklin's suggestion that
+  it is of electrical origin--Its close association with terrestrial
+  magnetism--Evaporation, cloud-formation, and dew--Dalton's demonstration
+  that water exists in the air as an independent gas--Hutton's theory of
+  rain--Luke Howard's paper on clouds--Observations on dew, by Professor
+  Wilson and Mr. Six--Dr. Wells's essay on dew--His observations
+  on several appearances connected with dew--Isotherms and ocean
+  currents--Humboldt and the-science of comparative climatology--His
+  studies of ocean currents--Maury's theory that gravity is the cause
+  of ocean currents--Dr. Croll on Climate and Time--Cyclones and
+  anti-cyclones,--Dove's studies in climatology--Professor Ferrel's
+  mathematical law of the deflection of winds--Tyndall's estimate of
+  the amount of heat given off by the liberation of a pound of
+  vapor--Meteorological observations and weather predictions.
+
+  CHAPTER VI. MODERN THEORIES OF HEAT AND LIGHT
+
+  Josiah Wedgwood and the clay pyrometer--Count Rumford and the vibratory
+  theory of heat--His experiments with boring cannon to determine the
+  nature of heat--Causing water to boil by the friction of the borer--His
+  final determination that heat is a form of motion--Thomas Young and the
+  wave theory of light--His paper on the theory of light and colors--His
+  exposition of the colors of thin plates--Of the colors of thick
+  plates, and of striated surfaces,--Arago and Fresnel champion the wave
+  theory--opposition to the theory by Biot--The French Academy's tacit
+  acceptance of the correctness of the theory by its admission of Fresnel
+  as a member.
+
+  CHAPTER VII. THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM
+
+  Galvani and the beginning of modern electricity--The construction of
+  the voltaic pile--Nicholson's and Carlisle's discovery that the galvanic
+  current decomposes water--Decomposition of various substances by Sir
+  Humphry Davy--His construction of an arc-light--The deflection of the
+  magnetic needle by electricity demonstrated by Oersted--Effect of
+  this important discovery--Ampere creates the science of
+  electro-dynamics--Joseph Henry's studies of electromagnets--Michael
+  Faraday begins his studies of electromagnetic induction--His famous
+  paper before the Royal Society, in 1831, in which he demonstrates
+  electro-magnetic induction--His explanation of Arago's
+  rotating disk--The search for a satisfactory method of storing
+  electricity--Roentgen rays, or X-rays.
+
+  CHAPTER VIII. THE CONSERVATION OF ENERGY
+
+  Faraday narrowly misses the discovery of the doctrine of
+  conservation--Carnot's belief that a definite quantity of work can be
+  transformed into a definite quantity of heat--The work of James Prescott
+  Joule--Investigations begun by Dr. Mayer--Mayer's paper of 1842--His
+  statement of the law of the conservation of energy--Mayer and
+  Helmholtz--Joule's paper of 1843--Joule or Mayer--Lord Kelvin and the
+  dissipation of energy-The final unification.
+
+
+  CHAPTER IX. THE ETHER AND PONDERABLE MATTER
+
+  James Clerk-Maxwell's conception of ether--Thomas Young and
+  "Luminiferous ether,"--Young's and Fresnel's conception of transverse
+  luminiferous undulations--Faraday's experiments pointing to the
+  existence of ether--Professor Lodge's suggestion of two ethers--Lord
+  Kelvin's calculation of the probable density of ether--The vortex theory
+  of atoms--Helmholtz's calculations in vortex motions--Professor
+  Tait's apparatus for creating vortex rings in the air---The ultimate
+  constitution of matter as conceived by Boscovich--Davy's speculations
+  as to the changes that occur in the substance of matter at different
+  temperatures--Clausius's and Maxwell's investigations of the
+  kinetic theory of gases--Lord Kelvin's estimate of the size of the
+  molecule--Studies of the potential energy of molecules--Action of gases
+  at low temperatures.
+
+  APPENDIX
+
+
+
+A HISTORY OF SCIENCE
+
+
+
+
+BOOK III. MODERN DEVELOPMENT OF THE PHYSICAL SCIENCES
+
+With the present book we enter the field of the distinctively modern.
+There is no precise date at which we take up each of the successive
+stories, but the main sweep of development has to do in each case with
+the nineteenth century. We shall see at once that this is a time both
+of rapid progress and of great differentiation. We have heard almost
+nothing hitherto of such sciences as paleontology, geology, and
+meteorology, each of which now demands full attention. Meantime,
+astronomy and what the workers of the elder day called natural
+philosophy become wonderfully diversified and present numerous
+phases that would have been startling enough to the star-gazers and
+philosophers of the earlier epoch.
+
+Thus, for example, in the field of astronomy, Herschel is able, thanks
+to his perfected telescope, to discover a new planet and then to reach
+out into the depths of space and gain such knowledge of stars and
+nebulae as hitherto no one had more than dreamed of. Then, in rapid
+sequence, a whole coterie of hitherto unsuspected minor planets is
+discovered, stellar distances are measured, some members of the starry
+galaxy are timed in their flight, the direction of movement of the solar
+system itself is investigated, the spectroscope reveals the chemical
+composition even of suns that are unthinkably distant, and a tangible
+theory is grasped of the universal cycle which includes the birth and
+death of worlds.
+
+Similarly the new studies of the earth's surface reveal secrets of
+planetary formation hitherto quite inscrutable. It becomes known that
+the strata of the earth's surface have been forming throughout untold
+ages, and that successive populations differing utterly from one another
+have peopled the earth in different geological epochs. The entire point
+of view of thoughtful men becomes changed in contemplating the history
+of the world in which we live--albeit the newest thought harks back to
+some extent to those days when the inspired thinkers of early Greece
+dreamed out the wonderful theories with which our earlier chapters have
+made our readers familiar.
+
+In the region of natural philosophy progress is no less pronounced and
+no less striking. It suffices here, however, by way of anticipation,
+simply to name the greatest generalization of the century in physical
+science--the doctrine of the conservation of energy.
+
+
+
+
+I. THE SUCCESSORS OF NEWTON IN ASTRONOMY
+
+
+HEVELIUS AND HALLEY
+
+STRANGELY enough, the decade immediately following Newton was one of
+comparative barrenness in scientific progress, the early years of the
+eighteenth century not being as productive of great astronomers as the
+later years of the seventeenth, or, for that matter, as the later years
+of the eighteenth century itself. Several of the prominent astronomers
+of the later seventeenth century lived on into the opening years of the
+following century, however, and the younger generation soon developed
+a coterie of astronomers, among whom Euler, Lagrange, Laplace, and
+Herschel, as we shall see, were to accomplish great things in this field
+before the century closed.
+
+One of the great seventeenth-century astronomers, who died just before
+the close of the century, was Johannes Hevelius (1611-1687), of Dantzig,
+who advanced astronomy by his accurate description of the face and
+the spots of the moon. But he is remembered also for having retarded
+progress by his influence in refusing to use telescopic sights in his
+observations, preferring until his death the plain sights long before
+discarded by most other astronomers. The advantages of these telescope
+sights have been discussed under the article treating of Robert Hooke,
+but no such advantages were ever recognized by Hevelius. So great was
+Hevelius's reputation as an astronomer that his refusal to recognize the
+advantage of the telescope sights caused many astronomers to hesitate
+before accepting them as superior to the plain; and even the famous
+Halley, of whom we shall speak further in a moment, was sufficiently
+in doubt over the matter to pay the aged astronomer a visit to test his
+skill in using the old-style sights. Side by side, Hevelius and Halley
+made their observations, Hevelius with his old instrument and Halley
+with the new. The results showed slightly in the younger man's favor,
+but not enough to make it an entirely convincing demonstration. The
+explanation of this, however, did not lie in the lack of superiority
+of the telescopic instrument, but rather in the marvellous skill of the
+aged Hevelius, whose dexterity almost compensated for the defect of his
+instrument. What he might have accomplished could he have been induced
+to adopt the telescope can only be surmised.
+
+Halley himself was by no means a tyro in matters astronomical at that
+time. As the only son of a wealthy soap-boiler living near London, he
+had been given a liberal education, and even before leaving college
+made such novel scientific observations as that of the change in the
+variation of the compass. At nineteen years of age he discovered a new
+method of determining the elements of the planetary orbits which was a
+distinct improvement over the old. The year following he sailed for the
+Island of St, Helena to make observations of the heavens in the southern
+hemisphere.
+
+It was while in St. Helena that Halley made his famous observation
+of the transit of Mercury over the sun's disk, this observation being
+connected, indirectly at least, with his discovery of a method of
+determining the parallax of the planets. By parallax is meant the
+apparent change in the position of an object, due really to a change in
+the position of the observer. Thus, if we imagine two astronomers making
+observations of the sun from opposite sides of the earth at the same
+time, it is obvious that to these observers the sun will appear to be
+at two different points in the sky. Half the angle measuring this
+difference would be known as the sun's parallax. This would depend,
+then, upon the distance of the earth from the sun and the length of
+the earth's radius. Since the actual length of this radius has been
+determined, the parallax of any heavenly body enables the astronomer to
+determine its exact distance.
+
+The parallaxes can be determined equally well, however, if two observers
+are separated by exactly known distances, several hundreds or thousands
+of miles apart. In the case of a transit of Venus across the sun's
+disk, for example, an observer at New York notes the image of the planet
+moving across the sun's disk, and notes also the exact time of this
+observation. In the same manner an observer at London makes similar
+observations. Knowing the distance between New York and London, and
+the different time of the passage, it is thus possible to calculate the
+difference of the parallaxes of the sun and a planet crossing its disk.
+The idea of thus determining the parallax of the planets originated, or
+at least was developed, by Halley, and from this phenomenon he thought
+it possible to conclude the dimensions of all the planetary orbits. As
+we shall see further on, his views were found to be correct by later
+astronomers.
+
+In 1721 Halley succeeded Flamsteed as astronomer royal at the Greenwich
+Observatory. Although sixty-four years of age at that time his activity
+in astronomy continued unabated for another score of years. At Greenwich
+he undertook some tedious observations of the moon, and during those
+observations was first to detect the acceleration of mean motion. He
+was unable to explain this, however, and it remained for Laplace in the
+closing years of the century to do so, as we shall see later.
+
+Halley's book, the Synopsis Astronomiae Cometicae, is one of the most
+valuable additions to astronomical literature since the time of Kepler.
+He was first to attempt the calculation of the orbit of a comet, having
+revived the ancient opinion that comets belong to the solar system,
+moving in eccentric orbits round the sun, and his calculation of the
+orbit of the comet of 1682 led him to predict correctly the return of
+that comet in 1758. Halley's Study of Meteors.
+
+Like other astronomers of his time he was greatly puzzled over the
+well-known phenomena of shooting-stars, or meteors, making many
+observations himself, and examining carefully the observations of other
+astronomers. In 1714 he gave his views as to the origin and composition
+of these mysterious visitors in the earth's atmosphere. As this
+subject will be again referred to in a later chapter, Halley's views,
+representing the most advanced views of his age, are of interest.
+
+"The theory of the air seemeth at present," he says, "to be perfectly
+well understood, and the differing densities thereof at all altitudes;
+for supposing the same air to occupy spaces reciprocally proportional to
+the quantity of the superior or incumbent air, I have elsewhere proved
+that at forty miles high the air is rarer than at the surface of
+the earth at three thousand times; and that the utmost height of the
+atmosphere, which reflects light in the Crepusculum, is not fully
+forty-five miles, notwithstanding which 'tis still manifest that some
+sort of vapors, and those in no small quantity, arise nearly to that
+height. An instance of this may be given in the great light the society
+had an account of (vide Transact. Sep., 1676) from Dr. Wallis, which was
+seen in very distant counties almost over all the south part of England.
+Of which though the doctor could not get so particular a relation as was
+requisite to determine the height thereof, yet from the distant places
+it was seen in, it could not but be very many miles high.
+
+"So likewise that meteor which was seen in 1708, on the 31st of July,
+between nine and ten o'clock at night, was evidently between forty and
+fifty miles perpendicularly high, and as near as I can gather, over
+Shereness and the buoy on the Nore. For it was seen at London moving
+horizontally from east by north to east by south at least fifty degrees
+high, and at Redgrove, in Suffolk, on the Yarmouth road, about twenty
+miles from the east coast of England, and at least forty miles to the
+eastward of London, it appeared a little to the westward of the south,
+suppose south by west, and was seen about thirty degrees high, sliding
+obliquely downward. I was shown in both places the situation thereof,
+which was as described, but could wish some person skilled in
+astronomical matters bad seen it, that we might pronounce concerning its
+height with more certainty. Yet, as it is, we may securely conclude
+that it was not many more miles westerly than Redgrove, which, as I
+said before, is about forty miles more easterly than London. Suppose it,
+therefore, where perpendicular, to have been thirty-five miles east
+from London, and by the altitude it appeared at in London--viz., fifty
+degrees, its tangent will be forty-two miles, for the height of the
+meteor above the surface of the earth; which also is rather of the
+least, because the altitude of the place shown me is rather more than
+less than fifty degrees; and the like may be concluded from the altitude
+it appeared in at Redgrove, near seventy miles distant. Though at this
+very great distance, it appeared to move with an incredible velocity,
+darting, in a very few seconds of time, for about twelve degrees of
+a great circle from north to south, being very bright at its first
+appearance; and it died away at the east of its course, leaving for some
+time a pale whiteness in the place, with some remains of it in the track
+where it had gone; but no hissing sound as it passed, or bounce of an
+explosion were heard.
+
+"It may deserve the honorable society's thoughts, how so great a
+quantity of vapor should be raised to the top of the atmosphere, and
+there collected, so as upon its ascension or otherwise illumination, to
+give a light to a circle of above one hundred miles diameter, not much
+inferior to the light of the moon; so as one might see to take a pin
+from the ground in the otherwise dark night. 'Tis hard to conceive what
+sort of exhalations should rise from the earth, either by the action
+of the sun or subterranean heat, so as to surmount the extreme cold
+and rareness of the air in those upper regions: but the fact is
+indisputable, and therefore requires a solution."
+
+From this much of the paper it appears that there was a general belief
+that this burning mass was heated vapor thrown off from the earth in
+some mysterious manner, yet this is unsatisfactory to Halley, for after
+citing various other meteors that have appeared within his knowledge, he
+goes on to say:
+
+"What sort of substance it must be, that could be so impelled and
+ignited at the same time; there being no Vulcano or other Spiraculum of
+subterraneous fire in the northeast parts of the world, that we ever yet
+heard of, from whence it might be projected.
+
+"I have much considered this appearance, and think it one of the hardest
+things to account for that I have yet met with in the phenomena of
+meteors, and I am induced to think that it must be some collection of
+matter formed in the aether, as it were, by some fortuitous concourse
+of atoms, and that the earth met with it as it passed along in its orb,
+then but newly formed, and before it had conceived any great impetus of
+descent towards the sun. For the direction of it was exactly opposite to
+that of the earth, which made an angle with the meridian at that time
+of sixty-seven gr., that is, its course was from west southwest to east
+northeast, wherefore the meteor seemed to move the contrary way. And
+besides falling into the power of the earth's gravity, and losing its
+motion from the opposition of the medium, it seems that it descended
+towards the earth, and was extinguished in the Tyrrhene Sea, to the
+west southwest of Leghorn. The great blow being heard upon its first
+immersion into the water, and the rattling like the driving of a cart
+over stones being what succeeded upon its quenching; something like this
+is always heard upon quenching a very hot iron in water. These facts
+being past dispute, I would be glad to have the opinion of the learned
+thereon, and what objection can be reasonably made against the above
+hypothesis, which I humbly submit to their censure."(1)
+
+These few paragraphs, coming as they do from a leading
+eighteenth-century astronomer, convey more clearly than any comment the
+actual state of the meteorological learning at that time. That this ball
+of fire, rushing "at a greater velocity than the swiftest cannon-ball,"
+was simply a mass of heated rock passing through our atmosphere, did not
+occur to him, or at least was not credited. Nor is this surprising when
+we reflect that at that time universal gravitation had been but recently
+discovered; heat had not as yet been recognized as simply a form of
+motion; and thunder and lightning were unexplained mysteries, not to
+be explained for another three-quarters of a century. In the chapter on
+meteorology we shall see how the solution of this mystery that puzzled
+Halley and his associates all their lives was finally attained.
+
+
+BRADLEY AND THE ABERRATION OF LIGHT
+
+Halley was succeeded as astronomer royal by a man whose useful additions
+to the science were not to be recognized or appreciated fully until
+brought to light by the Prussian astronomer Bessel early in the
+nineteenth century. This was Dr. James Bradley, an ecclesiastic, who
+ranks as one of the most eminent astronomers of the eighteenth century.
+His most remarkable discovery was the explanation of a peculiar motion
+of the pole-star, first observed, but not explained, by Picard a
+century before. For many years a satisfactory explanation was sought
+unsuccessfully by Bradley and his fellow-astronomers, but at last he was
+able to demonstrate that the stary Draconis, on which he was making his
+observations, described, or appeared to describe, a small ellipse.
+If this observation was correct, it afforded a means of computing the
+aberration of any star at all times. The explanation of the physical
+cause of this aberration, as Bradley thought, and afterwards
+demonstrated, was the result of the combination of the motion of light
+with the annual motion of the earth. Bradley first formulated this
+theory in 1728, but it was not until 1748--twenty years of continuous
+struggle and observation by him--that he was prepared to communicate the
+results of his efforts to the Royal Society. This remarkable paper is
+thought by the Frenchman, Delambre, to entitle its author to a place in
+science beside such astronomers as Hipparcbus and Kepler.
+
+Bradley's studies led him to discover also the libratory motion of the
+earth's axis. "As this appearance of Draconis indicated a diminution
+of the inclination of the earth's axis to the plane of the ecliptic,"
+he says; "and as several astronomers have supposed THAT inclination to
+diminish regularly; if this phenomenon depended upon such a cause, and
+amounted to 18" in nine years, the obliquity of the ecliptic would, at
+that rate, alter a whole minute in thirty years; which is much
+faster than any observations, before made, would allow. I had reason,
+therefore, to think that some part of this motion at the least, if not
+the whole, was owing to the moon's action upon the equatorial parts of
+the earth; which, I conceived, might cause a libratory motion of
+the earth's axis. But as I was unable to judge, from only nine years
+observations, whether the axis would entirely recover the same position
+that it had in the year 1727, I found it necessary to continue my
+observations through a whole period of the moon's nodes; at the end of
+which I had the satisfaction to see, that the stars, returned into the
+same position again; as if there had been no alteration at all in the
+inclination of the earth's axis; which fully convinced me that I had
+guessed rightly as to the cause of the phenomena. This circumstance
+proves likewise, that if there be a gradual diminution of the obliquity
+of the ecliptic, it does not arise only from an alteration in the
+position of the earth's axis, but rather from some change in the plane
+of the ecliptic itself; because the stars, at the end of the period
+of the moon's nodes, appeared in the same places, with respect to the
+equator, as they ought to have done, if the earth's axis had retained
+the same inclination to an invariable plane."(2)
+
+
+FRENCH ASTRONOMERS
+
+Meanwhile, astronomers across the channel were by no means idle. In
+France several successful observers were making many additions to the
+already long list of observations of the first astronomer of the Royal
+Observatory of Paris, Dominic Cassini (1625-1712), whose reputation
+among his contemporaries was much greater than among succeeding
+generations of astronomers. Perhaps the most deserving of these
+successors was Nicolas Louis de Lacaille (1713-1762), a theologian who
+had been educated at the expense of the Duke of Bourbon, and who, soon
+after completing his clerical studies, came under the patronage of
+Cassini, whose attention had been called to the young man's interest in
+the sciences. One of Lacaille's first under-takings was the remeasuring
+of the French are of the meridian, which had been incorrectly measured
+by his patron in 1684. This was begun in 1739, and occupied him for
+two years before successfully completed. As a reward, however, he was
+admitted to the academy and appointed mathematical professor in Mazarin
+College.
+
+In 1751 he went to the Cape of Good Hope for the purpose of determining
+the sun's parallax by observations of the parallaxes of Mars and Venus,
+and incidentally to make observations on the other southern hemisphere
+stars. The results of this undertaking were most successful, and were
+given in his Coelum australe stelligerum, etc., published in 1763. In
+this he shows that in the course of a single year he had observed some
+ten thousand stars, and computed the places of one thousand nine hundred
+and forty-two of them, measured a degree of the meridian, and made
+many observations of the moon--productive industry seldom equalled in
+a single year in any field. These observations were of great service to
+the astronomers, as they afforded the opportunity of comparing the stars
+of the southern hemisphere with those of the northern, which were being
+observed simultaneously by Lelande at Berlin.
+
+Lacaille's observations followed closely upon the determination of an
+absorbing question which occupied the attention of the astronomers in
+the early part of the century. This question was as to the shape of the
+earth--whether it was actually flattened at the poles. To settle this
+question once for all the Academy of Sciences decided to make the
+actual measurement of the length of two degrees, one as near the pole
+as possible, the other at the equator. Accordingly, three astronomers,
+Godin, Bouguer, and La Condamine, made the journey to a spot on the
+equator in Peru, while four astronomers, Camus, Clairaut, Maupertuis,
+and Lemonnier, made a voyage to a place selected in Lapland. The result
+of these expeditions was the determination that the globe is oblately
+spheroidal.
+
+A great contemporary and fellow-countryman of Lacaille was Jean Le Rond
+d'Alembert (1717-1783), who, although not primarily an astronomer, did
+so much with his mathematical calculations to aid that science that
+his name is closely connected with its progress during the eighteenth
+century. D'Alembert, who became one of the best-known men of science of
+his day, and whose services were eagerly sought by the rulers of Europe,
+began life as a foundling, having been exposed in one of the markets of
+Paris. The sickly infant was adopted and cared for in the family of a
+poor glazier, and treated as a member of the family. In later years,
+however, after the foundling had become famous throughout Europe, his
+mother, Madame Tencin, sent for him, and acknowledged her relationship.
+It is more than likely that the great philosopher believed her story,
+but if so he did not allow her the satisfaction of knowing his belief,
+declaring always that Madame Tencin could "not be nearer than a
+step-mother to him, since his mother was the wife of the glazier."
+
+D'Alembert did much for the cause of science by his example as well
+as by his discoveries. By living a plain but honest life, declining
+magnificent offers of positions from royal patrons, at the same time
+refusing to grovel before nobility, he set a worthy example to other
+philosophers whose cringing and pusillanimous attitude towards persons
+of wealth or position had hitherto earned them the contempt of the upper
+classes.
+
+His direct additions to astronomy are several, among others the
+determination of the mutation of the axis of the earth. He also
+determined the ratio of the attractive forces of the sun and moon,
+which he found to be about as seven to three. From this he reached the
+conclusion that the earth must be seventy times greater than the moon.
+The first two volumes of his Researches on the Systems of the World,
+published in 1754, are largely devoted to mathematical and astronomical
+problems, many of them of little importance now, but of great interest
+to astronomers at that time.
+
+Another great contemporary of D'Alembert, whose name is closely
+associated and frequently confounded with his, was Jean Baptiste Joseph
+Delambre (1749-1822). More fortunate in birth as also in his educational
+advantages, Delambre as a youth began his studies under the celebrated
+poet Delille. Later he was obliged to struggle against poverty,
+supporting himself for a time by making translations from Latin, Greek,
+Italian, and English, and acting as tutor in private families. The
+turning-point of his fortune came when the attention of Lalande was
+called to the young man by his remarkable memory, and Lalande soon
+showed his admiration by giving Delambre certain difficult astronomical
+problems to solve. By performing these tasks successfully his future as
+an astronomer became assured. At that time the planet Uranus had just
+been discovered by Herschel, and the Academy of Sciences offered as the
+subject for one of its prizes the determination of the planet's orbit.
+Delambre made this determination and won the prize--a feat that brought
+him at once into prominence.
+
+By his writings he probably did as much towards perfecting modern
+astronomy as any one man. His History of Astronomy is not merely a
+narrative of progress of astronomy but a complete abstract of all the
+celebrated works written on the subject. Thus he became famous as an
+historian as well as an astronomer.
+
+
+LEONARD EULER
+
+Still another contemporary of D'Alembert and Delambre, and somewhat
+older than either of them, was Leonard Euler (1707-1783), of Basel,
+whose fame as a philosopher equals that of either of the great
+Frenchmen. He is of particular interest here in his capacity of
+astronomer, but astronomy was only one of the many fields of science in
+which he shone. Surely something out of the ordinary was to be expected
+of the man who could "repeat the AEneid of Virgil from the beginning
+to the end without hesitation, and indicate the first and last line of
+every page of the edition which he used." Something was expected, and he
+fulfilled these expectations.
+
+In early life he devoted himself to the study of theology and the
+Oriental languages, at the request of his father, but his love of
+mathematics proved too strong, and, with his father's consent, he
+finally gave up his classical studies and turned to his favorite study,
+geometry. In 1727 he was invited by Catharine I. to reside in St.
+Petersburg, and on accepting this invitation he was made an associate
+of the Academy of Sciences. A little later he was made professor of
+physics, and in 1733 professor of mathematics. In 1735 he solved a
+problem in three days which some of the eminent mathematicians would not
+undertake under several months. In 1741 Frederick the Great invited him
+to Berlin, where he soon became a member of the Academy of Sciences and
+professor of mathematics; but in 1766 he returned to St. Petersburg.
+Towards the close of his life he became virtually blind, being obliged
+to dictate his thoughts, sometimes to persons entirely ignorant of the
+subject in hand. Nevertheless, his remarkable memory, still further
+heightened by his blindness, enabled him to carry out the elaborate
+computations frequently involved.
+
+Euler's first memoir, transmitted to the Academy of Sciences of Paris
+in 1747, was on the planetary perturbations. This memoir carried off the
+prize that had been offered for the analytical theory of the motions of
+Jupiter and Saturn. Other memoirs followed, one in 1749 and another in
+1750, with further expansions of the same subject. As some slight
+errors were found in these, such as a mistake in some of the formulae
+expressing the secular and periodic inequalities, the academy proposed
+the same subject for the prize of 1752. Euler again competed, and won
+this prize also. The contents of this memoir laid the foundation for
+the subsequent demonstration of the permanent stability of the planetary
+system by Laplace and Lagrange.
+
+It was Euler also who demonstrated that within certain fixed limits
+the eccentricities and places of the aphelia of Saturn and Jupiter are
+subject to constant variation, and he calculated that after a lapse
+of about thirty thousand years the elements of the orbits of these two
+planets recover their original values.
+
+
+
+
+II. THE PROGRESS OF MODERN ASTRONOMY
+
+
+A NEW epoch in astronomy begins with the work of William Herschel, the
+Hanoverian, whom England made hers by adoption. He was a man with a
+positive genius for sidereal discovery. At first a mere amateur in
+astronomy, he snatched time from his duties as music-teacher to grind
+him a telescopic mirror, and began gazing at the stars. Not content with
+his first telescope, he made another and another, and he had such genius
+for the work that he soon possessed a better instrument than was ever
+made before. His patience in grinding the curved reflective surface was
+monumental. Sometimes for sixteen hours together he must walk steadily
+about the mirror, polishing it, without once removing his hands.
+Meantime his sister, always his chief lieutenant, cheered him with her
+presence, and from time to time put food into his mouth. The telescope
+completed, the astronomer turned night into day, and from sunset to
+sunrise, year in and year out, swept the heavens unceasingly, unless
+prevented by clouds or the brightness of the moon. His sister sat always
+at his side, recording his observations. They were in the open air,
+perched high at the mouth of the reflector, and sometimes it was so cold
+that the ink froze in the bottle in Caroline Herschel's hand; but the
+two enthusiasts hardly noticed a thing so common-place as terrestrial
+weather. They were living in distant worlds.
+
+The results? What could they be? Such enthusiasm would move mountains.
+But, after all, the moving of mountains seems a liliputian task compared
+with what Herschel really did with those wonderful telescopes. He moved
+worlds, stars, a universe--even, if you please, a galaxy of universes;
+at least he proved that they move, which seems scarcely less wonderful;
+and he expanded the cosmos, as man conceives it, to thousands of times
+the dimensions it had before. As a mere beginning, he doubled the
+diameter of the solar system by observing the great outlying planet
+which we now call Uranus, but which he christened Georgium Sidus,
+in honor of his sovereign, and which his French contemporaries, not
+relishing that name, preferred to call Herschel.
+
+This discovery was but a trifle compared with what Herschel did later
+on, but it gave him world-wide reputation none the less. Comets and
+moons aside, this was the first addition to the solar system that had
+been made within historic times, and it created a veritable furor of
+popular interest and enthusiasm. Incidentally King George was flattered
+at having a world named after him, and he smiled on the astronomer, and
+came with his court to have a look at his namesake. The inspection
+was highly satisfactory; and presently the royal favor enabled the
+astronomer to escape the thraldom of teaching music and to devote his
+entire time to the more congenial task of star-gazing.
+
+Thus relieved from the burden of mundane embarrassments, he turned with
+fresh enthusiasm to the skies, and his discoveries followed one another
+in bewildering profusion. He found various hitherto unseen moons of our
+sister planets; he made special studies of Saturn, and proved that this
+planet, with its rings, revolves on its axis; he scanned the spots on
+the sun, and suggested that they influence the weather of our earth; in
+short, he extended the entire field of solar astronomy. But very soon
+this field became too small for him, and his most important researches
+carried him out into the regions of space compared with which the span
+of our solar system is a mere point. With his perfected telescopes he
+entered abysmal vistas which no human eve ever penetrated before, which
+no human mind had hitherto more than vaguely imagined. He tells us that
+his forty-foot reflector will bring him light from a distance of "at
+least eleven and three-fourths millions of millions of millions of
+miles"--light which left its source two million years ago. The smallest
+stars visible to the unaided eye are those of the sixth magnitude; this
+telescope, he thinks, has power to reveal stars of the 1342d magnitude.
+
+But what did Herschel learn regarding these awful depths of space and
+the stars that people them? That was what the world wished to know.
+Copernicus, Galileo, Kepler, had given us a solar system, but the stars
+had been a mystery. What says the great reflector--are the stars points
+of light, as the ancients taught, and as more than one philosopher of
+the eighteenth century has still contended, or are they suns, as others
+hold? Herschel answers, they are suns, each and every one of all the
+millions--suns, many of them, larger than the one that is the centre of
+our tiny system. Not only so, but they are moving suns. Instead of
+being fixed in space, as has been thought, they are whirling in gigantic
+orbits about some common centre. Is our sun that centre? Far from it.
+Our sun is only a star like all the rest, circling on with its attendant
+satellites--our giant sun a star, no different from myriad other stars,
+not even so large as some; a mere insignificant spark of matter in an
+infinite shower of sparks.
+
+Nor is this all. Looking beyond the few thousand stars that are visible
+to the naked eye, Herschel sees series after series of more distant
+stars, marshalled in galaxies of millions; but at last he reaches a
+distance beyond which the galaxies no longer increase. And yet--so he
+thinks--he has not reached the limits of his vision. What then? He has
+come to the bounds of the sidereal system--seen to the confines of the
+universe. He believes that he can outline this system, this universe,
+and prove that it has the shape of an irregular globe, oblately
+flattened to almost disklike proportions, and divided at one edge--a
+bifurcation that is revealed even to the naked eye in the forking of the
+Milky Way.
+
+This, then, is our universe as Herschel conceives it--a vast galaxy
+of suns, held to one centre, revolving, poised in space. But even
+here those marvellous telescopes do not pause. Far, far out beyond the
+confines of our universe, so far that the awful span of our own system
+might serve as a unit of measure, are revealed other systems, other
+universes, like our own, each composed, as he thinks, of myriads of
+suns, clustered like our galaxy into an isolated system--mere islands of
+matter in an infinite ocean of space. So distant from our universe are
+these now universes of Herschel's discovery that their light reaches
+us only as a dim, nebulous glow, in most cases invisible to the unaided
+eye. About a hundred of these nebulae were known when Herschel began
+his studies. Before the close of the century he had discovered about
+two thousand more of them, and many of these had been resolved by his
+largest telescopes into clusters of stars. He believed that the farthest
+of these nebulae that he could see was at least three hundred thousand
+times as distant from us as the nearest fixed star. Yet that nearest
+star--so more recent studies prove--is so remote that its light,
+travelling one hundred and eighty thousand miles a second, requires
+three and one-half years to reach our planet.
+
+As if to give the finishing touches to this novel scheme of cosmology,
+Herschel, though in the main very little given to unsustained
+theorizing, allows himself the privilege of one belief that he cannot
+call upon his telescope to substantiate. He thinks that all the myriad
+suns of his numberless systems are instinct with life in the human
+sense. Giordano Bruno and a long line of his followers had held that
+some of our sister planets may be inhabited, but Herschel extends
+the thought to include the moon, the sun, the stars--all the heavenly
+bodies. He believes that he can demonstrate the habitability of our own
+sun, and, reasoning from analogy, he is firmly convinced that all the
+suns of all the systems are "well supplied with inhabitants." In this,
+as in some other inferences, Herschel is misled by the faulty physics
+of his time. Future generations, working with perfected instruments, may
+not sustain him all along the line of his observations, even, let alone
+his inferences. But how one's egotism shrivels and shrinks as one grasps
+the import of his sweeping thoughts!
+
+Continuing his observations of the innumerable nebulae, Herschel is led
+presently to another curious speculative inference. He notes that some
+star groups are much more thickly clustered than others, and he is
+led to infer that such varied clustering tells of varying ages of the
+different nebulae. He thinks that at first all space may have been
+evenly sprinkled with the stars and that the grouping has resulted from
+the action of gravitation.
+
+"That the Milky Way is a most extensive stratum of stars of various
+sizes admits no longer of lasting doubt," he declares, "and that our sun
+is actually one of the heavenly bodies belonging to it is as evident. I
+have now viewed and gauged this shining zone in almost every direction
+and find it composed of stars whose number... constantly increases and
+decreases in proportion to its apparent brightness to the naked eye.
+
+"Let us suppose numberless stars of various sizes, scattered over an
+indefinite portion of space in such a manner as to be almost equally
+distributed throughout the whole. The laws of attraction which no doubt
+extend to the remotest regions of the fixed stars will operate in such a
+manner as most probably to produce the following effects:
+
+"In the first case, since we have supposed the stars to be of various
+sizes, it will happen that a star, being considerably larger than its
+neighboring ones, will attract them more than they will be attracted by
+others that are immediately around them; by which means they will be,
+in time, as it were, condensed about a centre, or, in other words, form
+themselves into a cluster of stars of almost a globular figure, more
+or less regular according to the size and distance of the surrounding
+stars....
+
+"The next case, which will also happen almost as frequently as the
+former, is where a few stars, though not superior in size to the rest,
+may chance to be rather nearer one another than the surrounding ones,...
+and this construction admits of the utmost variety of shapes....
+
+"From the composition and repeated conjunction of both the foregoing
+formations, a third may be derived when many large stars, or combined
+small ones, are spread in long, extended, regular, or crooked rows,
+streaks, or branches; for they will also draw the surrounding stars, so
+as to produce figures of condensed stars curiously similar to the former
+which gave rise to these condensations.
+
+"We may likewise admit still more extensive combinations; when, at the
+same time that a cluster of stars is forming at the one part of
+space, there may be another collection in a different but perhaps not
+far-distant quarter, which may occasion a mutual approach towards their
+own centre of gravity.
+
+"In the last place, as a natural conclusion of the former cases, there
+will be formed great cavities or vacancies by the retreating of the
+stars towards the various centres which attract them."(1)
+
+
+Looking forward, it appears that the time must come when all the suns
+of a system will be drawn together and destroyed by impact at a common
+centre. Already, it seems to Herschel, the thickest clusters have
+"outlived their usefulness" and are verging towards their doom.
+
+But again, other nebulae present an appearance suggestive of an opposite
+condition. They are not resolvable into stars, but present an almost
+uniform appearance throughout, and are hence believed to be composed of
+a shining fluid, which in some instances is seen to be condensed at the
+centre into a glowing mass. In such a nebula Herschel thinks he sees a
+sun in process of formation.
+
+
+THE NEBULAR HYPOTHESIS OF KANT
+
+Taken together, these two conceptions outline a majestic cycle of world
+formation and world destruction--a broad scheme of cosmogony, such as
+had been vaguely adumbrated two centuries before by Kepler and in
+more recent times by Wright and Swedenborg. This so-called "nebular
+hypothesis" assumes that in the beginning all space was uniformly filled
+with cosmic matter in a state of nebular or "fire-mist" diffusion,
+"formless and void." It pictures the condensation--coagulation, if
+you will--of portions of this mass to form segregated masses, and the
+ultimate development out of these masses of the sidereal bodies that we
+see.
+
+Perhaps the first elaborate exposition of this idea was that given by
+the great German philosopher Immanuel Kant (born at Konigsberg in 1724,
+died in 1804), known to every one as the author of the Critique of Pure
+Reason. Let us learn from his own words how the imaginative philosopher
+conceived the world to have come into existence.
+
+"I assume," says Kant, "that all the material of which the globes
+belonging to our solar system--all the planets and comets--consist, at
+the beginning of all things was decomposed into its primary elements,
+and filled the whole space of the universe in which the bodies formed
+out of it now revolve. This state of nature, when viewed in and by
+itself without any reference to a system, seems to be the very simplest
+that can follow upon nothing. At that time nothing has yet been formed.
+The construction of heavenly bodies at a distance from one another,
+their distances regulated by their attraction, their form arising out of
+the equilibrium of their collected matter, exhibit a later state.... In
+a region of space filled in this manner, a universal repose could last
+only a moment. The elements have essential forces with which to put
+each other in motion, and thus are themselves a source of life. Matter
+immediately begins to strive to fashion itself. The scattered elements
+of a denser kind, by means of their attraction, gather from a sphere
+around them all the matter of less specific gravity; again, these
+elements themselves, together with the material which they have united
+with them, collect in those points where the particles of a still denser
+kind are found; these in like manner join still denser particles, and
+so on. If we follow in imagination this process by which nature fashions
+itself into form through the whole extent of chaos, we easily perceive
+that all the results of the process would consist in the formation of
+divers masses which, when their formation was complete, would by the
+equality of their attraction be at rest and be forever unmoved.
+
+"But nature has other forces in store which are specially exerted when
+matter is decomposed into fine particles. They are those forces by which
+these particles repel one another, and which, by their conflict with
+attractions, bring forth that movement which is, as it were, the lasting
+life of nature. This force of repulsion is manifested in the elasticity
+of vapors, the effluences of strong-smelling bodies, and the diffusion
+of all spirituous matters. This force is an uncontestable phenomenon of
+matter. It is by it that the elements, which may be falling to the point
+attracting them, are turned sideways promiscuously from their movement
+in a straight line; and their perpendicular fall thereby issues in
+circular movements, which encompass the centre towards which they were
+falling. In order to make the formation of the world more distinctly
+conceivable, we will limit our view by withdrawing it from the infinite
+universe of nature and directing it to a particular system, as the
+one which belongs to our sun. Having considered the generation of this
+system, we shall be able to advance to a similar consideration of the
+origin of the great world-systems, and thus to embrace the infinitude of
+the whole creation in one conception.
+
+"From what has been said, it will appear that if a point is situated in
+a very large space where the attraction of the elements there situated
+acts more strongly than elsewhere, then the matter of the elementary
+particles scattered throughout the whole region will fall to that point.
+The first effect of this general fall is the formation of a body at this
+centre of attraction, which, so to speak, grows from an infinitely
+small nucleus by rapid strides; and in the proportion in which this mass
+increases, it also draws with greater force the surrounding particles
+to unite with it. When the mass of this central body has grown so great
+that the velocity with which it draws the particles to itself with great
+distances is bent sideways by the feeble degree of repulsion with which
+they impede one another, and when it issues in lateral movements which
+are capable by means of the centrifugal force of encompassing the
+central body in an orbit, then there are produced whirls or vortices
+of particles, each of which by itself describes a curved line by the
+composition of the attracting force and the force of revolution that had
+been bent sideways. These kinds of orbits all intersect one another,
+for which their great dispersion in this space gives place. Yet these
+movements are in many ways in conflict with one another, and they
+naturally tend to bring one another to a uniformity--that is, into a
+state in which one movement is as little obstructive to the other as
+possible. This happens in two ways: first by the particles limiting
+one another's movement till they all advance in one direction; and,
+secondly, in this way, that the particles limit their vertical movements
+in virtue of which they are approaching the centre of attraction, till
+they all move horizontally--i. e., in parallel circles round the sun as
+their centre, no longer intercept one another, and by the centrifugal
+force becoming equal with the falling force they keep themselves
+constantly in free circular orbits at the distance at which they move.
+The result, finally, is that only those particles continue to move in
+this region of space which have acquired by their fall a velocity, and
+through the resistance of the other particles a direction, by which they
+can continue to maintain a FREE CIRCULAR MOVEMENT....
+
+"The view of the formation of the planets in this system has the
+advantage over every other possible theory in holding that the origin
+of the movements, and the position of the orbits in arising at that same
+point of time--nay, more, in showing that even the deviations from the
+greatest possible exactness in their determinations, as well as the
+accordances themselves, become clear at a glance. The planets are formed
+out of particles which, at the distance at which they move, have exact
+movements in circular orbits; and therefore the masses composed out of
+them will continue the same movements and at the same rate and in the
+same direction."(2)
+
+
+It must be admitted that this explanation leaves a good deal to be
+desired. It is the explanation of a metaphysician rather than that of
+an experimental scientist. Such phrases as "matter immediately begins to
+strive to fashion itself," for example, have no place in the reasoning
+of inductive science. Nevertheless, the hypothesis of Kant is a
+remarkable conception; it attempts to explain along rational lines
+something which hitherto had for the most part been considered
+altogether inexplicable.
+
+But there are various questions that at once suggest themselves which
+the Kantian theory leaves unanswered. How happens it, for example, that
+the cosmic mass which gave birth to our solar system was divided into
+several planetary bodies instead of remaining a single mass? Were the
+planets struck from the sun by the chance impact of comets, as Buffon
+has suggested? or thrown out by explosive volcanic action, in accordance
+with the theory of Dr. Darwin? or do they owe their origin to some
+unknown law? In any event, how chanced it that all were projected in
+nearly the same plane as we now find them?
+
+
+LAPLACE AND THE NEBULAR HYPOTHESIS
+
+It remained for a mathematical astronomer to solve these puzzles. The
+man of all others competent to take the subject in hand was the French
+astronomer Laplace. For a quarter of a century he had devoted his
+transcendent mathematical abilities to the solution of problems of
+motion of the heavenly bodies. Working in friendly rivalry with his
+countryman Lagrange, his only peer among the mathematicians of the age,
+he had taken up and solved one by one the problems that Newton left
+obscure. Largely through the efforts of these two men the last lingering
+doubts as to the solidarity of the Newtonian hypothesis of universal
+gravitation had been removed. The share of Lagrange was hardly less than
+that of his co-worker; but Laplace will longer be remembered, because
+he ultimately brought his completed labors into a system, and,
+incorporating with them the labors of his contemporaries, produced
+in the Mecanique Celeste the undisputed mathematical monument of the
+century, a fitting complement to the Principia of Newton, which it
+supplements and in a sense completes.
+
+In the closing years of the eighteenth century Laplace took up the
+nebular hypothesis of cosmogony, to which we have just referred, and
+gave it definite proportions; in fact, made it so thoroughly his own
+that posterity will always link it with his name. Discarding the crude
+notions of cometary impact and volcanic eruption, Laplace filled up the
+gaps in the hypothesis with the aid of well-known laws of gravitation
+and motion. He assumed that the primitive mass of cosmic matter which
+was destined to form our solar system was revolving on its axis even at
+a time when it was still nebular in character, and filled all space to
+a distance far beyond the present limits of the system. As this vaporous
+mass contracted through loss of heat, it revolved more and more swiftly,
+and from time to time, through balance of forces at its periphery, rings
+of its substance were whirled off and left revolving there, subsequently
+to become condensed into planets, and in their turn whirl off minor
+rings that became moons. The main body of the original mass remains in
+the present as the still contracting and rotating body which we call the
+sun.
+
+Let us allow Laplace to explain all this in detail:
+
+"In order to explain the prime movements of the planetary system,"
+he says, "there are the five following phenomena: The movement of the
+planets in the same direction and very nearly in the same plane; the
+movement of the satellites in the same direction as that of the planets;
+the rotation of these different bodies and the sun in the same
+direction as their revolution, and in nearly the same plane; the slight
+eccentricity of the orbits of the planets and of the satellites; and,
+finally, the great eccentricity of the orbits of the comets, as if their
+inclinations had been left to chance.
+
+"Buffon is the only man I know who, since the discovery of the true
+system of the world, has endeavored to show the origin of the planets
+and their satellites. He supposes that a comet, in falling into the sun,
+drove from it a mass of matter which was reassembled at a distance in
+the form of various globes more or less large, and more or less removed
+from the sun, and that these globes, becoming opaque and solid, are now
+the planets and their satellites.
+
+"This hypothesis satisfies the first of the five preceding phenomena;
+for it is clear that all the bodies thus formed would move very nearly
+in the plane which passed through the centre of the sun, and in the
+direction of the torrent of matter which was produced; but the four
+other phenomena appear to be inexplicable to me by this means. Indeed,
+the absolute movement of the molecules of a planet ought then to be in
+the direction of the movement of its centre of gravity; but it does not
+at all follow that the motion of the rotation of the planets should be
+in the same direction. Thus the earth should rotate from east to west,
+but nevertheless the absolute movement of its molecules should be
+from east to west; and this ought also to apply to the movement of the
+revolution of the satellites, in which the direction, according to the
+hypothesis which he offers, is not necessarily the same as that of the
+progressive movement of the planets.
+
+"A phenomenon not only very difficult to explain under this hypothesis,
+but one which is even contrary to it, is the slight eccentricity of the
+planetary orbits. We know, by the theory of central forces, that if
+a body moves in a closed orbit around the sun and touches it, it also
+always comes back to that point at every revolution; whence it follows
+that if the planets were originally detached from the sun, they would
+touch it at each return towards it, and their orbits, far from being
+circular, would be very eccentric. It is true that a mass of matter
+driven from the sun cannot be exactly compared to a globe which touches
+its surface, for the impulse which the particles of this mass receive
+from one another and the reciprocal attractions which they exert among
+themselves, could, in changing the direction of their movements, remove
+their perihelions from the sun; but their orbits would be always most
+eccentric, or at least they would not have slight eccentricities except
+by the most extraordinary chance. Thus we cannot see, according to
+the hypothesis of Buffon, why the orbits of more than a hundred comets
+already observed are so elliptical. This hypothesis is therefore
+very far from satisfying the preceding phenomena. Let us see if it is
+possible to trace them back to their true cause.
+
+"Whatever may be its ultimate nature, seeing that it has caused or
+modified the movements of the planets, it is necessary that this cause
+should embrace every body, and, in view of the enormous distances which
+separate them, it could only have been a fluid of immense extent.
+In order to have given them an almost circular movement in the same
+direction around the sun, it is necessary that this fluid should
+have enveloped the sun as in an atmosphere. The consideration of the
+planetary movements leads us then to think that, on account of excessive
+heat, the atmosphere of the sun originally extended beyond the orbits of
+all the planets, and that it was successively contracted to its present
+limits.
+
+"In the primitive condition in which we suppose the sun to have been, it
+resembled a nebula such as the telescope shows is composed of a nucleus
+more or less brilliant, surrounded by a nebulosity which, on condensing
+itself towards the centre, forms a star. If it is conceived by analogy
+that all the stars were formed in this manner, it is possible to imagine
+their previous condition of nebulosity, itself preceded by other states
+in which the nebulous matter was still more diffused, the nucleus being
+less and less luminous. By going back as far as possible, we thus
+arrive at a nebulosity so diffused that its existence could hardly be
+suspected.
+
+"For a long time the peculiar disposition of certain stars, visible
+to the unaided eye, has struck philosophical observers. Mitchell
+has already remarked how little probable it is that the stars in the
+Pleiades, for example, could have been contracted into the small
+space which encloses them by the fortuity of chance alone, and he has
+concluded that this group of stars, and similar groups which the skies
+present to us, are the necessary result of the condensation of a nebula,
+with several nuclei, and it is evident that a nebula, by continually
+contracting, towards these various nuclei, at length would form a group
+of stars similar to the Pleiades. The condensation of a nebula with two
+nuclei would form a system of stars close together, turning one upon
+the other, such as those double stars of which we already know the
+respective movements.
+
+"But how did the solar atmosphere determine the movements of the
+rotation and revolution of the planets and satellites? If these bodies
+had penetrated very deeply into this atmosphere, its resistance would
+have caused them to fall into the sun. We can therefore conjecture that
+the planets were formed at their successive limits by the condensation
+of a zone of vapors which the sun, on cooling, left behind, in the plane
+of his equator.
+
+"Let us recall the results which we have given in a preceding chapter.
+The atmosphere of the sun could not have extended indefinitely. Its
+limit was the point where the centrifugal force due to its movement
+of rotation balanced its weight. But in proportion as the cooling
+contracted the atmosphere, and those molecules which were near to them
+condensed upon the surface of the body, the movement of the rotation
+increased; for, on account of the Law of Areas, the sum of the areas
+described by the vector of each molecule of the sun and its atmosphere
+and projected in the plane of the equator being always the same, the
+rotation should increase when these molecules approach the centre of the
+sun. The centrifugal force due to this movement becoming thus larger,
+the point where the weight is equal to it is nearer the sun. Supposing,
+then, as it is natural to admit, that the atmosphere extended at some
+period to its very limits, it should, on cooling, leave molecules behind
+at this limit and at limits successively occasioned by the increased
+rotation of the sun. The abandoned molecules would continue to revolve
+around this body, since their centrifugal force was balanced by their
+weight. But this equilibrium not arising in regard to the atmospheric
+molecules parallel to the solar equator, the latter, on account of their
+weight, approached the atmosphere as they condensed, and did not cease
+to belong to it until by this motion they came upon the equator.
+
+"Let us consider now the zones of vapor successively left behind. These
+zones ought, according to appearance, by the condensation and mutual
+attraction of their molecules, to form various concentric rings of vapor
+revolving around the sun. The mutual gravitational friction of each ring
+would accelerate some and retard others, until they had all acquired the
+same angular velocity. Thus the actual velocity of the molecules most
+removed from the sun would be the greatest. The following cause would
+also operate to bring about this difference of speed. The molecules
+farthest from the sun, and which by the effects of cooling and
+condensation approached one another to form the outer part of the ring,
+would have always described areas proportional to the time since the
+central force by which they were controlled has been constantly directed
+towards this body. But this constancy of areas necessitates an increase
+of velocity proportional to the distance. It is thus seen that the same
+cause would diminish the velocity of the molecules which form the inner
+part of the ring.
+
+"If all the molecules of the ring of vapor continued to condense without
+disuniting, they would at length form a ring either solid or fluid. But
+this formation would necessitate such a regularity in every part of the
+ring, and in its cooling, that this phenomenon is extremely rare; and
+the solar system affords us, indeed, but one example--namely, in the
+ring of Saturn. In nearly every case the ring of vapor was broken into
+several masses, each moving at similar velocities, and continuing to
+rotate at the same distance around the sun. These masses would take
+a spheroid form with a rotatory movement in the direction of the
+revolution, because their inner molecules had less velocity than the
+outer. Thus were formed so many planets in a condition of vapor. But
+if one of them were powerful enough to reunite successively by its
+attraction all the others around its centre of gravity, the ring of
+vapor would be thus transformed into a single spheroidical mass of
+vapor revolving around the sun with a rotation in the direction of its
+revolution. The latter case has been that which is the most common, but
+nevertheless the solar system affords us an instance of the first case
+in the four small planets which move between Jupiter and Mars; at least,
+if we do not suppose, as does M. Olbers, that they originally formed a
+single planet which a mighty explosion broke up into several portions
+each moving at different velocities.
+
+"According to our hypothesis, the comets are strangers to our planetary
+system. In considering them, as we have done, as minute nebulosities,
+wandering from solar system to solar system, and formed by the
+condensation of the nebulous matter everywhere existent in profusion in
+the universe, we see that when they come into that part of the heavens
+where the sun is all-powerful, he forces them to describe orbits either
+elliptical or hyperbolic, their paths being equally possible in all
+directions, and at all inclinations of the ecliptic, conformably to what
+has been observed. Thus the condensation of nebulous matter, by which
+we have at first explained the motions of the rotation and revolution
+of the planets and their satellites in the same direction, and in nearly
+approximate planes, explains also why the movements of the comets escape
+this general law."(3)
+
+
+The nebular hypothesis thus given detailed completion by Laplace is a
+worthy complement of the grand cosmologic scheme of Herschel. Whether
+true or false, the two conceptions stand as the final contributions
+of the eighteenth century to the history of man's ceaseless efforts to
+solve the mysteries of cosmic origin and cosmic structure. The world
+listened eagerly and without prejudice to the new doctrines; and that
+attitude tells of a marvellous intellectual growth of our race. Mark the
+transition. In the year 1600, Bruno was burned at the stake for teaching
+that our earth is not the centre of the universe. In 1700, Newton was
+pronounced "impious and heretical" by a large school of philosophers
+for declaring that the force which holds the planets in their orbits
+is universal gravitation. In 1800, Laplace and Herschel are honored for
+teaching that gravitation built up the system which it still controls;
+that our universe is but a minor nebula, our sun but a minor star, our
+earth a mere atom of matter, our race only one of myriad races peopling
+an infinity of worlds. Doctrines which but the span of two human lives
+before would have brought their enunciators to the stake were now
+pronounced not impious, but sublime.
+
+
+ASTEROIDS AND SATELLITES
+
+The first day of the nineteenth century was fittingly signalized by the
+discovery of a new world. On the evening of January 1, 1801, an Italian
+astronomer, Piazzi, observed an apparent star of about the eighth
+magnitude (hence, of course, quite invisible to the unaided eye), which
+later on was seen to have moved, and was thus shown to be vastly nearer
+the earth than any true star. He at first supposed, as Herschel had
+done when he first saw Uranus, that the unfamiliar body was a comet; but
+later observation proved it a tiny planet, occupying a position in space
+between Mars and Jupiter. It was christened Ceres, after the tutelary
+goddess of Sicily.
+
+Though unpremeditated, this discovery was not unexpected, for
+astronomers had long surmised the existence of a planet in the wide
+gap between Mars and Jupiter. Indeed, they were even preparing to make
+concerted search for it, despite the protests of philosophers, who
+argued that the planets could not possibly exceed the magic number
+seven, when Piazzi forestalled their efforts. But a surprise came
+with the sequel; for the very next year Dr. Olbers, the wonderful
+physician-astronomer of Bremen, while following up the course of Ceres,
+happened on another tiny moving star, similarly located, which soon
+revealed itself as planetary. Thus two planets were found where only one
+was expected.
+
+The existence of the supernumerary was a puzzle, but Olbers solved it
+for the moment by suggesting that Ceres and Pallas, as he called his
+captive, might be fragments of a quondam planet, shattered by internal
+explosion or by the impact of a comet. Other similar fragments, he
+ventured to predict, would be found when searched for. William Herschel
+sanctioned this theory, and suggested the name asteroids for the tiny
+planets. The explosion theory was supported by the discovery of another
+asteroid, by Harding, of Lilienthal, in 1804, and it seemed clinched
+when Olbers himself found a fourth in 1807. The new-comers were named
+Juno and Vesta respectively.
+
+There the case rested till 1845, when a Prussian amateur astronomer
+named Hencke found another asteroid, after long searching, and opened a
+new epoch of discovery. From then on the finding of asteroids became a
+commonplace. Latterly, with the aid of photography, the list has been
+extended to above four hundred, and as yet there seems no dearth in the
+supply, though doubtless all the larger members have been revealed. Even
+these are but a few hundreds of miles in diameter, while the smaller
+ones are too tiny for measurement. The combined bulk of these minor
+planets is believed to be but a fraction of that of the earth.
+
+Olbers's explosion theory, long accepted by astronomers, has been
+proven open to fatal objections. The minor planets are now believed to
+represent a ring of cosmical matter, cast off from the solar nebula
+like the rings that went to form the major planets, but prevented
+from becoming aggregated into a single body by the perturbing mass of
+Jupiter.
+
+
+The Discovery of Neptune
+
+As we have seen, the discovery of the first asteroid confirmed a
+conjecture; the other important planetary discovery of the nineteenth
+century fulfilled a prediction. Neptune was found through scientific
+prophecy. No one suspected the existence of a trans-Uranian planet till
+Uranus itself, by hair-breadth departures from its predicted orbit, gave
+out the secret. No one saw the disturbing planet till the pencil of the
+mathematician, with almost occult divination, had pointed out its place
+in the heavens. The general predication of a trans-Uranian planet was
+made by Bessel, the great Konigsberg astronomer, in 1840; the analysis
+that revealed its exact location was undertaken, half a decade later,
+by two independent workers--John Couch Adams, just graduated senior
+wrangler at Cambridge, England, and U. J. J. Leverrier, the leading
+French mathematician of his generation.
+
+Adams's calculation was first begun and first completed. But it had one
+radical defect--it was the work of a young and untried man. So it found
+lodgment in a pigeon-hole of the desk of England's Astronomer Royal, and
+an opportunity was lost which English astronomers have never ceased to
+mourn. Had the search been made, an actual planet would have been seen
+shining there, close to the spot where the pencil of the mathematician
+had placed its hypothetical counterpart. But the search was not made,
+and while the prophecy of Adams gathered dust in that regrettable
+pigeon-hole, Leverrier's calculation was coming on, his tentative
+results meeting full encouragement from Arago and other French savants.
+At last the laborious calculations proved satisfactory, and, confident
+of the result, Leverrier sent to the Berlin observatory, requesting that
+search be made for the disturber of Uranus in a particular spot of the
+heavens. Dr. Galle received the request September 23, 1846. That very
+night he turned his telescope to the indicated region, and there, within
+a single degree of the suggested spot, he saw a seeming star, invisible
+to the unaided eye, which proved to be the long-sought planet,
+henceforth to be known as Neptune. To the average mind, which finds
+something altogether mystifying about abstract mathematics, this was a
+feat savoring of the miraculous.
+
+Stimulated by this success, Leverrier calculated an orbit for an
+interior planet from perturbations of Mercury, but though prematurely
+christened Vulcan, this hypothetical nursling of the sun still haunts
+the realm of the undiscovered, along with certain equally hypothetical
+trans-Neptunian planets whose existence has been suggested by "residual
+perturbations" of Uranus, and by the movements of comets. No other
+veritable additions of the sun's planetary family have been made in our
+century, beyond the finding of seven small moons, which chiefly attest
+the advance in telescopic powers. Of these, the tiny attendants of our
+Martian neighbor, discovered by Professor Hall with the great Washington
+refractor, are of greatest interest, because of their small size and
+extremely rapid flight. One of them is poised only six thousand
+miles from Mars, and whirls about him almost four times as fast as he
+revolves, seeming thus, as viewed by the Martian, to rise in the west
+and set in the east, and making the month only one-fourth as long as the
+day.
+
+
+The Rings of Saturn
+
+The discovery of the inner or crape ring of Saturn, made simultaneously
+in 1850 by William C. Bond, at the Harvard observatory, in America,
+and the Rev. W. R. Dawes in England, was another interesting optical
+achievement; but our most important advances in knowledge of Saturn's
+unique system are due to the mathematician. Laplace, like his
+predecessors, supposed these rings to be solid, and explained their
+stability as due to certain irregularities of contour which Herschel
+bad pointed out. But about 1851 Professor Peirce, of Harvard, showed
+the untenability of this conclusion, proving that were the rings such as
+Laplace thought them they must fall of their own weight. Then Professor
+J. Clerk-Maxwell, of Cambridge, took the matter in hand, and his
+analysis reduced the puzzling rings to a cloud of meteoric particles--a
+"shower of brickbats"--each fragment of which circulates exactly as if
+it were an independent planet, though of course perturbed and jostled
+more or less by its fellows. Mutual perturbations, and the disturbing
+pulls of Saturn's orthodox satellites, as investigated by Maxwell,
+explain nearly all the phenomena of the rings in a manner highly
+satisfactory.
+
+After elaborate mathematical calculations covering many pages of his
+paper entitled "On the Stability of Saturn's Rings," he summarizes his
+deductions as follows:
+
+"Let us now gather together the conclusions we have been able to draw
+from the mathematical theory of various kinds of conceivable rings.
+
+"We found that the stability of the motion of a solid ring depended
+on so delicate an adjustment, and at the same time so unsymmetrical a
+distribution of mass, that even if the exact conditions were fulfilled,
+it could scarcely last long, and, if it did, the immense preponderance
+of one side of the ring would be easily observed, contrary to
+experience. These considerations, with others derived from the
+mechanical structure of so vast a body, compel us to abandon any theory
+of solid rings.
+
+"We next examined the motion of a ring of equal satellites, and found
+that if the mass of the planet is sufficient, any disturbances produced
+in the arrangement of the ring will be propagated around it in the form
+of waves, and will not introduce dangerous confusion. If the satellites
+are unequal, the propagations of the waves will no longer be regular,
+but disturbances of the ring will in this, as in the former case,
+produce only waves, and not growing confusion. Supposing the ring to
+consist, not of a single row of large satellites, but a cloud of evenly
+distributed unconnected particles, we found that such a cloud must
+have a very small density in order to be permanent, and that this is
+inconsistent with its outer and inner parts moving with the same angular
+velocity. Supposing the ring to be fluid and continuous, we found that
+it will be necessarily broken up into small portions.
+
+"We conclude, therefore, that the rings must consist of disconnected
+particles; these must be either solid or liquid, but they must be
+independent. The entire system of rings must, therefore, consist either
+of a series of many concentric rings each moving with its own velocity
+and having its own system of waves, or else of a confused multitude of
+revolving particles not arranged in rings and continually coming into
+collision with one another.
+
+"Taking the first case, we found that in an indefinite number of
+possible cases the mutual perturbations of two rings, stable in
+themselves, might mount up in time to a destructive magnitude, and that
+such cases must continually occur in an extensive system like that of
+Saturn, the only retarding cause being the irregularity of the rings.
+
+"The result of long-continued disturbance was found to be the
+spreading-out of the rings in breadth, the outer rings pressing outward,
+while the inner rings press inward.
+
+"The final result, therefore, of the mechanical theory is that the only
+system of rings which can exist is one composed of an indefinite number
+of unconnected particles, revolving around the planet with different
+velocities, according to their respective distances. These particles
+may be arranged in series of narrow rings, or they may move through one
+another irregularly. In the first case the destruction of the system
+will be very slow, in the second case it will be more rapid, but there
+may be a tendency towards arrangement in narrow rings which may retard
+the process.
+
+"We are not able to ascertain by observation the constitution of the two
+outer divisions of the system of rings, but the inner ring is certainly
+transparent, for the limb of Saturn has been observed through it. It is
+also certain that though the space occupied by the ring is transparent,
+it is not through the material parts of it that the limb of Saturn is
+seen, for his limb was observed without distortion; which shows that
+there was no refraction, and, therefore, that the rays did not pass
+through a medium at all, but between the solar or liquid particles of
+which the ring is composed. Here, then, we have an optical argument
+in favor of the theory of independent particles as the material of
+the rings. The two outer rings may be of the same nature, but not
+so exceedingly rare that a ray of light can pass through their whole
+thickness without encountering one of the particles.
+
+"Finally, the two outer rings have been observed for two hundred years,
+and it appears, from the careful analysis of all the observations of M.
+Struve, that the second ring is broader than when first observed, and
+that its inner edge is nearer the planet than formerly. The inner ring
+also is suspected to be approaching the planet ever since its discovery
+in 1850. These appearances seem to indicate the same slow progress of
+the rings towards separation which we found to be the result of theory,
+and the remark that the inner edge of the inner ring is more distinct
+seems to indicate that the approach towards the planet is less rapid
+near the edge, as we had reason to conjecture. As to the apparent
+unchangeableness of the exterior diameter of the outer ring, we must
+remember that the outer rings are certainly far more dense than the
+inner one, and that a small change in the outer rings must balance a
+great change in the inner one. It is possible, however, that some of the
+observed changes may be due to the existence of a resisting medium.
+If the changes already suspected should be confirmed by repeated
+observations with the same instruments, it will be worth while to
+investigate more carefully whether Saturn's rings are permanent or
+transitory elements of the solar system, and whether in that part of
+the heavens we see celestial immutability or terrestrial corruption
+and generation, and the old order giving place to the new before our
+eyes."(4)
+
+
+Studies of the Moon
+
+But perhaps the most interesting accomplishments of mathematical
+astronomy--from a mundane standpoint, at any rate--are those that refer
+to the earth's own satellite. That seemingly staid body was long ago
+discovered to have a propensity to gain a little on the earth, appearing
+at eclipses an infinitesimal moment ahead of time. Astronomers were
+sorely puzzled by this act of insubordination; but at last Laplace and
+Lagrange explained it as due to an oscillatory change in the earth's
+orbit, thus fully exonerating the moon, and seeming to demonstrate the
+absolute stability of our planetary system, which the moon's misbehavior
+had appeared to threaten.
+
+This highly satisfactory conclusion was an orthodox belief of celestial
+mechanics until 1853, when Professor Adams of Neptunian fame, with whom
+complex analyses were a pastime, reviewed Laplace's calculation, and
+discovered an error which, when corrected, left about half the moon's
+acceleration unaccounted for. This was a momentous discrepancy, which at
+first no one could explain. But presently Professor Helmholtz, the great
+German physicist, suggested that a key might be found in tidal friction,
+which, acting as a perpetual brake on the earth's rotation, and
+affecting not merely the waters but the entire substance of our planet,
+must in the long sweep of time have changed its rate of rotation. Thus
+the seeming acceleration of the moon might be accounted for as actual
+retardation of the earth's rotation--a lengthening of the day instead of
+a shortening of the month.
+
+Again the earth was shown to be at fault, but this time the moon could
+not be exonerated, while the estimated stability of our system, instead
+of being re-established, was quite upset. For the tidal retardation is
+not an oscillatory change which will presently correct itself, like the
+orbital wobble, but a perpetual change, acting always in one direction.
+Unless fully counteracted by some opposing reaction, therefore (as
+it seems not to be), the effect must be cumulative, the ultimate
+consequences disastrous. The exact character of these consequences was
+first estimated by Professor G. H. Darwin in 1879. He showed that tidal
+friction, in retarding the earth, must also push the moon out from the
+parent planet on a spiral orbit. Plainly, then, the moon must formerly
+have been nearer the earth than at present. At some very remote period
+it must have actually touched the earth; must, in other words, have been
+thrown off from the then plastic mass of the earth, as a polyp buds out
+from its parent polyp. At that time the earth was spinning about in a
+day of from two to four hours.
+
+Now the day has been lengthened to twenty-four hours, and the moon has
+been thrust out to a distance of a quarter-million miles; but the end is
+not yet. The same progress of events must continue, till, at some remote
+period in the future, the day has come to equal the month, lunar tidal
+action has ceased, and one face of the earth looks out always at the
+moon with that same fixed stare which even now the moon has been brought
+to assume towards her parent orb. Should we choose to take even greater
+liberties with the future, it may be made to appear (though some
+astronomers dissent from this prediction) that, as solar tidal action
+still continues, the day must finally exceed the month, and lengthen out
+little by little towards coincidence with the year; and that the moon
+meantime must pause in its outward flight, and come swinging back on a
+descending spiral, until finally, after the lapse of untold aeons, it
+ploughs and ricochets along the surface of the earth, and plunges to
+catastrophic destruction.
+
+But even though imagination pause far short of this direful culmination,
+it still is clear that modern calculations, based on inexorable tidal
+friction, suffice to revolutionize the views formerly current as to the
+stability of the planetary system. The eighteenth-century mathematician
+looked upon this system as a vast celestial machine which had been in
+existence about six thousand years, and which was destined to run on
+forever. The analyst of to-day computes both the past and the future of
+this system in millions instead of thousands of years, yet feels well
+assured that the solar system offers no contradiction to those laws of
+growth and decay which seem everywhere to represent the immutable order
+of nature.
+
+
+COMETS AND METEORS
+
+Until the mathematician ferreted out the secret, it surely never could
+have been suspected by any one that the earth's serene attendant,
+
+     "That orbed maiden, with white fire laden,
+     Whom mortals call the moon,"
+
+could be plotting injury to her parent orb. But there is another
+inhabitant of the skies whose purposes have not been similarly free from
+popular suspicion. Needless to say I refer to the black sheep of the
+sidereal family, that "celestial vagabond" the comet.
+
+Time out of mind these wanderers have been supposed to presage war,
+famine, pestilence, perhaps the destruction of the world. And little
+wonder. Here is a body which comes flashing out of boundless space into
+our system, shooting out a pyrotechnic tail some hundreds of millions of
+miles in length; whirling, perhaps, through the very atmosphere of the
+sun at a speed of three or four hundred miles a second; then darting off
+on a hyperbolic orbit that forbids it ever to return, or an elliptical
+one that cannot be closed for hundreds or thousands of years; the tail
+meantime pointing always away from the sun, and fading to nothingness as
+the weird voyager recedes into the spatial void whence it came. Not many
+times need the advent of such an apparition coincide with the outbreak
+of a pestilence or the death of a Caesar to stamp the race of comets as
+an ominous clan in the minds of all superstitious generations.
+
+It is true, a hard blow was struck at the prestige of these alleged
+supernatural agents when Newton proved that the great comet of 1680
+obeyed Kepler's laws in its flight about the sun; and an even harder
+one when the same visitant came back in 1758, obedient to Halley's
+prediction, after its three-quarters of a century of voyaging but in
+the abyss of space. Proved thus to bow to natural law, the celestial
+messenger could no longer fully, sustain its role. But long-standing
+notoriety cannot be lived down in a day, and the comet, though proved a
+"natural" object, was still regarded as a very menacing one for
+another hundred years or so. It remained for the nineteenth century to
+completely unmask the pretender and show how egregiously our forebears
+had been deceived.
+
+The unmasking began early in the century, when Dr. Olbers, then the
+highest authority on the subject, expressed the opinion that
+the spectacular tail, which had all along been the comet's chief
+stock-in-trade as an earth-threatener, is in reality composed of
+the most filmy vapors, repelled from the cometary body by the sun,
+presumably through electrical action, with a velocity comparable to that
+of light. This luminous suggestion was held more or less in abeyance for
+half a century. Then it was elaborated by Zollner, and particularly by
+Bredichin, of the Moscow observatory, into what has since been regarded
+as the most plausible of cometary theories. It is held that comets
+and the sun are similarly electrified, and hence mutually repulsive.
+Gravitation vastly outmatches this repulsion in the body of the comet,
+but yields to it in the case of gases, because electrical force varies
+with the surface, while gravitation varies only with the mass. From
+study of atomic weights and estimates of the velocity of thrust of
+cometary tails, Bredichin concluded that the chief components of the
+various kinds of tails are hydrogen, hydrocarbons, and the vapor of
+iron; and spectroscopic analysis goes far towards sustaining these
+assumptions.
+
+But, theories aside, the unsubstantialness of the comet's tail has been
+put to a conclusive test. Twice during the nineteenth century the
+earth has actually plunged directly through one of these threatening
+appendages--in 1819, and again in 1861, once being immersed to a depth
+of some three hundred thousand miles in its substance. Yet nothing
+dreadful happened to us. There was a peculiar glow in the atmosphere,
+so the more imaginative observers thought, and that was all. After such
+fiascos the cometary train could never again pose as a world-destroyer.
+
+But the full measure of the comet's humiliation is not yet told. The
+pyrotechnic tail, composed as it is of portions of the comet's actual
+substance, is tribute paid the sun, and can never be recovered. Should
+the obeisance to the sun be many times repeated, the train-forming
+material will be exhausted, and the comet's chiefest glory will have
+departed. Such a fate has actually befallen a multitude of comets which
+Jupiter and the other outlying planets have dragged into our system and
+helped the sun to hold captive here. Many of these tailless comets were
+known to the eighteenth-century astronomers, but no one at that time
+suspected the true meaning of their condition. It was not even known how
+closely some of them are enchained until the German astronomer Encke,
+in 1822, showed that one which he had rediscovered, and which has
+since borne his name, was moving in an orbit so contracted that it must
+complete its circuit in about three and a half years. Shortly afterwards
+another comet, revolving in a period of about six years, was discovered
+by Biela, and given his name. Only two more of these short-period comets
+were discovered during the first half of last century, but latterly they
+have been shown to be a numerous family. Nearly twenty are known
+which the giant Jupiter holds so close that the utmost reach of their
+elliptical tether does not let them go beyond the orbit of Saturn. These
+aforetime wanderers have adapted themselves wonderfully to planetary
+customs, for all of them revolve in the same direction with the planets,
+and in planes not wide of the ecliptic.
+
+Checked in their proud hyperbolic sweep, made captive in a planetary
+net, deprived of their trains, these quondam free-lances of the heavens
+are now mere shadows of their former selves. Considered as to mere
+bulk, they are very substantial shadows, their extent being measured in
+hundreds of thousands of miles; but their actual mass is so slight that
+they are quite at the mercy of the gravitation pulls of their captors.
+And worse is in store for them. So persistently do sun and planets tug
+at them that they are doomed presently to be torn into shreds.
+
+Such a fate has already overtaken one of them, under the very eyes of
+the astronomers, within the relatively short period during which these
+ill-fated comets have been observed. In 1832 Biela's comet passed quite
+near the earth, as astronomers measure distance, and in doing so created
+a panic on our planet. It did no greater harm than that, of course, and
+passed on its way as usual. The very next time it came within telescopic
+hail it was seen to have broken into two fragments. Six years later
+these fragments were separated by many millions of miles; and in 1852,
+when the comet was due again, astronomers looked for it in vain. It had
+been completely shattered.
+
+What had become of the fragments? At that time no one positively knew.
+But the question was to be answered presently. It chanced that just at
+this period astronomers were paying much attention to a class of bodies
+which they had hitherto somewhat neglected, the familiar shooting-stars,
+or meteors. The studies of Professor Newton, of Yale, and Professor
+Adams, of Cambridge, with particular reference to the great
+meteor-shower of November, 1866, which Professor Newton had predicted
+and shown to be recurrent at intervals of thirty-three years, showed
+that meteors are not mere sporadic swarms of matter flying at random,
+but exist in isolated swarms, and sweep about the sun in regular
+elliptical orbits.
+
+Presently it was shown by the Italian astronomer Schiaparelli that
+one of these meteor swarms moves in the orbit of a previously observed
+comet, and other coincidences of the kind were soon forthcoming. The
+conviction grew that meteor swarms are really the debris of comets; and
+this conviction became a practical certainty when, in November, 1872,
+the earth crossed the orbit of the ill-starred Biela, and a shower of
+meteors came whizzing into our atmosphere in lieu of the lost comet.
+
+And so at last the full secret was out. The awe-inspiring comet, instead
+of being the planetary body it had all along been regarded, is really
+nothing more nor less than a great aggregation of meteoric particles,
+which have become clustered together out in space somewhere, and which
+by jostling one another or through electrical action become luminous. So
+widely are the individual particles separated that the cometary body as
+a whole has been estimated to be thousands of times less dense than the
+earth's atmosphere at sea-level. Hence the ease with which the comet may
+be dismembered and its particles strung out into streaming swarms.
+
+So thickly is the space we traverse strewn with this cometary dust
+that the earth sweeps up, according to Professor Newcomb's estimate, a
+million tons of it each day. Each individual particle, perhaps no larger
+than a millet seed, becomes a shooting-star, or meteor, as it burns to
+vapor in the earth's upper atmosphere. And if one tiny planet sweeps
+up such masses of this cosmic matter, the amount of it in the entire
+stretch of our system must be beyond all estimate. What a story it tells
+of the myriads of cometary victims that have fallen prey to the sun
+since first he stretched his planetary net across the heavens!
+
+
+THE FIXED STARS
+
+When Biela's comet gave the inhabitants of the earth such a fright in
+1832, it really did not come within fifty millions of miles of us. Even
+the great comet through whose filmy tail the earth passed in 1861 was
+itself fourteen millions of miles away. The ordinary mind, schooled to
+measure space by the tiny stretches of a pygmy planet, cannot grasp the
+import of such distances; yet these are mere units of measure compared
+with the vast stretches of sidereal space. Were the comet which hurtles
+past us at a speed of, say, a hundred miles a second to continue its
+mad flight unchecked straight into the void of space, it must fly on its
+frigid way eight thousand years before it could reach the very nearest
+of our neighbor stars; and even then it would have penetrated but a
+mere arm's-length into the vistas where lie the dozen or so of sidereal
+residents that are next beyond. Even to the trained mind such distances
+are only vaguely imaginable. Yet the astronomer of our century has
+reached out across this unthinkable void and brought back many a secret
+which our predecessors thought forever beyond human grasp.
+
+A tentative assault upon this stronghold of the stars was being made
+by Herschel at the beginning of the century. In 1802 that greatest of
+observing astronomers announced to the Royal Society his discovery that
+certain double stars had changed their relative positions towards one
+another since he first carefully charted them twenty years before.
+Hitherto it had been supposed that double stars were mere optical
+effects. Now it became clear that some of them, at any rate, are
+true "binary systems," linked together presumably by gravitation and
+revolving about one another. Halley had shown, three-quarters of a
+century before, that the stars have an actual or "proper" motion in
+space; Herschel himself had proved that the sun shares this motion
+with the other stars. Here was another shift of place, hitherto quite
+unsuspected, to be reckoned with by the astronomer in fathoming sidereal
+secrets.
+
+
+Double Stars
+
+When John Herschel, the only son and the worthy successor of the great
+astronomer, began star-gazing in earnest, after graduating senior
+wrangler at Cambridge, and making two or three tentative professional
+starts in other directions to which his versatile genius impelled him,
+his first extended work was the observation of his father's double
+stars. His studies, in which at first he had the collaboration of Mr.
+James South, brought to light scores of hitherto unrecognized pairs, and
+gave fresh data for the calculation of the orbits of those longer
+known. So also did the independent researches of F. G. W. Struve,
+the enthusiastic observer of the famous Russian observatory at the
+university of Dorpat, and subsequently at Pulkowa. Utilizing data
+gathered by these observers, M. Savary, of Paris, showed, in 1827, that
+the observed elliptical orbits of the double stars are explicable by
+the ordinary laws of gravitation, thus confirming the assumption that
+Newton's laws apply to these sidereal bodies. Henceforth there could be
+no reason to doubt that the same force which holds terrestrial objects
+on our globe pulls at each and every particle of matter throughout the
+visible universe.
+
+The pioneer explorers of the double stars early found that the systems
+into which the stars are linked are by no means confined to single
+pairs. Often three or four stars are found thus closely connected into
+gravitation systems; indeed, there are all gradations between binary
+systems and great clusters containing hundreds or even thousands of
+members. It is known, for example, that the familiar cluster of the
+Pleiades is not merely an optical grouping, as was formerly supposed,
+but an actual federation of associated stars, some two thousand five
+hundred in number, only a few of which are visible to the unaided eve.
+And the more carefully the motions of the stars are studied, the more
+evident it becomes that widely separated stars are linked together into
+infinitely complex systems, as yet but little understood. At the same
+time, all instrumental advances tend to resolve more and more seemingly
+single stars into close pairs and minor clusters. The two Herschels
+between them discovered some thousands of these close multiple systems;
+Struve and others increased the list to above ten thousand; and Mr.
+S. W. Burnham, of late years the most enthusiastic and successful of
+double-star pursuers, added a thousand new discoveries while he was
+still an amateur in astronomy, and by profession the stenographer of a
+Chicago court. Clearly the actual number of multiple stars is beyond all
+present estimate.
+
+The elder Herschel's early studies of double stars were undertaken in
+the hope that these objects might aid him in ascertaining the actual
+distance of a star, through measurement of its annual parallax--that
+is to say, of the angle which the diameter of the earth's orbit would
+subtend as seen from the star. The expectation was not fulfilled. The
+apparent shift of position of a star as viewed from opposite sides of
+the earth's orbit, from which the parallax might be estimated, is so
+extremely minute that it proved utterly inappreciable, even to the
+almost preternaturally acute vision of Herschel, with the aid of any
+instrumental means then at command. So the problem of star distance
+allured and eluded him to the end, and he died in 1822 without seeing
+it even in prospect of solution. His estimate of the minimum distance of
+the nearest star, based though it was on the fallacious test of apparent
+brilliancy, was a singularly sagacious one, but it was at best a
+scientific guess, not a scientific measurement.
+
+
+The Distance of the Stars
+
+Just about this time, however, a great optician came to the aid of the
+astronomers. Joseph Fraunhofer perfected the refracting telescope,
+as Herschel had perfected the reflector, and invented a wonderfully
+accurate "heliometer," or sun-measurer. With the aid of these
+instruments the old and almost infinitely difficult problem of star
+distance was solved. In 1838 Bessel announced from the Konigsberg
+observatory that he had succeeded, after months of effort, in detecting
+and measuring the parallax of a star. Similar claims had been made often
+enough before, always to prove fallacious when put to further test; but
+this time the announcement carried the authority of one of the greatest
+astronomers of the age, and scepticism was silenced.
+
+Nor did Bessel's achievement long await corroboration. Indeed, as so
+often happens in fields of discovery, two other workers had almost
+simultaneously solved the same problem--Struve at Pulkowa, where the
+great Russian observatory, which so long held the palm over all others,
+had now been established; and Thomas Henderson, then working at the
+Cape of Good Hope, but afterwards the Astronomer Royal of Scotland.
+Henderson's observations had actual precedence in point of time, but
+Bessel's measurements were so much more numerous and authoritative that
+he has been uniformly considered as deserving the chief credit of the
+discovery, which priority of publication secured him.
+
+By an odd chance, the star on which Henderson's observations were made,
+and consequently the first star the parallax of which was ever measured,
+is our nearest neighbor in sidereal space, being, indeed, some ten
+billions of miles nearer than the one next beyond. Yet even this nearest
+star is more than two hundred thousand times as remote from us as the
+sun. The sun's light flashes to the earth in eight minutes, and to
+Neptune in about three and a half hours, but it requires three and a
+half years to signal Alpha Centauri. And as for the great majority of
+the stars, had they been blotted out of existence before the Christian
+era, we of to-day should still receive their light and seem to see them
+just as we do. When we look up to the sky, we study ancient history;
+we do not see the stars as they ARE, but as they WERE years, centuries,
+even millennia ago.
+
+The information derived from the parallax of a star by no means halts
+with the disclosure of the distance of that body. Distance known, the
+proper motion of the star, hitherto only to be reckoned as so many
+seconds of arc, may readily be translated into actual speed of progress;
+relative brightness becomes absolute lustre, as compared with the sun;
+and in the case of the double stars the absolute mass of the components
+may be computed from the laws of gravitation. It is found that stars
+differ enormously among themselves in all these regards. As to speed,
+some, like our sun, barely creep through space--compassing ten or twenty
+miles a second, it is true, yet even at that rate only passing through
+the equivalent of their own diameter in a day. At the other extreme,
+among measured stars, is one that moves two hundred miles a second; yet
+even this "flying star," as seen from the earth, seems to change its
+place by only about three and a half lunar diameters in a thousand
+years. In brightness, some stars yield to the sun, while others surpass
+him as the arc-light surpasses a candle. Arcturus, the brightest
+measured star, shines like two hundred suns; and even this giant orb is
+dim beside those other stars which are so distant that their parallax
+cannot be measured, yet which greet our eyes at first magnitude. As to
+actual bulk, of which apparent lustre furnishes no adequate test, some
+stars are smaller than the sun, while others exceed him hundreds or
+perhaps thousands of times. Yet one and all, so distant are they, remain
+mere disklike points of light before the utmost powers of the modern
+telescope.
+
+
+Revelations of the Spectroscope
+
+All this seems wonderful enough, but even greater things were in store.
+In 1859 the spectroscope came upon the scene, perfected by Kirchhoff
+and Bunsen, along lines pointed out by Fraunhofer almost half a century
+before. That marvellous instrument, by revealing the telltale lines
+sprinkled across a prismatic spectrum, discloses the chemical nature
+and physical condition of any substance whose light is submitted to it,
+telling its story equally well, provided the light be strong enough,
+whether the luminous substance be near or far--in the same room or at
+the confines of space. Clearly such an instrument must prove a veritable
+magic wand in the hands of the astronomer.
+
+Very soon eager astronomers all over the world were putting the
+spectroscope to the test. Kirchhoff himself led the way, and Donati and
+Father Secchi in Italy, Huggins and Miller in England, and Rutherfurd in
+America, were the chief of his immediate followers. The results exceeded
+the dreams of the most visionary. At the very outset, in 1860, it was
+shown that such common terrestrial substances as sodium, iron, calcium,
+magnesium, nickel, barium, copper, and zinc exist in the form of glowing
+vapors in the sun, and very soon the stars gave up a corresponding
+secret. Since then the work of solar and sidereal analysis has gone on
+steadily in the hands of a multitude of workers (prominent among whom,
+in this country, are Professor Young of Princeton, Professor Langley of
+Washington, and Professor Pickering of Harvard), and more than half
+the known terrestrial elements have been definitely located in the sun,
+while fresh discoveries are in prospect.
+
+It is true the sun also contains some seeming elements that are unknown
+on the earth, but this is no matter for surprise. The modern chemist
+makes no claim for his elements except that they have thus far resisted
+all human efforts to dissociate them; it would be nothing strange if
+some of them, when subjected to the crucible of the sun, which is seen
+to vaporize iron, nickel, silicon, should fail to withstand the test.
+But again, chemistry has by no means exhausted the resources of the
+earth's supply of raw material, and the substance which sends its
+message from a star may exist undiscovered in the dust we tread or in
+the air we breathe. In the year 1895 two new terrestrial elements were
+discovered; but one of these had for years been known to the astronomer
+as a solar and suspected as a stellar element, and named helium because
+of its abundance in the sun. The spectroscope had reached out millions
+of miles into space and brought back this new element, and it took the
+chemist a score of years to discover that he had all along had samples
+of the same substance unrecognized in his sublunary laboratory. There
+is hardly a more picturesque fact than that in the entire history of
+science.
+
+But the identity in substance of earth and sun and stars was not more
+clearly shown than the diversity of their existing physical conditions.
+It was seen that sun and stars, far from being the cool, earthlike,
+habitable bodies that Herschel thought them (surrounded by glowing
+clouds, and protected from undue heat by other clouds), are in truth
+seething caldrons of fiery liquid, or gas made viscid by condensation,
+with lurid envelopes of belching flames. It was soon made clear, also,
+particularly by the studies of Rutherfurd and of Secchi, that stars
+differ among themselves in exact constitution or condition. There are
+white or Sirian stars, whose spectrum revels in the lines of hydrogen;
+yellow or solar stars (our sun being the type), showing various metallic
+vapors; and sundry red stars, with banded spectra indicative of carbon
+compounds; besides the purely gaseous stars of more recent discovery,
+which Professor Pickering had specially studied. Zollner's famous
+interpretation of these diversities, as indicative of varying stages
+of cooling, has been called in question as to the exact sequence it
+postulates, but the general proposition that stars exist under widely
+varying conditions of temperature is hardly in dispute.
+
+The assumption that different star types mark varying stages of cooling
+has the further support of modern physics, which has been unable to
+demonstrate any way in which the sun's radiated energy may be restored,
+or otherwise made perpetual, since meteoric impact has been shown to
+be--under existing conditions, at any rate--inadequate. In accordance
+with the theory of Helmholtz, the chief supply of solar energy is held
+to be contraction of the solar mass itself; and plainly this must
+have its limits. Therefore, unless some means as yet unrecognized is
+restoring the lost energy to the stellar bodies, each of them must
+gradually lose its lustre, and come to a condition of solidification,
+seeming sterility, and frigid darkness. In the case of our own
+particular star, according to the estimate of Lord Kelvin, such a
+culmination appears likely to occur within a period of five or six
+million years.
+
+
+The Astronomy of the Invisible
+
+But by far the strongest support of such a forecast as this is furnished
+by those stellar bodies which even now appear to have cooled to the
+final stage of star development and ceased to shine. Of this class
+examples in miniature are furnished by the earth and the smaller of its
+companion planets. But there are larger bodies of the same type out
+in stellar space--veritable "dark stars"--invisible, of course, yet
+nowadays clearly recognized.
+
+The opening up of this "astronomy of the invisible" is another of the
+great achievements of the nineteenth century, and again it is Bessel
+to whom the honor of discovery is due. While testing his stars
+for parallax; that astute observer was led to infer, from certain
+unexplained aberrations of motion, that various stars, Sirius himself
+among the number, are accompanied by invisible companions, and in
+1840 he definitely predicated the existence of such "dark stars." The
+correctness of the inference was shown twenty years later, when Alvan
+Clark, Jr., the American optician, while testing a new lens, discovered
+the companion of Sirius, which proved thus to be faintly luminous. Since
+then the existence of other and quite invisible star companions has been
+proved incontestably, not merely by renewed telescopic observations, but
+by the curious testimony of the ubiquitous spectroscope.
+
+One of the most surprising accomplishments of that instrument is the
+power to record the flight of a luminous object directly in the line of
+vision. If the luminous body approaches swiftly, its Fraunhofer lines
+are shifted from their normal position towards the violet end of the
+spectrum; if it recedes, the lines shift in the opposite direction. The
+actual motion of stars whose distance is unknown may be measured in this
+way. But in certain cases the light lines are seen to oscillate on the
+spectrum at regular intervals. Obviously the star sending such light
+is alternately approaching and receding, and the inference that it is
+revolving about a companion is unavoidable. From this extraordinary test
+the orbital distance, relative mass, and actual speed of revolution of
+the absolutely invisible body may be determined. Thus the spectroscope,
+which deals only with light, makes paradoxical excursions into the
+realm of the invisible. What secrets may the stars hope to conceal when
+questioned by an instrument of such necromantic power?
+
+But the spectroscope is not alone in this audacious assault upon the
+strongholds of nature. It has a worthy companion and assistant in
+the photographic film, whose efficient aid has been invoked by the
+astronomer even more recently. Pioneer work in celestial photography
+was, indeed, done by Arago in France and by the elder Draper in America
+in 1839, but the results then achieved were only tentative, and it was
+not till forty years later that the method assumed really important
+proportions. In 1880, Dr. Henry Draper, at Hastings-on-the-Hudson, made
+the first successful photograph of a nebula. Soon after, Dr. David
+Gill, at the Cape observatory, made fine photographs of a comet, and the
+flecks of starlight on his plates first suggested the possibilities of
+this method in charting the heavens.
+
+Since then star-charting with the film has come virtually to supersede
+the old method. A concerted effort is being made by astronomers in
+various parts of the world to make a complete chart of the heavens, and
+before the close of our century this work will be accomplished, some
+fifty or sixty millions of visible stars being placed on record with a
+degree of accuracy hitherto unapproachable. Moreover, other millions of
+stars are brought to light by the negative, which are too distant or
+dim to be visible with any telescopic powers yet attained--a fact
+which wholly discredits all previous inferences as to the limits of
+our sidereal system. Hence, notwithstanding the wonderful instrumental
+advances of the nineteenth century, knowledge of the exact form and
+extent of our universe seems more unattainable than it seemed a century
+ago.
+
+
+The Structure of Nebulae
+
+Yet the new instruments, while leaving so much untold, have revealed
+some vastly important secrets of cosmic structure. In particular, they
+have set at rest the long-standing doubts as to the real structure and
+position of the mysterious nebulae--those lazy masses, only two or
+three of them visible to the unaided eye, which the telescope reveals
+in almost limitless abundance, scattered everywhere among the stars,
+but grouped in particular about the poles of the stellar stream or disk
+which we call the Milky Way.
+
+Herschel's later view, which held that some at least of the nebulae are
+composed of a "shining fluid," in process of condensation to form stars,
+was generally accepted for almost half a century. But in 1844, when
+Lord Rosse's great six-foot reflector--the largest telescope ever yet
+constructed--was turned on the nebulae, it made this hypothesis seem
+very doubtful. Just as Galileo's first lens had resolved the Milky Way
+into stars, just as Herschel had resolved nebulae that resisted all
+instruments but his own, so Lord Rosse's even greater reflector resolved
+others that would not yield to Herschel's largest mirror. It seemed
+a fair inference that with sufficient power, perhaps some day to be
+attained, all nebulae would yield, hence that all are in reality what
+Herschel had at first thought them--vastly distant "island universes,"
+composed of aggregations of stars, comparable to our own galactic
+system.
+
+But the inference was wrong; for when the spectroscope was first applied
+to a nebula in 1864, by Dr. Huggins, it clearly showed the spectrum not
+of discrete stars, but of a great mass of glowing gases, hydrogen among
+others. More extended studies showed, it is true, that some nebulae give
+the continuous spectrum of solids or liquids, but the different types
+intermingle and grade into one another. Also, the closest affinity
+is shown between nebulae and stars. Some nebulae are found to contain
+stars, singly or in groups, in their actual midst; certain condensed
+"planetary" nebulae are scarcely to be distinguished from stars of the
+gaseous type; and recently the photographic film has shown the presence
+of nebulous matter about stars that to telescopic vision differ in no
+respect from the generality of their fellows in the galaxy. The familiar
+stars of the Pleiades cluster, for example, appear on the negative
+immersed in a hazy blur of light. All in all, the accumulated
+impressions of the photographic film reveal a prodigality of nebulous
+matter in the stellar system not hitherto even conjectured.
+
+And so, of course, all question of "island universes" vanishes, and the
+nebulae are relegated to their true position as component parts of the
+one stellar system--the one universe--that is open to present human
+inspection. And these vast clouds of world-stuff have been found by
+Professor Keeler, of the Lick observatory, to be floating through space
+at the starlike speed of from ten to thirty-eight miles per second.
+
+The linking of nebulae with stars, so clearly evidenced by all these
+modern observations, is, after all, only the scientific corroboration of
+what the elder Herschel's later theories affirmed. But the nebulae have
+other affinities not until recently suspected; for the spectra of some
+of them are practically identical with the spectra of certain comets.
+The conclusion seems warranted that comets are in point of fact minor
+nebulae that are drawn into our system; or, putting it otherwise, that
+the telescopic nebulae are simply gigantic distant comets.
+
+
+Lockyer's Meteoric Hypothesis
+
+Following up the surprising clews thus suggested, Sir Norman Lockyer,
+of London, has in recent years elaborated what is perhaps the most
+comprehensive cosmogonic guess that has ever been attempted. His theory,
+known as the "meteoric hypothesis," probably bears the same relation
+to the speculative thought of our time that the nebular hypothesis of
+Laplace bore to that of the eighteenth century. Outlined in a few words,
+it is an attempt to explain all the major phenomena of the universe
+as due, directly or indirectly, to the gravitational impact of such
+meteoric particles, or specks of cosmic dust, as comets are composed
+of. Nebulae are vast cometary clouds, with particles more or less widely
+separated, giving off gases through meteoric collisions, internal or
+external, and perhaps glowing also with electrical or phosphorescent
+light. Gravity eventually brings the nebular particles into closer
+aggregations, and increased collisions finally vaporize the entire mass,
+forming planetary nebulae and gaseous stars. Continued condensation
+may make the stellar mass hotter and more luminous for a time, but
+eventually leads to its liquefaction, and ultimate consolidation--the
+aforetime nebulae becoming in the end a dark or planetary star.
+
+The exact correlation which Lockyer attempts to point out between
+successive stages of meteoric condensation and the various types of
+observed stellar bodies does not meet with unanimous acceptance. Mr.
+Ranyard, for example, suggests that the visible nebulae may not be
+nascent stars, but emanations from stars, and that the true pre-stellar
+nebulae are invisible until condensed to stellar proportions. But such
+details aside, the broad general hypothesis that all the bodies of the
+universe are, so to speak, of a single species--that nebulae (including
+comets), stars of all types, and planets, are but varying stages in the
+life history of a single race or type of cosmic organisms--is accepted
+by the dominant thought of our time as having the highest warrant of
+scientific probability.
+
+All this, clearly, is but an amplification of that nebular hypothesis
+which, long before the spectroscope gave us warrant to accurately judge
+our sidereal neighbors, had boldly imagined the development of stars out
+of nebulae and of planets out of stars. But Lockyer's hypothesis does
+not stop with this. Having traced the developmental process from the
+nebular to the dark star, it sees no cause to abandon this dark star to
+its fate by assuming, as the original speculation assumed, that this is
+a culminating and final stage of cosmic existence. For the dark star,
+though its molecular activities have come to relative stability and
+impotence, still retains the enormous potentialities of molar motion;
+and clearly, where motion is, stasis is not. Sooner or later, in its
+ceaseless flight through space, the dark star must collide with some
+other stellar body, as Dr. Croll imagines of the dark bodies which his
+"pre-nebular theory" postulates. Such collision may be long delayed; the
+dark star may be drawn in comet-like circuit about thousands of other
+stellar masses, and be hurtled on thousands of diverse parabolic or
+elliptical orbits, before it chances to collide--but that matters not:
+"billions are the units in the arithmetic of eternity," and sooner
+or later, we can hardly doubt, a collision must occur. Then without
+question the mutual impact must shatter both colliding bodies into
+vapor, or vapor combined with meteoric fragments; in short, into a
+veritable nebula, the matrix of future worlds. Thus the dark star, which
+is the last term of one series of cosmic changes, becomes the first term
+of another series--at once a post-nebular and a pre-nebular condition;
+and the nebular hypothesis, thus amplified, ceases to be a mere linear
+scale, and is rounded out to connote an unending series of cosmic
+cycles, more nearly satisfying the imagination.
+
+In this extended view, nebulae and luminous stars are but the infantile
+and adolescent stages of the life history of the cosmic individual; the
+dark star, its adult stage, or time of true virility. Or we may think of
+the shrunken dark star as the germ-cell, the pollen-grain, of the cosmic
+organism. Reduced in size, as becomes a germ-cell, to a mere fraction
+of the nebular body from which it sprang, it yet retains within
+its seemingly non-vital body all the potentialities of the original
+organism, and requires only to blend with a fellow-cell to bring a new
+generation into being. Thus may the cosmic race, whose aggregate census
+makes up the stellar universe, be perpetuated--individual solar systems,
+such as ours, being born, and growing old, and dying to live again in
+their descendants, while the universe as a whole maintains its unified
+integrity throughout all these internal mutations--passing on, it may
+be, by infinitesimal stages, to a culmination hopelessly beyond human
+comprehension.
+
+
+
+
+III. THE NEW SCIENCE OF PALEONTOLOGY
+
+
+WILLIAM SMITH AND FOSSIL SHELLS
+
+Ever since Leonardo da Vinci first recognized the true character of
+fossils, there had been here and there a man who realized that the
+earth's rocky crust is one gigantic mausoleum. Here and there a
+dilettante had filled his cabinets with relics from this monster crypt;
+here and there a philosopher had pondered over them--questioning whether
+perchance they had once been alive, or whether they were not mere
+abortive souvenirs of that time when the fertile matrix of the earth was
+supposed to have
+
+              "teemed at a birth
+     Innumerous living creatures, perfect forms,
+     Limbed and full grown."
+
+Some few of these philosophers--as Robert Hooke and Steno in the
+seventeenth century, and Moro, Leibnitz, Buffon, Whitehurst, Werner,
+Hutton, and others in the eighteenth--had vaguely conceived the
+importance of fossils as records of the earth's ancient history, but the
+wisest of them no more suspected the full import of the story written
+in the rocks than the average stroller in a modern museum suspects the
+meaning of the hieroglyphs on the case of a mummy.
+
+It was not that the rudiments of this story are so very hard to
+decipher--though in truth they are hard enough--but rather that the
+men who made the attempt had all along viewed the subject through an
+atmosphere of preconception, which gave a distorted image. Before this
+image could be corrected it was necessary that a man should appear who
+could see without prejudice, and apply sound common-sense to what he
+saw. And such a man did appear towards the close of the century, in the
+person of William Smith, the English surveyor. He was a self-taught man,
+and perhaps the more independent for that, and he had the gift, besides
+his sharp eyes and receptive mind, of a most tenacious memory. By
+exercising these faculties, rare as they are homely, he led the way to
+a science which was destined, in its later developments, to shake the
+structure of established thought to its foundations.
+
+Little enough did William Smith suspect, however, that any such dire
+consequences were to come of his act when he first began noticing the
+fossil shells that here and there are to be found in the stratified
+rocks and soils of the regions over which his surveyor's duties led him.
+Nor, indeed, was there anything of such apparent revolutionary character
+in the facts which he unearthed; yet in their implications these facts
+were the most disconcerting of any that had been revealed since the days
+of Copernicus and Galileo. In its bald essence, Smith's discovery was
+simply this: that the fossils in the rocks, instead of being scattered
+haphazard, are arranged in regular systems, so that any given stratum
+of rock is labelled by its fossil population; and that the order of
+succession of such groups of fossils is always the same in any vertical
+series of strata in which they occur. That is to say, if fossil A
+underlies fossil B in any given region, it never overlies it in any
+other series; though a kind of fossils found in one set of strata may
+be quite omitted in another. Moreover, a fossil once having disappeared
+never reappears in any later stratum.
+
+From these novel facts Smith drew the commonsense inference that the
+earth had had successive populations of creatures, each of which in
+its turn had become extinct. He partially verified this inference by
+comparing the fossil shells with existing species of similar orders,
+and found that such as occur in older strata of the rocks had no
+counterparts among living species. But, on the whole, being eminently
+a practical man, Smith troubled himself but little about the inferences
+that might be drawn from his facts. He was chiefly concerned in using
+the key he had discovered as an aid to the construction of the first
+geological map of England ever attempted, and he left to others the
+untangling of any snarls of thought that might seem to arise from his
+discovery of the succession of varying forms of life on the globe.
+
+He disseminated his views far and wide, however, in the course of his
+journeyings--quite disregarding the fact that peripatetics went out of
+fashion when the printing-press came in--and by the beginning of the
+nineteenth century he had begun to have a following among the geologists
+of England. It must not for a moment be supposed, however, that his
+contention regarding the succession of strata met with immediate or
+general acceptance. On the contrary, it was most bitterly antagonized.
+For a long generation after the discovery was made, the generality of
+men, prone as always to strain at gnats and swallow camels, preferred to
+believe that the fossils, instead of being deposited in successive ages,
+had been swept all at once into their present positions by the current
+of a mighty flood--and that flood, needless to say, the Noachian deluge.
+Just how the numberless successive strata could have been laid down
+in orderly sequence to the depth of several miles in one such fell
+cataclysm was indeed puzzling, especially after it came to be admitted
+that the heaviest fossils were not found always at the bottom; but to
+doubt that this had been done in some way was rank heresy in the early
+days of the nineteenth century.
+
+
+CUVIER AND FOSSIL VERTEBRATES
+
+But once discovered, William Smith's unique facts as to the succession
+of forms in the rocks would not down. There was one most vital point,
+however, regarding which the inferences that seem to follow from
+these facts needed verification--the question, namely, whether the
+disappearance of a fauna from the register in the rocks really implies
+the extinction of that fauna. Everything really depended upon the answer
+to that question, and none but an accomplished naturalist could answer
+it with authority. Fortunately, the most authoritative naturalist of the
+time, George Cuvier, took the question in hand--not, indeed, with the
+idea of verifying any suggestion of Smith's, but in the course of his
+own original studies--at the very beginning of the century, when Smith's
+views were attracting general attention.
+
+Cuvier and Smith were exact contemporaries, both men having been born in
+1769, that "fertile year" which gave the world also Chateaubriand, Von
+Humboldt, Wellington, and Napoleon. But the French naturalist was of
+very different antecedents from the English surveyor. He was brilliantly
+educated, had early gained recognition as a scientist, and while yet a
+young man had come to be known as the foremost comparative anatomist of
+his time. It was the anatomical studies that led him into the realm of
+fossils. Some bones dug out of the rocks by workmen in a quarry were
+brought to his notice, and at once his trained eye told him that they
+were different from anything he had seen before. Hitherto such bones,
+when not entirely ignored, had been for the most part ascribed to
+giants of former days, or even to fallen angels. Cuvier soon showed
+that neither giants nor angels were in question, but elephants of an
+unrecognized species. Continuing his studies, particularly with material
+gathered from gypsum beds near Paris, he had accumulated, by the
+beginning of the nineteenth century, bones of about twenty-five species
+of animals that he believed to be different from any now living on the
+globe.
+
+The fame of these studies went abroad, and presently fossil bones poured
+in from all sides, and Cuvier's conviction that extinct forms of animals
+are represented among the fossils was sustained by the evidence of many
+strange and anomalous forms, some of them of gigantic size. In 1816
+the famous Ossements Fossiles, describing these novel objects, was
+published, and vertebrate paleontology became a science. Among
+other things of great popular interest the book contained the first
+authoritative description of the hairy elephant, named by Cuvier the
+mammoth, the remains of which bad been found embedded in a mass of
+ice in Siberia in 1802, so wonderfully preserved that the dogs of the
+Tungusian fishermen actually ate its flesh. Bones of the same species
+had been found in Siberia several years before by the naturalist Pallas,
+who had also found the carcass of a rhinoceros there, frozen in a
+mud-bank; but no one then suspected that these were members of an
+extinct population--they were supposed to be merely transported relics
+of the flood.
+
+Cuvier, on the other hand, asserted that these and the other creatures
+he described had lived and died in the region where their remains were
+found, and that most of them have no living representatives upon the
+globe. This, to be sure, was nothing more than William Smith had tried
+all along to establish regarding lower forms of life; but flesh and
+blood monsters appeal to the imagination in a way quite beyond the power
+of mere shells; so the announcement of Cuvier's discoveries aroused the
+interest of the entire world, and the Ossements Fossiles was accorded a
+popular reception seldom given a work of technical science--a reception
+in which the enthusiastic approval of progressive geologists was mingled
+with the bitter protests of the conservatives.
+
+
+"Naturalists certainly have neither explored all the continents," said
+Cuvier, "nor do they as yet even know all the quadrupeds of those parts
+which have been explored. New species of this class are discovered from
+time to time; and those who have not examined with attention all the
+circumstances belonging to these discoveries may allege also that the
+unknown quadrupeds, whose fossil bones have been found in the strata
+of the earth, have hitherto remained concealed in some islands not yet
+discovered by navigators, or in some of the vast deserts which occupy
+the middle of Africa, Asia, the two Americas, and New Holland.
+
+"But if we carefully attend to the kind of quadrupeds that have been
+recently discovered, and to the circumstances of their discovery, we
+shall easily perceive that there is very little chance indeed of our
+ever finding alive those which have only been seen in a fossil state.
+
+"Islands of moderate size, and at a considerable distance from the large
+continents, have very few quadrupeds. These must have been carried
+to them from other countries. Cook and Bougainville found no other
+quadrupeds besides hogs and dogs in the South Sea Islands; and the
+largest quadruped of the West India Islands, when first discovered, was
+the agouti, a species of the cavy, an animal apparently between the rat
+and the rabbit.
+
+"It is true that the great continents, as Asia, Africa, the two
+Americas, and New Holland, have large quadrupeds, and, generally
+speaking, contain species common to each; insomuch, that upon
+discovering countries which are isolated from the rest of the world,
+the animals they contain of the class of quadruped were found entirely
+different from those which existed in other countries. Thus, when the
+Spaniards first penetrated into South America, they did not find it to
+contain a single quadruped exactly the same with those of Europe, Asia,
+and Africa. The puma, the jaguar, the tapir, the capybara, the llama,
+or glama, and vicuna, and the whole tribe of sapajous, were to them
+entirely new animals, of which they had not the smallest idea....
+
+"If there still remained any great continent to be discovered, we
+might perhaps expect to be made acquainted with new species of large
+quadrupeds, among which some might be found more or less similar to
+those of which we find the exuviae in the bowels of the earth. But it
+is merely sufficient to glance the eye over the maps of the world and
+observe the innumerable directions in which navigators have traversed
+the ocean, in order to be satisfied that there does not remain any large
+land to be discovered, unless it may be situated towards the Antarctic
+Pole, where eternal ice necessarily forbids the existence of animal
+life."(1)
+
+Cuvier then points out that the ancients were well acquainted with
+practically all the animals on the continents of Europe, Asia, and
+Africa now known to scientists. He finds little grounds, therefore, for
+belief in the theory that at one time there were monstrous animals on
+the earth which it was necessary to destroy in order that the present
+fauna and men might flourish. After reviewing these theories and beliefs
+in detail, he takes up his Inquiry Respecting the Fabulous Animals
+of the Ancients. "It is easy," he says, "to reply to the foregoing
+objections, by examining the descriptions that are left us by the
+ancients of those unknown animals, and by inquiring into their origins.
+Now that the greater number of these animals have an origin, the
+descriptions given of them bear the most unequivocal marks; as in almost
+all of them we see merely the different parts of known animals united by
+an unbridled imagination, and in contradiction to every established law
+of nature."(2)
+
+Having shown how the fabulous monsters of ancient times and of foreign
+nations, such as the Chinese, were simply products of the imagination,
+having no prototypes in nature, Cuvier takes up the consideration of the
+difficulty of distinguishing the fossil bones of quadrupeds.
+
+We shall have occasion to revert to this part of Cuvier's paper in
+another connection. Here it suffices to pass at once to the final
+conclusion that the fossil bones in question are the remains of an
+extinct fauna, the like of which has no present-day representation on
+the earth. Whatever its implications, this conclusion now seemed to
+Cuvier to be fully established.
+
+In England the interest thus aroused was sent to fever-heat in 1821 by
+the discovery of abundant beds of fossil bones in the stalagmite-covered
+floor of a cave at Kirkdale, Yorkshire which went to show that England,
+too, had once had her share of gigantic beasts. Dr. Buckland, the
+incumbent of the chair of geology at Oxford, and the most authoritative
+English geologist of his day, took these finds in hand and showed that
+the bones belonged to a number of species, including such alien forms as
+elephants, rhinoceroses, hippopotami, and hyenas. He maintained that all
+of these creatures had actually lived in Britain, and that the caves in
+which their bones were found had been the dens of hyenas.
+
+The claim was hotly disputed, as a matter of course. As late as 1827
+books were published denouncing Buckland, doctor of divinity though he
+was, as one who had joined in an "unhallowed cause," and reiterating the
+old cry that the fossils were only remains of tropical species washed
+thither by the deluge. That they were found in solid rocks or in caves
+offered no difficulty, at least not to the fertile imagination of
+Granville Penn, the leader of the conservatives, who clung to the old
+idea of Woodward and Cattcut that the deluge had dissolved the entire
+crust of the earth to a paste, into which the relics now called fossils
+had settled. The caves, said Mr. Penn, are merely the result of gases
+given off by the carcasses during decomposition--great air-bubbles, so
+to speak, in the pasty mass, becoming caverns when the waters receded
+and the paste hardened to rocky consistency.
+
+But these and such-like fanciful views were doomed even in the day of
+their utterance. Already in 1823 other gigantic creatures, christened
+ichthyosaurus and plesiosaurus by Conybeare, had been found in deeper
+strata of British rocks; and these, as well as other monsters whose
+remains were unearthed in various parts of the world, bore such strange
+forms that even the most sceptical could scarcely hope to find their
+counterparts among living creatures. Cuvier's contention that all the
+larger vertebrates of the existing age are known to naturalists was
+borne out by recent explorations, and there seemed no refuge from the
+conclusion that the fossil records tell of populations actually extinct.
+But if this were admitted, then Smith's view that there have been
+successive rotations of population could no longer be denied. Nor could
+it be in doubt that the successive faunas, whose individual remains have
+been preserved in myriads, representing extinct species by thousands
+and tens of thousands, must have required vast periods of time for the
+production and growth of their countless generations.
+
+As these facts came to be generally known, and as it came to be
+understood in addition that the very matrix of the rock in which fossils
+are imbedded is in many cases one gigantic fossil, composed of the
+remains of microscopic forms of life, common-sense, which, after all,
+is the final tribunal, came to the aid of belabored science. It was
+conceded that the only tenable interpretation of the record in the rocks
+is that numerous populations of creatures, distinct from one another and
+from present forms, have risen and passed away; and that the geologic
+ages in which these creatures lived were of inconceivable length. The
+rank and file came thus, with the aid of fossil records, to realize
+the import of an idea which James Hutton, and here and there another
+thinker, had conceived with the swift intuition of genius long
+before the science of paleontology came into existence. The Huttonian
+proposition that time is long had been abundantly established, and by
+about the close of the first third of the last century geologists had
+begun to speak of "ages" and "untold aeons of time" with a familiarity
+which their predecessors had reserved for days and decades.
+
+
+CHARLES LYELL COMBATS CATASTROPHISM
+
+And now a new question pressed for solution. If the earth has been
+inhabited by successive populations of beings now extinct, how have
+all these creatures been destroyed? That question, however, seemed to
+present no difficulties. It was answered out of hand by the application
+of an old idea. All down the centuries, whatever their varying phases of
+cosmogonic thought, there had been ever present the idea that past times
+were not as recent times; that in remote epochs the earth had been the
+scene of awful catastrophes that have no parallel in "these degenerate
+days." Naturally enough, this thought, embalmed in every cosmogonic
+speculation of whatever origin, was appealed to in explanation of the
+destruction of these hitherto unimagined hosts, which now, thanks to
+science, rose from their abysmal slumber as incontestable, but also as
+silent and as thought-provocative, as Sphinx or pyramid. These ancient
+hosts, it was said, have been exterminated at intervals of odd millions
+of years by the recurrence of catastrophes of which the Mosaic deluge is
+the latest, but perhaps not the last.
+
+This explanation had fullest warrant of scientific authority. Cuvier had
+prefaced his classical work with a speculative disquisition whose
+very title (Discours sur les Revolutions du Globe) is ominous of
+catastrophism, and whose text fully sustains the augury. And Buckland,
+Cuvier's foremost follower across the Channel, had gone even beyond
+the master, naming the work in which he described the Kirkdale fossils,
+Reliquiae Diluvianae, or Proofs of a Universal Deluge.
+
+Both these authorities supposed the creatures whose remains they studied
+to have perished suddenly in the mighty flood whose awful current, as
+they supposed, gouged out the modern valleys and hurled great blocks of
+granite broadcast over the land. And they invoked similar floods for the
+extermination of previous populations.
+
+It is true these scientific citations had met with only qualified
+approval at the time of their utterance, because then the conservative
+majority of mankind did not concede that there had been a plurality of
+populations or revolutions; but now that the belief in past geologic
+ages had ceased to be a heresy, the recurring catastrophes of the great
+paleontologists were accepted with acclaim. For the moment science and
+tradition were at one, and there was a truce to controversy, except
+indeed in those outlying skirmish-lines of thought whither news from
+headquarters does not permeate till it has become ancient history at its
+source.
+
+The truce, however, was not for long. Hardly had contemporary
+thought begun to adjust itself to the conception of past ages of
+incomprehensible extent, each terminated by a catastrophe of the
+Noachian type, when a man appeared who made the utterly bewildering
+assertion that the geological record, instead of proving numerous
+catastrophic revolutions in the earth's past history, gives no warrant
+to the pretensions of any universal catastrophe whatever, near or
+remote.
+
+This iconoclast was Charles Lyell, the Scotchman, who was soon to be
+famous as the greatest geologist of his time. As a young man he had
+become imbued with the force of the Huttonian proposition, that present
+causes are one with those that produced the past changes of the
+globe, and he carried that idea to what he conceived to be its logical
+conclusion. To his mind this excluded the thought of catastrophic
+changes in either inorganic or organic worlds.
+
+But to deny catastrophism was to suggest a revolution in current
+thought. Needless to say, such revolution could not be effected without
+a long contest. For a score of years the matter was argued pro and con.,
+often with most unscientific ardor. A mere outline of the controversy
+would fill a volume; yet the essential facts with which Lyell at last
+established his proposition, in its bearings on the organic world, may
+be epitomized in a few words. The evidence which seems to tell of past
+revolutions is the apparently sudden change of fossils from one stratum
+to another of the rocks. But Lyell showed that this change is not always
+complete. Some species live on from one alleged epoch into the next. By
+no means all the contemporaries of the mammoth are extinct, and numerous
+marine forms vastly more ancient still have living representatives.
+
+Moreover, the blanks between strata in any particular vertical series
+are amply filled in with records in the form of thick strata in some
+geographically distant series. For example, in some regions Silurian
+rocks are directly overlaid by the coal measures; but elsewhere this
+sudden break is filled in with the Devonian rocks that tell of a great
+"age of fishes." So commonly are breaks in the strata in one region
+filled up in another that we are forced to conclude that the
+record shown by any single vertical series is of but local
+significance--telling, perhaps, of a time when that particular sea-bed
+oscillated above the water-line, and so ceased to receive sediment until
+some future age when it had oscillated back again. But if this be
+the real significance of the seemingly sudden change from stratum to
+stratum, then the whole case for catastrophism is hopelessly lost; for
+such breaks in the strata furnish the only suggestion geology can offer
+of sudden and catastrophic changes of wide extent.
+
+Let us see how Lyell elaborates these ideas, particularly with reference
+to the rotation of species.(2)
+
+"I have deduced as a corollary," he says, "that the species existing at
+any particular period must, in the course of ages, become extinct, one
+after the other. 'They must die out,' to borrow an emphatic expression
+from Buffon, 'because Time fights against them.' If the views which I
+have taken are just, there will be no difficulty in explaining why
+the habitations of so many species are now restrained within exceeding
+narrow limits. Every local revolution tends to circumscribe the range
+of some species, while it enlarges that of others; and if we are led
+to infer that new species originate in one spot only, each must require
+time to diffuse itself over a wide area. It will follow, therefore, from
+the adoption of our hypothesis that the recent origin of some species
+and the high antiquity of others are equally consistent with the general
+fact of their limited distribution, some being local because they have
+not existed long enough to admit of their wide dissemination; others,
+because circumstances in the animate or inanimate world have occurred to
+restrict the range within which they may once have obtained....
+
+"If the reader should infer, from the facts laid before him, that the
+successive extinction of animals and plants may be part of the constant
+and regular course of nature, he will naturally inquire whether there
+are any means provided for the repair of these losses? Is it possible as
+a part of the economy of our system that the habitable globe should to a
+certain extent become depopulated, both in the ocean and on the land, or
+that the variety of species should diminish until some new era arrives
+when a new and extraordinary effort of creative energy is to be
+displayed? Or is it possible that new species can be called into being
+from time to time, and yet that so astonishing a phenomenon can escape
+the naturalist?
+
+"In the first place, it is obviously more easy to prove that a species
+once numerously represented in a given district has ceased to be
+than that some other which did not pre-exist had made its
+appearance--assuming always, for reasons before stated, that single
+stocks only of each animal and plant are originally created, and that
+individuals of new species did not suddenly start up in many different
+places at once.
+
+"So imperfect has the science of natural history remained down to our
+own times that, within the memory of persons now living, the numbers
+of known animals and plants have doubled, or even quadrupled, in many
+classes. New and often conspicuous species are annually discovered in
+parts of the old continent long inhabited by the most civilized nations.
+Conscious, therefore, of the limited extent of our information, we
+always infer, when such discoveries are made, that the beings in
+question bad previously eluded our research, or had at least existed
+elsewhere, and only migrated at a recent period into the territories
+where we now find them.
+
+"What kind of proofs, therefore, could we reasonably expect to find of
+the origin at a particular period of a new species?
+
+"Perhaps, it may be said in reply, that within the last two or three
+centuries some forest tree or new quadruped might have been observed to
+appear suddenly in those parts of England or France which had been most
+thoroughly investigated--that naturalists might have been able to show
+that no such being inhabited any other region of the globe, and that
+there was no tradition of anything similar having been observed in the
+district where it had made its appearance.
+
+"Now, although this objection may seem plausible, yet its force will be
+found to depend entirely on the rate of fluctuation which we suppose
+to prevail in the animal world, and on the proportions which such
+conspicuous subjects of the animal and vegetable kingdoms bear to those
+which are less known and escape our observation. There are perhaps
+more than a million species of plants and animals, exclusive of the
+microscopic and infusory animalcules, now inhabiting the terraqueous
+globe, so that if only one of these were to become extinct annually, and
+one new one were to be every year called into being, much more than a
+million of years might be required to bring about a complete revolution
+of organic life.
+
+"I am not hazarding at present any hypothesis as to the probable rate
+of change, but none will deny that when the annual birth and the annual
+death of one species on the globe is proposed as a mere speculation,
+this, at least, is to imagine no slight degree of instability in the
+animate creation. If we divide the surface of the earth into twenty
+regions of equal area, one of these might comprehend a space of land and
+water about equal in dimensions to Europe, and might contain a twentieth
+part of the million of species which may be assumed to exist in the
+animal kingdom. In this region one species only could, according to the
+rate of mortality before assumed, perish in twenty years, or only five
+out of fifty thousand in the course of a century. But as a considerable
+portion of the whole world belongs to the aquatic classes, with which
+we have a very imperfect acquaintance, we must exclude them from our
+consideration, and, if they constitute half of the entire number, then
+one species only might be lost in forty years among the terrestrial
+tribes. Now the mammalia, whether terrestrial or aquatic, bear so small
+a proportion to other classes of animals, forming less, perhaps, than
+a thousandth part of a whole, that, if the longevity of species in the
+different orders were equal, a vast period must elapse before it would
+come to the turn of this conspicuous class to lose one of their number.
+If one species only of the whole animal kingdom died out in forty years,
+no more than one mammifer might disappear in forty thousand years, in a
+region of the dimensions of Europe.
+
+"It is easy, therefore, to see that in a small portion of such an area,
+in countries, for example, of the size of England and France, periods
+of much greater duration must elapse before it would be possible
+to authenticate the first appearance of one of the larger plants or
+animals, assuming the annual birth and death of one species to be the
+rate of vicissitude in the animal creation throughout the world."(3)
+
+
+In a word, then, said Lyell, it becomes clear that the numberless
+species that have been exterminated in the past have died out one by
+one, just as individuals of a species die, not in vast shoals; if
+whole populations have passed away, it has been not by instantaneous
+extermination, but by the elimination of a species now here, now there,
+much as one generation succeeds another in the life history of any
+single species. The causes which have brought about such gradual
+exterminations, and in the long lapse of ages have resulted in rotations
+of population, are the same natural causes that are still in operation.
+Species have died out in the past as they are dying out in the present,
+under influence of changed surroundings, such as altered climate, or
+the migration into their territory of more masterful species. Past and
+present causes are one--natural law is changeless and eternal.
+
+Such was the essence of the Huttonian doctrine, which Lyell adopted and
+extended, and with which his name will always be associated. Largely
+through his efforts, though of course not without the aid of many other
+workers after a time, this idea--the doctrine of uniformitarianism, it
+came to be called--became the accepted dogma of the geologic world not
+long after the middle of the nineteenth century. The catastrophists,
+after clinging madly to their phantom for a generation, at last
+capitulated without terms: the old heresy became the new orthodoxy, and
+the way was paved for a fresh controversy.
+
+
+THE ORIGIN OF SPECIES
+
+The fresh controversy followed quite as a matter of course. For the idea
+of catastrophism had not concerned the destruction of species merely,
+but their introduction as well. If whole faunas had been extirpated
+suddenly, new faunas had presumably been introduced with equal
+suddenness by special creation; but if species die out gradually,
+the introduction of new species may be presumed to be correspondingly
+gradual. Then may not the new species of a later geological epoch be
+the modified lineal descendants of the extinct population of an earlier
+epoch?
+
+The idea that such might be the case was not new. It had been suggested
+when fossils first began to attract conspicuous attention; and such
+sagacious thinkers as Buffon and Kant and Goethe and Erasmus Darwin
+had been disposed to accept it in the closing days of the eighteenth
+century. Then, in 1809, it had been contended for by one of the early
+workers in systematic paleontology--Jean Baptiste Lamarck, who
+had studied the fossil shells about Paris while Cuvier studied the
+vertebrates, and who had been led by these studies to conclude that
+there had been not merely a rotation but a progression of life on the
+globe. He found the fossil shells--the fossils of invertebrates, as he
+himself had christened them--in deeper strata than Cuvier's vertebrates;
+and he believed that there had been long ages when no higher forms than
+these were in existence, and that in successive ages fishes, and then
+reptiles, had been the highest of animate creatures, before mammals,
+including man, appeared. Looking beyond the pale of his bare facts,
+as genius sometimes will, he had insisted that these progressive
+populations had developed one from another, under influence of changed
+surroundings, in unbroken series.
+
+Of course such a thought as this was hopelessly misplaced in a
+generation that doubted the existence of extinct species, and hardly
+less so in the generation that accepted catastrophism; but it had been
+kept alive by here and there an advocate like Geoffrey Saint-Hilaire,
+and now the banishment of catastrophism opened the way for its more
+respectful consideration. Respectful consideration was given it by Lyell
+in each recurring edition of his Principles, but such consideration led
+to its unqualified rejection. In its place Lyell put forward a modified
+hypothesis of special creation. He assumed that from time to time,
+as the extirpation of a species had left room, so to speak, for a new
+species, such new species had been created de novo; and he supposed that
+such intermittent, spasmodic impulses of creation manifest themselves
+nowadays quite as frequently as at any time in the past. He did not say
+in so many words that no one need be surprised to-day were he to see a
+new species of deer, for example, come up out of the ground before him,
+"pawing to get free," like Milton's lion, but his theory implied as
+much. And that theory, let it be noted, was not the theory of Lyell
+alone, but of nearly all his associates in the geologic world. There is
+perhaps no other fact that will bring home to one so vividly the advance
+in thought of our own generation as the recollection that so crude, so
+almost unthinkable a conception could have been the current doctrine of
+science less than half a century ago.
+
+This theory of special creation, moreover, excluded the current doctrine
+of uniformitarianism as night excludes day, though most thinkers of the
+time did not seem to be aware of the incompatibility of the two ideas.
+It may be doubted whether even Lyell himself fully realized it. If he
+did, he saw no escape from the dilemma, for it seemed to him that
+the record in the rocks clearly disproved the alternative Lamarckian
+hypothesis. And almost with one accord the paleontologists of the
+time sustained the verdict. Owen, Agassiz, Falconer, Barrande, Pictet,
+Forbes, repudiated the idea as unqualifiedly as their great predecessor
+Cuvier had done in the earlier generation. Some of them did, indeed,
+come to believe that there is evidence of a progressive development of
+life in the successive ages, but no such graded series of fossils had
+been discovered as would give countenance to the idea that one species
+had ever been transformed into another. And to nearly every one this
+objection seemed insuperable.
+
+But in 1859 appeared a book which, though not dealing primarily with
+paleontology, yet contained a chapter that revealed the geological
+record in an altogether new light. The book was Charles Darwin's Origin
+of Species, the chapter that wonderful citation of the "Imperfections of
+the Geological Record." In this epoch-making chapter Darwin shows what
+conditions must prevail in any given place in order that fossils shall
+be formed, how unusual such conditions are, and how probable it is that
+fossils once imbedded in sediment of a sea-bed will be destroyed by
+metamorphosis of the rocks, or by denudation when the strata are raised
+above the water-level. Add to this the fact that only small territories
+of the earth have been explored geologically, he says, and it becomes
+clear that the paleontological record as we now possess it shows but
+a mere fragment of the past history of organisms on the earth. It is
+a history "imperfectly kept and written in a changing dialect. Of this
+history we possess the last volume alone, relating only to two or three
+countries. Of this volume only here and there a short chapter has been
+preserved, and of each page only here and there a few lines." For a
+paleontologist to dogmatize from such a record would be as rash, he
+thinks, as "for a naturalist to land for five minutes on a barren point
+of Australia and then discuss the number and range of its productions."
+
+This citation of observations, which when once pointed out seemed almost
+self-evident, came as a revelation to the geological world. In the
+clarified view now possible old facts took on a new meaning. It was
+recalled that Cuvier had been obliged to establish a new order for some
+of the first fossil creatures he examined, and that Buckland had noted
+that the nondescript forms were intermediate in structure between
+allied existing orders. More recently such intermediate forms had been
+discovered over and over; so that, to name but one example, Owen had
+been able, with the aid of extinct species, to "dissolve by gradations
+the apparently wide interval between the pig and the camel." Owen,
+moreover, had been led to speak repeatedly of the "generalized forms"
+of extinct animals, and Agassiz had called them "synthetic or prophetic
+types," these terms clearly implying "that such forms are in fact
+intermediate or connecting links." Darwin himself had shown some years
+before that the fossil animals of any continent are closely related to
+the existing animals of that continent--edentates predominating, for
+example, in South America, and marsupials in Australia. Many observers
+had noted that recent strata everywhere show a fossil fauna more nearly
+like the existing one than do more ancient strata; and that fossils from
+any two consecutive strata are far more closely related to each other
+than are the fossils of two remote formations, the fauna of each
+geological formation being, indeed, in a wide view, intermediate between
+preceding and succeeding faunas.
+
+So suggestive were all these observations that Lyell, the admitted
+leader of the geological world, after reading Darwin's citations, felt
+able to drop his own crass explanation of the introduction of species
+and adopt the transmutation hypothesis, thus rounding out the doctrine
+of uniformitarianism to the full proportions in which Lamarck had
+conceived it half a century before. Not all paleontologists could follow
+him at once, of course; the proof was not yet sufficiently demonstrative
+for that; but all were shaken in the seeming security of their former
+position, which is always a necessary stage in the progress of thought.
+And popular interest in the matter was raised to white heat in a
+twinkling.
+
+So, for the third time in this first century of its existence,
+paleontology was called upon to play a leading role in a controversy
+whose interest extended far beyond the bounds of staid truth-seeking
+science. And the controversy waged over the age of the earth had not
+been more bitter, that over catastrophism not more acrimonious, than
+that which now raged over the question of the transmutation of species.
+The question had implications far beyond the bounds of paleontology, of
+course. The main evidence yet presented had been drawn from quite other
+fields, but by common consent the record in the rocks might furnish a
+crucial test of the truth or falsity of the hypothesis. "He who rejects
+this view of the imperfections of the geological record," said Darwin,
+"will rightly reject the whole theory."
+
+With something more than mere scientific zeal, therefore,
+paleontologists turned anew to the records in the rocks, to inquire what
+evidence in proof or refutation might be found in unread pages of the
+"great stone book." And, as might have been expected, many minds being
+thus prepared to receive new evidence, such evidence was not long
+withheld.
+
+
+FOSSIL MAN
+
+Indeed, at the moment of Darwin's writing a new and very instructive
+chapter of the geologic record was being presented to the public--a
+chapter which for the first time brought man into the story. In 1859
+Dr. Falconer, the distinguished British paleontologist, made a visit
+to Abbeville, in the valley of the Somme, incited by reports that for
+a decade before bad been sent out from there by M. Boucher de Perthes.
+These reports had to do with the alleged finding of flint implements,
+clearly the work of man, in undisturbed gravel-beds, in the midst of
+fossil remains of the mammoth and other extinct animals. What Falconer
+saw there and what came of his visit may best be told in his own words:
+
+"In September of 1856 I made the acquaintance of my distinguished friend
+M. Boucher de Perthes," wrote Dr. Falconer, "on the introduction of M.
+Desnoyers at Paris, when he presented to me the earlier volume of his
+Antiquites celtiques, etc., with which I thus became acquainted for the
+first time. I was then fresh from the examination of the Indian fossil
+remains of the valley of the Jumna; and the antiquity of the human race
+being a subject of interest to both, we conversed freely about it,
+each from a different point of view. M. de Perthes invited me to visit
+Abbeville, in order to examine his antediluvian collection, fossil and
+geological, gleaned from the valley of the Somme. This I was unable to
+accomplish then, but I reserved it for a future occasion.
+
+"In October, 1856, having determined to proceed to Sicily, I arranged
+by correspondence with M. Boucher de Perthes to visit Abbeville on my
+journey through France. I was at the time in constant communication
+with Mr. Prestwich about the proofs of the antiquity of the human race
+yielded by the Broxham Cave, in which he took a lively interest; and
+I engaged to communicate to him the opinions at which I should arrive,
+after my examination of the Abbeville collection. M. de Perthes gave me
+the freest access to his materials, with unreserved explanations of all
+the facts of the case that had come under his observation; and having
+considered his Menchecourt Section, taken with such scrupulous care, and
+identified the molars of elephas primigenius, which he had exhumed with
+his own hands deep in that section, along with flint weapons, presenting
+the same character as some of those found in the Broxham Cave, I arrived
+at the conviction that they were of contemporaneous age, although I
+was not prepared to go along with M. de Perthes in all his inferences
+regarding the hieroglyphics and in an industrial interpretation of the
+various other objects which he had met with."(4)
+
+
+That Dr. Falconer was much impressed by the collection of M. de
+Perthes is shown in a communication which he sent at once to his friend
+Prestwich:
+
+"I have been richly rewarded," he exclaims. "His collection of wrought
+flint implements, and of the objects of every description associated
+with them, far exceeds everything I expected to have seen, especially
+from a single locality. He has made great additions, since the
+publication of his first volume, in the second, which I now have by
+me. He showed me flint hatchets which HE HAD DUG UP with his own hands,
+mixed INDISCRIMINATELY with molars of elephas primigenius. I examined
+and identified plates of the molars and the flint objects which were
+got along with them. Abbeville is an out-of-the-way place, very little
+visited; and the French savants who meet him in Paris laugh at Monsieur
+de Perthes and his researches. But after devoting the greater part of
+a day to his vast collection, I am perfectly satisfied that there is
+a great deal of fair presumptive evidence in favor of many of his
+speculations regarding the remote antiquity of these industrial objects
+and their association with animals now extinct. M. Boucher's hotel
+is, from the ground floor to garret, a continued museum, filled with
+pictures, mediaeval art, and Gaulish antiquities, including antediluvian
+flint-knives, fossil-bones, etc. If, during next summer, you should
+happen to be paying a visit to France, let me strongly recommend you to
+come to Abbeville. I am sure you would be richly rewarded."(5)
+
+
+This letter aroused the interest of the English geologists, and in the
+spring of 1859 Prestwich and Mr. (afterwards Sir John) Evans made a
+visit to Abbeville to see the specimens and examine at first hand the
+evidences as pointed out by Dr. Falconer. "The evidence yielded by the
+valley of the Somme," continues Falconer, in speaking of this visit,
+"was gone into with the scrupulous care and severe and exhaustive
+analysis which are characteristic of Mr. Prestwich's researches. The
+conclusions to which he was conducted were communicated to the Royal
+Society on May 12, 1859, in his celebrated memoir, read on May 26th and
+published in the Philosophical Transactions of 1860, which, in addition
+to researches made in the valley of the Somme, contained an account of
+similar phenomena presented by the valley of the Waveney, near Hoxne, in
+Suffolk. Mr. Evans communicated to the Society of Antiquaries a memoir
+on the character and geological position of the 'Flint Implements in the
+Drift,' which appeared in the Archaeologia for 1860. The results arrived
+at by Mr. Prestwich were expressed as follows:
+
+"First. That the flint implements are the result of design and the work
+of man.
+
+"Second. That they are found in beds of gravel, sand, and clay, which
+have never been artificially disturbed.
+
+"Third. That they occur associated with the remains of land,
+fresh-water, and marine testacea, of species now living, and most of
+them still common in the same neighborhood, and also with the remains of
+various mammalia--a few species now living, but more of extinct forms.
+
+"Fourth. That the period at which their entombment took place was
+subsequent to the bowlder-clay period, and to that extent post-glacial;
+and also that it was among the latest in geological time--one apparently
+anterior to the surface assuming its present form, so far as it regards
+some of the minor features."(6)
+
+
+These reports brought the subject of the very significant human fossils
+at Abbeville prominently before the public; whereas the publications of
+the original discoverer, Boucher de Perthes, bearing date of 1847, had
+been altogether ignored. A new aspect was thus given to the current
+controversy.
+
+As Dr. Falconer remarked, geology was now passing through the same
+ordeal that astronomy passed in the age of Galileo. But the times were
+changed since the day when the author of the Dialogues was humbled
+before the Congregation of the Index, and now no Index Librorum
+Prohibitorum could avail to hide from eager human eyes such pages of
+the geologic story as Nature herself had spared. Eager searchers were
+turning the leaves with renewed zeal everywhere, and with no small
+measure of success. In particular, interest attached just at this
+time to a human skull which Dr. Fuhlrott had discovered in a cave at
+Neanderthal two or three years before--a cranium which has ever since
+been famous as the Neanderthal skull, the type specimen of what modern
+zoologists are disposed to regard as a distinct species of man, Homo
+neanderthalensis. Like others of the same type since discovered at Spy,
+it is singularly simian in character--low-arched, with receding forehead
+and enormous, protuberant eyebrows. When it was first exhibited to the
+scientists at Berlin by Dr. Fuhlrott, in 1857, its human character was
+doubted by some of the witnesses; of that, however, there is no present
+question.
+
+This interesting find served to recall with fresh significance some
+observations that had been made in France and Belgium a long generation
+earlier, but whose bearings had hitherto been ignored. In 1826 MM.
+Tournal and Christol had made independent discoveries of what they
+believed to be human fossils in the caves of the south of France; and
+in 1827 Dr. Schmerling had found in the cave of Engis, in Westphalia,
+fossil bones of even greater significance. Schmerling's explorations
+had been made with the utmost care, and patience. At Engis he had
+found human bones, including skulls, intermingled with those of extinct
+mammals of the mammoth period in a way that left no doubt in his mind
+that all dated from the same geological epoch. He bad published a full
+account of his discoveries in an elaborate monograph issued in 1833.
+
+But at that time, as it chanced, human fossils were under a ban as
+effectual as any ever pronounced by canonical index, though of far
+different origin. The oracular voice of Cuvier had declared against the
+authenticity of all human fossils. Some of the bones brought him for
+examination the great anatomist had pettishly pitched out of the window,
+declaring them fit only for a cemetery, and that had settled the matter
+for a generation: the evidence gathered by lesser workers could avail
+nothing against the decision rendered at the Delphi of Science. But no
+ban, scientific or canonical, can longer resist the germinative power of
+a fact, and so now, after three decades of suppression, the truth which
+Cuvier had buried beneath the weight of his ridicule burst its bonds,
+and fossil man stood revealed, if not as a flesh-and-blood, at least as
+a skeletal entity.
+
+The reception now accorded our prehistoric ancestor by the progressive
+portion of the scientific world amounted to an ovation; but the
+unscientific masses, on the other hand, notwithstanding their usual
+fondness for tracing remote genealogies, still gave the men of Engis
+and Neanderthal the cold shoulder. Nor were all of the geologists quite
+agreed that the contemporaneity of these human fossils with the animals
+whose remains had been mingled with them had been fully established. The
+bare possibility that the bones of man and of animals that long preceded
+him had been swept together into the eaves in successive ages, and
+in some mysterious way intermingled there, was clung to by the
+conservatives as a last refuge. But even this small measure of security
+was soon to be denied them, for in 1865 two associated workers,
+M. Edouard Lartet and Mr. Henry Christy, in exploring the caves of
+Dordogne, unearthed a bit of evidence against which no such objection
+could be urged. This momentous exhibit was a bit of ivory, a fragment
+of the tusk of a mammoth, on which was scratched a rude but unmistakable
+outline portrait of the mammoth itself. If all the evidence as to man's
+antiquity before presented was suggestive merely, here at last was
+demonstration; for the cave-dwelling man could not well have drawn the
+picture of the mammoth unless he had seen that animal, and to admit that
+man and the mammoth had been contemporaries was to concede the entire
+case. So soon, therefore, as the full import of this most instructive
+work of art came to be realized, scepticism as to man's antiquity was
+silenced for all time to come.
+
+In the generation that has elapsed since the first drawing of the
+cave-dweller artist was discovered, evidences of the wide-spread
+existence of man in an early epoch have multiplied indefinitely, and
+to-day the paleontologist traces the history of our race back beyond the
+iron and bronze ages, through a neolithic or polished-stone age, to
+a paleolithic or rough-stone age, with confidence born of unequivocal
+knowledge. And he looks confidently to the future explorer of the
+earth's fossil records to extend the history back into vastly more
+remote epochs, for it is little doubted that paleolithic man, the most
+ancient of our recognized progenitors, is a modern compared to those
+generations that represented the real childhood of our race.
+
+
+THE FOSSIL-BEDS OF AMERICA
+
+Coincidently with the discovery of these highly suggestive pages of the
+geologic story, other still more instructive chapters were being brought
+to light in America. It was found that in the Rocky Mountain region, in
+strata found in ancient lake beds, records of the tertiary period, or
+age of mammals, had been made and preserved with fulness not approached
+in any other region hitherto geologically explored. These records were
+made known mainly by Professors Joseph Leidy, O. C. Marsh, and E. D.
+Cope, working independently, and more recently by numerous younger
+paleontologists.
+
+The profusion of vertebrate remains thus brought to light quite beggars
+all previous exhibits in point of mere numbers. Professor Marsh, for
+example, who was first in the field, found three hundred new tertiary
+species between the years 1870 and 1876. Meanwhile, in cretaceous
+strata, he unearthed remains of about two hundred birds with teeth, six
+hundred pterodactyls, or flying dragons, some with a spread of wings
+of twenty-five feet, and one thousand five hundred mosasaurs of the
+sea-serpent type, some of them sixty feet or more in length. In a single
+bed of Jurassic rock, not larger than a good-sized lecture-room, he
+found the remains of one hundred and sixty individuals of mammals,
+representing twenty species and nine genera; while beds of the same age
+have yielded three hundred reptiles, varying from the size of a rabbit
+to sixty or eighty feet in length.
+
+But the chief interest of these fossils from the West is not their
+number but their nature; for among them are numerous illustrations of
+just such intermediate types of organisms as must have existed in the
+past if the succession of life on the globe has been an unbroken lineal
+succession. Here are reptiles with bat-like wings, and others with
+bird-like pelves and legs adapted for bipedal locomotion. Here are
+birds with teeth, and other reptilian characters. In short, what with
+reptilian birds and birdlike reptiles, the gap between modern reptiles
+and birds is quite bridged over. In a similar way, various diverse
+mammalian forms, as the tapir, the rhinoceros, and the horse, are linked
+together by fossil progenitors. And, most important of all, Professor
+Marsh has discovered a series of mammalian remains, occurring in
+successive geological epochs, which are held to represent beyond cavil
+the actual line of descent of the modern horse; tracing the lineage
+of our one-toed species back through two and three toed forms, to an
+ancestor in the eocene or early tertiary that had four functional toes
+and the rudiment of a fifth. This discovery is too interesting and too
+important not to be detailed at length in the words of the discoverer.
+
+
+Marsh Describes the Fossil Horse
+
+"It is a well-known fact," says Professor Marsh, "that the Spanish
+discoverers of America discovered no horses on this continent, and that
+the modern horse (Equus caballus, Linn.) was subsequently introduced
+from the Old World. It is, however, not so generally known that these
+animals had formerly been abundant here, and that long before, in
+tertiary time, near relatives of the horse, and probably his ancestors,
+existed in the far West in countless numbers and in a marvellous variety
+of forms. The remains of equine mammals, now known from the tertiary and
+quaternary deposits of this country, already represent more than double
+the number of genera and species hitherto found in the strata of the
+eastern hemisphere, and hence afford most important aid in tracing out
+the genealogy of the horses still existing.
+
+"The animals of this group which lived in America during the three
+diversions of the tertiary period were especially numerous in the Rocky
+Mountain regions, and their remains are well preserved in the old lake
+basins which then covered so much of that country. The most ancient
+of these lakes--which extended over a considerable part of the present
+territories of Wyoming and Utah--remained so long in eocene times that
+the mud and sand, slowly deposited in it, accumulated to more than a
+mile in vertical thickness. In these deposits vast numbers of tropical
+animals were entombed, and here the oldest equine remains occur,
+four species of which have been described. These belong to the genus
+Orohippus (Marsh), and are all of a diminutive size, hardly bigger than
+a fox. The skeletons of these animals resemble that of the horse in many
+respects, much more indeed than any other existing species, but, instead
+of the single toe on each foot, so characteristic of all modern equines,
+the various species of Orohippus had four toes before and three behind,
+all of which reached the ground. The skull, too, was proportionately
+shorter, and the orbit was not enclosed behind by a bridge of bone.
+There were fifty four teeth in all, and the premolars were larger than
+the molars. The crowns of these teeth were very short. The canine teeth
+were developed in both sexes, and the incisors did not have the "mark"
+which indicates the age of the modern horse. The radius and ulna were
+separate, and the latter was entire through the whole length. The tibia
+and fibula were distinct. In the forefoot all the digits except the
+pollex, or first, were well developed. The third digit is the largest,
+and its close resemblance to that of the horse is clearly marked. The
+terminal phalanx, or coffin-bone, has a shallow median bone in front,
+as in many species of this group in the later tertiary. The fourth digit
+exceeds the second in size, and the second is much the shortest of all.
+Its metacarpal bone is considerably curved outward. In the hind-foot
+of this genus there are but three digits. The fourth metatarsal is much
+larger than the second.
+
+"The larger number of equine mammals now known from the tertiary
+deposits of this country, and their regular distributions through the
+subdivisions of this formation, afford a good opportunity to ascertain
+the probable descent of the modern horse. The American representative of
+the latter is the extinct Equus fraternus (Leidy), a species almost, if
+not wholly, identical with the Old World Equus caballus (Linnaeus), to
+which our recent horse belongs. Huxley has traced successfully the later
+genealogy of the horse through European extinct forms, but the line in
+America was probably a more direct one, and the record is more complete.
+Taking, then, as the extreme of a series, Orohippus agilis (Marsh),
+from the eocene, and Equus fraternus (Leidy), from the quaternary,
+intermediate forms may be intercalated with considerable certainty
+from thirty or more well-marked species that lived in the intervening
+periods. The natural line of descent would seem to be through the
+following genera: Orohippus, of the eocene; Miohippus and Anchitherium,
+of the miocene; Anchippus, Hipparion, Protohippus, Phohippus, of the
+pliocene; and Equus, quaternary and recent.
+
+"The most marked changes undergone by the successive equine genera are
+as follows: First, increase in size; second, increase in speed, through
+concentration of limb bones; third, elongation of head and neck, and
+modifications of skull. The eocene Orohippus was the size of a fox.
+Miohippus and Anchitherium, from the miocene, were about as large as a
+sheep. Hipparion and Pliohippus, of the pliocene, equalled the ass in
+height; while the size of the quaternary Equus was fully up to that of a
+modern horse.
+
+"The increase of speed was equally well marked, and was a direct
+result of the gradual formation of the limbs. The latter were slowly
+concentrated by the reduction of their lateral elements and enlargement
+of the axial bone, until the force exerted by each limb came to act
+directly through its axis in the line of motion. This concentration is
+well seen--e.g., in the fore-limb. There was, first, a change in the
+scapula and humerus, especially in the latter, which facilitated motion
+in one line only; second, an expansion of the radius and reduction of
+the ulna, until the former alone remained entire and effective; third,
+a shortening of all the carpal bones and enlargement of the median ones,
+insuring a firmer wrist; fourth, an increase of size of the third digit,
+at the expense of those of each side, until the former alone supported
+the limb.
+
+"Such is, in brief, a general outline of the more marked changes that
+seemed to have produced in America the highly specialized modern Equus
+from his diminutive four-toed predecessor, the eocene Orohippus. The
+line of descent appears to have been direct, and the remains now known
+supply every important intermediate form. It is, of course, impossible
+to say with certainty through which of the three-toed genera of the
+pliocene that lived together the succession came. It is not impossible
+that the latter species, which appear generically identical, are the
+descendants of more distinct pliocene types, as the persistent tendency
+in all the earlier forms was in the same direction. Considering the
+remarkable development of the group through the tertiary period, and
+its existence even later, it seems very strange that none of the species
+should have survived, and that we are indebted for our present horse to
+the Old World."(7)
+
+
+PALEONTOLOGY OF EVOLUTION
+
+These and such-like revelations have come to light in our own time--are,
+indeed, still being disclosed. Needless to say, no index of any sort now
+attempts to conceal them; yet something has been accomplished towards
+the same end by the publication of the discoveries in Smithsonian
+bulletins and in technical memoirs of government surveys. Fortunately,
+however, the results have been rescued from that partial oblivion by
+such interpreters as Professors Huxley and Cope, so the unscientific
+public has been allowed to gain at least an inkling of the wonderful
+progress of paleontology in our generation.
+
+The writings of Huxley in particular epitomize the record. In 1862 he
+admitted candidly that the paleontological record as then known, so far
+as it bears on the doctrine of progressive development, negatives
+that doctrine. In 1870 he was able to "soften somewhat the Brutus-like
+severity" of his former verdict, and to assert that the results of
+recent researches seem "to leave a clear balance in favor of the
+doctrine of the evolution of living forms one from another." Six years
+later, when reviewing the work of Marsh in America and of Gaudry
+in Pikermi, he declared that, "on the evidence of paleontology, the
+evolution of many existing forms of animal life from their predecessors
+is no longer an hypothesis, but an historical fact." In 1881 he
+asserted that the evidence gathered in the previous decade had been so
+unequivocal that, had the transmutation hypothesis not existed, "the
+paleontologist would have had to invent it."
+
+Since then the delvers after fossils have piled proof on proof in
+bewildering profusion. The fossil-beds in the "bad lands" of western
+America seem inexhaustible. And in the Connecticut River Valley near
+relatives of the great reptiles which Professor Marsh and others
+have found in such profusion in the West left their tracks on the
+mud-flats--since turned to sandstone; and a few skeletons also have been
+found. The bodies of a race of great reptiles that were the lords of
+creation of their day have been dissipated to their elements, while the
+chance indentations of their feet as they raced along the shores, mere
+footprints on the sands, have been preserved among the most imperishable
+of the memory-tablets of the world.
+
+Of the other vertebrate fossils that have been found in the eastern
+portions of America, among the most abundant and interesting are the
+skeletons of mastodons. Of these one of the largest and most complete is
+that which was unearthed in the bed of a drained lake near Newburg, New
+York, in 1845. This specimen was larger than the existing elephants,
+and had tusks eleven feet in length. It was mounted and described by Dr.
+John C. Warren, of Boston, and has been famous for half a century as the
+"Warren mastodon."
+
+But to the student of racial development as recorded by the fossils all
+these sporadic finds have but incidental interest as compared with the
+rich Western fossil-beds to which we have already referred. From records
+here unearthed, the racial evolution of many mammals has in the past few
+years been made out in greater or less detail. Professor Cope has traced
+the ancestry of the camels (which, like the rhinoceroses, hippopotami,
+and sundry other forms now spoken of as "Old World," seem to have had
+their origin here) with much completeness.
+
+A lemuroid form of mammal, believed to be of the type from which man
+has descended, has also been found in these beds. It is thought that the
+descendants of this creature, and of the other "Old-World" forms
+above referred to, found their way to Asia, probably, as suggested by
+Professor Marsh, across a bridge at Bering Strait, to continue their
+evolution on the other hemisphere, becoming extinct in the land of their
+nativity. The ape-man fossil found in the tertiary strata of the island
+of Java in 1891 by the Dutch surgeon Dr. Eugene Dubois, and named
+Pithecanthropus erectus, may have been a direct descendant of the
+American tribe of primitive lemurs, though this is only a conjecture.
+
+Not all the strange beasts which have left their remains in our "bad
+lands" are represented by living descendants. The titanotheres, or
+brontotheridae, for example, a gigantic tribe, offshoots of the
+same stock which produced the horse and rhinoceros, represented the
+culmination of a line of descent. They developed rapidly in a geological
+sense, and flourished about the middle of the tertiary period; then,
+to use Agassiz's phrase," time fought against them." The story of their
+evolution has been worked out by Professors Leidy, Marsh, Cope, and H.
+F. Osborne.
+
+A recent bit of paleontological evidence bearing on the question of
+the introduction of species is that presented by Dr. J. L. Wortman in
+connection with the fossil lineage of the edentates. It was suggested by
+Marsh, in 1877, that these creatures, whose modern representatives are
+all South American, originated in North America long before the two
+continents had any land connection. The stages of degeneration by which
+these animals gradually lost the enamel from their teeth, coming finally
+to the unique condition of their modern descendants of the sloth tribe,
+are illustrated by strikingly graded specimens now preserved in the
+American Museum of Natural History, as shown by Dr. Wortman.
+
+All these and a multitude of other recent observations that cannot be
+even outlined here tell the same story. With one accord paleontologists
+of our time regard the question of the introduction of new species as
+solved. As Professor Marsh has said, "to doubt evolution today is to
+doubt science; and science is only another name for truth."
+
+Thus the third great battle over the meaning of the fossil records has
+come to a conclusion. Again there is a truce to controversy, and it may
+seem to the casual observer that the present stand of the science of
+fossils is final and impregnable. But does this really mean that a full
+synopsis of the story of paleontology has been told? Or do we only await
+the coming of the twentieth-century Lamarck or Darwin, who shall
+attack the fortified knowledge of to-day with the batteries of a new
+generalization?
+
+
+
+
+IV. THE ORIGIN AND DEVELOPMENT OF MODERN GEOLOGY
+
+
+JAMES HUTTON
+
+One might naturally suppose that the science of the earth which lies at
+man's feet would at least have kept pace with the science of the distant
+stars. But perhaps the very obviousness of the phenomena delayed the
+study of the crust of the earth. It is the unattainable that allures and
+mystifies and enchants the developing mind. The proverbial child spurns
+its toys and cries for the moon.
+
+So in those closing days of the eighteenth century, when astronomers had
+gone so far towards explaining the mysteries of the distant portions
+of the universe, we find a chaos of opinion regarding the structure
+and formation of the earth. Guesses were not wanting to explain the
+formation of the world, it is true, but, with one or two exceptions,
+these are bizarre indeed. One theory supposed the earth to have been at
+first a solid mass of ice, which became animated only after a comet had
+dashed against it. Other theories conceived the original globe as a mass
+of water, over which floated vapors containing the solid elements, which
+in due time were precipitated as a crust upon the waters. In a word, the
+various schemes supposed the original mass to have been ice, or water,
+or a conglomerate of water and solids, according to the random fancies
+of the theorists; and the final separation into land and water was
+conceived to have taken place in all the ways which fancy, quite
+unchecked by any tenable data, could invent.
+
+Whatever important changes in the general character of the surface of
+the globe were conceived to have taken place since its creation were
+generally associated with the Mosaic: deluge, and the theories which
+attempted to explain this catastrophe were quite on a par with
+those which dealt with a remoter period of the earth's history. Some
+speculators, holding that the interior of the globe is a great abyss
+of waters, conceived that the crust had dropped into this chasm and had
+thus been inundated. Others held that the earth had originally revolved
+on a vertical axis, and that the sudden change to its present position
+bad caused the catastrophic shifting of its oceans. But perhaps the
+favorite theory was that which supposed a comet to have wandered near
+the earth, and in whirling about it to have carried the waters, through
+gravitation, in a vast tide over the continents.
+
+Thus blindly groped the majority of eighteenth-century philosophers in
+their attempts to study what we now term geology. Deluded by the old
+deductive methods, they founded not a science, but the ghost of a
+science, as immaterial and as unlike anything in nature as any other
+phantom that could be conjured from the depths of the speculative
+imagination. And all the while the beckoning earth lay beneath the feet
+of these visionaries; but their eyes were fixed in air.
+
+At last, however, there came a man who had the penetration to see that
+the phantom science of geology needed before all else a body corporeal,
+and who took to himself the task of supplying it. This was Dr.
+James Hutton, of Edinburgh, physician, farmer, and manufacturing
+chemist--patient, enthusiastic, level-headed devotee of science.
+Inspired by his love of chemistry to study the character of rocks and
+soils, Hutton had not gone far before the earth stood revealed to him
+in a new light. He saw, what generations of predecessors had blindly
+refused to see, that the face of nature everywhere, instead of being
+rigid and immutable, is perennially plastic, and year by year is
+undergoing metamorphic changes. The solidest rocks are day by day
+disintegrated slowly, but none the less surely, by wind and rain and
+frost, by mechanical attrition and chemical decomposition, to form the
+pulverized earth and clay. This soil is being swept away by perennial
+showers, and carried off to the oceans. The oceans themselves beat on
+their shores, and eat insidiously into the structure of sands and rocks.
+Everywhere, slowly but surely, the surface of the land is being worn
+away; its substance is being carried to burial in the seas.
+
+Should this denudation continue long enough, thinks Hutton, the entire
+surface of the continents must be worn away. Should it be continued LONG
+ENOUGH! And with that thought there flashes on his mind an inspiring
+conception--the idea that solar time is long, indefinitely long. That
+seems a simple enough thought--almost a truism--to the twentieth-century
+mind; but it required genius to conceive it in the eighteenth. Hutton
+pondered it, grasped its full import, and made it the basis of his
+hypothesis, his "theory of the earth."
+
+
+MODERN GEOLOGY
+
+The hypothesis is this--that the observed changes of the surface of
+the earth, continued through indefinite lapses of time, must result in
+conveying all the land at last to the sea; in wearing continents away
+till the oceans overflow them. What then? Why, as the continents wear
+down, the oceans are filling up. Along their bottoms the detritus of
+wasted continents is deposited in strata, together with the bodies of
+marine animals and vegetables. Why might not this debris solidify to
+form layers of rocks--the basis of new continents? Why not, indeed?
+
+But have we any proof that such formation of rocks in an ocean-bed has,
+in fact, occurred? To be sure we have. It is furnished by every bed
+of limestone, every outcropping fragment of fossil-bearing rock, every
+stratified cliff. How else than through such formation in an ocean-bed
+came these rocks to be stratified? How else came they to contain the
+shells of once living organisms imbedded in their depths? The ancients,
+finding fossil shells imbedded in the rocks, explained them as mere
+freaks of "nature and the stars." Less superstitious generations had
+repudiated this explanation, but had failed to give a tenable solution
+of the mystery. To Hutton it is a mystery no longer. To him it seems
+clear that the basis of the present continents was laid in ancient
+sea-beds, formed of the detritus of continents yet more ancient.
+
+But two links are still wanting to complete the chain of Hutton's
+hypothesis. Through what agency has the ooze of the ocean-bed been
+transformed into solid rock? and through what agency has this rock been
+lifted above the surface of the water to form new continents? Hutton
+looks about him for a clew, and soon he finds it. Everywhere about us
+there are outcropping rocks that are not stratified, but which give
+evidence to the observant eye of having once been in a molten state.
+Different minerals are mixed together; pebbles are scattered through
+masses of rock like plums in a pudding; irregular crevices in otherwise
+solid masses of rock--so-called veinings--are seen to be filled with
+equally solid granite of a different variety, which can have gotten
+there in no conceivable way, so Hutton thinks, but by running in while
+molten, as liquid metal is run into the moulds of the founder. Even
+the stratified rocks, though they seemingly have not been melted, give
+evidence in some instances of having been subjected to the action of
+heat. Marble, for example, is clearly nothing but calcined limestone.
+
+With such evidence before him, Hutton is at no loss to complete his
+hypothesis. The agency which has solidified the ocean-beds, he says,
+is subterranean heat. The same agency, acting excessively, has produced
+volcanic cataclysms, upheaving ocean-beds to form continents. The rugged
+and uneven surfaces of mountains, the tilted and broken character
+of stratified rocks everywhere, are the standing witnesses of these
+gigantic upheavals.
+
+And with this the imagined cycle is complete. The continents, worn away
+and carried to the sea by the action of the elements, have been made
+over into rocks again in the ocean-beds, and then raised once more into
+continents. And this massive cycle, In Hutton's scheme, is supposed
+to have occurred not once only, but over and over again, times without
+number. In this unique view ours is indeed a world without beginning
+and without end; its continents have been making and unmaking in endless
+series since time began.
+
+Hutton formulated his hypothesis while yet a young man, not long after
+the middle of the century. He first gave it publicity in 1781, in a
+paper before the Royal Society of Edinburgh:
+
+"A solid body of land could not have answered the purpose of a habitable
+world," said Hutton, "for a soil is necessary to the growth of plants,
+and a soil is nothing but the material collected from the destruction of
+the solid land. Therefore the surface of this land inhabited by man, and
+covered by plants and animals, is made by nature to decay, in dissolving
+from that hard and compact state in which it is found; and this soil
+is necessarily washed away by the continual circulation of the water
+running from the summits of the mountains towards the general receptacle
+of that fluid.
+
+"The heights of our land are thus levelled with our shores, our fertile
+plains are formed from the ruins of the mountains; and those travelling
+materials are still pursued by the moving water, and propelled along the
+inclined surface of the earth. These movable materials, delivered into
+the sea, cannot, for a long continuance, rest upon the shore, for by the
+agitation of the winds, the tides, and the currents every movable thing
+is carried farther and farther along the shelving bottom of the sea,
+towards the unfathomable regions of the ocean.
+
+"If the vegetable soil is thus constantly removed from the surface of
+the land, and if its place is then to be supplied from the dissolution
+of the solid earth as here represented, we may perceive an end to this
+beautiful machine; an end arising from no error in its constitution as
+a world, but from that destructibility of its land which is so necessary
+in the system of the globe, in the economy of life and vegetation.
+
+"The immense time necessarily required for the total destruction of
+the land must not be opposed to that view of future events which is
+indicated by the surest facts and most approved principles. Time, which
+measures everything in our idea, and is often deficient to our schemes,
+is to nature endless and as nothing; it cannot limit that by which alone
+it has existence; and as the natural course of time, which to us seems
+infinite, cannot be bounded by any operation that may have an end, the
+progress of things upon this globe that in the course of nature cannot
+be limited by time must proceed in a continual succession. We are,
+therefore, to consider as inevitable the destruction of our land, so far
+as effected by those operations which are necessary in the purpose of
+the globe, considered as a habitable world, and so far as we have
+not examined any other part of the economy of nature, in which other
+operations and a different intention might appear.
+
+"We have now considered the globe of this earth as a machine,
+constructed upon chemical as well as mechanical principles, by which its
+different parts are all adapted, in form, in quality, and quantity, to a
+certain end--an end attained with certainty of success, and an end from
+which we may perceive wisdom in contemplating the means employed.
+
+"But is this world to be considered thus merely as a machine, to last no
+longer than its parts retain their present position, their proper forms
+and qualities? Or may it not be also considered as an organized body
+such as has a constitution, in which the necessary decay of the machine
+is naturally repaired in the exertion of those productive powers by
+which it has been formed?
+
+"This is the view in which we are now to examine the globe; to see if
+there be, in the constitution of the world, a reproductive operation
+by which a ruined constitution may be again repaired and a duration of
+stability thus procured to the machine considered as a world containing
+plants and animals.
+
+"If no such reproductive power, or reforming operation, after due
+inquiry, is to be found in the constitution of this world, we should
+have reason to conclude that the system of this earth has either been
+intentionally made imperfect or has not been the work of infinite power
+and wisdom."(1)
+
+
+This, then, was the important question to be answered--the question of
+the constitution of the globe. To accomplish this, it was necessary,
+first of all, to examine without prejudice the material already in hand,
+adding such new discoveries from time to time as might be made, but
+always applying to the whole unvarying scientific principles and
+inductive methods of reasoning.
+
+"If we are to take the written history of man for the rule by which we
+should judge of the time when the species first began," said Hutton,
+"that period would be but little removed from the present state of
+things. The Mosaic history places this beginning of man at no great
+distance; and there has not been found, in natural history, any document
+by which high antiquity might be attributed to the human race. But
+this is not the case with regard to the inferior species of animals,
+particularly those which inhabit the ocean and its shores. We find
+in natural history monuments which prove that those animals had long
+existed; and we thus procure a measure for the computation of a period
+of time extremely remote, though far from being precisely ascertained.
+
+"In examining things present, we have data from which to reason with
+regard to what has been; and from what actually has been we have
+data for concluding with regard to that which is to happen hereafter.
+Therefore, upon the supposition that the operations of nature are
+equable and steady, we find, in natural appearances, means for
+concluding a certain portion of time to have necessarily elapsed in the
+production of those events of which we see the effects.
+
+"It is thus that, in finding the relics of sea animals of every kind
+in the solid body of our earth, a natural history of those animals
+is formed, which includes a certain portion of time; and for the
+ascertaining this portion of time we must again have recourse to the
+regular operations of this world. We shall thus arrive at facts which
+indicate a period to which no other species of chronology is able to
+remount.
+
+"We find the marks of marine animals in the most solid parts of the
+earth, consequently those solid parts have been formed after the ocean
+was inhabited by those animals which are proper to that fluid medium.
+If, therefore, we knew the natural history of these solid parts, and
+could trace the operations of the globe by which they have been formed,
+we would have some means for computing the time through which those
+species of animals have continued to live. But how shall we describe a
+process which nobody has seen performed and of which no written history
+gives any account? This is only to be investigated, first, in examining
+the nature of those solid bodies the history of which we want to know;
+and, secondly, in examining the natural operations of the globe, in
+order to see if there now exist such operations as, from the nature of
+the solid bodies, appear to have been necessary for their formation.
+
+"There are few beds of marble or limestone in which may not be found
+some of those objects which indicate the marine object of the mass. If,
+for example, in a mass of marble taken from a quarry upon the top of the
+Alps or Andes there shall be found one cockle-shell or piece of coral,
+it must be concluded that this bed of stone has been originally formed
+at the bottom of the sea, as much as another bed which is evidently
+composed almost altogether of cockle-shells and coral. If one bed of
+limestone is thus found to have been of marine origin, every concomitant
+bed of the same kind must be also concluded to have been formed in the
+same manner.
+
+"In those calcareous strata, which are evidently of marine origin,
+there are many parts which are of sparry structure--that is to say, the
+original texture of those beds in such places has been dissolved, and a
+new structure has been assumed which is peculiar to a certain state of
+the calcareous earth. This change is produced by crystallization, in
+consequence of a previous state of fluidity, which has so disposed
+the concerting parts as to allow them to assume a regular shape and
+structure proper to that substance. A body whose external form has
+been modified by this process is called a CRYSTAL; one whose internal
+arrangement of parts is determined by it is said to be of a SPARRY
+STRUCTURE, and this is known from its fracture.
+
+"There are, in all the regions of the earth, huge masses of calcareous
+matter in that crystalline form or sparry state in which, perhaps, no
+vestige can be found of any organized body, nor any indication that such
+calcareous matter has belonged to animals; but as in other masses this
+sparry structure or crystalline state is evidently assumed by the marine
+calcareous substances in operations which are natural to the globe,
+and which are necessary to the consolidation of the strata, it does not
+appear that the sparry masses in which no figured body is formed
+have been originally different from other masses, which, being only
+crystallized in part, and in part still retaining their original form,
+have ample evidence of their marine origin.
+
+"We are led, in this manner, to conclude that all the strata of the
+earth, not only those consisting of such calcareous masses, but others
+superincumbent upon these, have had their origin at the bottom of the
+sea.
+
+"The general amount of our reasoning is this, that nine-tenths, perhaps,
+or ninety-nine-hundredths, of this earth, so far as we see, have been
+formed by natural operations of the globe in collecting loose materials
+and depositing them at the bottom of the sea; consolidating those
+collections in various degrees, and either elevating those consolidated
+masses above the level on which they were formed or lowering the level
+of that sea.
+
+"Let us now consider how far the other proposition of strata being
+elevated by the power of heat above the level of the sea may be
+confirmed from the examination of natural appearances. The strata formed
+at the bottom of the ocean are necessarily horizontal in their position,
+or nearly so, and continuous in their horizontal direction or extent.
+They may be changed and gradually assume the nature of each other, so
+far as concerns the materials of which they are formed, but there cannot
+be any sudden change, fracture, or displacement naturally in the body
+of a stratum. But if the strata are cemented by the heat of fusion,
+and erected with an expansive power acting below, we may expect to find
+every species of fracture, dislocation, and contortion in those bodies
+and every degree of departure from a horizontal towards a vertical
+position.
+
+"The strata of the globe are actually found in every possible position:
+for from horizontal they are frequently found vertical; from continuous
+they are broken and separated in every possible direction; and from a
+plane they are bent and doubled. It is impossible that they could have
+originally been formed, by the known laws of nature, in their present
+state and position; and the power that has been necessarily required
+for their change has not been inferior to that which might have been
+required for their elevation from the place in which they have been
+formed."(2)
+
+
+From all this, therefore, Hutton reached the conclusion that the
+elevation of the bodies of land above the water on the earth's surface
+had been effected by the same force which had acted in consolidating the
+strata and giving them stability. This force he conceived to be exerted
+by the expansion of heated matter.
+
+"We have," he said, "been now supposing that the beginning of our
+present earth had been laid in the bottom of the ocean, at the
+completion of the former land, but this was only for the sake of
+distinctness. The just view is this, that when the former land of the
+globe had been complete, so as to begin to waste and be impaired by
+the encroachment of the sea, the present land began to appear above the
+surface of the ocean. In this manner we suppose a due proportion to be
+always preserved of land and water upon the surface of the globe, for
+the purpose of a habitable world such as this which we possess. We
+thus also allow time and opportunity for the translation of animals and
+plants to occupy the earth.
+
+"But if the earth on which we live began to appear in the ocean at
+the time when the LAST began to be resolved, it could not be from the
+materials of the continent immediately preceding this which we examine
+that the present earth has been constructed; for the bottom of the ocean
+must have been filled with materials before land could be made to appear
+above its surface.
+
+"Let us suppose that the continent which is to succeed our land is at
+present beginning to appear above the water in the middle of the Pacific
+Ocean; it must be evident that the materials of this great body, which
+is formed and ready to be brought forth, must have been collected from
+the destruction of an earth which does not now appear. Consequently,
+in this true statement of the case there is necessarily required the
+destruction of an animal and vegetable earth prior to the former land;
+and the materials of that earth which is first in our account must have
+been collected at the bottom of the ocean, and begun to be concocted for
+the production of the present earth, when the land immediately preceding
+the present had arrived at its full extent.
+
+"We have now got to the end of our reasoning; we have no data further
+to conclude immediately from that which actually is; but we have got
+enough; we have the satisfaction to find that in nature there are
+wisdom, system, and consistency. For having in the natural history of
+the earth seen a succession of worlds, we may from this conclude that
+there is a system in nature; in like manner as, from seeing revolutions
+of the planets, it is concluded that there is a system by which they are
+intended to continue those revolutions. But if the succession of worlds
+is established in the system of nature, it is in vain to look for
+anything higher in the origin of the earth. The result, therefore,
+of our present inquiry is that we find no vestige of a beginning--no
+prospect of an end."
+
+
+Altogether remarkable as this paper seems in the light of later
+knowledge, neither friend nor foe deigned to notice it at the moment.
+It was not published in book form until the last decade of the century,
+when Hutton had lived with and worked over his theory for almost fifty
+years. Then it caught the eye of the world. A school of followers
+expounded the Huttonian doctrines; a rival school under Werner in
+Germany opposed some details of the hypothesis, and the educated world
+as a whole viewed the disputants askance. The very novelty of the new
+views forbade their immediate acceptance. Bitter attacks were made upon
+the "heresies," and that was meant to be a soberly tempered judgment
+which in 1800 pronounced Hutton's theories "not only hostile to sacred
+history, but equally hostile to the principles of probability, to the
+results of the ablest observations on the mineral kingdom, and to the
+dictates of rational philosophy." And all this because Hutton's theory
+presupposed the earth to have been in existence more than six thousand
+years.
+
+Thus it appears that though the thoughts of men had widened, in those
+closing days of the eighteenth century, to include the stars, they had
+not as yet expanded to receive the most patent records that are written
+everywhere on the surface of the earth. Before Hutton's views could be
+accepted, his pivotal conception that time is long must be established
+by convincing proofs. The evidence was being gathered by William Smith,
+Cuvier, and other devotees of the budding science of paleontology in
+the last days of the century, but their labors were not brought to
+completion till a subsequent epoch.
+
+
+NEPTUNISTS VERSUS PLUTONISTS
+
+In the mean time, James Hutton's theory that continents wear away and
+are replaced by volcanic upheaval gained comparatively few adherents.
+Even the lucid Illustrations of the Huttonian Theory, which Playfair,
+the pupil and friend of the great Scotchman, published in 1802, did not
+at once prove convincing. The world had become enamoured of the rival
+theory of Hutton's famous contemporary, Werner of Saxony--the theory
+which taught that "in the beginning" all the solids of the earth's
+present crust were dissolved in the heated waters of a universal sea.
+Werner affirmed that all rocks, of whatever character, had been formed
+by precipitation from this sea as the waters cooled; that even veins
+have originated in this way; and that mountains are gigantic crystals,
+not upheaved masses. In a word, he practically ignored volcanic action,
+and denied in toto the theory of metamorphosis of rocks through the
+agency of heat.
+
+The followers of Werner came to be known as Neptunists; the Huttonians
+as Plutonists. The history of geology during the first quarter of the
+nineteenth century is mainly a recital of the intemperate controversy
+between these opposing schools; though it should not be forgotten that,
+meantime, the members of the Geological Society of London were making
+an effort to hunt for facts and avoid compromising theories. Fact and
+theory, however, were too closely linked to be thus divorced.
+
+The brunt of the controversy settled about the unstratified
+rocks--granites and their allies--which the Plutonists claimed as of
+igneous origin. This contention had the theoretical support of the
+nebular hypothesis, then gaining ground, which supposed the earth to be
+a cooling globe. The Plutonists laid great stress, too, on the observed
+fact that the temperature of the earth increases at a pretty constant
+ratio as descent towards its centre is made in mines. But in particular
+they appealed to the phenomena of volcanoes.
+
+The evidence from this source was gathered and elaborated by Mr. G.
+Poulett Scrope, secretary of the Geological Society of England, who, in
+1823, published a classical work on volcanoes in which he claimed that
+volcanic mountains, including some of the highest-known peaks, are
+merely accumulated masses of lava belched forth from a crevice in the
+earth's crust.
+
+"Supposing the globe to have had any irregular shape when detached from
+the sun," said Scrope, "the vaporization of its surface, and, of course,
+of its projecting angles, together with its rotatory motion on its axis
+and the liquefaction of its outer envelope, would necessarily occasion
+its actual figure of an oblate spheroid. As the process of expansion
+proceeded in depth, the original granitic beds were first partially
+disaggregated, next disintegrated, and more or less liquefied,
+the crystals being merged in the elastic vehicle produced by the
+vaporization of the water contained between the laminae.
+
+"Where this fluid was produced in abundance by great dilatation--that
+is, in the outer and highly disintegrated strata, the superior specific
+gravity of the crystals forced it to ooze upward, and thus a great
+quantity of aqueous vapor was produced on the surface of the globe. As
+this elastic fluid rose into outer space, its continually increasing
+expansion must have proportionately lowered its temperature; and, in
+consequence, a part was recondensed into water and sank back towards the
+more solid surface of the globe.
+
+"And in this manner, for a certain time, a violent reciprocation of
+atmospheric phenomena must have continued--torrents of vapor rising
+outwardly, while equally tremendous torrents of condensed vapor, or
+rain, fell towards the earth. The accumulation of the latter on the
+yet unstable and unconsolidated surface of the globe constituted the
+primeval ocean. The surface of this ocean was exposed to continued
+vaporization owing to intense heat; but this process, abstracting
+caloric from the stratum of the water below, by partially cooling it,
+tended to preserve the remainder in a liquid form. The ocean will have
+contained, both in solution and suspension, many of the matters carried
+upward from the granitic bed in which the vapors from whose condensation
+it proceeded were produced, and which they had traversed in their rise.
+The dissolved matters will have been silex, carbonates, and sulphates
+of lime, and those other mineral substances which water at an intense
+temperature and under such circumstances was enabled to hold in
+solution. The suspended substances will have been all the lighter and
+finer particles of the upper beds where the disintegration had been
+extreme; and particularly their mica, which, owing to the tenuity of its
+plate-shaped crystals, would be most readily carried up by the ascending
+fluid, and will have remained longest in suspension.
+
+"But as the torrents of vapor, holding these various matters in
+solution and suspension, were forced upward, the greater part of the
+disintegrated crystals by degrees subsided; those of felspar and quartz
+first, the mica being, as observed above, from the form of its plates,
+of peculiar buoyancy, and therefore held longest in suspension.
+
+"The crystals of felspar and quartz as they subsided, together with a
+small proportion of mica, would naturally arrange themselves so as to
+have their longest dimensions more or less parallel to the surface on
+which they rest; and this parallelism would be subsequently increased,
+as we shall see hereafter, by the pressure of these beds sustained
+between the weight of the supported column of matter and the expansive
+force beneath them. These beds I conceive, when consolidated, to
+constitute the gneiss formation.
+
+"The farther the process of expansion proceeded in depth, the more was
+the column of liquid matter lengthened, which, gravitating towards
+the centre of the globe, tended to check any further expansion. It is,
+therefore, obvious that after the globe settled into its actual orbit,
+and thenceforward lost little of its enveloping matter, the whole
+of which began from that moment to gravitate towards its centre, the
+progress of expansion inwardly would continually increase in rapidity;
+and a moment must have at length arrived hen the forces of expansion and
+repression had reached an equilibrium and the process was stopped from
+progressing farther inwardly by the great pressure of the gravitating
+column of liquid.
+
+"This column may be considered as consisting of different strata, though
+the passage from one extremity of complete solidity to the other of
+complete expansion, in reality, must have been perfectly gradual. The
+lowest stratum, immediately above the extreme limit of expansion, will
+have been granite barely DISAGGREGATED, and rendered imperfectly liquid
+by the partial vaporization of its contained water.
+
+"The second stratum was granite DISINTEGRATED; aqueous vapor, having
+been produced in such abundance as to be enabled to rise upward,
+partially disintegrating the crystals of felspar and mica, and
+superficially dissolving those of quartz. This mass would reconsolidate
+into granite, though of a smaller grain than the preceding rock.
+
+"The third stratum was so disintegrated that a greater part of the mica
+had been carried up by the escaping vapor IN SUSPENSION, and that of
+quartz in solution; the felspar crystals, with the remaining quartz and
+mica, SUBSIDING by their specific gravity and arranging themselves in
+horizontal planes.
+
+"The consolidation of this stratum produced the gneiss formation.
+
+"The fourth zone will have been composed of the ocean of turbid and
+heated water, holding mica, etc., in suspension, and quartz, carbonate
+of lime, etc., in solution, and continually traversed by reciprocating
+bodies of heated water rising from below, and of cold fluid sinking from
+the surface, by reason of their specific gravities.
+
+"The disturbance thus occasioned will have long retarded the deposition
+of the suspended particles. But this must by degrees have taken place,
+the quartz grains and the larger and coarser plates of mica subsiding
+first and the finest last.
+
+"But the fragments of quartz and mica were not deposited alone; a great
+proportion of the quartz held in SOLUTION must have been precipitated
+at the same time as the water cooled, and therefore by degrees lost
+its faculty of so much in solution. Thus was gradually produced the
+formation of mica-schist, the mica imperfectly recrystallizing or being
+merely aggregated together in horizontal plates, between which the
+quartz either spread itself generally in minute grains or unified into
+crystalline nuclei. On other spots, instead of silex, carbonate of lime
+was precipitated, together with more or less of the nucaceous sediment,
+and gave rise to saccharoidal limestones. At a later period, when the
+ocean was yet further cooled down, rock-salt and sulphate of lime were
+locally precipitated in a similar mode.
+
+"The fifth stratum was aeriform, and consisted in great part of
+aqueous vapors; the remainder being a compound of other elastic fluids
+(permanent gases) which had been formed probably from the volatilization
+of some of the substances contained in the primitive granite and carried
+upward with the aqueous vapor from below. These gases will have
+been either mixed together or otherwise disposed, according to their
+different specific gravities or chemical affinities, and this stratum
+constituted the atmosphere or aerial envelope of the globe.
+
+"When, in this manner, the general and positive expansion of the globe,
+occasioned by the sudden reduction of outward pressure, had ceased (in
+consequence of the REPRESSIVE FORCE, consisting of the weight of its
+fluid envelope, having reached an equilibrium with the EXPANSIVE FORCE,
+consisting of the caloric of the heated nucleus), the rapid superficial
+evaporation of the ocean continued; and, by gradually reducing its
+temperature, occasioned the precipitation of a proportionate quantity
+of the minerals it held in solution, particularly its silex. These
+substances falling to the bottom, accompanied by a large proportion of
+the matters held in solution, particularly the mica, in consequence of
+the greater comparative tranquillity of the ocean, agglomerated these
+into more or less compact beds of rock (the mica-schist formation),
+producing the first crust or solid envelope of the globe. Upon this,
+other stratified rocks, composed sometimes of a mixture, sometimes of
+an alternation of precipitations, sediments, and occasionally of
+conglomerates, were by degrees deposited, giving rise to the TRANSITION
+formations.
+
+"Beneath this crust a new process now commenced. The outer zones of
+crystalline matter having been suddenly refrigerated by the rapid
+vaporization and partial escape of the water they contained, abstracted
+caloric from the intensely heated nucleus of the globe. These
+crystalline zones were of unequal density, the expansion they had
+suffered diminishing from above downward.
+
+"Their expansive force was, however, equal at all points, their
+temperature everywhere bearing an inverse ratio to their density. But
+when by the accession of caloric from the inner and unliquefied nucleus
+the temperature, and consequently the expansive force of the lower
+strata of dilated crystalline matter, was augmented, it acted upon the
+upper and more liquefied strata. These being prevented from yielding
+OUTWARDLY by the tenacity and weight of the solid involucrum of
+precipitated and sedimental deposits which overspread them, sustained
+a pressure out of proportion to their expansive force, and were in
+consequence proportionately condensed, and by the continuance of the
+process, where the overlying strata were sufficiently resistant, finally
+consolidated.
+
+"This process of consolidation must have progressed from above downward,
+with the increase of the expansive force in the lower strata, commencing
+from the upper surface, which, its temperature being lowest, offered the
+least resistance to the force of compression.
+
+"By this process the upper zone of crystalline matter, which had
+intumesced so far as to allow of the escape of its aqueous vapor and of
+much of its mica and quartz, was resolidified, the component crystals
+arranging themselves in planes perpendicular to the direction of the
+pressure by which the mass was consolidated--that is, to the radius of
+the globe. The gneiss formation, as already observed, was the result.
+
+"The inferior zone of barely disintegrated granite, from which only
+a part of the steam and quartz and none of the mica had escaped,
+reconsolidated in a confused or granitoidal manner; but exhibits marks
+of the process it had undergone in its broken crystals of felspar and
+mica, its rounded and superficially dissolved grains of quartz, its
+imbedded fragments (broken from the more solid parts of the mass, as it
+rose, and enveloped by the softer parts), its concretionary nodules and
+new minerals, etc.
+
+"Beneath this, the granite which had been simply disintegrated was again
+solidified, and returned in all respects to its former condition. The
+temperature, however, and with it the expansive force of the inferior
+zone, was continually on the increase, the caloric of the interior of
+the globe still endeavoring to put itself in equilibrio by passing off
+towards the less-intensely heated crust.
+
+"This continually increasing expansive force must at length have
+overcome the resistance opposed by the tenacity and weight of the
+overlying consolidated strata. It is reasonable to suppose that this
+result took place contemporaneously, or nearly so, on many spots,
+wherever accidental circumstances in the texture or composition of the
+oceanic deposits led them to yield more readily; and in this manner
+were produced those original fissures in the primeval crust of the earth
+through some of which (fissures of elevation) were intruded portions of
+interior crystalline zones in a solid or nearly solid state, together
+with more or less of the intumescent granite, in the manner
+above described; while others (fissures of eruption) gave rise to
+extravasations of the heated crystalline matter, in the form of
+lavas--that is, still further liquefied by the greater comparative
+reduction of the pressure they endured."(3)
+
+
+The Neptunists stoutly contended for the aqueous origin of volcanic as
+of other mountains. But the facts were with Scrope, and as time went
+on it came to be admitted that not merely volcanoes, but many "trap"
+formations not taking the form of craters, had been made by the
+obtrusion of molten rock through fissures in overlying strata. Such,
+for example, to cite familiar illustrations, are Mount Holyoke, in
+Massachusetts, and the well-known formation of the Palisades along the
+Hudson.
+
+But to admit the "Plutonic" origin of such widespread formations was
+practically to abandon the Neptunian hypothesis. So gradually the
+Huttonian explanation of the origin of granites and other "igneous"
+rocks, whether massed or in veins, came to be accepted. Most geologists
+then came to think of the earth as a molten mass, on which the crust
+rests as a mere film. Some, indeed, with Lyell, preferred to believe
+that the molten areas exist only as lakes in a solid crust, heated to
+melting, perhaps, by electrical or chemical action, as Davy suggested.
+More recently a popular theory attempts to reconcile geological facts
+with the claim of the physicists, that the earth's entire mass is at
+least as rigid as steel, by supposing that a molten film rests between
+the observed solid crust and the alleged solid nucleus. But be that
+as it may, the theory that subterranean heat has been instrumental in
+determining the condition of "primary" rocks, and in producing many
+other phenomena of the earth's crust, has never been in dispute since
+the long controversy between the Neptunists and the Plutonists led to
+its establishment.
+
+
+LYELL AND UNIFORMITARIANISM
+
+If molten matter exists beneath the crust of the earth, it must contract
+in cooling, and in so doing it must disturb the level of the portion of
+the crust already solidified. So a plausible explanation of the upheaval
+of continents and mountains was supplied by the Plutonian theory, as
+Hutton had from the first alleged. But now an important difference
+of opinion arose as to the exact rationale of such upheavals. Hutton
+himself, and practically every one else who accepted his theory, had
+supposed that there are long periods of relative repose, during which
+the level of the crust is undisturbed, followed by short periods of
+active stress, when continents are thrown up with volcanic suddenness,
+as by the throes of a gigantic earthquake. But now came Charles Lyell
+with his famous extension of the "uniformitarian" doctrine, claiming
+that past changes of the earth's surface have been like present changes
+in degree as well as in kind. The making of continents and mountains,
+he said, is going on as rapidly to-day as at any time in the past. There
+have been no gigantic cataclysmic upheavals at any time, but all
+changes in level of the strata as a whole have been gradual, by slow
+oscillation, or at most by repeated earthquake shocks such as are still
+often experienced.
+
+In support of this very startling contention Lyell gathered a mass
+of evidence of the recent changes in level of continental areas. He
+corroborated by personal inspection the claim which had been made by
+Playfair in 1802, and by Von Buch in 1807, that the coast-line of Sweden
+is rising at the rate of from a few inches to several feet in a
+century. He cited Darwin's observations going to prove that Patagonia is
+similarly rising, and Pingel's claim that Greenland is slowly sinking.
+Proof as to sudden changes of level of several feet, over large areas,
+due to earthquakes, was brought forward in abundance. Cumulative
+evidence left it no longer open to question that such oscillatory
+changes of level, either upward or downward, are quite the rule, and
+it could not be denied that these observed changes, if continued long
+enough in one direction, would produce the highest elevations. The
+possibility that the making of even the highest ranges of mountains had
+been accomplished without exaggerated catastrophic action came to be
+freely admitted.
+
+It became clear that the supposedly stable-land surfaces are in
+reality much more variable than the surface of the "shifting sea"; that
+continental masses, seemingly so fixed, are really rising and falling
+in billows thousands of feet in height, ages instead of moments being
+consumed in the sweep between crest and hollow.
+
+These slow oscillations of land surfaces being understood, many
+geological enigmas were made clear--such as the alternation of marine
+and fresh-water formations in a vertical series, which Cuvier and
+Brongniart had observed near Paris; or the sandwiching of layers of
+coal, of subaerial formation, between layers of subaqueous clay or
+sandstone, which may be observed everywhere in the coal measures. In
+particular, the extreme thickness of the sedimentary strata as a whole,
+many times exceeding the depth of the deepest known sea, was for the
+first time explicable when it was understood that such strata had formed
+in slowly sinking ocean-beds.
+
+All doubt as to the mode of origin of stratified rocks being thus
+removed, the way was opened for a more favorable consideration of
+that other Huttonian doctrine of the extremely slow denudation of land
+surfaces. The enormous amount of land erosion will be patent to any
+one who uses his eyes intelligently in a mountain district. It will be
+evident in any region where the strata are tilted--as, for example, the
+Alleghanies--that great folds of strata which must once have risen miles
+in height have in many cases been worn entirely away, so that now a
+valley marks the location of the former eminence. Where the strata are
+level, as in the case of the mountains of Sicily, the Scotch Highlands,
+and the familiar Catskills, the evidence of denudation is, if possible,
+even more marked; for here it is clear that elevation and valley have
+been carved by the elements out of land that rose from the sea as level
+plateaus.
+
+But that this herculean labor of land-sculpturing could have been
+accomplished by the slow action of wind and frost and shower was an
+idea few men could grasp within the first half-century after Hutton
+propounded it; nor did it begin to gain general currency until Lyell's
+crusade against catastrophism, begun about 1830, had for a quarter of a
+century accustomed geologists to the thought of slow, continuous changes
+producing final results of colossal proportions. And even long after
+that it was combated by such men as Murchison, Director-General of
+the Geological Survey of Great Britain, then accounted the foremost
+field-geologist of his time, who continued to believe that the existing
+valleys owe their main features to subterranean forces of upheaval.
+Even Murchison, however, made some recession from the belief of the
+Continental authorities, Elie de Beaumont and Leopold von Buch,
+who contended that the mountains had sprung up like veritable
+jacks-in-the-box. Von Buch, whom his friend and fellow-pupil Von
+Humboldt considered the foremost geologist of the time, died in 1853,
+still firm in his early faith that the erratic bowlders found high on
+the Jura had been hurled there, like cannon-balls, across the valley of
+Geneva by the sudden upheaval of a neighboring mountain-range.
+
+
+AGASSIZ AND THE GLACIAL THEORY
+
+The bowlders whose presence on the crags of the Jura the old Gerinan
+accounted for in a manner so theatrical had long been a source of
+contention among geologists. They are found not merely on the Jura,
+but on numberless other mountains in all north-temperate latitudes, and
+often far out in the open country, as many a farmer who has broken his
+plough against them might testify. The early geologists accounted for
+them, as for nearly everything else, with their supposititious Deluge.
+Brongniart and Cuvier and Buckland and their contemporaries appeared
+to have no difficulty in conceiving that masses of granite weighing
+hundreds of tons had been swept by this current scores or hundreds
+of miles from their source. But, of course, the uniformitarian faith
+permitted no such explanation, nor could it countenance the projection
+idea; so Lyell was bound to find some other means of transportation for
+the puzzling erratics.
+
+The only available medium was ice, but, fortunately, this one seemed
+quite sufficient. Icebergs, said Lyell, are observed to carry all manner
+of debris, and deposit it in the sea-bottoms. Present land surfaces
+have often been submerged beneath the sea. During the latest of these
+submergences icebergs deposited the bowlders now scattered here
+and there over the land. Nothing could be simpler or more clearly
+uniformitarian. And even the catastrophists, though they met Lyell
+amicably on almost no other theoretical ground, were inclined to admit
+the plausibility of his theory of erratics. Indeed, of all Lyell's
+nonconformist doctrines, this seemed the one most likely to meet with
+general acceptance.
+
+Yet, even as this iceberg theory loomed large and larger before the
+geological world, observations were making in a different field that
+were destined to show its fallacy. As early as 1815 a sharp-eyed
+chamois-hunter of the Alps, Perraudin by name, had noted the existence
+of the erratics, and, unlike most of his companion hunters, had puzzled
+his head as to how the bowlders got where he saw them. He knew nothing
+of submerged continents or of icebergs, still less of upheaving
+mountains; and though he doubtless had heard of the Flood, he had no
+experience of heavy rocks floating like corks in water. Moreover, he
+had never observed stones rolling uphill and perching themselves on
+mountain-tops, and he was a good enough uniformitarian (though he would
+have been puzzled indeed had any one told him so) to disbelieve that
+stones in past times had disported themselves differently in this regard
+from stones of the present. Yet there the stones are. How did they get
+there?
+
+The mountaineer thought that he could answer that question. He saw about
+him those gigantic serpent-like streams of ice called glaciers, "from
+their far fountains slow rolling on," carrying with them blocks of
+granite and other debris to form moraine deposits. If these glaciers had
+once been much more extensive than they now are, they might have carried
+the bowlders and left them where we find them. On the other hand, no
+other natural agency within the sphere of the chamois-hunter's knowledge
+could have accomplished this, ergo the glaciers must once have been more
+extensive. Perraudin would probably have said that common-sense drove
+him to this conclusion; but be that as it may, he had conceived one of
+the few truly original and novel ideas of which the nineteenth century
+can boast.
+
+Perraudin announced his idea to the greatest scientist in his little
+world--Jean de Charpentier, director of the mines at Bex, a skilled
+geologist who had been a fellow-pupil of Von Buch and Von Humboldt
+under Werner at the Freiberg School of Mines. Charpentier laughed at
+the mountaineer's grotesque idea, and thought no more about it. And ten
+years elapsed before Perraudin could find any one who treated his notion
+with greater respect. Then he found a listener in M. Venetz, a civil
+engineer, who read a paper on the novel glacial theory before a local
+society in 1823. This brought the matter once more to the attention of
+De Charpentier, who now felt that there might be something in it worth
+investigation.
+
+A survey of the field in the light of the new theory soon convinced
+Charpentier that the chamois-hunter had all along been right. He became
+an enthusiastic supporter of the idea that the Alps had once been
+imbedded in a mass of ice, and in 1836 he brought the notion to the
+attention of Louis Agassiz, who was spending the summer in the Alps.
+Agassiz was sceptical at first, but soon became a convert.
+
+In 1840 Agassiz published a paper in which the results of his Alpine
+studies were elaborated.
+
+"Let us consider," he says, "those more considerable changes to which
+glaciers are subject, or rather, the immense extent which they had in
+the prehistoric period. This former immense extension, greater than any
+that tradition has preserved, is proved, in the case of nearly every
+valley in the Alps, by facts which are both many and well established.
+The study of these facts is even easy if the student is looking out for
+them, and if he will seize the least indication of their presence; and,
+if it were a long time before they were observed and connected with
+glacial action, it is because the evidences are often isolated and occur
+at places more or less removed from the glacier which originated them.
+If it be true that it is the prerogative of the scientific observer to
+group in the field of his mental vision those facts which appear to be
+without connection to the vulgar herd, it is, above all, in such a case
+as this that he is called upon to do so. I have often compared these
+feeble effects, produced by the glacial action of former ages, with the
+appearance of the markings upon a lithographic stone, prepared for the
+purpose of preservation, and upon which one cannot see the lines of the
+draughtsman's work unless it is known beforehand where and how to search
+for them.
+
+"The fact of the former existence of glaciers which have now disappeared
+is proved by the survival of the various phenomena which always
+accompany them, and which continue to exist even after the ice has
+melted. These phenomena are as follows:
+
+"1. Moraines.--The disposition and composition of moraines enable them
+to be always recognized, even when they are no longer adjacent to a
+glacier nor immediately surround its lower extremities. I may remark
+that lateral and terminal moraines alone enable us to recognize with
+certainty the limits of glacial extension, because they can be easily
+distinguished from the dikes and irregularly distributed stones carried
+down by the Alpine torrents, The lateral moraines deposited upon the
+sides of valleys are rarely affected by the larger torrents, but they
+are, however, often cut by the small streams which fall down the side of
+a mountain, and which, by interfering with their continuity, make them
+so much more difficult to recognize.
+
+"2. The Perched Bowlders.--It often happens that glaciers encounter
+projecting points of rock, the sides of which become rounded, and around
+which funnel-like cavities are formed with more or less profundity. When
+glaciers diminish and retire, the blocks which have fallen into these
+funnels often remain perched upon the top of the projecting rocky point
+within it, in such a state of equilibrium that any idea of a current of
+water as the cause of their transportation is completely inadmissible
+on account of their position. When such points of rock project above
+the surface of the glacier or appear as a more considerable islet in
+the midst of its mass (such as is the case in the Jardin of the Mer de
+Glace, above Montavert), such projections become surrounded on all
+sides by stones which ultimately form a sort of crown around the summit
+whenever the glaciers decrease or retire completely. Water currents
+never produce anything like this; but, on the contrary, whenever a
+stream breaks itself against a projecting rock, the stones which it
+carries down are turned aside and form a more or less regular trail.
+Never, under such circumstances, can the stones remain either at the
+top or at the sides of the rock, for, if such a thing were possible,
+the rapidity of the current would be accelerated by the increased
+resistance, and the moving bowlders would be carried beyond the
+obstruction before they were finally deposited.
+
+"3. The polished and striated rocks, such as have been described in
+Chapter XIV., afford yet further evidence of the presence of a glacier;
+for, as has been said already, neither a current nor the action of waves
+upon an extensive beach produces such effects. The general direction of
+the channels and furrows indicates the direction of the general movement
+of the glacier, and the streaks which vary more or less from this
+direction are produced by the local effects of oscillation and retreat,
+as we shall presently see.
+
+"4. The Lapiaz, or Lapiz, which the inhabitants of German Switzerland
+call Karrenfelder, cannot always be distinguished from erosions,
+because, both produced as they are by water, they do not differ in their
+exterior characteristics, but only in their positions. Erosions due to
+torrents are always found in places more or less depressed, and never
+occur upon large inclined surfaces. The Lapiaz, on the contrary, are
+frequently found upon the projecting parts of the sides of valleys in
+places where it is not possible to suppose that water has ever formed
+a current. Some geologists, in their embarrassment to explain these
+phenomena, have supposed that they were due to the infiltration of
+acidulated water, but this hypothesis is purely gratuitous.
+
+"We will now describe the remains of these various phenomena as they are
+found in the Alps outside the actual glacial limits, in order to prove
+that at a certain epoch glaciers were much larger than they are to-day.
+
+"The ancient moraines, situated as they are at a great distance from
+those of the present day, are nowhere so distinct or so frequent as
+in Valais, where MM. Venetz and J. de Charpentier noticed them for the
+first time; but as their observations are as yet unpublished, and they
+themselves gave me the information, it would be an appropriation of
+their discovery if I were to describe them here in detail. I will limit
+myself to say that there can be found traces, more or less distinct, of
+ancient terminal moraines in the form of vaulted dikes at the foot of
+every glacier, at a distance of a few minutes' walk, a quarter of an
+hour, a half-hour, an hour, and even of several leagues from their
+present extremities. These traces become less distinct in proportion
+to their distance from the glacier, and, since they are also often
+traversed by torrents, they are not as continuous as the moraines which
+are nearer to the glaciers. The farther these ancient moraines are
+removed from the termination of a glacier, the higher up they reach upon
+the sides of the valley, which proves to us that the thickness of the
+glacier must have been greater when its size was larger. At the same
+time, their number indicates so many stopping-places in the retreat of
+the glacier, or so many extreme limits of its extension--limits which
+were never reached again after it had retired. I insist upon this point,
+because if it is true that all these moraines demonstrate a larger
+extent of the glacier, they also prove that their retreat into their
+present boundaries, far from having been catastrophic, was marked on the
+contrary by periods of repose more or less frequent, which caused the
+formation of a series of concentric moraines which even now indicate
+their retrogression.
+
+"The remains of longitudinal moraines are less frequent, less distinct,
+and more difficult to investigate, because, indicating as they do the
+levels to which the edges of the glacier reached at different epochs,
+it is generally necessary to look for them above the line of the
+paths along the escarpments of the valleys, and hence it is not always
+possible to follow them along a valley. Often, also, the sides of a
+valley which enclosed a glacier are so steep that it is only here and
+there that the stones have remained in place. They are, nevertheless,
+very distinct in the lower part of the valley of the Rhone, between
+Martigny and the Lake of Geneva, where several parallel ridges can be
+observed, one above the other, at a height of one thousand, one thousand
+two hundred, and even one thousand five hundred feet above the Rhone.
+It is between St. Maurice and the cascade of Pissevache, close to the
+hamlet of Chaux-Fleurie, that they are most accessible, for at this
+place the sides of the valley at different levels ascend in little
+terraces, upon which the moraines have been preserved. They are also
+very distinct above the Bains de Lavey, and above the village of Monthey
+at the entrance of the Val d'Illiers, where the sides of the valley are
+less inclined than in many other places.
+
+"The perched bowlders which are found in the Alpine valleys, at
+considerable distances from the glaciers, occupy at times positions so
+extraordinary that they excite in a high degree the curiosity of those
+who see them. For instance, when one sees an angular stone perched upon
+the top of an isolated pyramid, or resting in some way in a very steep
+locality, the first inquiry of the mind is, When and how have these
+stones been placed in such positions, where the least shock would seem
+to turn them over? But this phenomenon is not in the least astonishing
+when it is seen to occur also within the limits of actual glaciers, and
+it is recalled by what circumstances it is occasioned.
+
+"The most curious examples of perched stones which can be cited are
+those which command the northern part of the cascade of Pissevache,
+close to Chaux-Fleurie, and those above the Bains de Lavey, close to the
+village of Morcles; and those, even more curious, which I have seen in
+the valley of St. Nicolas and Oberhasli. At Kirchet, near Meiringen, can
+be seen some very remarkable crowns of bowlders around several domes
+of rock which appear to have been projected above the surface of the
+glacier which surrounded them. Something very similar can be seen around
+the top of the rock of St. Triphon.
+
+"The extraordinary phenomenon of perched stones could not escape the
+observing eye of De Saussure, who noticed several at Saleve, of which
+he described the positions in the following manner: 'One sees,' said he,
+'upon the slope of an inclined meadow, two of these great bowlders of
+granite, elevated one upon the other, above the grass at a height of two
+or three feet, upon a base of limestone rock on which both rest. This
+base is a continuation of the horizontal strata of the mountain, and is
+even united with it visibly on its lower face, being cut perpendicularly
+upon the other sides, and is not larger than the stone which it
+supports.' But seeing that the entire mountain is composed of the same
+limestone, De Saussure naturally concluded that it would be absurd to
+think that it was elevated precisely and only beneath the blocks of
+granite. But, on the other hand, since he did not know the manner in
+which these perched stones are deposited in our days by glacial action,
+he had recourse to another explanation: He supposes that the rock was
+worn away around its base by the continual erosion of water and air,
+while the portion of the rock which served as the base for the granite
+had been protected by it. This explanation, although very ingenious,
+could no longer be admitted after the researches of M. Elie de Beaumont
+had proved that the action of atmospheric agencies was not by a good
+deal so destructive as was theretofore supposed. De Saussure speaks
+also of a detached bowlder, situated upon the opposite side of the
+Tete-Noire, 'which is,' he says, 'of so great a size that one is tempted
+to believe that it was formed in the place it occupies; and it is called
+Barme russe, because it is worn away beneath in the form of a cave which
+can afford accommodation for more than thirty persons at a time."(4)
+
+But the implications of the theory of glaciers extend, so Agassiz has
+come to believe, far beyond the Alps. If the Alps had been covered with
+an ice sheet, so had many other regions of the northern hemisphere.
+Casting abroad for evidences of glacial action, Agassiz found them
+everywhere in the form of transported erratics, scratched and polished
+outcropping rocks, and moraine-like deposits. Finally, he became
+convinced that the ice sheet that covered the Alps had spread over the
+whole of the higher latitudes of the northern hemisphere, forming an
+ice cap over the globe. Thus the common-sense induction of the
+chamois-hunter blossomed in the mind of Agassiz into the conception of a
+universal ice age.
+
+In 1837 Agassiz had introduced his theory to the world, in a paper read
+at Neuchatel, and three years later he published his famous Etudes sur
+les Glaciers, from which we have just quoted. Never did idea make a more
+profound disturbance in the scientific world. Von Buch treated it
+with alternate ridicule, contempt, and rage; Murchison opposed it with
+customary vigor; even Lyell, whose most remarkable mental endowment was
+an unfailing receptiveness to new truths, could not at once discard
+his iceberg theory in favor of the new claimant. Dr. Buckland, however,
+after Agassiz had shown him evidence of former glacial action in his own
+Scotland, became a convert--the more readily, perhaps, as it seemed to
+him to oppose the uniformitarian idea. Gradually others fell in line,
+and after the usual imbittered controversy and the inevitable full
+generation of probation, the idea of an ice age took its place among
+the accepted tenets of geology. All manner of moot points still demanded
+attention--the cause of the ice age, the exact extent of the ice sheet,
+the precise manner in which it produced its effects, and the exact
+nature of these effects; and not all of these have even yet been
+determined. But, details aside, the ice age now has full recognition
+from geologists as an historical period. There may have been many ice
+ages, as Dr. Croll contends; there was surely one; and the conception
+of such a period is one of the very few ideas of our century that no
+previous century had even so much as faintly adumbrated.
+
+
+THE GEOLOGICAL AGES
+
+But, for that matter, the entire subject of historical geology is
+one that had but the barest beginning before our century. Until the
+paleontologist found out the key to the earth's chronology, no one--not
+even Hutton--could have any definite idea as to the true story of the
+earth's past. The only conspicuous attempt to classify the strata was
+that made by Werner, who divided the rocks into three systems, based on
+their supposed order of deposition, and called primary, transition, and
+secondary.
+
+Though Werner's observations were confined to the small province of
+Saxony, he did not hesitate to affirm that all over the world the
+succession of strata would be found the same as there, the concentric
+layers, according to this conception, being arranged about the earth
+with the regularity of layers on an onion. But in this Werner was
+as mistaken as in his theoretical explanation of the origin of the
+"primary" rocks. It required but little observation to show that the
+exact succession of strata is never precisely the same in any widely
+separated regions. Nevertheless, there was a germ of truth in Werner's
+system. It contained the idea, however faultily interpreted, of a
+chronological succession of strata; and it furnished a working outline
+for the observers who were to make out the true story of geological
+development. But the correct interpretation of the observed facts could
+only be made after the Huttonian view as to the origin of strata had
+gained complete acceptance.
+
+When William Smith, having found the true key to this story, attempted
+to apply it, the territory with which he had to deal chanced to be one
+where the surface rocks are of that later series which Werner termed
+secondary. He made numerous subdivisions within this system, based
+mainly on the fossils. Meantime it was found that, judged by the
+fossils, the strata that Brongniart and Cuvier studied near Paris were
+of a still more recent period (presumed at first to be due to the latest
+deluge), which came to be spoken of as tertiary. It was in these beds,
+some of which seemed to have been formed in fresh-water lakes, that many
+of the strange mammals which Cuvier first described were found.
+
+But the "transition" rocks, underlying the "secondary" system that Smith
+studied, were still practically unexplored when, along in the thirties,
+they were taken in hand by Roderick Impey Murchison, the reformed
+fox-hunter and ex-captain, who had turned geologist to such notable
+advantage, and Adam Sedgwick, the brilliant Woodwardian professor at
+Cambridge.
+
+Working together, these two friends classified the
+
+transition rocks into chronological groups, since familiar to every one
+in the larger outlines as the Silurian system (age of invertebrates) and
+the Devonian system (age of fishes)--names derived respectively from the
+country of the ancient Silures, in Wales and Devonshire, England. It
+was subsequently discovered that these systems of strata, which crop out
+from beneath newer rocks in restricted areas in Britain, are spread out
+into broad, undisturbed sheets over thousands of miles in continental
+Europe and in America. Later on Murchison studied them in Russia,
+and described them, conjointly with Verneuil and Von Kerserling, in
+a ponderous and classical work. In America they were studied by Hall,
+Newberry, Whitney, Dana, Whitfield, and other pioneer geologists, who
+all but anticipated their English contemporaries.
+
+The rocks that are of still older formation than those studied by
+Murchison and Sedgwick (corresponding in location to the "primary" rocks
+of Werner's conception) are the surface feature of vast areas in Canada,
+and were first prominently studied there by William I. Logan, of the
+Canadian Government Survey, as early as 1846, and later on by Sir
+William Dawson. These rocks--comprising the Laurentian system--were
+formerly supposed to represent parts of the original crust of the earth,
+formed on first cooling from a molten state; but they are now more
+generally regarded as once-stratified deposits metamorphosed by the
+action of heat.
+
+Whether "primitive" or metamorphic, however, these Canadian rocks, and
+analogous ones beneath the fossiliferous strata of other countries,
+are the oldest portions of the earth's crust of which geology has any
+present knowledge. Mountains of this formation, as the Adirondacks and
+the Storm King range, overlooking the Hudson near West Point, are the
+patriarchs of their kind, beside which Alleghanies and Sierra Nevadas
+are recent upstarts, and Rockies, Alps, and Andes are mere parvenus of
+yesterday.
+
+The Laurentian rocks were at first spoken of as representing "Azoic"
+time; but in 1846 Dawson found a formation deep in their midst which was
+believed to b e the fossil relic of a very low form of life, and after
+that it became customary to speak of the system as "Eozoic." Still more
+recently the title of Dawson's supposed fossil to rank as such has been
+questioned, and Dana's suggestion that the early rocks be termed merely
+Archman has met with general favor. Murchison and Sedgwick's Silurian,
+Devonian, and Carboniferous groups (the ages of invertebrates, of
+fishes, and of coal plants, respectively) are together spoken of as
+representing Paleozoic time. William Smith's system of strata, next
+above these, once called "secondary," represents Mesozoic time, or
+the age of reptiles. Still higher, or more recent, are Cuvier and
+Brongniart's tertiary rocks, representing the age of mammals. Lastly,
+the most recent formations, dating back, however, to a period far enough
+from recent in any but a geological sense, are classed as quaternary,
+representing the age of man.
+
+It must not be supposed, however, that the successive "ages" of the
+geologist are shut off from one another in any such arbitrary way as
+this verbal classification might seem to suggest. In point of fact,
+these "ages" have no better warrant for existence than have the
+"centuries" and the "weeks" of every-day computation. They are
+convenient, and they may even stand for local divisions in the strata,
+but they are bounded by no actual gaps in the sweep of terrestrial
+events.
+
+Moreover, it must be understood that the "ages" of different continents,
+though described under the same name, are not necessarily of exact
+contemporaneity. There is no sure test available by which it could be
+shown that the Devonian age, for instance, as outlined in the strata of
+Europe, did not begin millions of years earlier or later than the period
+whose records are said to represent the Devonian age in America. In
+attempting to decide such details as this, mineralogical data fail us
+utterly. Even in rocks of adjoining regions identity of structure is no
+proof of contemporaneous origin; for the veritable substance of the
+rock of one age is ground up to build the rocks of subsequent ages.
+Furthermore, in seas where conditions change but little the same form
+of rock may be made age after age. It is believed that chalk-beds still
+forming in some of our present seas may form one continuous mass dating
+back to earliest geologic ages. On the other hand, rocks different in
+character maybe formed at the same time in regions not far apart--say
+a sandstone along shore, a coral limestone farther seaward, and a
+chalk-bed beyond. This continuous stratum, broken in the process of
+upheaval, might seem the record of three different epochs.
+
+Paleontology, of course, supplies far better chronological tests, but
+even these have their limitations. There has been no time since rocks
+now in existence were formed, if ever, when the earth had a uniform
+climate and a single undiversified fauna over its entire land surface,
+as the early paleontologists supposed. Speaking broadly, the same
+general stages have attended the evolution of organic forms everywhere,
+but there is nothing to show that equal periods of time witnessed
+corresponding changes in diverse regions, but quite the contrary.
+To cite but a single illustration, the marsupial order, which is the
+dominant mammalian type of the living fauna of Australia to-day,
+existed in Europe and died out there in the tertiary age. Hence a future
+geologist might think the Australia of to-day contemporaneous with a
+period in Europe which in reality antedated it by perhaps millions of
+years.
+
+All these puzzling features unite to render the subject of historical
+geology anything but the simple matter the fathers of the science
+esteemed it. No one would now attempt to trace the exact sequence of
+formation of all the mountains of the globe, as Elie de Beaumont did
+a half-century ago. Even within the limits of a single continent, the
+geologist must proceed with much caution in attempting to chronicle the
+order in which its various parts rose from the matrix of the sea. The
+key to this story is found in the identification of the strata that
+are the surface feature in each territory. If Devonian rocks are at
+the surface in any given region, for example, it would appear that this
+region became a land surface in the Devonian age, or just afterwards.
+But a moment's consideration shows that there is an element of
+uncertainty about this, due to the steady denudation that all land
+surfaces undergo. The Devonian rocks may lie at the surface simply
+because the thousands of feet of carboniferous strata that once lay
+above them have been worn away. All that the cautious geologist dare
+assert, therefore, is that the region in question did not become
+permanent land surface earlier than the Devonian age.
+
+But to know even this is much--sufficient, indeed, to establish the
+chronological order of elevation, if not its exact period, for all parts
+of any continent that have been geologically explored--understanding
+always that there must be no scrupling about a latitude of a few
+millions or perhaps tens of millions of years here and there.
+
+Regarding our own continent, for example, we learn through the
+researches of a multitude of workers that in the early day it was a mere
+archipelago. Its chief island--the backbone of the future continent--was
+a great V-shaped area surrounding what is now Hudson Bay, an area built
+tip, perhaps, through denudation of a yet more ancient polar continent,
+whose existence is only conjectured. To the southeast an island that
+is now the Adirondack Mountains, and another that is now the Jersey
+Highlands rose above the waste of waters, and far to the south stretched
+probably a line of islands now represented by the Blue Ridge Mountains.
+Far off to the westward another line of islands foreshadowed our present
+Pacific border. A few minor islands in the interior completed the
+archipelago.
+
+From this bare skeleton the continent grew, partly by the deposit of
+sediment from the denudation of the original islands (which once towered
+miles, perhaps, where now they rise thousands of feet), but largely also
+by the deposit of organic remains, especially in the interior sea, which
+teemed with life. In the Silurian ages, invertebrates--brachiopods and
+crinoids and cephalopods--were the dominant types. But very early--no
+one knows just when--there came fishes of many strange forms, some of
+the early ones enclosed in turtle-like shells. Later yet, large spaces
+within the interior sea having risen to the surface, great marshes or
+forests of strange types of vegetation grew and deposited their remains
+to form coal-beds. Many times over such forests were formed, only to be
+destroyed by the oscillations of the land surface. All told, the strata
+of this Paleozoic period aggregate several miles in thickness, and the
+time consumed in their formation stands to all later time up to the
+present, according to Professor Dana's estimate, as three to one.
+
+Towards the close of this Paleozoic era the Appalachian Mountains
+were slowly upheaved in great convoluted folds, some of them probably
+reaching three or four miles above the sea-level, though the tooth
+of time has since gnawed them down to comparatively puny limits. The
+continental areas thus enlarged were peopled during the ensuing Mesozoic
+time with multitudes of strange reptiles, many of them gigantic in size.
+The waters, too, still teeming with invertebrates and fishes, had their
+quota of reptilian monsters; and in the air were flying reptiles, some
+of which measured twenty-five feet from tip to tip of their batlike
+wings. During this era the Sierra Nevada Mountains rose. Near the
+eastern border of the forming continent the strata were perhaps now too
+thick and stiff to bend into mountain folds, for they were rent into
+great fissures, letting out floods of molten lava, remnants of which are
+still in evidence after ages of denudation, as the Palisades along the
+Hudson, and such elevations as Mount Holyoke in western Massachusetts.
+
+Still there remained a vast interior sea, which later on, in the
+tertiary age, was to be divided by the slow uprising of the land, which
+only yesterday--that is to say, a million, or three or five or ten
+million, years ago--became the Rocky Mountains. High and erect these
+young mountains stand to this day, their sharp angles and rocky contours
+vouching for their youth, in strange contrast with the shrunken forms
+of the old Adirondacks, Green Mountains, and Appalachians, whose lowered
+heads and rounded shoulders attest the weight of ages. In the vast lakes
+which still remained on either side of the Rocky range, tertiary
+strata were slowly formed to the ultimate depth of two or three miles,
+enclosing here and there those vertebrate remains which were to be
+exposed again to view by denudation when the land rose still higher,
+and then, in our own time, to tell so wonderful a story to the
+paleontologist.
+
+Finally, the interior seas were filled, and the shore lines of the
+continent assumed nearly their present outline.
+
+Then came the long winter of the glacial epoch--perhaps of a succession
+of glacial epochs. The ice sheet extended southward to about the
+fortieth parallel, driving some animals before it, and destroying those
+that were unable to migrate. At its fulness, the great ice mass lay
+almost a mile in depth over New England, as attested by the scratched
+and polished rock surfaces and deposited erratics in the White
+Mountains. Such a mass presses down with a weight of about one hundred
+and twenty-five tons to the square foot, according to Dr. Croll's
+estimate. It crushed and ground everything beneath it more or less, and
+in some regions planed off hilly surfaces into prairies. Creeping slowly
+forward, it carried all manner of debris with it. When it melted away
+its terminal moraine built up the nucleus of the land masses now known
+as Long Island and Staten Island; other of its deposits formed the
+"drumlins" about Boston famous as Bunker and Breed's hills; and it left
+a long, irregular line of ridges of "till" or bowlder clay and scattered
+erratics clear across the country at about the latitude of New York
+city.
+
+As the ice sheet slowly receded it left minor moraines all along its
+course. Sometimes its deposits dammed up river courses or inequalities
+in the surface, to form the lakes which everywhere abound over Northern
+territories. Some glacialists even hold the view first suggested by
+Ramsey, of the British Geological Survey, that the great glacial sheets
+scooped out the basins of many lakes, including the system that feeds
+the St. Lawrence. At all events, it left traces of its presence all
+along the line of its retreat, and its remnants exist to this day as
+mountain glaciers and the polar ice cap. Indeed, we live on the border
+of the last glacial epoch, for with the closing of this period the long
+geologic past merges into the present.
+
+
+PAST, PRESENT, AND FUTURE
+
+And the present, no less than the past, is a time of change. This is the
+thought which James Hutton conceived more than a century ago, but which
+his contemporaries and successors were so very slow to appreciate. Now,
+however, it has become axiomatic--one can hardly realize that it was
+ever doubted. Every new scientific truth, says Agassiz, must pass
+through three stages--first, men say it is not true; then they declare
+it hostile to religion; finally, they assert that every one has known
+it always. Hutton's truth that natural law is changeless and eternal
+has reached this final stage. Nowhere now could you find a scientist
+who would dispute the truth of that text which Lyell, quoting from
+Playfair's Illustrations of the Huttonian Theory, printed on the
+title-page of his Principles: "Amid all the revolutions of the globe
+the economy of Nature has been uniform, and her laws are the only things
+that have resisted the general movement. The rivers and the rocks, the
+seas and the continents, have been changed in all their parts; but
+the laws which direct those changes, and the rules to which they are
+subject, have remained invariably the same."
+
+But, on the other hand, Hutton and Playfair, and in particular Lyell,
+drew inferences from this principle which the modern physicist can by no
+means admit. To them it implied that the changes on the surface of the
+earth have always been the same in degree as well as in kind, and must
+so continue while present forces hold their sway. In other words, they
+thought of the world as a great perpetual-motion machine. But the
+modern physicist, given truer mechanical insight by the doctrines of the
+conservation and the dissipation of energy, will have none of that. Lord
+Kelvin, in particular, has urged that in the periods of our earth's in
+fancy and adolescence its developmental changes must have been, like
+those of any other infant organism, vastly more rapid and pronounced
+than those of a later day; and to every clear thinker this truth also
+must now seem axiomatic.
+
+Whoever thinks of the earth as a cooling globe can hardly doubt that its
+crust, when thinner, may have heaved under strain of the moon's tidal
+pull--whether or not that body was nearer--into great billows, daily
+rising and falling, like waves of the present seas vastly magnified.
+
+Under stress of that same lateral pressure from contraction which now
+produces the slow depression of the Jersey coast, the slow rise of
+Sweden, the occasional belching of an insignificant volcano, the jetting
+of a geyser, or the trembling of an earthquake, once large areas were
+rent in twain, and vast floods of lava flowed over thousands of square
+miles of the earth's surface, perhaps, at a single jet; and, for aught
+we know to the contrary, gigantic mountains may have heaped up their
+contorted heads in cataclysms as spasmodic as even the most ardent
+catastrophist of the elder day of geology could have imagined.
+
+The atmosphere of that early day, filled with vast volumes of carbon,
+oxygen, and other chemicals that have since been stored in beds of coal,
+limestone, and granites, may have worn down the rocks on the one hand
+and built up organic forms on the other, with a rapidity that would now
+seem hardly conceivable.
+
+And yet while all these anomalous things went on, the same laws held
+sway that now are operative; and a true doctrine of uniformitarianism
+would make no unwonted concession in conceding them all--though most of
+the imbittered geological controversies of the middle of the nineteenth
+century were due to the failure of both parties to realize that simple
+fact.
+
+And as of the past and present, so of the future. The same forces will
+continue to operate; and under operation of these unchanging forces each
+day will differ from every one that has preceded it. If it be true,
+as every physicist believes, that the earth is a cooling globe, then,
+whatever its present stage of refrigeration, the time must come when its
+surface contour will assume a rigidity of level not yet attained. Then,
+just as surely, the slow action of the elements will continue to wear
+away the land surfaces, particle by particle, and transport them to the
+ocean, as it does to-day, until, compensation no longer being afforded
+by the upheaval of the continents, the last foot of dry land will sink
+for the last time beneath the water, the last mountain-peak melting
+away, and our globe, lapsing like any other organism into its second
+childhood, will be on the surface--as presumably it was before the first
+continent rose--one vast "waste of waters." As puny man conceives time
+and things, an awful cycle will have lapsed; in the sweep of the cosmic
+life, a pulse-beat will have throbbed.
+
+
+
+
+V. THE NEW SCIENCE OF METEOROLOGY
+
+
+METEORITES
+
+"An astonishing miracle has just occurred in our district," wrote M.
+Marais, a worthy if undistinguished citizen of France, from his home at
+L'Aigle, under date of "the 13th Floreal, year 11"--a date which outside
+of France would be interpreted as meaning May 3, 1803. This "miracle"
+was the appearance of a "fireball" in broad daylight--"perhaps it was
+wildfire," says the naive chronicle--which "hung over the meadow," being
+seen by many people, and then exploded with a loud sound, scattering
+thousands of stony fragments over the surface of a territory some miles
+in extent.
+
+Such a "miracle" could not have been announced at a more opportune time.
+For some years the scientific world had been agog over the question
+whether such a form of lightning as that reported--appearing in a clear
+sky, and hurling literal thunderbolts--had real existence. Such
+cases had been reported often enough, it is true. The "thunderbolts"
+themselves were exhibited as sacred relics before many an altar, and
+those who doubted their authenticity had been chided as having "an
+evil heart of unbelief." But scientific scepticism had questioned the
+evidence, and late in the eighteenth century a consensus of opinion
+in the French Academy had declined to admit that such stones had been
+"conveyed to the earth by lightning," let alone any more miraculous
+agency.
+
+In 1802, however, Edward Howard had read a paper before the Royal
+Society in which, after reviewing the evidence recently put forward,
+he had reached the conclusion that the fall of stones from the sky,
+sometimes or always accompanied by lightning, must be admitted as
+an actual phenomenon, however inexplicable. So now, when the great
+stone-fall at L'Aigle was announced, the French Academy made haste to
+send the brilliant young physicist Jean Baptiste Biot to investigate
+it, that the matter might, if possible, be set finally at rest.
+The investigation was in all respects successful, and Biot's report
+transferred the stony or metallic lightning-bolt--the aerolite or
+meteorite--from the realm of tradition and conjecture to that of
+accepted science.
+
+But how explain this strange phenomenon? At once speculation was rife.
+One theory contended that the stony masses had not actually fallen, but
+had been formed from the earth by the action of the lightning; but this
+contention was early abandoned. The chemists were disposed to believe
+that the aerolites had been formed by the combination of elements
+floating in the upper atmosphere. Geologists, on the other hand, thought
+them of terrestrial origin, urging that they might have been thrown up
+by volcanoes. The astronomers, as represented by Olbers and Laplace,
+modified this theory by suggesting that the stones might, indeed, have
+been cast out by volcanoes, but by volcanoes situated not on the earth,
+but on the moon.
+
+And one speculator of the time took a step even more daring, urging that
+the aerolites were neither of telluric nor selenitic origin, nor yet
+children of the sun, as the old Greeks had, many of them, contended,
+but that they are visitants from the depths of cosmic space. This bold
+speculator was the distinguished German physicist Ernst F. F. Chladni,
+a man of no small repute in his day. As early as 1794 he urged his
+cosmical theory of meteorites, when the very existence of meteorites was
+denied by most scientists. And he did more: he declared his belief
+that these falling stones were really one in origin and kind with those
+flashing meteors of the upper atmosphere which are familiar everywhere
+as "shooting-stars."
+
+Each of these coruscating meteors, he affirmed, must tell of the
+ignition of a bit of cosmic matter entering the earth's atmosphere. Such
+wandering bits of matter might be the fragments of shattered worlds, or,
+as Chladni thought more probable, merely aggregations of "world stuff"
+never hitherto connected with any large planetary mass.
+
+Naturally enough, so unique a view met with very scant favor.
+Astronomers at that time saw little to justify it; and the
+non-scientific world rejected it with fervor as being "atheistic and
+heretical," because its acceptance would seem to imply that the universe
+is not a perfect mechanism.
+
+Some light was thrown on the moot point presently by the observations of
+Brandes and Benzenberg, which tended to show that falling-stars travel
+at an actual speed of from fifteen to ninety miles a second. This
+observation tended to discredit the selenitic theory, since an object,
+in order to acquire such speed in falling merely from the moon, must
+have been projected with an initial velocity not conceivably to be given
+by any lunar volcanic impulse. Moreover, there was a growing conviction
+that there are no active volcanoes on the moon, and other considerations
+of the same tenor led to the complete abandonment of the selenitic
+theory.
+
+But the theory of telluric origin of aerolites was by no means so easily
+disposed of. This was an epoch when electrical phenomena were exciting
+unbounded and universal interest, and there was a not unnatural tendency
+to appeal to electricity in explanation of every obscure phenomenon; and
+in this case the seeming similarity between a lightning flash and the
+flash of an aerolite lent color to the explanation. So we find Thomas
+Forster, a meteorologist of repute, still adhering to the atmospheric
+theory of formation of aerolites in his book published in 1823; and,
+indeed, the prevailing opinion of the time seemed divided between
+various telluric theories, to the neglect of any cosmical theory
+whatever.
+
+But in 1833 occurred a phenomenon which set the matter finally at
+rest. A great meteoric shower occurred in November of that year, and
+in observing it Professor Denison Olmstead, of Yale, noted that all
+the stars of the shower appeared to come from a single centre or
+vanishing-point in the heavens, and that this centre shifted its
+position with the stars, and hence was not telluric. The full
+significance of this observation was at once recognized by astronomers;
+it demonstrated beyond all cavil the cosmical origin of the
+shooting-stars. Some conservative meteorologists kept up the argument
+for the telluric origin for some decades to come, as a matter of
+course--such a band trails always in the rear of progress. But even
+these doubters were silenced when the great shower of shooting-stars
+appeared again in 1866, as predicted by Olbers and Newton, radiating
+from the same point of the heavens as before.
+
+Since then the spectroscope has added its confirmatory evidence as to
+the identity of meteorite and shooting-star, and, moreover, has linked
+these atmospheric meteors with such distant cosmic residents as comets
+and nebulae. Thus it appears that Chladni's daring hypothesis of
+1794 has been more than verified, and that the fragments of matter
+dissociated from planetary connection--which be postulated and was
+declared atheistic for postulating--have been shown to be billions
+of times more numerous than any larger cosmic bodies of which we have
+cognizance--so widely does the existing universe differ from man's
+preconceived notions as to what it should be.
+
+Thus also the "miracle" of the falling stone, against which the
+scientific scepticism of yesterday presented "an evil heart of
+unbelief," turns out to be the most natural phenomena, inasmuch as it is
+repeated in our atmosphere some millions of times each day.
+
+
+THE AURORA BOREALIS
+
+If fire-balls were thought miraculous and portentous in days of yore,
+what interpretation must needs have been put upon that vastly more
+picturesque phenomenon, the aurora? "Through all the city," says the
+Book of Maccabees, "for the space of almost forty days, there were seen
+horsemen running in the air, in cloth of gold, armed with lances, like
+a band of soldiers: and troops of horsemen in array encountering and
+running one against another, with shaking of shields and multitude of
+pikes, and drawing of swords, and casting of darts, and glittering of
+golden ornaments and harness." Dire omens these; and hardly less ominous
+the aurora seemed to all succeeding generations that observed it down
+well into the eighteenth century--as witness the popular excitement in
+England in 1716 over the brilliant aurora of that year, which became
+famous through Halley's description.
+
+But after 1752, when Franklin dethroned the lightning, all spectacular
+meteors came to be regarded as natural phenomena, the aurora among the
+rest. Franklin explained the aurora--which was seen commonly enough in
+the eighteenth century, though only recorded once in the seventeenth--as
+due to the accumulation of electricity on the surface of polar snows,
+and its discharge to the equator through the upper atmosphere. Erasmus
+Darwin suggested that the luminosity might be due to the ignition of
+hydrogen, which was supposed by many philosophers to form the upper
+atmosphere. Dalton, who first measured the height of the aurora,
+estimating it at about one hundred miles, thought the phenomenon due
+to magnetism acting on ferruginous particles in the air, and his
+explanation was perhaps the most popular one at the beginning of the
+last century.
+
+Since then a multitude of observers have studied the aurora, but the
+scientific grasp has found it as elusive in fact as it seems to casual
+observation, and its exact nature is as undetermined to-day as it was a
+hundred years ago. There has been no dearth of theories concerning it,
+however. Blot, who studied it in the Shetland Islands in 1817, thought
+it due to electrified ferruginous dust, the origin of which he ascribed
+to Icelandic volcanoes. Much more recently the idea of ferruginous
+particles has been revived, their presence being ascribed not to
+volcanoes, but to the meteorites constantly being dissipated in the
+upper atmosphere. Ferruginous dust, presumably of such origin, has been
+found on the polar snows, as well as on the snows of mountain-tops, but
+whether it could produce the phenomena of auroras is at least an open
+question.
+
+Other theorists have explained the aurora as due to the accumulation of
+electricity on clouds or on spicules of ice in the upper air. Yet others
+think it due merely to the passage of electricity through rarefied air
+itself. Humboldt considered the matter settled in yet another way when
+Faraday showed, in 1831, that magnetism may produce luminous effects.
+But perhaps the prevailing theory of to-day assumes that the aurora is
+due to a current of electricity generated at the equator and passing
+through upper regions of space, to enter the earth at the magnetic
+poles--simply reversing the course which Franklin assumed.
+
+The similarity of the auroral light to that generated in a vacuum
+bulb by the passage of electricity lends support to the long-standing
+supposition that the aurora is of electrical origin, but the subject
+still awaits complete elucidation. For once even that mystery-solver the
+spectroscope has been baffled, for the line it sifts from the aurora is
+not matched by that of any recognized substance. A like line is found
+in the zodiacal light, it is true, but this is of little aid, for the
+zodiacal light, though thought by some astronomers to be due to meteor
+swarms about the sun, is held to be, on the whole, as mysterious as the
+aurora itself.
+
+Whatever the exact nature of the aurora, it has long been known to
+be intimately associated with the phenomena of terrestrial magnetism.
+Whenever a brilliant aurora is visible, the world is sure to be visited
+with what Humboldt called a magnetic storm--a "storm" which manifests
+itself to human senses in no way whatsoever except by deflecting the
+magnetic needle and conjuring with the electric wire. Such magnetic
+storms are curiously associated also with spots on the sun--just how no
+one has explained, though the fact itself is unquestioned. Sun-spots,
+too, seem directly linked with auroras, each of these phenomena passing
+through periods of greatest and least frequency in corresponding cycles
+of about eleven years' duration.
+
+It was suspected a full century ago by Herschel that the variations in
+the number of sun-spots had a direct effect upon terrestrial weather,
+and he attempted to demonstrate it by using the price of wheat as a
+criterion of climatic conditions, meantime making careful observation
+of the sun-spots. Nothing very definite came of his efforts in this
+direction, the subject being far too complex to be determined without
+long periods of observation. Latterly, however, meteorologists,
+particularly in the tropics, are disposed to think they find evidence
+of some such connection between sun-spots and the weather as Herschel
+suspected. Indeed, Mr. Meldrum declares that there is a positive
+coincidence between periods of numerous sun-spots and seasons of
+excessive rain in India.
+
+That some such connection does exist seems intrinsically probable. But
+the modern meteorologist, learning wisdom of the past, is extremely
+cautious about ascribing casual effects to astronomical phenomena.
+He finds it hard to forget that until recently all manner of climatic
+conditions were associated with phases of the moon; that not so very
+long ago showers of falling-stars were considered "prognostic" of
+certain kinds of weather; and that the "equinoctial storm" had
+been accepted as a verity by every one, until the unfeeling hand of
+statistics banished it from the earth.
+
+Yet, on the other hand, it is easily within the possibilities that the
+science of the future may reveal associations between the weather and
+sun-spots, auroras, and terrestrial magnetism that as yet are hardly
+dreamed of. Until such time, however, these phenomena must feel
+themselves very grudgingly admitted to the inner circle of meteorology.
+More and more this science concerns itself, in our age of concentration
+and specialization, with weather and climate. Its votaries no
+longer concern themselves with stars or planets or comets or
+shooting-stars--once thought the very essence of guides to weather
+wisdom; and they are even looking askance at the moon, and asking her
+to show cause why she also should not be excluded from their domain.
+Equally little do they care for the interior of the earth, since they
+have learned that the central emanations of heat which Mairan imagined
+as a main source of aerial warmth can claim no such distinction. Even
+such problems as why the magnetic pole does not coincide with the
+geographical, and why the force of terrestrial magnetism decreases from
+the magnetic poles to the magnetic equator, as Humboldt first discovered
+that it does, excite them only to lukewarm interest; for magnetism,
+they say, is not known to have any connection whatever with climate or
+weather.
+
+
+EVAPORATION, CLOUD FORMATION, AND DEW
+
+There is at least one form of meteor, however, of those that interested
+our forebears whose meteorological importance they did not overestimate.
+This is the vapor of water. How great was the interest in this familiar
+meteor at the beginning of the century is attested by the number of
+theories then extant regarding it; and these conflicting theories bear
+witness also to the difficulty with which the familiar phenomenon of the
+evaporation of water was explained.
+
+Franklin had suggested that air dissolves water much as water dissolves
+salt, and this theory was still popular, though Deluc had disproved it
+by showing that water evaporates even more rapidly in a vacuum than
+in air. Deluc's own theory, borrowed from earlier chemists, was that
+evaporation is the chemical union of particles of water with particles
+of the supposititious element heat. Erasmus Darwin combined the two
+theories, suggesting that the air might hold a variable quantity of
+vapor in mere solution, and in addition a permanent moiety in chemical
+combination with caloric.
+
+Undisturbed by these conflicting views, that strangely original
+genius, John Dalton, afterwards to be known as perhaps the greatest
+of theoretical chemists, took the question in hand, and solved it by
+showing that water exists in the air as an utterly independent gas. He
+reached a partial insight into the matter in 1793, when his first volume
+of meteorological essays was published; but the full elucidation of
+the problem came to him in 1801. The merit of his studies was at once
+recognized, but the tenability of his hypothesis was long and ardently
+disputed.
+
+While the nature of evaporation was in dispute, as a matter of course
+the question of precipitation must be equally undetermined. The most
+famous theory of the period was that formulated by Dr. Hutton in a paper
+read before the Royal Society of Edinburgh, and published in the volume
+of transactions which contained also the same author's epoch-making
+paper on geology. This "theory of rain" explained precipitation as due
+to the cooling of a current of saturated air by contact with a colder
+current, the assumption being that the surplusage of moisture was
+precipitated in a chemical sense, just as the excess of salt dissolved
+in hot water is precipitated when the water cools. The idea that the
+cooling of the saturated air causes the precipitation of its moisture
+is the germ of truth that renders this paper of Hutton's important. All
+correct later theories build on this foundation.
+
+"Let us suppose the surface of this earth wholly covered with water,"
+said Hutton, "and that the sun were stationary, being always vertical in
+one place; then, from the laws of heat and rarefaction, there would be
+formed a circulation in the atmosphere, flowing from the dark and cold
+hemisphere to the heated and illuminated place, in all directions,
+towards the place of the greatest cold.
+
+"As there is for the atmosphere of this earth a constant cooling cause,
+this fluid body could only arrive at a certain degree of heat; and this
+would be regularly decreasing from the centre of illumination to the
+opposite point of the globe, most distant from the light and heat.
+Between these two regions of extreme heat and cold there would, in every
+place, be found two streams of air following in opposite directions. If
+those streams of air, therefore, shall be supposed as both sufficiently
+saturated with humidity, then, as they are of different temperatures,
+there would be formed a continual condensation of aqueous vapor, in some
+middle region of the atmosphere, by the commixtion of part of those two
+opposite streams.
+
+"Hence there is reason to believe that in this supposed case there would
+be formed upon the surface of the globe three different regions--the
+torrid region, the temperate, and the frigid. These three regions would
+continue stationary; and the operations of each would be continual. In
+the torrid region, nothing but evaporation and heat would take place;
+no cloud could be formed, because in changing the transparency of the
+atmosphere to opacity it would be heated immediately by the operation of
+light, and thus the condensed water would be again evaporated. But this
+power of the sun would have a termination; and it is these that would
+begin the region of temperate heat and of continual rain. It is not
+probable that the region of temperance would reach far beyond the region
+of light; and in the hemisphere of darkness there would be found a
+region of extreme cold and perfect dryness.
+
+"Let us now suppose the earth as turning on its axis in the equinoctial
+situation. The torrid region would thus be changed into a zone, in
+which there would be night and day; consequently, here would be much
+temperance, compared with the torrid region now considered; and here
+perhaps there would be formed periodical condensation and evaporation of
+humidity, corresponding to the seasons of night and day. As temperance
+would thus be introduced into the region of torrid extremity, so would
+the effect of this change be felt over all the globe, every part of
+which would now be illuminated, consequently heated in some degree. Thus
+we would have a line of great heat and evaporation, graduating each way
+into a point of great cold and congelation. Between these two extremes
+of heat and cold there would be found in each hemisphere a region
+of much temperance, in relation to heat, but of much humidity in the
+atmosphere, perhaps of continual rain and condensation.
+
+"The supposition now formed must appear extremely unfit for making this
+globe a habitable world in every part; but having thus seen the effect
+of night and day in temperating the effects of heat and cold in every
+place, we are now prepared to contemplate the effects of supposing this
+globe to revolve around the sun with a certain inclination of its axis.
+By this beautiful contrivance, that comparatively uninhabited globe is
+now divided into two hemispheres, each of which is thus provided with
+a summer and a winter season. But our present view is limited to the
+evaporation and condensation of humidity; and, in this contrivance of
+the seasons, there must appear an ample provision for those alternate
+operations in every part; for as the place of the vertical sun is moved
+alternately from one tropic to the other, heat and cold, the original
+causes of evaporation and condensation, must be carried over all the
+globe, producing either annual seasons of rain or diurnal seasons of
+condensation and evaporation, or both these seasons, more or less--that
+is, in some degree.
+
+"The original cause of motion in the atmosphere is the influence of the
+sun heating the surface of the earth exposed to that luminary. We have
+not supposed that surface to have been of one uniform shape and similar
+substance; from whence it has followed that the annual propers of
+the sun, perhaps also the diurnal propers, would produce a regular
+condensation of rain in certain regions, and the evaporation of humidity
+in others; and this would have a regular progress in certain determined
+seasons, and would not vary. But nothing can be more distant from this
+supposition, that is the natural constitution of the earth; for the
+globe is composed of sea and land, in no regular shape or mixture, while
+the surface of the land is also irregular with respect to its elevations
+and depressions, and various with regard to the humidity and dryness of
+that part which is exposed to heat as the cause of evaporation. Hence a
+source of the most valuable motions in the fluid atmosphere with aqueous
+vapor, more or less, so far as other natural operations will admit; and
+hence a source of the most irregular commixture of the several parts of
+this elastic fluid, whether saturated or not with aqueous vapor.
+
+"According to the theory, nothing is required for the production of rain
+besides the mixture of portions of the atmosphere with humidity, and of
+mixing the parts that are in different degrees of heat. But we have seen
+the causes of saturating every portion of the atmosphere with humidity
+and of mixing the parts which are in different degrees of heat.
+Consequently, over all the surface of the globe there should happen
+occasionally rain and evaporation, more or less; and also, in every
+place, those vicissitudes should be observed to take place with some
+tendency to regularity, which, however, may be so disturbed as to be
+hardly distinguishable upon many occasions. Variable winds and variable
+rains should be found in proportion as each place is situated in an
+irregular mixture of land and water; whereas regular winds should be
+found in proportion to the uniformity of the surface; and regular rains
+in proportion to the regular changes of those winds by which the mixture
+of the atmosphere necessary to the rain may be produced. But as it will
+be acknowledged that this is the case in almost all this earth where
+rain appears according to the conditions here specified, the theory is
+found to be thus in conformity with nature, and natural appearances are
+thus explained by the theory."(1)
+
+
+The next ambitious attempt to explain the phenomena of aqueous meteors
+was made by Luke Howard, in his remarkable paper on clouds, published in
+the Philosophical Magazine in 1803--the paper in which the names cirrus,
+cumulus, stratus, etc., afterwards so universally adopted, were first
+proposed. In this paper Howard acknowledges his indebtedness to Dalton
+for the theory of evaporation; yet he still clings to the idea that
+the vapor, though independent of the air, is combined with particles of
+caloric. He holds that clouds are composed of vapor that has previously
+risen from the earth, combating the opinions of those who believe
+that they are formed by the union of hydrogen and oxygen existing
+independently in the air; though he agrees with these theorists that
+electricity has entered largely into the modus operandi of cloud
+formation. He opposes the opinion of Deluc and De Saussure that clouds
+are composed of particles of water in the form of hollow vesicles
+(miniature balloons, in short, perhaps filled with hydrogen), which
+untenable opinion was a revival of the theory as to the formation of all
+vapor which Dr. Halley had advocated early in the eighteenth century.
+
+Of particular interest are Howard's views as to the formation of dew,
+which he explains as caused by the particles of caloric forsaking the
+vapor to enter the cool body, leaving the water on the surface. This
+comes as near the truth, perhaps, as could be expected while the old
+idea as to the materiality of heat held sway. Howard believed, however,
+that dew is usually formed in the air at some height, and that it
+settles to the surface, opposing the opinion, which had gained vogue
+in France and in America (where Noah Webster prominently advocated it),
+that dew ascends from the earth.
+
+The complete solution of the problem of dew formation--which really
+involved also the entire question of precipitation of watery vapor in
+any form--was made by Dr. W. C. Wells, a man of American birth, whose
+life, however, after boyhood, was spent in Scotland (where as a young
+man he enjoyed the friendship of David Hume) and in London. Inspired,
+no doubt, by the researches of Mack, Hutton, and their confreres of
+that Edinburgh school, Wells made observations on evaporation and
+precipitation as early as 1784, but other things claimed his attention;
+and though he asserts that the subject was often in his mind, he did not
+take it up again in earnest until about 1812.
+
+Meantime the observations on heat of Rumford and Davy and Leslie had
+cleared the way for a proper interpretation of the facts--about the
+facts themselves there had long been practical unanimity of opinion. Dr.
+Black, with his latent-heat observations, had really given the clew to
+all subsequent discussions of the subject of precipitation of vapor;
+and from this time on it had been known that heat is taken up when water
+evaporates, and given out again when it condenses. Dr. Darwin had shown
+in 1788, in a paper before the Royal Society, that air gives off heat
+on contracting and takes it up on expanding; and Dalton, in his essay
+of 1793, had explained this phenomenon as due to the condensation and
+vaporization of the water contained in the air.
+
+But some curious and puzzling observations which Professor Patrick
+Wilson, professor of astronomy in the University of Glasgow, had
+communicated to the Royal Society of Edinburgh in 1784, and some similar
+ones made by Mr. Six, of Canterbury, a few years later, had remained
+unexplained. Both these gentlemen observed that the air is cooler where
+dew is forming than the air a few feet higher, and they inferred
+that the dew in forming had taken up heat, in apparent violation of
+established physical principles.
+
+It remained for Wells, in his memorable paper of 1816, to show that
+these observers had simply placed the cart before the horse. He made it
+clear that the air is not cooler because the dew is formed, but that
+the dew is formed because the air is cooler--having become so through
+radiation of heat from the solids on which the dew forms. The dew
+itself, in forming, gives out its latent heat, and so tends to equalize
+the temperature.
+
+Wells's paper is so admirable an illustration of the lucid presentation
+of clearly conceived experiments and logical conclusions that we should
+do it injustice not to present it entire. The author's mention of
+the observations of Six and Wilson gives added value to his own
+presentation.
+
+
+Dr. Wells's Essay on Dew
+
+"I was led in the autumn of 1784, by the event of a rude experiment,
+to think it probable that the formation of dew is attended with the
+production of cold. In 1788, a paper on hoar-frost, by Mr. Patrick
+Wilson, of Glasgow, was published in the first volume of the
+Transactions of the Royal Society of Edinburgh, by which it appeared
+that this opinion bad been entertained by that gentleman before it
+had occurred to myself. In the course of the same year, Mr. Six, of
+Canterbury, mentioned in a paper communicated to the Royal Society
+that on clear and dewy nights he always found the mercury lower in a
+thermometer laid upon the ground in a meadow in his neighborhood than
+it was in a similar thermometer suspended in the air six feet above the
+former; and that upon one night the difference amounted to five degrees
+of Fahrenheit's scale. Mr. Six, however, did not suppose, agreeably to
+the opinion of Mr. Wilson and myself, that the cold was occasioned by
+the formation of dew, but imagined that it proceeded partly from the
+low temperature of the air, through which the dew, already formed in the
+atmosphere, had descended, and partly from the evaporation of moisture
+from the ground, on which his thermometer had been placed. The
+conjecture of Mr. Wilson and the observations of Mr. Six, together
+with many facts which I afterwards learned in the course of reading,
+strengthened my opinion; but I made no attempt, before the autumn of
+1811, to ascertain by experiment if it were just, though it had in
+the mean time almost daily occurred to my thoughts. Happening, in
+that season, to be in that country in a clear and calm night, I laid a
+thermometer upon grass wet with dew, and suspended a second in the air,
+two feet above the other. An hour afterwards the thermometer on the
+grass was found to be eight degrees lower, by Fahrenheit's division,
+than the one in the air. Similar results having been obtained from
+several similar experiments, made during the same autumn, I determined
+in the next spring to prosecute the subject with some degree of
+steadiness, and with that view went frequently to the house of one of my
+friends who lives in Surrey.
+
+"At the end of two months I fancied that I had collected information
+worthy of being published; but, fortunately, while preparing an account
+of it I met by accident with a small posthumous work by Mr. Six, printed
+at Canterbury in 1794, in which are related differences observed on dewy
+nights between thermometers placed upon grass and others in the air that
+are much greater than those mentioned in the paper presented by him to
+the Royal Society in 1788. In this work, too, the cold of the grass is
+attributed, in agreement with the opinion of Mr. Wilson, altogether to
+the dew deposited upon it. The value of my own observations appearing to
+me now much diminished, though they embraced many points left untouched
+by Mr. Six, I gave up my intentions of making them known. Shortly after,
+however, upon considering the subject more closely, I began to suspect
+that Mr. Wilson, Mr. Six, and myself had all committed an error
+regarding the cold which accompanies dew as an effect of the formation
+of that fluid. I therefore resumed my experiments, and having by means
+of them, I think, not only established the justness of my suspicions,
+but ascertained the real cause both of dew and of several other natural
+appearances which have hitherto received no sufficient explanation, I
+venture now to submit to the consideration of the learned an account
+of some of my labors, without regard to the order of time in which they
+were performed, and of various conclusions which may be drawn from them,
+mixed with facts and opinions already published by others:
+
+"There are various occurrences in nature which seem to me strictly
+allied to dew, though their relation to it be not always at first sight
+perceivable. The statement and explanation of several of these will form
+the concluding part of the present essay.
+
+"1. I observed one morning, in winter, that the insides of the panes of
+glass in the windows of my bedchamber were all of them moist, but that
+those which had been covered by an inside shutter during the night were
+much more so than the others which had been uncovered. Supposing that
+this diversity of appearance depended upon a difference of temperature,
+I applied the naked bulbs of two delicate thermometers to a covered
+and uncovered pane; on which I found that the former was three degrees
+colder than the latter. The air of the chamber, though no fire was kept
+in it, was at this time eleven and one-half degrees warmer than that
+without. Similar experiments were made on many other mornings, the
+results of which were that the warmth of the internal air exceeded that
+of the external from eight to eighteen degrees, the temperature of the
+covered panes would be from one to five degrees less than the uncovered;
+that the covered were sometimes dewed, while the uncovered were dry;
+that at other times both were free from moisture; that the outsides of
+the covered and uncovered panes had similar differences with respect to
+heat, though not so great as those of the inner surfaces; and that no
+variation in the quantity of these differences was occasioned by the
+weather's being cloudy or fair, provided the heat of the internal air
+exceeded that of the external equally in both of those states of the
+atmosphere.
+
+"The remote reason of these differences did not immediately present
+itself. I soon, however, saw that the closed shutter shielded the glass
+which it covered from the heat that was radiated to the windows by
+the walls and furniture of the room, and thus kept it nearer to the
+temperature of the external air than those parts could be which, from
+being uncovered, received the heat emitted to them by the bodies just
+mentioned.
+
+"In making these experiments, I seldom observed the inside of any pane
+to be more than a little damped, though it might be from eight to twelve
+degrees colder than the general mass of the air in the room; while, in
+the open air, I had often found a great dew to form on substances
+only three or four degrees colder than the atmosphere. This at first
+surprised me; but the cause now seems plain. The air of the chamber had
+once been a portion of the external atmosphere, and had afterwards
+been heated, when it could receive little accessories to its original
+moisture. It constantly required being cooled considerably before it
+was even brought back to its former nearness to repletion with water;
+whereas the whole external air is commonly, at night, nearly replete
+with moisture, and therefore readily precipitates dew on bodies only a
+little colder than itself.
+
+"When the air of a room is warmer than the external atmosphere, the
+effect of an outside shutter on the temperature of the glass of the
+window will be directly opposite to what has just been stated; since
+it must prevent the radiation, into the atmosphere, of the heat of the
+chamber transmitted through the glass.
+
+"2. Count Rumford appears to have rightly conjectured that the
+inhabitants of certain hot countries, who sleep at nights on the tops of
+their houses, are cooled during this exposure by the radiation of their
+heat to the sky; or, according to his manner of expression, by receiving
+frigorific rays from the heavens. Another fact of this kind seems to be
+the greater chill which we often experience upon passing at night from
+the cover of a house into the air than might have been expected from the
+cold of the external atmosphere. The cause, indeed, is said to be the
+quickness of transition from one situation to another. But if this were
+the whole reason, an equal chill would be felt in the day, when the
+difference, in point of heat, between the internal and external air was
+the same as at night, which is not the case. Besides, if I can trust my
+own observation, the feeling of cold from this cause is more remarkable
+in a clear than in a cloudy night, and in the country than in towns. The
+following appears to be the manner in which these things are chiefly to
+be explained:
+
+"During the day our bodies while in the open air, although not
+immediately exposed to the sun's rays, are yet constantly deriving
+heat from them by means of the reflection of the atmosphere. This heat,
+though it produces little change on the temperature of the air which it
+traverses, affords us some compensation for the heat which we radiate to
+the heavens. At night, also, if the sky be overcast, some compensation
+will be made to us, both in the town and in the country, though in a
+less degree than during the day, as the clouds will remit towards the
+earth no inconsiderable quantity of heat. But on a clear night, in an
+open part of the country, nothing almost can be returned to us from
+above in place of the heat which we radiate upward. In towns, however,
+some compensation will be afforded even on the clearest nights for the
+heat which we lose in the open air by that which is radiated to us from
+the sun round buildings.
+
+"To our loss of heat by radiation at times that we derive little
+compensation from the radiation of other bodies is probably to be
+attributed a great part of the hurtful effects of the night air.
+Descartes says that these are not owing to dew, as was the common
+opinion of his contemporaries, but to the descent of certain noxious
+vapors which have been exhaled from the earth during the heat of the
+day, and are afterwards condensed by the cold of a serene night. The
+effects in question certainly cannot be occasioned by dew, since that
+fluid does not form upon a healthy human body in temperate climates; but
+they may, notwithstanding, arise from the same cause that produces dew
+on those substances which do not, like the human body, possess the power
+of generating heat for the supply of what they lose by radiation or any
+other means."(2)
+
+
+This explanation made it plain why dew forms on a clear night, when
+there are no clouds to reflect the radiant heat. Combined with Dalton's
+theory that vapor is an independent gas, limited in quantity in any
+given space by the temperature of that space, it solved the problem of
+the formation of clouds, rain, snow, and hoar-frost. Thus this paper
+of Wells's closed the epoch of speculation regarding this field of
+meteorology, as Hutton's paper of 1784 had opened it. The fact that the
+volume containing Hutton's paper contained also his epoch-making paper
+on geology finds curiously a duplication in the fact that Wells's volume
+contained also his essay on Albinism, in which the doctrine of natural
+selection was for the first time formulated, as Charles Darwin freely
+admitted after his own efforts had made the doctrine famous.
+
+
+ISOTHERMS AND OCEAN CURRENTS
+
+The very next year after Dr. Wells's paper was published there appeared
+in France the third volume of the Memoires de Physique et de Chimie de
+la Societe d'Arcueil, and a new epoch in meteorology was inaugurated.
+The society in question was numerically an inconsequential band, listing
+only a dozen members; but every name was a famous one: Arago, Berard,
+Berthollet, Biot, Chaptal, De Candolle, Dulong, Gay-Lussac, Humboldt,
+Laplace, Poisson, and Thenard--rare spirits every one. Little danger
+that the memoirs of such a band would be relegated to the dusty shelves
+where most proceedings of societies belong--no milk-for-babes fare would
+be served to such a company.
+
+The particular paper which here interests us closes this third and
+last volume of memoirs. It is entitled "Des Lignes Isothermes et de
+la Distribution de la Chaleursurle Globe." The author is Alexander
+Humboldt. Needless to say, the topic is handled in a masterly
+manner. The distribution of heat on the surface of the globe, on the
+mountain-sides, in the interior of the earth; the causes that regulate
+such distribution; the climatic results--these are the topics discussed.
+But what gives epochal character to the paper is the introduction of
+those isothermal lines circling the earth in irregular course, joining
+together places having the same mean annual temperature, and thus laying
+the foundation for a science of comparative climatology.
+
+It is true the attempt to study climates comparatively was not new.
+Mairan had attempted it in those papers in which he developed his
+bizarre ideas as to central emanations of heat. Euler had brought
+his profound mathematical genius to bear on the topic, evolving the
+"extraordinary conclusion that under the equator at midnight the
+cold ought to be more rigorous than at the poles in winter." And
+in particular Richard Kirwan, the English chemist, had combined the
+mathematical and the empirical methods and calculated temperatures for
+all latitudes. But Humboldt differs from all these predecessors in that
+he grasps the idea that the basis of all such computations should be
+not theory, but fact. He drew his isothermal lines not where some occult
+calculation would locate them on an ideal globe, but where practical
+tests with the thermometer locate them on our globe as it is. London,
+for example, lies in the same latitude as the southern extremity of
+Hudson Bay; but the isotherm of London, as Humboldt outlines it, passes
+through Cincinnati.
+
+Of course such deviations of climatic conditions between places in the
+same latitude had long been known. As Humboldt himself observes,
+the earliest settlers of America were astonished to find themselves
+subjected to rigors of climate for which their European experience had
+not at all prepared them. Moreover, sagacious travellers, in particular
+Cook's companion on his second voyage, young George Forster, had
+noted as a general principle that the western borders of continents
+in temperate regions are always warmer than corresponding latitudes of
+their eastern borders; and of course the general truth of temperatures
+being milder in the vicinity of the sea than in the interior of
+continents had long been familiar. But Humboldt's isothermal lines for
+the first time gave tangibility to these ideas, and made practicable a
+truly scientific study of comparative climatology.
+
+In studying these lines, particularly as elaborated by further
+observations, it became clear that they are by no means haphazard in
+arrangement, but are dependent upon geographical conditions which in
+most cases are not difficult to determine. Humboldt himself pointed out
+very clearly the main causes that tend to produce deviations from the
+average--or, as Dove later on called it, the normal--temperature of any
+given latitude. For example, the mean annual temperature of a region
+(referring mainly to the northern hemisphere) is raised by the proximity
+of a western coast; by a divided configuration of the continent into
+peninsulas; by the existence of open seas to the north or of radiating
+continental surfaces to the south; by mountain ranges to shield from
+cold winds; by the infrequency of swamps to become congealed; by the
+absence of woods in a dry, sandy soil; and by the serenity of sky in the
+summer months and the vicinity of an ocean current bringing water which
+is of a higher temperature than that of the surrounding sea.
+
+Conditions opposite to these tend, of course, correspondingly to lower
+the temperature. In a word, Humboldt says the climatic distribution of
+heat depends on the relative distribution of land and sea, and on the
+"hypsometrical configuration of the continents"; and he urges that
+"great meteorological phenomena cannot be comprehended when considered
+independently of geognostic relations"--a truth which, like most other
+general principles, seems simple enough once it is pointed out.
+
+With that broad sweep of imagination which characterized him, Humboldt
+speaks of the atmosphere as the "aerial ocean, in the lower strata
+and on the shoals of which we live," and he studies the atmospheric
+phenomena always in relation to those of that other ocean of water. In
+each of these oceans there are vast permanent currents, flowing
+always in determinate directions, which enormously modify the climatic
+conditions of every zone. The ocean of air is a vast maelstrom, boiling
+up always under the influence of the sun's heat at the equator, and
+flowing as an upper current towards either pole, while an undercurrent
+from the poles, which becomes the trade-winds, flows towards the equator
+to supply its place.
+
+But the superheated equatorial air, becoming chilled, descends to the
+surface in temperate latitudes, and continues its poleward journey as
+the anti-trade-winds. The trade-winds are deflected towards the west,
+because in approaching the equator they constantly pass over surfaces of
+the earth having a greater and greater velocity of rotation, and so, as
+it were, tend to lag behind--an explanation which Hadley pointed out in
+1735, but which was not accepted until Dalton independently worked it
+out and promulgated it in 1793. For the opposite reason, the anti-trades
+are deflected towards the east; hence it is that the western, borders
+of continents in temperate zones are bathed in moist sea-breezes, while
+their eastern borders lack this cold-dispelling influence.
+
+In the ocean of water the main currents run as more sharply
+circumscribed streams--veritable rivers in the sea. Of these the best
+known and most sharply circumscribed is the familiar Gulf Stream,
+which has its origin in an equatorial current, impelled westward by
+trade-winds, which is deflected northward in the main at Cape St. Roque,
+entering the Caribbean Sea and Gulf of Mexico, to emerge finally through
+the Strait of Florida, and journey off across the Atlantic to warm the
+shores of Europe.
+
+Such, at least, is the Gulf Stream as Humboldt understood it. Since his
+time, however, ocean currents in general, and this one in particular,
+have been the subject of no end of controversy, it being hotly disputed
+whether either causes or effects of the Gulf Stream are just what
+Humboldt, in common with others of his time, conceived them to be. About
+the middle of the century Lieutenant M. F. Maury, the distinguished
+American hydrographer and meteorologist, advocated a theory of
+gravitation as the chief cause of the currents, claiming that difference
+in density, due to difference in temperature and saltness, would
+sufficiently account for the oceanic circulation. This theory gained
+great popularity through the wide circulation of Maury's Physical
+Geography of the Sea, which is said to have passed through more editions
+than any other scientific book of the period; but it was ably and
+vigorously combated by Dr. James Croll, the Scottish geologist, in his
+Climate and Time, and latterly the old theory that ocean currents are
+due to the trade-winds has again come into favor. Indeed, very recently
+a model has been constructed, with the aid of which it is said to have
+been demonstrated that prevailing winds in the direction of the actual
+trade-winds would produce such a current as the Gulf Stream.
+
+Meantime, however, it is by no means sure that gravitation does not
+enter into the case to the extent of producing an insensible general
+oceanic circulation, independent of the Gulf Stream and similar marked
+currents, and similar in its larger outlines to the polar-equatorial
+circulation of the air. The idea of such oceanic circulation was first
+suggested in detail by Professor Lenz, of St. Petersburg, in 1845, but
+it was not generally recognized until Dr. Carpenter independently hit
+upon the idea more than twenty years later. The plausibility of the
+conception is obvious; yet the alleged fact of such circulation has been
+hotly disputed, and the question is still sub judice.
+
+But whether or not such general circulation of ocean water takes place,
+it is beyond dispute that the recognized currents carry an enormous
+quantity of heat from the tropics towards the poles. Dr. Croll, who has
+perhaps given more attention to the physics of the subject than almost
+any other person, computes that the Gulf Stream conveys to the North
+Atlantic one-fourth as much heat as that body receives directly from the
+sun, and he argues that were it not for the transportation of heat by
+this and similar Pacific currents, only a narrow tropical region of the
+globe would be warm enough for habitation by the existing faunas. Dr.
+Croll argues that a slight change in the relative values of northern
+and southern trade-winds (such as he believes has taken place at various
+periods in the past) would suffice to so alter the equatorial current
+which now feeds the Gulf Stream that its main bulk would be deflected
+southward instead of northward, by the angle of Cape St. Roque. Thus the
+Gulf Stream would be nipped in the bud, and, according to Dr. Croll's
+estimates, the results would be disastrous for the northern hemisphere.
+The anti-trades, which now are warmed by the Gulf Stream, would then
+blow as cold winds across the shores of western Europe, and in all
+probability a glacial epoch would supervene throughout the northern
+hemisphere.
+
+The same consequences, so far as Europe is concerned at least, would
+apparently ensue were the Isthmus of Panama to settle into the sea,
+allowing the Caribbean current to pass into the Pacific. But the
+geologist tells us that this isthmus rose at a comparatively recent
+geological period, though it is hinted that there had been some time
+previously a temporary land connection between the two continents. Are
+we to infer, then, that the two Americas in their unions and disunions
+have juggled with the climate of the other hemisphere? Apparently so, if
+the estimates made of the influence of the Gulf Stream be tenable. It is
+a far cry from Panama to Russia. Yet it seems within the possibilities
+that the meteorologist may learn from the geologist of Central America
+something that will enable him to explain to the paleontologist of
+Europe how it chanced that at one time the mammoth and rhinoceros roamed
+across northern Siberia, while at another time the reindeer and musk-ox
+browsed along the shores of the Mediterranean.
+
+Possibilities, I said, not probabilities. Yet even the faint glimmer of
+so alluring a possibility brings home to one with vividness the truth
+of Humboldt's perspicuous observation that meteorology can be properly
+comprehended only when studied in connection with the companion
+sciences. There are no isolated phenomena in nature.
+
+
+CYCLONES AND ANTI-CYCLONES
+
+Yet, after all, it is not to be denied that the chief concern of the
+meteorologist must be with that other medium, the "ocean of air, on
+the shoals of which we live." For whatever may be accomplished by water
+currents in the way of conveying heat, it is the wind currents that
+effect the final distribution of that heat. As Dr. Croll has urged, the
+waters of the Gulf Stream do not warm the shores of Europe by direct
+contact, but by warming the anti-trade-winds, which subsequently blow
+across the continent. And everywhere the heat accumulated by water
+becomes effectual in modifying climate, not so much by direct radiation
+as by diffusion through the medium of the air.
+
+This very obvious importance of aerial currents led to their practical
+study long before meteorology had any title to the rank of science, and
+Dalton's explanation of the trade-winds had laid the foundation for a
+science of wind dynamics before the beginning of the nineteenth century.
+But no substantial further advance in this direction was effected until
+about 1827, when Heinrich W. Dove, of Konigsberg, afterwards to be known
+as perhaps the foremost meteorologist of his generation, included
+the winds among the subjects of his elaborate statistical studies in
+climatology.
+
+Dove classified the winds as permanent, periodical, and variable. His
+great discovery was that all winds, of whatever character, and not
+merely the permanent winds, come under the influence of the earth's
+rotation in such a way as to be deflected from their course, and hence
+to take on a gyratory motion--that, in short, all local winds are minor
+eddies in the great polar-equatorial whirl, and tend to reproduce in
+miniature the character of that vast maelstrom. For the first time,
+then, temporary or variable winds were seen to lie within the province
+of law.
+
+A generation later, Professor William Ferrel, the American
+meteorologist, who had been led to take up the subject by a perusal of
+Maury's discourse on ocean winds, formulated a general mathematical law,
+to the effect that any body moving in a right line along the surface of
+the earth in any direction tends to have its course deflected, owing to
+the earth's rotation, to the right hand in the northern and to the left
+hand in the southern hemisphere. This law had indeed been stated as
+early as 1835 by the French physicist Poisson, but no one then thought
+of it as other than a mathematical curiosity; its true significance was
+only understood after Professor Ferrel had independently rediscovered it
+(just as Dalton rediscovered Hadley's forgotten law of the trade-winds)
+and applied it to the motion of wind currents.
+
+Then it became clear that here is a key to the phenomena of atmospheric
+circulation, from the great polar-equatorial maelstrom which manifests
+itself in the trade-winds to the most circumscribed riffle which is
+announced as a local storm. And the more the phenomena were studied,
+the more striking seemed the parallel between the greater maelstrom
+and these lesser eddies. Just as the entire atmospheric mass of each
+hemisphere is seen, when viewed as a whole, to be carried in a great
+whirl about the pole of that hemisphere, so the local disturbances
+within this great tide are found always to take the form of whirls about
+a local storm-centre--which storm-centre, meantime, is carried along
+in the major current, as one often sees a little whirlpool in the water
+swept along with the main current of the stream. Sometimes, indeed, the
+local eddy, caught as it were in an ancillary current of the great
+polar stream, is deflected from its normal course and may seem to travel
+against the stream; but such deviations are departures from the rule. In
+the great majority of cases, for example, in the north temperate zone, a
+storm-centre (with its attendant local whirl) travels to the northeast,
+along the main current of the anti-trade-wind, of which it is a part;
+and though exceptionally its course may be to the southeast instead, it
+almost never departs so widely from the main channel as to progress to
+the westward. Thus it is that storms sweeping over the United States can
+be announced, as a rule, at the seaboard in advance of their coming by
+telegraphic communication from the interior, while similar storms
+come to Europe off the ocean unannounced. Hence the more practical
+availability of the forecasts of weather bureaus in the former country.
+
+But these local whirls, it must be understood, are local only in a very
+general sense of the word, inasmuch as a single one may be more than
+a thousand miles in diameter, and a small one is two or three hundred
+miles across. But quite without regard to the size of the whirl, the air
+composing it conducts itself always in one of two ways. It never whirls
+in concentric circles; it always either rushes in towards the centre in
+a descending spiral, in which case it is called a cyclone, or it spreads
+out from the centre in a widening spiral, in which case it is called an
+anti-cyclone. The word cyclone is associated in popular phraseology with
+a terrific storm, but it has no such restriction in technical usage. A
+gentle zephyr flowing towards a "storm-centre" is just as much a
+cyclone to the meteorologist as is the whirl constituting a West-Indian
+hurricane. Indeed, it is not properly the wind itself that is called the
+cyclone in either case, but the entire system of whirls--including the
+storm-centre itself, where there may be no wind at all.
+
+What, then, is this storm-centre? Merely an area of low barometric
+pressure--an area where the air has become lighter than the air of
+surrounding regions. Under influence of gravitation the air seeks its
+level just as water does; so the heavy air comes flowing in from
+all sides towards the low-pressure area, which thus becomes a
+"storm-centre." But the inrushing currents never come straight to their
+mark. In accordance with Ferrel's law, they are deflected to the right,
+and the result, as will readily be seen, must be a vortex current, which
+whirls always in one direction--namely, from left to right, or in the
+direction opposite to that of the hands of a watch held with its face
+upward. The velocity of the cyclonic currents will depend largely upon
+the difference in barometric pressure between the storm-centre and the
+confines of the cyclone system. And the velocity of the currents will
+determine to some extent the degree of deflection, and hence the exact
+path of the descending spiral in which the wind approaches the centre.
+But in every case and in every part of the cyclone system it is true, as
+Buys Ballot's famous rule first pointed out, that a person standing with
+his back to the wind has the storm-centre at his left.
+
+The primary cause of the low barometric pressure which marks the
+storm-centre and establishes the cyclone is expansion of the air through
+excess of temperature. The heated air, rising into cold upper regions,
+has a portion of its vapor condensed into clouds, and now a new dynamic
+factor is added, for each particle of vapor, in condensing, gives up its
+modicum of latent heat. Each pound of vapor thus liberates, according
+to Professor Tyndall's estimate, enough heat to melt five pounds of cast
+iron; so the amount given out where large masses of cloud are forming
+must enormously add to the convection currents of the air, and hence to
+the storm-developing power of the forming cyclone. Indeed, one school
+of meteorologists, of whom Professor Espy was the leader, has held that,
+without such added increment of energy constantly augmenting the dynamic
+effects, no storm could long continue in violent action. And it is
+doubted whether any storm could ever attain, much less continue, the
+terrific force of that most dreaded of winds of temperate zones, the
+tornado--a storm which obeys all the laws of cyclones, but differs from
+ordinary cyclones in having a vortex core only a few feet or yards in
+diameter--without the aid of those great masses of condensing vapor
+which always accompany it in the form of storm-clouds.
+
+The anti-cyclone simply reverses the conditions of the cyclone. Its
+centre is an area of high pressure, and the air rushes out from it in
+all directions towards surrounding regions of low pressure. As before,
+all parts of the current will be deflected towards the right, and
+the result, clearly, is a whirl opposite in direction to that of the
+cyclone. But here there is a tendency to dissipation rather than to
+concentration of energy, hence, considered as a storm-generator, the
+anti-cyclone is of relative insignificance.
+
+In particular the professional meteorologist who conducts a "weather
+bureau"--as, for example, the chief of the United States signal-service
+station in New York--is so preoccupied with the observation of this
+phenomenon that cyclone-hunting might be said to be his chief pursuit.
+It is for this purpose, in the main, that government weather bureaus
+or signal-service departments have been established all over the world.
+Their chief work is to follow up cyclones, with the aid of telegraphic
+reports, mapping their course and recording the attendant meteorological
+conditions. Their so-called predictions or forecasts are essentially
+predications, gaining locally the effect of predictions because the
+telegraph outstrips the wind.
+
+At only one place on the globe has it been possible as yet for the
+meteorologist to make long-time forecasts meriting the title of
+predictions. This is in the middle Ganges Valley of northern India.
+In this country the climatic conditions are largely dependent upon the
+periodical winds called monsoons, which blow steadily landward from
+April to October, and seaward from October to April. The summer monsoons
+bring the all-essential rains; if they are delayed or restricted
+in extent, there will be drought and consequent famine. And such
+restriction of the monsoon is likely to result when there has been an
+unusually deep or very late snowfall on the Himalayas, because of the
+lowering of spring temperature by the melting snow. Thus here it is
+possible, by observing the snowfall in the mountains, to predict with
+some measure of success the average rainfall of the following summer.
+The drought of 1896, with the consequent famine and plague that
+devastated India the following winter, was thus predicted some months in
+advance.
+
+This is the greatest present triumph of practical meteorology. Nothing
+like it is yet possible anywhere in temperate zones. But no one can
+say what may not be possible in times to come, when the data now being
+gathered all over the world shall at last be co-ordinated, classified,
+and made the basis of broad inductions. Meteorology is pre-eminently a
+science of the future.
+
+
+
+
+VI. MODERN THEORIES OF HEAT AND LIGHT
+
+THE eighteenth-century philosopher made great strides in his studies
+of the physical properties of matter and the application of these
+properties in mechanics, as the steam-engine, the balloon, the optic
+telegraph, the spinning-jenny, the cotton-gin, the chronometer, the
+perfected compass, the Leyden jar, the lightning-rod, and a host of
+minor inventions testify. In a speculative way he had thought out more
+or less tenable conceptions as to the ultimate nature of matter, as
+witness the theories of Leibnitz and Boscovich and Davy, to which we
+may recur. But he had not as yet conceived the notion of a distinction
+between matter and energy, which is so fundamental to the physics of a
+later epoch. He did not speak of heat, light, electricity, as forms
+of energy or "force"; he conceived them as subtile forms of matter--as
+highly attenuated yet tangible fluids, subject to gravitation and
+chemical attraction; though he had learned to measure none of them but
+heat with accuracy, and this one he could test only within narrow limits
+until late in the century, when Josiah Wedgwood, the famous potter,
+taught him to gauge the highest temperatures with the clay pyrometer.
+
+He spoke of the matter of heat as being the most universally distributed
+fluid in nature; as entering in some degree into the composition of
+nearly all other substances; as being sometimes liquid, sometimes
+condensed or solid, and as having weight that could be detected with
+the balance. Following Newton, he spoke of light as a "corpuscular
+emanation" or fluid, composed of shining particles which possibly are
+transmutable into particles of heat, and which enter into chemical
+combination with the particles of other forms of matter. Electricity
+he considered a still more subtile kind of matter-perhaps an attenuated
+form of light. Magnetism, "vital fluid," and by some even a "gravic
+fluid," and a fluid of sound were placed in the same scale; and, taken
+together, all these supposed subtile forms of matter were classed as
+"imponderables."
+
+This view of the nature of the "imponderables" was in some measure a
+retrogression, for many seventeenth-century philosophers, notably
+Hooke and Huygens and Boyle, had held more correct views; but the
+materialistic conception accorded so well with the eighteenth-century
+tendencies of thought that only here and there a philosopher like Euler
+called it in question, until well on towards the close of the century.
+Current speech referred to the materiality of the "imponderables"
+unquestioningly. Students of meteorology--a science that was just
+dawning--explained atmospheric phenomena on the supposition that heat,
+the heaviest imponderable, predominated in the lower atmosphere, and
+that light, electricity, and magnetism prevailed in successively higher
+strata. And Lavoisier, the most philosophical chemist of the century,
+retained heat and light on a par with oxygen, hydrogen, iron, and the
+rest, in his list of elementary substances.
+
+
+COUNT RUMFORD AND THE VIBRATORY THEORY OF HEAT
+
+But just at the close of the century the confidence in the status of
+the imponderables was rudely shaken in the minds of philosophers by the
+revival of the old idea of Fra Paolo and Bacon and Boyle, that heat,
+at any rate, is not a material fluid, but merely a mode of motion or
+vibration among the particles of "ponderable" matter. The new champion
+of the old doctrine as to the nature of heat was a very distinguished
+philosopher and diplomatist of the time, who, it may be worth recalling,
+was an American. He was a sadly expatriated American, it is true, as his
+name, given all the official appendages, will amply testify; but he had
+been born and reared in a Massachusetts village none the less, and
+he seems always to have retained a kindly interest in the land of his
+nativity, even though he lived abroad in the service of other powers
+during all the later years of his life, and was knighted by England,
+ennobled by Bavaria, and honored by the most distinguished scientific
+bodies of Europe. The American, then, who championed the vibratory
+theory of heat, in opposition to all current opinion, in this closing
+era of the eighteenth century, was Lieutenant-General Sir Benjamin
+Thompson, Count Rumford, F.R.S.
+
+Rumford showed that heat may be produced in indefinite quantities by
+friction of bodies that do not themselves lose any appreciable matter
+in the process, and claimed that this proves the immateriality of heat.
+Later on he added force to the argument by proving, in refutation of the
+experiments of Bowditch, that no body either gains or loses weight in
+virtue of being heated or cooled. He thought he had proved that heat is
+only a form of motion.
+
+His experiment for producing indefinite quantities of heat by friction
+is recorded by him in his paper entitled, "Inquiry Concerning the Source
+of Heat Excited by Friction."
+
+"Being engaged, lately, in superintending the boring of cannon in the
+workshops of the military arsenal at Munich," he says, "I was struck
+with the very considerable degree of heat which a brass gun acquires in
+a short time in being bored; and with the still more intense heat (much
+greater than that of boiling water, as I found by experiment) of the
+metallic chips separated from it by the borer.
+
+"Taking a cannon (a brass six-pounder), cast solid, and rough, as it
+came from the foundry, and fixing it horizontally in a machine used
+for boring, and at the same time finishing the outside of the cannon by
+turning, I caused its extremity to be cut off; and by turning down
+the metal in that part, a solid cylinder was formed, 7 3/4 inches in
+diameter and 9 8/10 inches long; which, when finished, remained joined
+to the rest of the metal (that which, properly speaking, constituted the
+cannon) by a small cylindrical neck, only 2 1/5 inches in diameter and 3
+8/10 inches long.
+
+"This short cylinder, which was supported in its horizontal position,
+and turned round its axis by means of the neck by which it remained
+united to the cannon, was now bored with the horizontal borer used in
+boring cannon.
+
+"This cylinder being designed for the express purpose of generating heat
+by friction, by having a blunt borer forced against its solid bottom at
+the same time that it should be turned round its axis by the force of
+horses, in order that the heat accumulated in the cylinder might from
+time to time be measured, a small, round hole 0.37 of an inch only in
+diameter and 4.2 inches in depth, for the purpose of introducing a small
+cylindrical mercurial thermometer, was made in it, on one side, in a
+direction perpendicular to the axis of the cylinder, and ending in the
+middle of the solid part of the metal which formed the bottom of the
+bore.
+
+"At the beginning of the experiment, the temperature of the air in the
+shade, as also in the cylinder, was just sixty degrees Fahrenheit. At
+the end of thirty minutes, when the cylinder had made 960 revolutions
+about its axis, the horses being stopped, a cylindrical mercury
+thermometer, whose bulb was 32/100 of an inch in diameter and 3 1/4
+inches in length, was introduced into the hole made to receive it in
+the side of the cylinder, when the mercury rose almost instantly to one
+hundred and thirty degrees.
+
+"In order, by one decisive experiment, to determine whether the air
+of the atmosphere had any part or not in the generation of the heat, I
+contrived to repeat the experiment under circumstances in which it was
+evidently impossible for it to produce any effect whatever. By means
+of a piston exactly fitted to the mouth of the bore of the cylinder,
+through the middle of which piston the square iron bar, to the end of
+which the blunt steel borer was fixed, passed in a square hole made
+perfectly air-tight, the excess of the external air, to the inside of
+the bore of the cylinder, was effectually prevented. I did not find,
+however, by this experiment that the exclusion of the air diminished in
+the smallest degree the quantity of heat excited by the friction.
+
+"There still remained one doubt, which, though it appeared to me to be
+so slight as hardly to deserve any attention, I was, however, desirous
+to remove. The piston which choked the mouth of the bore of the
+cylinder, in order that it might be air-tight, was fitted into it with
+so much nicety, by means of its collars of leather, and pressed against
+it with so much force, that, notwithstanding its being oiled, it
+occasioned a considerable degree of friction when the hollow cylinder
+was turned round its axis. Was not the heat produced, or at least some
+part of it, occasioned by this friction of the piston? and, as the
+external air had free access to the extremity of the bore, where it came
+into contact with the piston, is it not possible that this air may have
+had some share in the generation of the heat produced?
+
+"A quadrangular oblong deal box, water-tight, being provided with
+holes or slits in the middle of each of its ends, just large enough to
+receive, the one the square iron rod to the end of which the blunt steel
+borer was fastened, the other the small cylindrical neck which joined
+the hollow cylinder to the cannon; when this box (which was occasionally
+closed above by a wooden cover or lid moving on hinges) was put into
+its place--that is to say, when, by means of the two vertical opening
+or slits in its two ends, the box was fixed to the machinery in such
+a manner that its bottom being in the plane of the horizon, its axis
+coincided with the axis of the hollow metallic cylinder, it is evident,
+from the description, that the hollow, metallic cylinder would occupy
+the middle of the box, without touching it on either side; and that,
+on pouring water into the box and filling it to the brim, the cylinder
+would be completely covered and surrounded on every side by that fluid.
+And, further, as the box was held fast by the strong, square iron rod
+which passed in a square hole in the centre of one of its ends, while
+the round or cylindrical neck which joined the hollow cylinder to the
+end of the cannon could turn round freely on its axis in the round hole
+in the centre of the other end of it, it is evident that the machinery
+could be put in motion without the least danger of forcing the box out
+of its place, throwing the water out of it, or deranging any part of the
+apparatus."
+
+Everything being thus ready, the box was filled with cold water, having
+been made water-tight by means of leather collars, and the machinery put
+in motion. "The result of this beautiful experiment," says Rumford, "was
+very striking, and the pleasure it afforded me amply repaid me for
+all the trouble I had had in contriving and arranging the complicated
+machinery used in making it. The cylinder, revolving at the rate of
+thirty-two times in a minute, had been in motion but a short time when I
+perceived, by putting my hand into the water and touching the outside
+of the cylinder, that heat was generated, and it was not long before the
+water which surrounded the cylinder began to be sensibly warm.
+
+"At the end of one hour I found, by plunging a thermometer into the
+box,... that its temperature had been raised no less than forty-seven
+degrees Fahrenheit, being now one hundred and seven degrees Fahrenheit.
+... One hour and thirty minutes after the machinery had been put in
+motion the heat of the water in the box was one hundred and forty-two
+degrees. At the end of two hours... it was raised to one hundred and
+seventy-eight degrees; and at two hours and thirty minutes it ACTUALLY
+BOILED!
+
+"It would be difficult to describe the surprise and astonishment
+expressed in the countenances of the bystanders on seeing so large a
+quantity of cold water heated, and actually made to boil, without any
+fire. Though there was, in fact, nothing that could justly be considered
+as a surprise in this event, yet I acknowledge fairly that it afforded
+me a degree of childish pleasure which, were I ambitious of the
+reputation of a GRAVE PHILOSOPHER, I ought most certainly rather to hide
+than to discover...."
+
+Having thus dwelt in detail on these experiments, Rumford comes now to
+the all-important discussion as to the significance of them--the
+subject that had been the source of so much speculation among the
+philosophers--the question as to what heat really is, and if there
+really is any such thing (as many believed) as an igneous fluid, or a
+something called caloric.
+
+"From whence came this heat which was continually given off in this
+manner, in the foregoing experiments?" asks Rumford. "Was it furnished
+by the small particles of metal detached from the larger solid masses
+on their being rubbed together? This, as we have already seen, could not
+possibly have been the case.
+
+"Was it furnished by the air? This could not have been the case; for,
+in three of the experiments, the machinery being kept immersed in water,
+the access of the air of the atmosphere was completely prevented.
+
+"Was it furnished by the water which surrounded the machinery? That this
+could not have been the case is evident: first, because this water was
+continually RECEIVING heat from the machinery, and could not, at the
+same time, be GIVING TO and RECEIVING HEAT FROM the same body; and,
+secondly, because there was no chemical decomposition of any part of
+this water. Had any such decomposition taken place (which, indeed, could
+not reasonably have been expected), one of its component elastic fluids
+(most probably hydrogen) must, at the same time, have been set at
+liberty, and, in making its escape into the atmosphere, would have been
+detected; but, though I frequently examined the water to see if any
+air-bubbles rose up through it, and had even made preparations for
+catching them if they should appear, I could perceive none; nor was
+there any sign of decomposition of any kind whatever, or other chemical
+process, going on in the water.
+
+"Is it possible that the heat could have been supplied by means of the
+iron bar to the end of which the blunt steel borer was fixed? Or by the
+small neck of gun-metal by which the hollow cylinder was united to the
+cannon? These suppositions seem more improbable even than either of
+the before-mentioned; for heat was continually going off, or OUT OF THE
+MACHINERY, by both these passages during the whole time the experiment
+lasted.
+
+"And in reasoning on this subject we must not forget to consider that
+most remarkable circumstance, that the source of the heat generated by
+friction in these experiments appeared evidently to be INEXHAUSTIBLE.
+
+"It is hardly necessary to add that anything which any INSULATED body,
+or system of bodies, can continue to furnish WITHOUT LIMITATION cannot
+possibly be a MATERIAL substance; and it appears to me to be extremely
+difficult, if not quite impossible, to form any distinct idea of
+anything capable of being excited and communicated, in the manner
+the heat was excited and communicated in these experiments, except in
+MOTION."(1)
+
+
+THOMAS YOUNG AND THE WAVE THEORY OF LIGHT
+
+But contemporary judgment, while it listened respectfully to Rumford,
+was little minded to accept his verdict. The cherished beliefs of a
+generation are not to be put down with a single blow. Where many minds
+have a similar drift, however, the first blow may precipitate a
+general conflict; and so it was here. Young Humphry Davy had duplicated
+Rumford's experiments, and reached similar conclusions; and soon others
+fell into line. Then, in 1800, Dr. Thomas Young--"Phenomenon Young" they
+called him at Cambridge, because he was reputed to know everything--took
+up the cudgels for the vibratory theory of light, and it began to be
+clear that the two "imponderables," heat and light, must stand or
+fall together; but no one as yet made a claim against the fluidity of
+electricity.
+
+Before we take up the details of the assault made by Young upon the
+old doctrine of the materiality of light, we must pause to consider the
+personality of Young himself. For it chanced that this Quaker physician
+was one of those prodigies who come but few times in a century, and
+the full list of whom in the records of history could be told on one's
+thumbs and fingers. His biographers tell us things about him that read
+like the most patent fairy-tales. As a mere infant in arms he had been
+able to read fluently. Before his fourth birthday came he had read the
+Bible twice through, as well as Watts's Hymns--poor child!--and when
+seven or eight he had shown a propensity to absorb languages much as
+other children absorb nursery tattle and Mother Goose rhymes. When
+he was fourteen, a young lady visiting the household of his tutor
+patronized the pretty boy by asking to see a specimen of his penmanship.
+The pretty boy complied readily enough, and mildly rebuked his
+interrogator by rapidly writing some sentences for her in fourteen
+languages, including such as, Arabian, Persian, and Ethiopic.
+
+Meantime languages had been but an incident in the education of the lad.
+He seems to have entered every available field of thought--mathematics,
+physics, botany, literature, music, painting, languages, philosophy,
+archaeology, and so on to tiresome lengths--and once he had entered any
+field he seldom turned aside until he had reached the confines of the
+subject as then known and added something new from the recesses of his
+own genius. He was as versatile as Priestley, as profound as Newton
+himself. He had the range of a mere dilettante, but everywhere the full
+grasp of the master. He took early for his motto the saying that what
+one man has done, another man may do. Granting that the other man has
+the brain of a Thomas Young, it is a true motto.
+
+Such, then, was the young Quaker who came to London to follow out
+the humdrum life of a practitioner of medicine in the year 1801. But
+incidentally the young physician was prevailed upon to occupy the
+interims of early practice by fulfilling the duties of the chair of
+Natural Philosophy at the Royal Institution, which Count Rumford
+had founded, and of which Davy was then Professor of Chemistry--the
+institution whose glories have been perpetuated by such names as Faraday
+and Tyndall, and which the Briton of to-day speaks of as the "Pantheon
+of Science." Here it was that Thomas Young made those studies which have
+insured him a niche in the temple of fame not far removed from that of
+Isaac Newton.
+
+As early as 1793, when he was only twenty, Young had begun to
+Communicate papers to the Royal Society of London, which were adjudged
+worthy to be printed in full in the Philosophical Transactions; so it
+is not strange that he should have been asked to deliver the Bakerian
+lecture before that learned body the very first year after he came to
+London. The lecture was delivered November 12, 1801. Its subject was
+"The Theory of Light and Colors," and its reading marks an epoch in
+physical science; for here was brought forward for the first time
+convincing proof of that undulatory theory of light with which every
+student of modern physics is familiar--the theory which holds that light
+is not a corporeal entity, but a mere pulsation in the substance of
+an all-pervading ether, just as sound is a pulsation in the air, or in
+liquids or solids.
+
+Young had, indeed, advocated this theory at an earlier date, but it was
+not until 1801 that he hit upon the idea which enabled him to bring it
+to anything approaching a demonstration. It was while pondering over the
+familiar but puzzling phenomena of colored rings into which white
+light is broken when reflected from thin films--Newton's rings, so
+called--that an explanation occurred to him which at once put the entire
+undulatory theory on a new footing. With that sagacity of insight which
+we call genius, he saw of a sudden that the phenomena could be explained
+by supposing that when rays of light fall on a thin glass, part of the
+rays being reflected from the upper surface, other rays, reflected from
+the lower surface, might be so retarded in their course through the
+glass that the two sets would interfere with one another, the forward
+pulsation of one ray corresponding to the backward pulsation of another,
+thus quite neutralizing the effect. Some of the component pulsations of
+the light being thus effaced by mutual interference, the remaining
+rays would no longer give the optical effect of white light; hence the
+puzzling colors.
+
+Here is Young's exposition of the subject:
+
+Of the Colors of Thin Plates
+
+"When a beam of light falls upon two refracting surfaces, the partial
+reflections coincide perfectly in direction; and in this case the
+interval of retardation taken between the surfaces is to their radius as
+twice the cosine of the angle of refraction to the radius.
+
+"Let the medium between the surfaces be rarer than the surrounding
+mediums; then the impulse reflected at the second surface, meeting a
+subsequent undulation at the first, will render the particles of the
+rarer medium capable of wholly stopping the motion of the denser and
+destroying the reflection, while they themselves will be more strongly
+propelled than if they had been at rest, and the transmitted light will
+be increased. So that the colors by reflection will be destroyed, and
+those by transmission rendered more vivid, when the double thickness or
+intervals of retardation are any multiples of the whole breadth of
+the undulations; and at intermediate thicknesses the effects will be
+reversed according to the Newtonian observation.
+
+"If the same proportions be found to hold good with respect to thin
+plates of a denser medium, which is, indeed, not improbable, it will be
+necessary to adopt the connected demonstrations of Prop. IV., but, at
+any rate, if a thin plate be interposed between a rarer and a denser
+medium, the colors by reflection and transmission may be expected to
+change places."
+
+
+OF THE COLORS OF THICK PLATES
+
+"When a beam of light passes through a refracting surface, especially
+if imperfectly polished, a portion of it is irregularly scattered, and
+makes the surface visible in all directions, but most conspicuously
+in directions not far distant from that of the light itself; and if a
+reflecting surface be placed parallel to the refracting surface, this
+scattered light, as well as the principal beam, will be reflected, and
+there will be also a new dissipation of light, at the return of the beam
+through the refracting surface. These two portions of scattered light
+will coincide in direction; and if the surfaces be of such a form as to
+collect the similar effects, will exhibit rings of colors. The interval
+of retardation is here the difference between the paths of the principal
+beam and of the scattered light between the two surfaces; of course,
+wherever the inclination of the scattered light is equal to that of the
+beam, although in different planes, the interval will vanish and all the
+undulations will conspire. At other inclinations, the interval will be
+the difference of the secants from the secant of the inclination, or
+angle of refraction of the principal beam. From these causes, all the
+colors of concave mirrors observed by Newton and others are necessary
+consequences; and it appears that their production, though somewhat
+similar, is by no means as Newton imagined, identical with the
+production of thin plates."(2)
+
+
+By following up this clew with mathematical precision, measuring the
+exact thickness of the plate and the space between the different rings
+of color, Young was able to show mathematically what must be the length
+of pulsation for each of the different colors of the spectrum. He
+estimated that the undulations of red light, at the extreme lower end
+of the visible spectrum, must number about thirty-seven thousand six
+hundred and forty to the inch, and pass any given spot at a rate of four
+hundred and sixty-three millions of millions of undulations in a second,
+while the extreme violet numbers fifty-nine thousand seven hundred and
+fifty undulations to the inch, or seven hundred and thirty-five millions
+of millions to the second.
+
+
+The Colors of Striated Surfaces
+
+Young similarly examined the colors that are produced by scratches on
+a smooth surface, in particular testing the light from "Mr. Coventry's
+exquisite micrometers," which consist of lines scratched on glass at
+measured intervals. These microscopic tests brought the same results as
+the other experiments. The colors were produced at certain definite
+and measurable angles, and the theory of interference of undulations
+explained them perfectly, while, as Young affirmed with confidence, no
+other hypothesis hitherto advanced would explain them at all. Here are
+his words:
+
+"Let there be in a given plane two reflecting points very near each
+other, and let the plane be so situated that the reflected image of a
+luminous object seen in it may appear to coincide with the points; then
+it is obvious that the length of the incident and reflected ray, taken
+together, is equal with respect to both points, considering them as
+capable of reflecting in all directions. Let one of the points be
+now depressed below the given plane; then the whole path of the
+light reflected from it will be lengthened by a line which is to the
+depression of the point as twice the cosine of incidence to the radius.
+
+"If, therefore, equal undulations of given dimensions be reflected
+from two points, situated near enough to appear to the eye but as one,
+whenever this line is equal to half the breadth of a whole undulation
+the reflection from the depressed point will so interfere with the
+reflection from the fixed point that the progressive motion of the one
+will coincide with the retrograde motion of the other, and they will
+both be destroyed; but when this line is equal to the whole breadth of
+an undulation, the effect will be doubled, and when to a breadth and
+a half, again destroyed; and thus for a considerable number of
+alternations, and if the reflected undulations be of a different kind,
+they will be variously affected, according to their proportions to the
+various length of the line which is the difference between the lengths
+of their two paths, and which may be denominated the interval of a
+retardation.
+
+"In order that the effect may be the more perceptible, a number of pairs
+of points must be united into two parallel lines; and if several such
+pairs of lines be placed near each other, they will facilitate the
+observation. If one of the lines be made to revolve round the other as
+an axis, the depression below the given plane will be as the sine of the
+inclination; and while the eye and the luminous object remain fixed the
+difference of the length of the paths will vary as this sine.
+
+"The best subjects for the experiment are Mr. Coventry's exquisite
+micrometers; such of them as consist of parallel lines drawn on glass,
+at a distance of one-five-hundredth of an inch, are the most convenient.
+Each of these lines appears under a microscope to consist of two or more
+finer lines, exactly parallel, and at a distance of somewhat more than
+a twentieth more than the adjacent lines. I placed one of these so as to
+reflect the sun's light at an angle of forty-five degrees, and fixed
+it in such a manner that while it revolved round one of the lines as an
+axis, I could measure its angular motion; I found that the longest red
+color occurred at the inclination 10 1/4 degrees, 20 3/4 degrees, 32
+degrees, and 45 degrees; of which the sines are as the numbers 1, 2, 3,
+and 4. At all other angles also, when the sun's light was reflected from
+the surface, the color vanished with the inclination, and was equal at
+equal inclinations on either side.
+
+This experiment affords a very strong confirmation of the theory. It is
+impossible to deduce any explanation of it from any hypothesis hitherto
+advanced; and I believe it would be difficult to invent any other
+that would account for it. There is a striking analogy between this
+separation of colors and the production of a musical note by successive
+echoes from equidistant iron palisades, which I have found to correspond
+pretty accurately with the known velocity of sound and the distances of
+the surfaces.
+
+"It is not improbable that the colors of the integuments of some
+insects, and of some other natural bodies, exhibiting in different
+lights the most beautiful versatility, may be found to be of this
+description, and not to be derived from thin plates. In some cases a
+single scratch or furrow may produce similar effects, by the reflection
+of its opposite edges."(3)
+
+
+This doctrine of interference of undulations was the absolutely novel
+part of Young's theory. The all-compassing genius of Robert Hooke had,
+indeed, very nearly apprehended it more than a century before, as Young
+himself points out, but no one else bad so much as vaguely conceived
+it; and even with the sagacious Hooke it was only a happy guess, never
+distinctly outlined in his own mind, and utterly ignored by all
+others. Young did not know of Hooke's guess until he himself had fully
+formulated the theory, but he hastened then to give his predecessor
+all the credit that could possibly be adjudged his due by the most
+disinterested observer. To Hooke's contemporary, Huygens, who was the
+originator of the general doctrine of undulation as the explanation of
+light, Young renders full justice also. For himself he claims only the
+merit of having demonstrated the theory which these and a few others of
+his predecessors had advocated without full proof.
+
+The following year Dr. Young detailed before the Royal Society
+other experiments, which threw additional light on the doctrine of
+interference; and in 1803 he cited still others, which, he affirmed,
+brought the doctrine to complete demonstration. In applying this
+demonstration to the general theory of light, he made the striking
+suggestion that "the luminiferous ether pervades the substance of all
+material bodies with little or no resistance, as freely, perhaps, as the
+wind passes through a grove of trees." He asserted his belief also that
+the chemical rays which Ritter had discovered beyond the violet end of
+the visible spectrum are but still more rapid undulations of the same
+character as those which produce light. In his earlier lecture he had
+affirmed a like affinity between the light rays and the rays of
+radiant heat which Herschel detected below the red end of the spectrum,
+suggesting that "light differs from heat only in the frequency of its
+undulations or vibrations--those undulations which are within certain
+limits with respect to frequency affecting the optic nerve and
+constituting light, and those which are slower and probably stronger
+constituting heat only." From the very outset he had recognized the
+affinity between sound and light; indeed, it had been this affinity that
+led him on to an appreciation of the undulatory theory of light.
+
+But while all these affinities seemed so clear to the great
+co-ordinating brain of Young, they made no such impression on the minds
+of his contemporaries. The immateriality of light had been substantially
+demonstrated, but practically no one save its author accepted the
+demonstration. Newton's doctrine of the emission of corpuscles was too
+firmly rooted to be readily dislodged, and Dr. Young had too many other
+interests to continue the assault unceasingly. He occasionally wrote
+something touching on his theory, mostly papers contributed to
+the Quarterly Review and similar periodicals, anonymously or
+under pseudonym, for he had conceived the notion that too great
+conspicuousness in fields outside of medicine would injure his practice
+as a physician. His views regarding light (including the original papers
+from the Philosophical Transactions of the Royal Society) were again
+given publicity in full in his celebrated volume on natural philosophy,
+consisting in part of his lectures before the Royal Institution,
+published in 1807; but even then they failed to bring conviction to
+the philosophic world. Indeed, they did not even arouse a controversial
+spirit, as his first papers had done.
+
+
+ARAGO AND FRESNEL CHAMPION THE WAVE THEORY
+
+So it chanced that when, in 1815, a young French military engineer,
+named Augustin Jean Fresnel, returning from the Napoleonic wars,
+became interested in the phenomena of light, and made some experiments
+concerning diffraction which seemed to him to controvert the accepted
+notions of the materiality of light, he was quite unaware that his
+experiments had been anticipated by a philosopher across the Channel.
+He communicated his experiments and results to the French Institute,
+supposing them to be absolutely novel. That body referred them to a
+committee, of which, as good fortune would have it, the dominating
+member was Dominique Francois Arago, a man as versatile as Young
+himself, and hardly less profound, if perhaps not quite so original.
+Arago at once recognized the merit of Fresnel's work, and soon became a
+convert to the theory. He told Fresnel that Young had anticipated him
+as regards the general theory, but that much remained to be done, and
+he offered to associate himself with Fresnel in prosecuting the
+investigation. Fresnel was not a little dashed to learn that his
+original ideas had been worked out by another while he was a lad, but he
+bowed gracefully to the situation and went ahead with unabated zeal.
+
+The championship of Arago insured the undulatory theory a hearing
+before the French Institute, but by no means sufficed to bring about
+its general acceptance. On the contrary, a bitter feud ensued, in which
+Arago was opposed by the "Jupiter Olympus of the Academy," Laplace, by
+the only less famous Poisson, and by the younger but hardly less able
+Biot. So bitterly raged the feud that a life-long friendship between
+Arago and Biot was ruptured forever. The opposition managed to delay the
+publication of Fresnel's papers, but Arago continued to fight with his
+customary enthusiasm and pertinacity, and at last, in 1823, the Academy
+yielded, and voted Fresnel into its ranks, thus implicitly admitting the
+value of his work.
+
+It is a humiliating thought that such controversies as this must mar
+the progress of scientific truth; but fortunately the story of the
+introduction of the undulatory theory has a more pleasant side. Three
+men, great both in character and in intellect, were concerned in
+pressing its claims--Young, Fresnel, and Arago--and the relations of
+these men form a picture unmarred by any of those petty jealousies that
+so often dim the lustre of great names. Fresnel freely acknowledged
+Young's priority so soon as his attention was called to it; and Young
+applauded the work of the Frenchman, and aided with his counsel in the
+application of the undulatory theory to the problems of polarization of
+light, which still demanded explanation, and which Fresnel's fertility
+of experimental resource and profundity of mathematical insight sufficed
+in the end to conquer.
+
+After Fresnel's admission to the Institute in 1823 the opposition
+weakened, and gradually the philosophers came to realize the merits of
+a theory which Young had vainly called to their attention a full
+quarter-century before. Now, thanks largely to Arago, both Young and
+Fresnel received their full meed of appreciation. Fresnel was given the
+Rumford medal of the Royal Society of England in 1825, and chosen one of
+the foreign members of the society two years later, while Young in turn
+was elected one of the eight foreign members of the French Academy. As
+a fitting culmination of the chapter of felicities between the three
+friends, it fell to the lot of Young, as Foreign Secretary of the
+Royal Society, to notify Fresnel of the honors shown him by England's
+representative body of scientists; while Arago, as Perpetual Secretary
+of the French Institute, conveyed to Young in the same year the
+notification that he had been similarly honored by the savants of
+France.
+
+A few months later Fresnel was dead, and Young survived him only two
+years. Both died prematurely, but their great work was done, and
+the world will remember always and link together these two names in
+connection with a theory which in its implications and importance ranks
+little below the theory of universal gravitation.
+
+
+
+
+VII. THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM
+
+GALVANI AND VOLTA
+
+The full importance of Young's studies of light might perhaps have
+gained earlier recognition had it not chanced that, at the time when
+they were made, the attention of the philosophic world was turned with
+the fixity and fascination of a hypnotic stare upon another field, which
+for a time brooked no rival. How could the old, familiar phenomenon,
+light, interest any one when the new agent, galvanism, was in view? As
+well ask one to fix attention on a star while a meteorite blazes across
+the sky.
+
+Galvanism was so called precisely as the Roentgen ray was christened at
+a later day--as a safe means of begging the question as to the nature of
+the phenomena involved. The initial fact in galvanism was the discovery
+of Luigi Galvani (1737-1798), a physician of Bologna, in 1791, that
+by bringing metals in contact with the nerves of a frog's leg violent
+muscular contractions are produced. As this simple little experiment led
+eventually to the discovery of galvanic electricity and the invention
+of the galvanic battery, it may be regarded as the beginning of modern
+electricity.
+
+The story is told that Galvani was led to his discovery while preparing
+frogs' legs to make a broth for his invalid wife. As the story runs, he
+had removed the skins from several frogs' legs, when, happening to touch
+the exposed muscles with a scalpel which had lain in close proximity to
+an electrical machine, violent muscular action was produced. Impressed
+with this phenomenon, he began a series of experiments which finally
+resulted in his great discovery. But be this story authentic or not, it
+is certain that Galvani experimented for several years upon frogs' legs
+suspended upon wires and hooks, until he finally constructed his arc
+of two different metals, which, when arranged so that one was placed
+in contact with a nerve and the other with a muscle, produced violent
+contractions.
+
+These two pieces of metal form the basic principle of the modern
+galvanic battery, and led directly to Alessandro Volta's invention
+of his "voltaic pile," the immediate ancestor of the modern galvanic
+battery. Volta's experiments were carried on at the same time as those
+of Galvani, and his invention of his pile followed close upon Galvani's
+discovery of the new form of electricity. From these facts the new form
+of electricity was sometimes called "galvanic" and sometimes "voltaic"
+electricity, but in recent years the term "galvanism" and "galvanic
+current" have almost entirely supplanted the use of the term voltaic.
+
+It was Volta who made the report of Galvani's wonderful discovery to
+the Royal Society of London, read on January 31, 1793. In this letter he
+describes Galvani's experiments in detail and refers to them in glowing
+terms of praise. He calls it one of the "most beautiful and important
+discoveries," and regarded it as the germ or foundation upon which other
+discoveries were to be made. The prediction proved entirely correct,
+Volta himself being the chief discoverer.
+
+Working along lines suggested by Galvani's discovery, Volta constructed
+an apparatus made up of a number of disks of two different kinds of
+metal, such as tin and silver, arranged alternately, a piece of some
+moist, porous substance, like paper or felt, being interposed between
+each pair of disks. With this "pile," as it was called, electricity
+was generated, and by linking together several such piles an electric
+battery could be formed.
+
+This invention took the world by storm. Nothing like the enthusiasm it
+created in the philosophic world had been known since the invention
+of the Leyden jar, more than half a century before. Within a few weeks
+after Volta's announcement, batteries made according to his plan were
+being experimented with in every important laboratory in Europe.
+
+As the century closed, half the philosophic world was speculating as to
+whether "galvanic influence" were a new imponderable, or only a form of
+electricity; and the other half was eagerly seeking to discover what new
+marvels the battery might reveal. The least imaginative man could see
+that here was an invention that would be epoch-making, but the most
+visionary dreamer could not even vaguely adumbrate the real measure of
+its importance.
+
+It was evident at once that almost any form of galvanic battery,
+despite imperfections, was a more satisfactory instrument for generating
+electricity than the frictional machine hitherto in use, the advantage
+lying in the fact that the current from the galvanic battery could
+be controlled practically at will, and that the apparatus itself
+was inexpensive and required comparatively little attention. These
+advantages were soon made apparent by the practical application of the
+electric current in several fields.
+
+It will be recalled that despite the energetic endeavors of such
+philosophers as Watson, Franklin, Galvani, and many others, the field
+of practical application of electricity was very limited at the close of
+the eighteenth century. The lightning-rod had come into general use, to
+be sure, and its value as an invention can hardly be overestimated. But
+while it was the result of extensive electrical discoveries, and is
+a most practical instrument, it can hardly be called one that puts
+electricity to practical use, but simply acts as a means of warding
+off the evil effects of a natural manifestation of electricity. The
+invention, however, had all the effects of a mechanism which turned
+electricity to practical account. But with the advent of the new kind of
+electricity the age of practical application began.
+
+
+DAVY AND ELECTRIC LIGHT
+
+Volta's announcement of his pile was scarcely two months old when two
+Englishmen, Messrs. Nicholson and Carlisle, made the discovery that
+the current from the galvanic battery had a decided effect upon certain
+chemicals, among other things decomposing water into its elements,
+hydrogen and oxygen. On May 7, 1800, these investigators arranged the
+ends of two brass wires connected with the poles of a voltaic pile,
+composed of alternate silver and zinc plates, so that the current coming
+from the pile was discharged through a small quantity of "New River
+water." "A fine stream of minute bubbles immediately began to flow from
+the point of the lower wire in the tube which communicated with the
+silver," wrote Nicholson, "and the opposite point of the upper wire
+became tarnished, first deep orange and then black...." The product of
+gas during two hours and a half was two-thirtieths of a cubic inch.
+"It was then mixed with an equal quantity of common air," continues
+Nicholson, "and exploded by the application of a lighted waxen thread."
+
+This demonstration was the beginning of the very important science of
+electro-chemistry.
+
+The importance of this discovery was at once recognized by Sir Humphry
+Davy, who began experimenting immediately in this new field. He
+constructed a series of batteries in various combinations, with which
+he attacked the "fixed alkalies," the composition of which was then
+unknown. Very shortly he was able to decompose potash into bright
+metallic globules, resembling quicksilver. This new substance he named
+"potassium." Then in rapid succession the elementary substances sodium,
+calcium, strontium, and magnesium were isolated.
+
+It was soon discovered, also, that the new electricity, like the old,
+possessed heating power under certain conditions, even to the fusing of
+pieces of wire. This observation was probably first made by Frommsdorff,
+but it was elaborated by Davy, who constructed a battery of two thousand
+cells with which he produced a bright light from points of carbon--the
+prototype of the modern arc lamp. He made this demonstration before the
+members of the Royal Institution in 1810. But the practical utility of
+such a light for illuminating purposes was still a thing of the future.
+The expense of constructing and maintaining such an elaborate battery,
+and the rapid internal destruction of its plates, together with the
+constant polarization, rendered its use in practical illumination out of
+the question. It was not until another method of generating electricity
+was discovered that Davy's demonstration could be turned to practical
+account.
+
+In Davy's own account of his experiment he says:
+
+"When pieces of charcoal about an inch long and one-sixth of an inch in
+diameter were brought near each other (within the thirtieth or fortieth
+of an inch), a bright spark was produced, and more than half the volume
+of the charcoal became ignited to whiteness; and, by withdrawing the
+points from each other, a constant discharge took place through the
+heated air, in a space equal to at least four inches, producing a most
+brilliant ascending arch of light, broad and conical in form in the
+middle. When any substance was introduced into this arch, it instantly
+became ignited; platina melted as readily in it as wax in a common
+candle; quartz, the sapphire, magnesia, lime, all entered into fusion;
+fragments of diamond and points of charcoal and plumbago seemed to
+evaporate in it, even when the connection was made in the receiver of an
+air-pump; but there was no evidence of their having previously undergone
+fusion. When the communication between the points positively and
+negatively electrified was made in the air rarefied in the receiver of
+the air-pump, the distance at which the discharge took place increased
+as the exhaustion was made; and when the atmosphere in the vessel
+supported only one-fourth of an inch of mercury in the barometrical
+gauge, the sparks passed through a space of nearly half an inch; and, by
+withdrawing the points from each other, the discharge was made through
+six or seven inches, producing a most brilliant coruscation of purple
+light; the charcoal became intensely ignited, and some platina wire
+attached to it fused with brilliant scintillations and fell in large
+globules upon the plate of the pump. All the phenomena of
+chemical decomposition were produced with intense rapidity by this
+combination."(1)
+
+But this experiment demonstrated another thing besides the possibility
+of producing electric light and chemical decomposition, this being the
+heating power capable of being produced by the electric current. Thus
+Davy's experiment of fusing substances laid the foundation of the modern
+electric furnaces, which are of paramount importance in several great
+commercial industries.
+
+While some of the results obtained with Davy's batteries were
+practically as satisfactory as could be obtained with modern cell
+batteries, the batteries themselves were anything but satisfactory. They
+were expensive, required constant care and attention, and, what was more
+important from an experimental standpoint at least, were not constant in
+their action except for a very limited period of time, the current soon
+"running down." Numerous experimenters, therefore, set about devising a
+satisfactory battery, and when, in 1836, John Frederick Daniell produced
+the cell that bears his name, his invention was epoch-making in the
+history of electrical progress. The Royal Society considered it of
+sufficient importance to bestow the Copley medal upon the inventor,
+whose device is the direct parent of all modern galvanic cells. From the
+time of the advent of the Daniell cell experiments in electricity were
+rendered comparatively easy. In the mean while, however, another great
+discovery was made.
+
+
+ELECTRICITY AND MAGNETISM
+
+For many years there had been a growing suspicion, amounting in
+many instances to belief in the close relationship existing between
+electricity and magnetism. Before the winter of 1815, however, it was
+a belief that was surmised but not demonstrated. But in that year it
+occurred to Jean Christian Oersted, of Denmark, to pass a current of
+electricity through a wire held parallel with, but not quite touching, a
+suspended magnetic needle. The needle was instantly deflected and swung
+out of its position.
+
+"The first experiments in connection with the subject which I am
+undertaking to explain," wrote Oersted, "were made during the course
+of lectures which I held last winter on electricity and magnetism. From
+those experiments it appeared that the magnetic needle could be moved
+from its position by means of a galvanic battery--one with a closed
+galvanic circuit. Since, however, those experiments were made with an
+apparatus of small power, I undertook to repeat and increase them with a
+large galvanic battery.
+
+"Let us suppose that the two opposite ends of the galvanic apparatus are
+joined by a metal wire. This I shall always call the conductor for
+the sake of brevity. Place a rectilinear piece of this conductor in
+a horizontal position over an ordinary magnetic needle so that it is
+parallel to it. The magnetic needle will be set in motion and will
+deviate towards the west under that part of the conductor which comes
+from the negative pole of the galvanic battery. If the wire is not more
+than four-fifths of an inch distant from the middle of this needle, this
+deviation will be about forty-five degrees. At a greater distance
+the angle of deviation becomes less. Moreover, the deviation varies
+according to the strength of the battery. The conductor can be moved
+towards the east or west, so long as it remains parallel to the needle,
+without producing any other result than to make the deviation smaller.
+
+"The conductor can consist of several combined wires or metal coils. The
+nature of the metal does not alter the result except, perhaps, to make
+it greater or less. We have used wires of platinum, gold, silver, brass,
+and iron, and coils of lead, tin, and quicksilver with the same result.
+If the conductor is interrupted by water, all effect is not cut off,
+unless the stretch of water is several inches long.
+
+"The conductor works on the magnetic needle through glass, metals, wood,
+water, and resin, through clay vessels and through stone, for when we
+placed a glass plate, a metal plate, or a board between the conductor
+and the needle the effect was not cut off; even the three together
+seemed hardly to weaken the effect, and the same was the case with an
+earthen vessel, even when it was full of water. Our experiments also
+demonstrated that the said effects were not altered when we used a
+magnetic needle which was in a brass case full of water.
+
+"When the conductor is placed in a horizontal plane under the magnetic
+needle all the effects we have described take place in precisely the
+same way, but in the opposite direction to what took place when the
+conductor was in a horizontal plane above the needle.
+
+"If the conductor is moved in a horizontal plane so that it gradually
+makes ever-increasing angles with the magnetic meridian, the deviation
+of the magnetic needle from the magnetic meridian is increased when the
+wire is turned towards the place of the needle; it decreases, on the
+other hand, when it is turned away from that place.
+
+"A needle of brass which is hung in the same way as the magnetic needle
+is not set in motion by the influence of the conductor. A needle of
+glass or rubber likewise remains static under similar experiments. Hence
+the electrical conductor affects only the magnetic parts of a substance.
+That the electrical current is not confined to the conducting wire,
+but is comparatively widely diffused in the surrounding space, is
+sufficiently demonstrated from the foregoing observations."(2)
+
+
+The effect of Oersted's demonstration is almost incomprehensible. By it
+was shown the close relationship between magnetism and electricity. It
+showed the way to the establishment of the science of electrodynamics;
+although it was by the French savant Andre Marie Ampere (1775-1836) that
+the science was actually created, and this within the space of one week
+after hearing of Oersted's experiment in deflecting the needle. Ampere
+first received the news of Oersted's experiment on September 11, 1820,
+and on the 18th of the same month he announced to the Academy the
+fundamental principles of the science of electro-dynamics--seven days of
+rapid progress perhaps unequalled in the history of science.
+
+Ampere's distinguished countryman, Arago, a few months later, gave
+the finishing touches to Oersted's and Ampere's discoveries, by
+demonstrating conclusively that electricity not only influenced a
+magnet, but actually produced magnetism under proper circumstances--a
+complemental fact most essential in practical mechanics.
+
+Some four years after Arago's discovery, Sturgeon made the first
+"electro-magnet" by winding a soft iron core with wire through which
+a current of electricity was passed. This study of electro-magnets was
+taken up by Professor Joseph Henry, of Albany, New York, who succeeded
+in making magnets of enormous lifting power by winding the iron core
+with several coils of wire. One of these magnets, excited by a single
+galvanic cell of less than half a square foot of surface, and containing
+only half a pint of dilute acids, sustained a weight of six hundred and
+fifty pounds.
+
+Thus by Oersted's great discovery of the intimate relationship of
+magnetism and electricity, with further elaborations and discoveries by
+Ampere, Volta, and Henry, and with the invention of Daniell's cell, the
+way was laid for putting electricity to practical use. Soon followed the
+invention and perfection of the electro-magnetic telegraph and a host of
+other but little less important devices.
+
+
+FARADAY AND ELECTRO-MAGNETIC INDUCTION
+
+With these great discoveries and inventions at hand, electricity became
+no longer a toy or a "plaything for philosophers," but of enormous
+and growing importance commercially. Still, electricity generated
+by chemical action, even in a very perfect cell, was both feeble and
+expensive, and, withal, only applicable in a comparatively limited
+field. Another important scientific discovery was necessary before such
+things as electric traction and electric lighting on a large scale were
+to become possible; but that discovery was soon made by Sir Michael
+Faraday.
+
+Faraday, the son of a blacksmith and a bookbinder by trade, had
+interested Sir Humphry Davy by his admirable notes on four of Davy's
+lectures, which he had been able to attend. Although advised by the
+great scientist to "stick to his bookbinding" rather than enter the
+field of science, Faraday became, at twenty-two years of age, Davy's
+assistant in the Royal Institution. There, for several years, he devoted
+all his spare hours to scientific investigations and experiments,
+perfecting himself in scientific technique.
+
+A few years later he became interested, like all the scientists of
+the time, in Arago's experiment of rotating a copper disk underneath a
+suspended compass-needle. When this disk was rotated rapidly, the
+needle was deflected, or even rotated about its axis, in a manner quite
+inexplicable. Faraday at once conceived the idea that the cause of this
+rotation was due to electricity, induced in the revolving disk--not only
+conceived it, but put his belief in writing. For several years, however,
+he was unable to demonstrate the truth of his assumption, although he
+made repeated experiments to prove it. But in 1831 he began a series
+of experiments that established forever the fact of electro-magnetic
+induction.
+
+In his famous paper, read before the Royal Society in 1831, Faraday
+describes the method by which he first demonstrated electro-magnetic
+induction, and then explained the phenomenon of Arago's revolving disk.
+
+"About twenty-six feet of copper wire, one-twentieth of an inch in
+diameter, were wound round a cylinder of wood as a helix," he said,
+"the different spires of which were prevented from touching by a thin
+interposed twine. This helix was covered with calico, and then a
+second wire applied in the same manner. In this way twelve helices were
+"superposed, each containing an average length of wire of twenty-seven
+feet, and all in the same direction. The first, third, fifth, seventh,
+ninth, and eleventh of these helices were connected at their extremities
+end to end so as to form one helix; the others were connected in a
+similar manner; and thus two principal helices were produced, closely
+interposed, having the same direction, not touching anywhere, and each
+containing one hundred and fifty-five feet in length of wire.
+
+One of these helices was connected with a galvanometer, the other with
+a voltaic battery of ten pairs of plates four inches square, with double
+coppers and well charged; yet not the slightest sensible deflection of
+the galvanometer needle could be observed.
+
+"A similar compound helix, consisting of six lengths of copper and six
+of soft iron wire, was constructed. The resulting iron helix contained
+two hundred and eight feet; but whether the current from the trough was
+passed through the copper or the iron helix, no effect upon the other
+could be perceived at the galvanometer.
+
+"In these and many similar experiments no difference in action of any
+kind appeared between iron and other metals.
+
+"Two hundred and three feet of copper wire in one length were passed
+round a large block of wood; other two hundred and three feet of similar
+wire were interposed as a spiral between the turns of the first, and
+metallic contact everywhere prevented by twine. One of these helices was
+connected with a galvanometer and the other with a battery of a hundred
+pairs of plates four inches square, with double coppers and well
+charged. When the contact was made, there was a sudden and very slight
+effect at the galvanometer, and there was also a similar slight effect
+when the contact with the battery was broken. But whilst the voltaic
+current was continuing to pass through the one helix, no galvanometrical
+appearances of any effect like induction upon the other helix could be
+perceived, although the active power of the battery was proved to be
+great by its heating the whole of its own helix, and by the brilliancy
+of the discharge when made through charcoal.
+
+"Repetition of the experiments with a battery of one hundred and twenty
+pairs of plates produced no other effects; but it was ascertained, both
+at this and at the former time, that the slight deflection of the needle
+occurring at the moment of completing the connection was always in one
+direction, and that the equally slight deflection produced when the
+contact was broken was in the other direction; and, also, that these
+effects occurred when the first helices were used.
+
+"The results which I had by this time obtained with magnets led me
+to believe that the battery current through one wire did, in reality,
+induce a similar current through the other wire, but that it continued
+for an instant only, and partook more of the nature of the electrical
+wave passed through from the shock of a common Leyden jar than of that
+from a voltaic battery, and, therefore, might magnetize a steel needle
+although it scarcely affected the galvanometer.
+
+"This expectation was confirmed; for on substituting a small hollow
+helix, formed round a glass tube, for the galvanometer, introducing
+a steel needle, making contact as before between the battery and the
+inducing wire, and then removing the needle before the battery contact
+was broken, it was found magnetized.
+
+"When the battery contact was first made, then an unmagnetized needle
+introduced, and lastly the battery contact broken, the needle was found
+magnetized to an equal degree apparently with the first; but the poles
+were of the contrary kinds."(3)
+
+To Faraday these experiments explained the phenomenon of Arago's
+rotating disk, the disk inducing the current from the magnet, and, in
+reacting, deflecting the needle. To prove this, he constructed a disk
+that revolved between the poles of an electro-magnet, connecting the
+axis and the edge of the disk with a galvanometer. "... A disk of
+copper, twelve inches in diameter, fixed upon a brass axis," he says,
+"was mounted in frames so as to be revolved either vertically or
+horizontally, its edge being at the same time introduced more or less
+between the magnetic poles. The edge of the plate was well amalgamated
+for the purpose of obtaining good but movable contact; a part round the
+axis was also prepared in a similar manner.
+
+"Conductors or collectors of copper and lead were constructed so as to
+come in contact with the edge of the copper disk, or with other forms
+of plates hereafter to be described. These conductors we're about four
+inches long, one-third of an inch wide, and one-fifth of an inch thick;
+one end of each was slightly grooved, to allow of more exact adaptation
+to the somewhat convex edge of the plates, and then amalgamated. Copper
+wires, one-sixteenth of an inch in thickness, attached in the ordinary
+manner by convolutions to the other ends of these conductors, passed
+away to the galvanometer.
+
+"All these arrangements being made, the copper disk was adjusted, the
+small magnetic poles being about one-half an inch apart, and the edge
+of the plate inserted about half their width between them. One of the
+galvanometer wires was passed twice or thrice loosely round the brass
+axis of the plate, and the other attached to a conductor, which itself
+was retained by the hand in contact with the amalgamated edge of the
+disk at the part immediately between the magnetic poles. Under these
+circumstances all was quiescent, and the galvanometer exhibited no
+effect. But the instant the plate moved the galvanometer was influenced,
+and by revolving the plate quickly the needle could be deflected ninety
+degrees or more."(4)
+
+
+This rotating disk was really a dynamo electric machine in miniature,
+the first ever constructed, but whose direct descendants are the
+ordinary dynamos. Modern dynamos range in power from little machines
+operating machinery requiring only fractions of a horsepower to great
+dynamos operating street-car lines and lighting cities; but all
+are built on the same principle as Faraday's rotating disk. By this
+discovery the use of electricity as a practical and economical motive
+power became possible.
+
+
+STORAGE BATTERIES
+
+When the discoveries of Faraday of electro-magnetic induction had made
+possible the means of easily generating electricity, the next natural
+step was to find a means of storing it or accumulating it. This,
+however, proved no easy matter, and as yet a practical storage or
+secondary battery that is neither too cumbersome, too fragile, nor too
+weak in its action has not been invented. If a satisfactory storage
+battery could be made, it is obvious that its revolutionary effects
+could scarcely be overestimated. In the single field of aeronautics, it
+would probably solve the question of aerial navigation. Little wonder,
+then, that inventors have sought so eagerly for the invention of
+satisfactory storage batteries. As early as 1803 Ritter had attempted to
+make such a secondary battery. In 1843 Grove also attempted it. But it
+was not until 1859, when Gaston Planche produced his invention, that
+anything like a reasonably satisfactory storage battery was made.
+Planche discovered that sheets of lead immersed in dilute sulphuric acid
+were very satisfactory for the production of polarization effects. He
+constructed a battery of sheets of lead immersed in sulphuric acid, and,
+after charging these for several hours from the cells of an ordinary
+Bunsen battery, was able to get currents of great strength and
+considerable duration. This battery, however, from its construction of
+lead, was necessarily heavy and cumbersome. Faure improved it somewhat
+by coating the lead plates with red-lead, thus increasing the capacity
+of the cell. Faure's invention gave a fresh impetus to inventors, and
+shortly after the market was filled with storage batteries of various
+kinds, most of them modifications of Planche's or Faure's. The ardor
+of enthusiastic inventors soon flagged, however, for all these storage
+batteries proved of little practical account in the end, as compared
+with other known methods of generating power.
+
+Three methods of generating electricity are in general use: static or
+frictional electricity is generated by "plate" or "static" machines;
+galvanic, generated by batteries based on Volta's discovery; and
+induced, or faradic, generated either by chemical or mechanical action.
+There is still another kind, thermo-electricity, that may be generated
+in a most simple manner. In 1821 Seebecle, of Berlin, discovered that
+when a circuit was formed of two wires of different metals, if there
+be a difference in temperature at the juncture of these two metals
+an electrical current will be established. In this way heat may
+be transmitted directly into the energy of the current without the
+interposition of the steam-engine. Batteries constructed in this way
+are of low resistance, however, although by arranging several of them
+in "series," currents of considerable strength can be generated. As yet,
+however, they are of little practical importance.
+
+About the middle of the century Clerk-Maxwell advanced the idea that
+light waves were really electro-magnetic waves. If this were true and
+light proved to be simply one form of electrical energy, then the same
+would be true of radiant heat. Maxwell advanced this theory, but failed
+to substantiate it by experimental confirmation. But Dr. Heinrich
+Hertz, a few years later, by a series of experiments, demonstrated the
+correctness of Maxwell's surmises. What are now called "Hertzian waves"
+are waves apparently identical with light waves, but of much lower
+pitch or period. In his experiments Hertz showed that, under proper
+conditions, electric sparks between polished balls were attended by
+ether waves of the same nature as those of light, but of a pitch of
+several millions of vibrations per second. These waves could be dealt
+with as if they were light waves--reflected, refracted, and polarized.
+These are the waves that are utilized in wireless telegraphy.
+
+
+ROENTGEN RAYS, OR X-RAYS
+
+In December of 1895 word came out of Germany of a scientific discovery
+that startled the world. It came first as a rumor, little credited; then
+as a pronounced report; at last as a demonstration. It told of a new
+manifestation of energy, in virtue of which the interior of opaque
+objects is made visible to human eyes. One had only to look into a tube
+containing a screen of a certain composition, and directed towards
+a peculiar electrical apparatus, to acquire clairvoyant vision more
+wonderful than the discredited second-sight of the medium. Coins within
+a purse, nails driven into wood, spectacles within a leather case,
+became clearly visible when subjected to the influence of this magic
+tube; and when a human hand was held before the tube, its bones stood
+revealed in weird simplicity, as if the living, palpitating flesh about
+them were but the shadowy substance of a ghost.
+
+Not only could the human eye see these astounding revelations, but the
+impartial evidence of inanimate chemicals could be brought forward to
+prove that the mind harbored no illusion. The photographic film recorded
+the things that the eye might see, and ghostly pictures galore soon gave
+a quietus to the doubts of the most sceptical. Within a month of the
+announcement of Professor Roentgen's experiments comment upon the
+"X-ray" and the "new photography" had become a part of the current
+gossip of all Christendom.
+
+It is hardly necessary to say that such a revolutionary thing as the
+discovery of a process whereby opaque objects became transparent, or
+translucent, was not achieved at a single bound with no intermediate
+discoveries. In 1859 the German physicist Julius Plucker (1801-1868)
+noticed that when there was an electrical discharge through an exhausted
+tube at a low pressure, on the surrounding walls of the tube near the
+negative pole, or cathode, appeared a greenish phosphorescence. This
+discovery was soon being investigated by a number of other scientists,
+among others Hittorf, Goldstein, and Professor (now Sir William)
+Crookes. The explanations given of this phenomenon by Professor Crookes
+concern us here more particularly, inasmuch as his views did not
+accord exactly with those held by the other two scientists, and as his
+researches were more directly concerned in the discovery of the
+Roentgen rays. He held that the heat and phosphorescence produced in a
+low-pressure tube were caused by streams of particles, projected from
+the cathode with great velocity, striking the sides of the glass tube.
+The composition of the glass seemed to enter into this phosphorescence
+also, for while lead glass produced blue phosphorescence, soda glass
+produced a yellowish green. The composition of the glass seemed to
+be changed by a long-continued pelting of these particles, the
+phosphorescence after a time losing its initial brilliancy, caused by
+the glass becoming "tired," as Professor Crookes said. Thus when some
+opaque substance, such as iron, is placed between the cathode and the
+sides of the glass tube so that it casts a shadow in a certain spot
+on the glass for some little time, it is found on removing the opaque
+substance or changing its position that the area of glass at first
+covered by the shadow now responded to the rays in a different manner
+from the surrounding glass.
+
+The peculiar ray's, now known as the cathode rays, not only cast a
+shadow, but are deflected by a magnet, so that the position of the
+phosphorescence on the sides of the tube may be altered by the proximity
+of a powerful magnet. From this it would seem that the rays are composed
+of particles charged with negative electricity, and Professor J. J.
+Thomson has modified the experiment of Perrin to show that negative
+electricity is actually associated with the rays. There is reason for
+believing, therefore, that the cathode rays are rapidly moving charges
+of negative electricity. It is possible, also, to determine the velocity
+at which these particles are moving by measuring the deflection produced
+by the magnetic field.
+
+From the fact that opaque substances cast a shadow in these rays it was
+thought at first that all solids were absolutely opaque to them. Hertz,
+however, discovered that a small amount of phosphorescence occurred on
+the glass even when such opaque substances as gold-leaf or aluminium
+foil were interposed between the cathode and the sides of the tube.
+Shortly afterwards Lenard discovered that the cathode rays can be made
+to pass from the inside of a discharge tube to the outside air. For
+convenience these rays outside the tube have since been known as "Lenard
+rays."
+
+In the closing days of December, 1895, Professor Wilhelm Konrad
+Roentgen, of Wurzburg, announced that he had made the discovery of the
+remarkable effect arising from the cathode rays to which reference
+was made above. He found that if a plate covered with a phosphorescent
+substance is placed near a discharge tube exhausted so highly that the
+cathode rays produced a green phosphorescence, this plate is made to
+glow in a peculiar manner. The rays producing this glow were not the
+cathode rays, although apparently arising from them, and are what have
+since been called the Roentgen rays, or X-rays.
+
+Roentgen found that a shadow is thrown upon the screen by substances
+held between it and the exhausted tube, the character of the shadow
+depending upon the density of the substance. Thus metals are almost
+completely opaque to the rays; such substances as bone much less so, and
+ordinary flesh hardly so at all. If a coin were held in the hand that
+had been interposed between the tube and the screen the picture formed
+showed the coin as a black shadow; and the bones of the hand, while
+casting a distinct shadow, showed distinctly lighter; while the soft
+tissues produced scarcely any shadow at all. The value of such a
+discovery was obvious from the first; and was still further enhanced by
+the discovery made shortly that, photographic plates are affected by the
+rays, thus making it possible to make permanent photographic records of
+pictures through what we know as opaque substances.
+
+What adds materially to the practical value of Roentgen's discovery is
+the fact that the apparatus for producing the X-rays is now so simple
+and relatively inexpensive that it is within the reach even of amateur
+scientists. It consists essentially of an induction coil attached either
+to cells or a street-current plug for generating the electricity, a
+focus tube, and a phosphorescence screen. These focus tubes are made in
+various shapes, but perhaps the most popular are in the form of a glass
+globe, not unlike an ordinary small-sized water-bottle, this tube being
+closed and exhausted, and having the two poles (anode and cathode)
+sealed into the glass walls, but protruding at either end for attachment
+to the conducting wires from the induction coil. This tube may be
+mounted on a stand at a height convenient for manipulation.
+The phosphorescence screen is usually a plate covered with some
+platino-cyanide and mounted in the end of a box of convenient size, the
+opposite end of which is so shaped that it fits the contour of the face,
+shutting out the light and allowing the eyes of the observer to focalize
+on the screen at the end. For making observations the operator has
+simply to turn on the current of electricity and apply the screen to
+his eyes, pointing it towards the glowing tube, when the shadow of any
+substance interposed between the tube and the screen will appear upon
+the phosphorescence plate.
+
+The wonderful shadow pictures produced on the phosphorescence screen,
+or the photographic plate, would seem to come from some peculiar form
+of light, but the exact nature of these rays is still an open question.
+Whether the Roentgen rays are really a form of light--that is, a form
+of "electro-magnetic disturbance propagated through ether," is not fully
+determined. Numerous experiments have been undertaken to determine this,
+but as yet no proof has been found that the rays are a form of light,
+although there appears to be nothing in their properties inconsistent
+with their being so. For the moment most investigators are content to
+admit that the term X-ray virtually begs the question as to the intimate
+nature of the form of energy involved.
+
+
+
+
+VIII. THE CONSERVATION OF ENERGY
+
+
+As we have seen, it was in 1831 that Faraday opened up the field of
+magneto-electricity. Reversing the experiments of his predecessors, who
+had found that electric currents may generate magnetism, he showed that
+magnets have power under certain circumstances to generate electricity;
+he proved, indeed, the interconvertibility of electricity and magnetism.
+Then he showed that all bodies are more or less subject to the influence
+of magnetism, and that even light may be affected by magnetism as to its
+phenomena of polarization. He satisfied himself completely of the
+true identity of all the various forms of electricity, and of the
+convertibility of electricity and chemical action. Thus he linked
+together light, chemical affinity, magnetism, and electricity. And,
+moreover, he knew full well that no one of these can be produced in
+indefinite supply from another. "Nowhere," he says, "is there a pure
+creation or production of power without a corresponding exhaustion of
+something to supply it."
+
+When Faraday wrote those words in 1840 he was treading on the very heels
+of a greater generalization than any which he actually formulated; nay,
+he had it fairly within his reach. He saw a great truth without fully
+realizing its import; it was left for others, approaching the same truth
+along another path, to point out its full significance.
+
+The great generalization which Faraday so narrowly missed is the truth
+which since then has become familiar as the doctrine of the conservation
+of energy--the law that in transforming energy from one condition to
+another we can never secure more than an equivalent quantity; that, in
+short, "to create or annihilate energy is as impossible as to create or
+annihilate matter; and that all the phenomena of the material universe
+consist in transformations of energy alone." Some philosophers think
+this the greatest generalization ever conceived by the mind of man. Be
+that as it may, it is surely one of the great intellectual landmarks
+of the nineteenth century. It stands apart, so stupendous and so
+far-reaching in its implications that the generation which first saw the
+law developed could little appreciate it; only now, through the vista of
+half a century, do we begin to see it in its true proportions.
+
+A vast generalization such as this is never a mushroom growth, nor does
+it usually spring full grown from the mind of any single man. Always a
+number of minds are very near a truth before any one mind fully grasps
+it. Pre-eminently true is this of the doctrine of the conservation of
+energy. Not Faraday alone, but half a dozen different men had an inkling
+of it before it gained full expression; indeed, every man who advocated
+the undulatory theory of light and heat was verging towards the goal.
+The doctrine of Young and Fresnel was as a highway leading surely on
+to the wide plain of conservation. The phenomena of electro-magnetism
+furnished another such highway. But there was yet another road which led
+just as surely and even more readily to the same goal. This was the road
+furnished by the phenomena of heat, and the men who travelled it were
+destined to outstrip their fellow-workers; though, as we have seen,
+wayfarers on other roads were within hailing distance when the leaders
+passed the mark.
+
+In order to do even approximate justice to the men who entered into
+the great achievement, we must recall that just at the close of the
+eighteenth century Count Rumford and Humphry Davy independently showed
+that labor may be transformed into heat; and correctly interpreted this
+fact as meaning the transformation of molar into molecular motion. We
+can hardly doubt that each of these men of genius realized--vaguely, at
+any rate--that there must be a close correspondence between the amount
+of the molar and the molecular motions; hence that each of them was in
+sight of the law of the mechanical equivalent of heat. But neither of
+them quite grasped or explicitly stated what each must vaguely have
+seen; and for just a quarter of a century no one else even came abreast
+their line of thought, let alone passing it.
+
+But then, in 1824, a French philosopher, Sadi Carnot, caught step with
+the great Englishmen, and took a long leap ahead by explicitly stating
+his belief that a definite quantity of work could be transformed into
+a definite quantity of heat, no more, no less. Carnot did not, indeed,
+reach the clear view of his predecessors as to the nature of heat, for
+he still thought it a form of "imponderable" fluid; but he reasoned none
+the less clearly as to its mutual convertibility with mechanical work.
+But important as his conclusions seem now that we look back upon
+them with clearer vision, they made no impression whatever upon his
+contemporaries. Carnot's work in this line was an isolated phenomenon
+of historical interest, but it did not enter into the scheme of the
+completed narrative in any such way as did the work of Rumford and Davy.
+
+The man who really took up the broken thread where Rumford and Davy had
+dropped it, and wove it into a completed texture, came upon the scene
+in 1840. His home was in Manchester, England; his occupation that of
+a manufacturer. He was a friend and pupil of the great Dr. Dalton.
+His name was James Prescott Joule. When posterity has done its final
+juggling with the names of the nineteenth century, it is not unlikely
+that the name of this Manchester philosopher will be a household word,
+like the names of Aristotle, Copernicus, and Newton.
+
+For Joule's work it was, done in the fifth decade of the century, which
+demonstrated beyond all cavil that there is a precise and absolute
+equivalence between mechanical work and heat; that whatever the form of
+manifestation of molar motion, it can generate a definite and measurable
+amount of heat, and no more. Joule found, for example, that at the
+sea-level in Manchester a pound weight falling through seven hundred and
+seventy-two feet could generate enough heat to raise the temperature
+of a pound of water one degree Fahrenheit. There was nothing haphazard,
+nothing accidental, about this; it bore the stamp of unalterable law.
+And Joule himself saw, what others in time were made to see, that this
+truth is merely a particular case within a more general law. If
+heat cannot be in any sense created, but only made manifest as a
+transformation of another kind of motion, then must not the same
+thing be true of all those other forms of "force"--light, electricity,
+magnetism--which had been shown to be so closely associated, so mutually
+convertible, with heat? All analogy seemed to urge the truth of
+this inference; all experiment tended to confirm it. The law of the
+mechanical equivalent of heat then became the main corner-stone of the
+greater law of the conservation of energy.
+
+But while this citation is fresh in mind, we must turn our attention
+with all haste to a country across the Channel--to Denmark, in
+short--and learn that even as Joule experimented with the transformation
+of heat, a philosopher of Copenhagen, Colding by name, had hit upon the
+same idea, and carried it far towards a demonstration. And then, without
+pausing, we must shift yet again, this time to Germany, and consider the
+work of three other men, who independently were on the track of the same
+truth, and two of whom, it must be admitted, reached it earlier than
+either Joule or Colding, if neither brought it to quite so clear a
+demonstration. The names of these three Germans are Mohr, Mayer,
+and Helmholtz. Their share in establishing the great doctrine of
+conservation must now claim our attention.
+
+As to Karl Friedrich Mohr, it may be said that his statement of the
+doctrine preceded that of any of his fellows, yet that otherwise it was
+perhaps least important. In 1837 this thoughtful German had grasped
+the main truth, and given it expression in an article published in the
+Zeitschrift fur Physik, etc. But the article attracted no attention
+whatever, even from Mohr's own countrymen. Still, Mohr's title to
+rank as one who independently conceived the great truth, and perhaps
+conceived it before any other man in the world saw it as clearly, even
+though he did not demonstrate its validity, is not to be disputed.
+
+It was just five years later, in 1842, that Dr. Julius Robert Mayer,
+practising physician in the little German town of Heilbronn, published a
+paper in Liebig's Annalen on "The Forces of Inorganic Nature," in which
+not merely the mechanical theory of heat, but the entire doctrine of
+the conservation of energy, is explicitly if briefly stated. Two years
+earlier Dr. Mayer, while surgeon to a Dutch India vessel cruising in the
+tropics, had observed that the venous blood of a patient seemed redder
+than venous blood usually is observed to be in temperate climates. He
+pondered over this seemingly insignificant fact, and at last reached
+the conclusion that the cause must be the lesser amount of oxidation
+required to keep up the body temperature in the tropics. Led by this
+reflection to consider the body as a machine dependent on outside forces
+for its capacity to act, he passed on into a novel realm of thought,
+which brought him at last to independent discovery of the mechanical
+theory of heat, and to the first full and comprehensive appreciation
+of the great law of conservation. Blood-letting, the modern physician
+holds, was a practice of very doubtful benefit, as a rule, to the
+subject; but once, at least, it led to marvellous results. No straw is
+go small that it may not point the receptive mind of genius to new and
+wonderful truths.
+
+
+MAYER'S PAPER OF 1842
+
+The paper in which Mayer first gave expression to his revolutionary
+ideas bore the title of "The Forces of Inorganic Nature," and was
+published in 1842. It is one of the gems of scientific literature, and
+fortunately it is not too long to be quoted in its entirety. Seldom if
+ever was a great revolutionary doctrine expounded in briefer compass:
+
+"What are we to understand by 'forces'? and how are different forces
+related to each other? The term force conveys for the most part the idea
+of something unknown, unsearchable, and hypothetical; while the term
+matter, on the other hand, implies the possession, by the object in
+question, of such definite properties as weight and extension. An
+attempt, therefore, to render the idea of force equally exact with that
+of matter is one which should be welcomed by all those who desire to
+have their views of nature clear and unencumbered by hypothesis.
+
+"Forces are causes; and accordingly we may make full application in
+relation to them of the principle causa aequat effectum. If the cause
+c has the effect e, then c = e; if, in its turn, e is the cause of a
+second effect of f, we have e = f, and so on: c = e = f... = c. In a
+series of causes and effects, a term or a part of a term can never, as
+is apparent from the nature of an equation, become equal to nothing.
+This first property of all causes we call their indestructibility.
+
+"If the given cause c has produced an effect e equal to itself, it has
+in that very act ceased to be--c has become e. If, after the production
+of e, c still remained in the whole or in part, there must be still
+further effects corresponding to this remaining cause: the total effect
+of c would thus be > e, which would be contrary to the supposition c =
+e. Accordingly, since c becomes e, and e becomes f, etc., we must regard
+these various magnitudes as different forms under which one and the same
+object makes its appearance. This capability of assuming various forms
+is the second essential property of all causes. Taking both properties
+together, we may say, causes an INDESTRUCTIBLE quantitatively, and
+quantitatively CONVERTIBLE objects.
+
+"There occur in nature two causes which apparently never pass one into
+the other," said Mayer. "The first class consists of such causes as
+possess the properties of weight and impenetrability. These are kinds of
+matter. The other class is composed of causes which are wanting in the
+properties just mentioned--namely, forces, called also imponderables,
+from the negative property that has been indicated. Forces are therefore
+INDESTRUCTIBLE, CONVERTIBLE, IMPONDERABLE OBJECTS.
+
+"As an example of causes and effects, take matter: explosive gas, H + O,
+and water, HO, are related to each other as cause and effect; therefore
+H + O = HO. But if H + O becomes HO, heat, cal., makes its appearance
+as well as water; this heat must likewise have a cause, x, and we have
+therefore H + O + X = HO + cal. It might be asked, however, whether H
++ O is really = HO, and x = cal., and not perhaps H + O = cal., and x =
+HO, whence the above equation could equally be deduced; and so in many
+other cases. The phlogistic chemists recognized the equation between
+cal. and x, or phlogiston as they called it, and in so doing made a
+great step in advance; but they involved themselves again in a system of
+mistakes by putting x in place of O. In this way they obtained H = HO +
+x.
+
+"Chemistry teaches us that matter, as a cause, has matter for its
+effect; but we may say with equal justification that to force as a cause
+corresponds force as effect. Since c = e, and e = c, it is natural to
+call one term of an equation a force, and the other an effect of force,
+or phenomenon, and to attach different notions to the expression force
+and phenomenon. In brief, then, if the cause is matter, the effect is
+matter; if the cause is a force, the effect is also a force.
+
+"The cause that brings about the raising of a weight is a force. The
+effect of the raised weight is, therefore, also a force; or, expressed
+in a more general form, SEPARATION IN SPACE OF PONDERABLE OBJECTS IS
+A FORCE; and since this force causes the fall of bodies, we call it
+FALLING FORCE. Falling force and fall, or, still more generally,
+falling force and motion, are forces related to each other as cause and
+effect--forces convertible into each other--two different forms of one
+and the same object. For example, a weight resting on the ground is not
+a force: it is neither the cause of motion nor of the lifting of another
+weight. It becomes so, however, in proportion as it is raised above the
+ground. The cause--that is, the distance between a weight and the earth,
+and the effect, or the quantity of motion produced, bear to each other,
+as shown by mechanics, a constant relation.
+
+"Gravity being regarded as the cause of the falling of bodies, a
+gravitating force is spoken of; and thus the ideas of PROPERTY and
+of FORCE are confounded with each other. Precisely that which is
+the essential attribute of every force--that is, the UNION of
+indestructibility with convertibility--is wanting in every property:
+between a property and a force, between gravity and motion, it is
+therefore impossible to establish the equation required for a rightly
+conceived causal relation. If gravity be called a force, a cause
+is supposed which produces effects without itself diminishing, and
+incorrect conceptions of the causal connections of things are thereby
+fostered. In order that a body may fall, it is just as necessary that it
+be lifted up as that it should be heavy or possess gravity. The fall of
+bodies, therefore, ought not to be ascribed to their gravity alone. The
+problem of mechanics is to develop the equations which subsist between
+falling force and motion, motion and falling force, and between
+different motions. Here is a case in point: The magnitude of the falling
+force v is directly proportional (the earth's radius being assumed--oo)
+to the magnitude of the mass m, and the height d, to which it is
+raised--that is, v = md. If the height d = l, to which the mass m is
+raised, is transformed into the final velocity c = l of this mass, we
+have also v = mc; but from the known relations existing between d and c,
+it results that, for other values of d or of c, the measure of the
+force v is mc squared; accordingly v = md = mcsquared. The law of the
+conservation of vis viva is thus found to be based on the general law of
+the indestructibility of causes.
+
+"In many cases we see motion cease without having caused another motion
+or the lifting of a weight. But a force once in existence cannot be
+annihilated--it can only change its form. And the question therefore
+arises, what other forms is force, which we have become acquainted with
+as falling force and motion, capable of assuming? Experience alone
+can lead us to a conclusion on this point. That we may experiment to
+advantage, we must select implements which, besides causing a real
+cessation of motion, are as little as possible altered by the objects
+to be examined. For example, if we rub together two metal plates, we see
+motion disappear, and heat, on the other hand, make its appearance, and
+there remains to be determined only whether MOTION is the cause of heat.
+In order to reach a decision on this point, we must discuss the question
+whether, in the numberless cases in which the expenditure of motion is
+accompanied by the appearance of heat, the motion has not some other
+effect than the production of heat, and the heat some other cause than
+the motion.
+
+"A serious attempt to ascertain the effects of ceasing motion has never
+been made. Without wishing to exclude a priori the hypothesis which
+it may be possible to establish, therefore, we observe only that, as a
+rule, this effect cannot be supposed to be an alteration in the state of
+aggregation of the moved (that is, rubbing, etc.) bodies. If we assume
+that a certain quantity of motion v is expended in the conversion of a
+rubbing substance m into n, we must then have m + v - n, and n = m + v;
+and when n is reconverted into m, v must appear again in some form or
+other.
+
+"By the friction of two metallic plates continued for a very long time,
+we can gradually cause the cessation of an immense quantity of movement;
+but would it ever occur to us to look for even the smallest trace of the
+force which has disappeared in the metallic dust that we could collect,
+and to try to regain it thence? We repeat, the motion cannot have been
+annihilated; and contrary, or positive and negative, motions cannot be
+regarded as = o any more than contrary motions can come out of nothing,
+or a weight can raise itself.
+
+"Without the recognition of a causal relation between motion and heat,
+it is just as difficult to explain the production of heat as it is
+to give any account of the motion that disappears. The heat cannot be
+derived from the diminution of the volume of the rubbing substances.
+It is well known that two pieces of ice may be melted by rubbing them
+together in vacuo; but let any one try to convert ice into water by
+pressure, however enormous. The author has found that water undergoes
+a rise of temperature when shaken violently. The water so heated (from
+twelve to thirteen degrees centigrade) has a greater bulk after being
+shaken than it had before. Whence now comes this quantity of heat, which
+by repeated shaking may be called into existence in the same apparatus
+as often as we please? The vibratory hypothesis of heat is an approach
+towards the doctrine of heat being the effect of motion, but it does not
+favor the admission of this causal relation in its full generality. It
+rather lays the chief stress on restless oscillations.
+
+"If it be considered as now established that in many cases no other
+effect of motion can be traced except heat, and that no other cause
+than motion can be found for the heat that is produced, we prefer the
+assumption that heat proceeds from motion to the assumption of a cause
+without effect and of an effect without a cause. Just as the chemist,
+instead of allowing oxygen and hydrogen to disappear without further
+investigation, and water to be produced in some inexplicable manner,
+establishes a connection between oxygen and hydrogen on the one hand,
+and water on the other.
+
+"We may conceive the natural connection existing between falling force,
+motion, and heat as follows: We know that heat makes its appearance
+when the separate particles of a body approach nearer to each other;
+condensation produces heat. And what applies to the smallest particles
+of matter, and the smallest intervals between them, must also apply to
+large masses and to measurable distances. The falling of a weight is a
+diminution of the bulk of the earth, and must therefore without doubt be
+related to the quantity of heat thereby developed; this quantity of heat
+must be proportional to the greatness of the weight and its distance
+from the ground. From this point of view we are easily led to the
+equations between falling force, motion, and heat that have already been
+discussed.
+
+"But just as little as the connection between falling force and motion
+authorizes the conclusion that the essence of falling force is motion,
+can such a conclusion be adopted in the case of heat. We are, on the
+contrary, rather inclined to infer that, before it can become heat,
+motion must cease to exist as motion, whether simple, or vibratory, as
+in the case of light and radiant heat, etc.
+
+"If falling force and motion are equivalent to heat, heat must also
+naturally be equivalent to motion and falling force. Just as heat
+appears as an EFFECT of the diminution of bulk and of the cessation
+of motion, so also does heat disappear as a CAUSE when its effects are
+produced in the shape of motion, expansion, or raising of weight.
+
+"In water-mills the continual diminution in bulk which the earth
+undergoes, owing to the fall of the water, gives rise to motion, which
+afterwards disappears again, calling forth unceasingly a great quantity
+of heat; and, inversely, the steam-engine serves to decompose heat again
+into motion or the raising of weights. A locomotive with its train may
+be compared to a distilling apparatus; the heat applied under the boiler
+passes off as motion, and this is deposited again as heat at the axles
+of the wheels."
+
+Mayer then closes his paper with the following deduction: "The solution
+of the equations subsisting between falling force and motion requires
+that the space fallen through in a given time--e. g., the first
+second--should be experimentally determined. In like manner, the
+solution of the equations subsisting between falling force and motion on
+the one hand and heat on the other requires an answer to the question,
+How great is the quantity of heat which corresponds to a given quantity
+of motion or falling force? For instance, we must ascertain how high a
+given weight requires to be raised above the ground in order that its
+falling force maybe equivalent to the raising of the temperature of
+an equal weight of water from 0 degrees to 1 degrees centigrade. The
+attempt to show that such an equation is the expression of a physical
+truth may be regarded as the substance of the foregoing remarks.
+
+"By applying the principles that have been set forth to the relations
+subsisting between the temperature and the volume of gases, we find
+that the sinking of a mercury column by which a gas is compressed is
+equivalent to the quantity of heat set free by the compression; and
+hence it follows, the ratio between the capacity for heat of air under
+constant pressure and its capacity under constant volume being taken as
+= 1.421, that the warming of a given weight of water from 0 degrees to
+ equal weight from the height of about three hundred and sixty-five
+metres. If we compare with this result the working of our best
+steam-engines, we see how small a part only of the heat applied under
+the boiler is really transformed into motion or the raising of weights;
+and this may serve as justification for the attempts at the profitable
+production of motion by some other method than the expenditure of the
+chemical difference between carbon and oxygen--more particularly by
+the transformation into motion of electricity obtained by chemical
+means."(1)
+
+
+MAYER AND HELMHOLTZ
+
+Here, then, was this obscure German physician, leading the humdrum life
+of a village practitioner, yet seeing such visions as no human being in
+the world had ever seen before.
+
+The great principle he had discovered became the dominating thought of
+his life, and filled all his leisure hours. He applied it far and wide,
+amid all the phenomena of the inorganic and organic worlds. It taught
+him that both vegetables and animals are machines, bound by the same
+laws that hold sway over inorganic matter, transforming energy, but
+creating nothing. Then his mind reached out into space and met a
+universe made up of questions. Each star that blinked down at him as he
+rode in answer to a night-call seemed an interrogation-point asking,
+How do I exist? Why have I not long since burned out if your theory
+of conservation be true? No one had hitherto even tried to answer that
+question; few had so much as realized that it demanded an answer. But
+the Heilbronn physician understood the question and found an answer.
+His meteoric hypothesis, published in 1848, gave for the first time a
+tenable explanation of the persistent light and heat of our sun and the
+myriad other suns--an explanation to which we shall recur in another
+connection.
+
+All this time our isolated philosopher, his brain aflame with the glow
+of creative thought, was quite unaware that any one else in the world
+was working along the same lines. And the outside world was equally
+heedless of the work of the Heilbronn physician. There was no friend to
+inspire enthusiasm and give courage, no kindred spirit to react on this
+masterful but lonely mind. And this is the more remarkable because there
+are few other cases where a master-originator in science has come upon
+the scene except as the pupil or friend of some other master-originator.
+Of the men we have noticed in the present connection, Young was the
+friend and confrere of Davy; Davy, the protege of Rumford; Faraday, the
+pupil of Davy; Fresnel, the co-worker with Arago; Colding, the confrere
+of Oersted; Joule, the pupil of Dalton. But Mayer is an isolated
+phenomenon--one of the lone mountain-peak intellects of the century.
+That estimate may be exaggerated which has called him the Galileo of the
+nineteenth century, but surely no lukewarm praise can do him justice.
+
+Yet for a long time his work attracted no attention whatever. In 1847,
+when another German physician, Hermann von Helmholtz, one of the most
+massive and towering intellects of any age, had been independently
+led to comprehension of the doctrine of the conservation of energy
+and published his treatise on the subject, he had hardly heard of his
+countryman Mayer. When he did hear of him, however, he hastened to
+renounce all claim to the doctrine of conservation, though the world at
+large gives him credit of independent even though subsequent discovery.
+
+
+JOULE'S PAPER OF 1843
+
+Meantime, in England, Joule was going on from one experimental
+demonstration to another, oblivious of his German competitors and almost
+as little noticed by his own countrymen. He read his first paper before
+the chemical section of the British Association for the Advancement of
+Science in 1843, and no one heeded it in the least. It is well worth our
+while, however, to consider it at length. It bears the title, "On the
+Calorific Effects of Magneto-Electricity, and the Mechanical Value
+of Heat." The full text, as published in the Report of the British
+Association, is as follows:
+
+"Although it has been long known that fine platinum wire can be ignited
+by magneto-electricity, it still remained a matter of doubt whether heat
+was evolved by the COILS in which the magneto-electricity was generated;
+and it seemed indeed not unreasonable to suppose that COLD was produced
+there in order to make up for the heat evolved by the other part of
+the circuit. The author therefore has endeavored to clear up this
+uncertainty by experiment. His apparatus consisted of a small compound
+electro-magnet, immersed in water, revolving between the poles of a
+powerful stationary magnet. The magneto-electricity developed in the
+coils of the revolving electro-magnet was measured by an accurate
+galvanometer; and the temperature of the water was taken before and
+after each experiment by a very delicate thermometer. The influence of
+the temperature of the surrounding atmospheric air was guarded against
+by covering the revolving tube with flannel, etc., and by the adoption
+of a system of interpolation. By an extensive series of experiments with
+the above apparatus the author succeeded in proving that heat is evolved
+by the coils of the magneto-electrical machine, as well as by any other
+part of the circuit, in proportion to the resistance to conduction
+of the wire and the square of the current; the magneto having, under
+comparable circumstances, the same calorific power as the voltaic
+electricity.
+
+"Professor Jacobi, of St. Petersburg, bad shown that the motion of an
+electro-magnetic machine generates magneto-electricity in opposition
+to the voltaic current of the battery. The author had observed the same
+phenomenon on arranging his apparatus as an electro-magnetic machine;
+but had found that no additional heat was evolved on account of the
+conflict of forces in the coil of the electro-magnet, and that the heat
+evolved by the coil remained, as before, proportional to the square of
+the current. Again, by turning the machine contrary to the direction of
+the attractive forces, so as to increase the intensity of the voltaic
+current by the assistance of the magneto-electricity, he found that the
+evolution of heat was still proportional to the square of the current.
+The author discovered, therefore, that the heat evolved by the voltaic
+current is invariably proportional to the square of the current, however
+the intensity of the current may be varied by magnetic induction. But
+Dr. Faraday has shown that the chemical effects of the current
+are simply as its quantity. Therefore he concluded that in the
+electro-magnetic engine a part of the heat due to the chemical actions
+of the battery is lost by the circuit, and converted into mechanical
+power; and that when the electro-magnetic engine is turned CONTRARY to
+the direction of the attractive forces, a greater quantity of heat is
+evolved by the circuit than is due to the chemical reactions of the
+battery, the over-plus quantity being produced by the conversion of the
+mechanical force exerted in turning the machine. By a dynamometrical
+apparatus attached to his machine, the author has ascertained that,
+in all the above cases, a quantity of heat, capable of increasing the
+temperature of a pound of water by one degree of Fahrenheit's scale, is
+equal to the mechanical force capable of raising a weight of about eight
+hundred and thirty pounds to the height of one foot."(2)
+
+
+JOULE OR MAYER?
+
+Two years later Joule wished to read another paper, but the chairman
+hinted that time was limited, and asked him to confine himself to
+a brief verbal synopsis of the results of his experiments. Had the
+chairman but known it, he was curtailing a paper vastly more important
+than all the other papers of the meeting put together. However, the
+synopsis was given, and one man was there to hear it who had the genius
+to appreciate its importance. This was William Thomson, the present
+Lord Kelvin, now known to all the world as among the greatest of natural
+philosophers, but then only a novitiate in science. He came to
+Joule's aid, started rolling the ball of controversy, and subsequently
+associated himself with the Manchester experimenter in pursuing his
+investigations.
+
+But meantime the acknowledged leaders of British science viewed the
+new doctrine askance. Faraday, Brewster, Herschel--those were the great
+names in physics at that day, and no one of them could quite accept
+the new views regarding energy. For several years no older physicist,
+speaking with recognized authority, came forward in support of the
+doctrine of conservation. This culminating thought of the first half
+of the nineteenth century came silently into the world, unheralded and
+unopposed. The fifth decade of the century had seen it elaborated and
+substantially demonstrated in at least three different countries, yet
+even the leaders of thought did not so much as know of its existence.
+In 1853 Whewell, the historian of the inductive sciences, published a
+second edition of his history, and, as Huxley has pointed out, he did
+not so much as refer to the revolutionizing thought which even then was
+a full decade old.
+
+By this time, however, the battle was brewing. The rising generation
+saw the importance of a law which their elders could not appreciate, and
+soon it was noised abroad that there were more than one claimant to the
+honor of discovery. Chiefly through the efforts of Professor Tyndall,
+the work of Mayer became known to the British public, and a most
+regrettable controversy ensued between the partisans of Mayer and those
+of Joule--a bitter controversy, in which Davy's contention that science
+knows no country was not always regarded, and which left its scars upon
+the hearts and minds of the great men whose personal interests were
+involved.
+
+And so to this day the question who is the chief discoverer of the law
+of the conservation of energy is not susceptible of a categorical answer
+that would satisfy all philosophers. It is generally held that the first
+choice lies between Joule and Mayer. Professor Tyndall has expressed the
+belief that in future each of these men will be equally remembered in
+connection with this work. But history gives us no warrant for such a
+hope. Posterity in the long run demands always that its heroes shall
+stand alone. Who remembers now that Robert Hooke contested with Newton
+the discovery of the doctrine of universal gravitation? The judgment of
+posterity is unjust, but it is inexorable. And so we can little doubt
+that a century from now one name will be mentioned as that of the
+originator of the great doctrine of the conservation of energy. The man
+whose name is thus remembered will perhaps be spoken of as the Galileo,
+the Newton, of the nineteenth century; but whether the name thus
+dignified by the final verdict of history will be that of Colding, Mohr,
+Mayer, Helmholtz, or Joule, is not as, yet decided.
+
+
+LORD KELVIN AND THE DISSIPATION OF ENERGY
+
+The gradual permeation of the field by the great doctrine of
+conservation simply repeated the history of the introduction of every
+novel and revolutionary thought. Necessarily the elder generation, to
+whom all forms of energy were imponderable fluids, must pass away before
+the new conception could claim the field. Even the word energy, though
+Young had introduced it in 1807, did not come into general use till some
+time after the middle of the century. To the generality of philosophers
+(the word physicist was even less in favor at this time) the various
+forms of energy were still subtile fluids, and never was idea
+relinquished with greater unwillingness than this. The experiments of
+Young and Fresnel had convinced a large number of philosophers that
+light is a vibration and not a substance; but so great an authority as
+Biot clung to the old emission idea to the end of his life, in 1862, and
+held a following.
+
+Meantime, however, the company of brilliant young men who had just
+served their apprenticeship when the doctrine of conservation came upon
+the scene had grown into authoritative positions, and were battling
+actively for the new ideas. Confirmatory evidence that energy is a
+molecular motion and not an "imponderable" form of matter accumulated
+day by day. The experiments of two Frenchmen, Hippolyte L. Fizeau and
+Leon Foucault, served finally to convince the last lingering sceptics
+that light is an undulation; and by implication brought heat into the
+same category, since James David Forbes, the Scotch physicist, had shown
+in 1837 that radiant heat conforms to the same laws of polarization
+and double refraction that govern light. But, for that matter, the
+experiments that had established the mechanical equivalent of
+heat hardly left room for doubt as to the immateriality of this
+"imponderable." Doubters had indeed, expressed scepticism as to
+the validity of Joule's experiments, but the further researches,
+experimental and mathematical, of such workers as Thomson (Lord Kelvin),
+Rankine, and Tyndall in Great Britain, of Helmholtz and Clausius in
+Germany, and of Regnault in France, dealing with various manifestations
+of heat, placed the evidence beyond the reach of criticism.
+
+Out of these studies, just at the middle of the century, to which
+the experiments of Mayer and Joule had led, grew the new science
+of thermo-dynamics. Out of them also grew in the mind of one of the
+investigators a new generalization, only second in importance to the
+doctrine of conservation itself. Professor William Thomson (Lord Kelvin)
+in his studies in thermodynamics was early impressed with the fact that
+whereas all the molar motion developed through labor or gravity could
+be converted into heat, the process is not fully reversible. Heat can,
+indeed, be converted into molar motion or work, but in the process a
+certain amount of the heat is radiated into space and lost. The same
+thing happens whenever any other form of energy is converted into molar
+motion. Indeed, every transmutation of energy, of whatever character,
+seems complicated by a tendency to develop heat, part of which is
+lost. This observation led Professor Thomson to his doctrine of the
+dissipation of energy, which he formulated before the Royal Society of
+Edinburgh in 1852, and published also in the Philosophical Magazine the
+same year, the title borne being, "On a Universal Tendency in Nature to
+the Dissipation of Mechanical Energy."
+
+From the principle here expressed Professor Thomson drew the startling
+conclusion that, "since any restoration of this mechanical energy
+without more than an equivalent dissipation is impossible," the
+universe, as known to us, must be in the condition of a machine
+gradually running down; and in particular that the world we live on has
+been within a finite time unfit for human habitation, and must again
+become so within a finite future. This thought seems such a commonplace
+to-day that it is difficult to realize how startling it appeared half a
+century ago. A generation trained, as ours has been, in the doctrines
+of the conservation and dissipation of energy as the very alphabet
+of physical science can but ill appreciate the mental attitude of a
+generation which for the most part had not even thought it problematical
+whether the sun could continue to give out heat and light forever. But
+those advance thinkers who had grasped the import of the doctrine of
+conservation could at once appreciate the force of Thomson's doctrine
+of dissipation, and realize the complementary character of the two
+conceptions.
+
+Here and there a thinker like Rankine did, indeed, attempt to fancy
+conditions under which the energy lost through dissipation might be
+restored to availability, but no such effort has met with success, and
+in time Professor Thomson's generalization and his conclusions as to the
+consequences of the law involved came to be universally accepted.
+
+The introduction of the new views regarding the nature of energy
+followed, as I have said, the course of every other growth of new ideas.
+Young and imaginative men could accept the new point of view; older
+philosophers, their minds channelled by preconceptions, could not get
+into the new groove. So strikingly true is this in the particular case
+now before us that it is worth while to note the ages at the time of the
+revolutionary experiments of the men whose work has been mentioned as
+entering into the scheme of evolution of the idea that energy is merely
+a manifestation of matter in motion. Such a list will tell the story
+better than a volume of commentary.
+
+Observe, then, that Davy made his epochal experiment of melting ice by
+friction when he was a youth of twenty. Young was no older when he
+made his first communication to the Royal Society, and was in his
+twenty-seventh year when he first actively espoused the undulatory
+theory. Fresnel was twenty-six when he made his first important
+discoveries in the same field; and Arago, who at once became his
+champion, was then but two years his senior, though for a decade he had
+been so famous that one involuntarily thinks of him as belonging to an
+elder generation.
+
+Forbes was under thirty when he discovered the polarization of heat,
+which pointed the way to Mohr, then thirty-one, to the mechanical
+equivalent. Joule was twenty-two in 1840, when his great work was
+begun; and Mayer, whose discoveries date from the same year, was then
+twenty-six, which was also the age of Helmholtz when he published his
+independent discovery of the same law. William Thomson was a youth just
+past his majority when he came to the aid of Joule before the British
+Society, and but seven years older when he formulated his own doctrine
+of the dissipation of energy. And Clausius and Rankine, who are usually
+mentioned with Thomson as the great developers of thermo-dynamics, were
+both far advanced with their novel studies before they were thirty.
+With such a list in mind, we may well agree with the father of inductive
+science that "the man who is young in years may be old in hours."
+
+Yet we must not forget that the shield has a reverse side. For was not
+the greatest of observing astronomers, Herschel, past thirty-five before
+he ever saw a telescope, and past fifty before he discovered the heat
+rays of the spectrum? And had not Faraday reached middle life before he
+turned his attention especially to electricity? Clearly, then, to make
+this phrase complete, Bacon should have added that "the man who is
+old in years may be young in imagination." Here, however, even more
+appropriate than in the other case--more's the pity--would have been the
+application of his qualifying clause: "but that happeneth rarely."
+
+
+THE FINAL UNIFICATION
+
+There are only a few great generalizations as yet thought out in any
+single field of science. Naturally, then, after a great generalization
+has found definitive expression, there is a period of lull before
+another forward move. In the case of the doctrines of energy, the
+lull has lasted half a century. Throughout this period, it is true, a
+multitude of workers have been delving in the field, and to the casual
+observer it might seem as if their activity had been boundless, while
+the practical applications of their ideas--as exemplified, for example,
+in the telephone, phonograph, electric light, and so on--have been
+little less than revolutionary. Yet the most competent of living
+authorities, Lord Kelvin, could assert in 1895 that in fifty years he
+had learned nothing new regarding the nature of energy.
+
+This, however, must not be interpreted as meaning that the world has
+stood still during these two generations. It means rather that the rank
+and file have been moving forward along the road the leaders had
+already travelled. Only a few men in the world had the range of thought
+regarding the new doctrine of energy that Lord Kelvin had at the middle
+of the century. The few leaders then saw clearly enough that if one
+form of energy is in reality merely an undulation or vibration among the
+particles of "ponderable" matter or of ether, all other manifestations
+of energy must be of the same nature. But the rank and file were not
+even within sight of this truth for a long time after they had partly
+grasped the meaning of the doctrine of conservation. When, late in
+the fifties, that marvellous young Scotchman, James Clerk-Maxwell,
+formulating in other words an idea of Faraday's, expressed his belief
+that electricity and magnetism are but manifestations of various
+conditions of stress and motion in the ethereal medium (electricity a
+displacement of strain, magnetism a whirl in the ether), the idea met
+with no immediate popularity. And even less cordial was the reception
+given the same thinker's theory, put forward in 1863, that the ethereal
+undulations producing the phenomenon we call light differ in no respect
+except in their wave-length from the pulsations of electro-magnetism.
+
+At about the same time Helmholtz formulated a somewhat similar
+electro-magnetic theory of light; but even the weight of this combined
+authority could not give the doctrine vogue until very recently, when
+the experiments of Heinrich Hertz, the pupil of Helmholtz, have shown
+that a condition of electrical strain may be developed into a wave
+system by recurrent interruptions of the electric state in the
+generator, and that such waves travel through the ether with the
+rapidity of light. Since then the electro-magnetic theory of light has
+been enthusiastically referred to as the greatest generalization of
+the century; but the sober thinker must see that it is really only
+what Hertz himself called it--one pier beneath the great arch of
+conservation. It is an interesting detail of the architecture, but the
+part cannot equal the size of the whole.
+
+More than that, this particular pier is as yet by no means a very firm
+one. It has, indeed, been demonstrated that waves of electro-magnetism
+pass through space with the speed of light, but as yet no one has
+developed electric waves even remotely approximating the shortness of
+the visual rays. The most that can positively be asserted, therefore,
+is that all the known forms of radiant energy-heat, light,
+electro-magnetism--travel through space at the same rate of speed, and
+consist of traverse vibrations--"lateral quivers," as Fresnel said of
+light--known to differ in length, and not positively known to differ
+otherwise. It has, indeed, been suggested that the newest form of
+radiant energy, the famous X-ray of Professor Roentgen's discovery, is
+a longitudinal vibration, but this is a mere surmise. Be that as it
+may, there is no one now to question that all forms of radiant energy,
+whatever their exact affinities, consist essentially of undulatory
+motions of one uniform medium.
+
+A full century of experiment, calculation, and controversy has thus
+sufficed to correlate the "imponderable fluids" of our forebears, and
+reduce them all to manifestations of motion among particles of matter.
+At first glimpse that seems an enormous change of view. And yet, when
+closely considered, that change in thought is not so radical as the
+change in phrase might seem to imply. For the nineteenth-century
+physicist, in displacing the "imponderable fluids" of many kinds--one
+each for light, heat, electricity, magnetism--has been obliged to
+substitute for them one all-pervading fluid, whose various quivers,
+waves, ripples, whirls or strains produce the manifestations which in
+popular parlance are termed forms of force. This all-pervading fluid the
+physicist terms the ether, and he thinks of it as having no weight. In
+effect, then, the physicist has dispossessed the many imponderables in
+favor of a single imponderable--though the word imponderable has been
+banished from his vocabulary. In this view the ether--which, considered
+as a recognized scientific verity, is essentially a nineteenth-century
+discovery--is about the most interesting thing in the universe.
+Something more as to its properties, real or assumed, we shall have
+occasion to examine as we turn to the obverse side of physics, which
+demands our attention in the next chapter.
+
+
+
+
+IX. THE ETHER AND PONDERABLE MATTER
+
+
+"Whatever difficulties we may have in forming a consistent idea of the
+constitution of the ether, there can be no doubt that the interplanetary
+and interstellar spaces are not empty, but are occupied by a material
+substance or body which is certainly the largest and probably the most
+uniform body of which we have any knowledge."
+
+Such was the verdict pronounced some thirty years ago by James
+Clerk-Maxwell, one of the very greatest of nineteenth-century
+physicists, regarding the existence of an all-pervading plenum in the
+universe, in which every particle of tangible matter is immersed.
+And this verdict may be said to express the attitude of the entire
+philosophical world of our day. Without exception, the authoritative
+physicists of our time accept this plenum as a verity, and reason about
+it with something of the same confidence they manifest in speaking of
+"ponderable" matter or of, energy. It is true there are those among them
+who are disposed to deny that this all-pervading plenum merits the name
+of matter. But that it is a something, and a vastly important something
+at that, all are agreed. Without it, they allege, we should know nothing
+of light, of radiant heat, of electricity or magnetism; without it there
+would probably be no such thing as gravitation; nay, they even hint that
+without this strange something, ether, there would be no such thing as
+matter in the universe. If these contentions of the modern physicist are
+justified, then this intangible ether is incomparably the most important
+as well as the "largest and most uniform substance or body" in the
+universe. Its discovery may well be looked upon as one of the most
+important feats of the nineteenth century.
+
+For a discovery of that century it surely is, in the sense that all
+the known evidences of its existence were gathered in that epoch.
+True dreamers of all ages have, for metaphysical reasons, imagined the
+existence of intangible fluids in space--they had, indeed, peopled
+space several times over with different kinds of ethers, as Maxwell
+remarks--but such vague dreamings no more constituted the discovery of
+the modern ether than the dream of some pre-Columbian visionary that
+land might lie beyond the unknown waters constituted the discovery
+of America. In justice it must be admitted that Huyghens, the
+seventeenth-century originator of the undulatory theory of light, caught
+a glimpse of the true ether; but his contemporaries and some eight
+generations of his successors were utterly deaf to his claims; so
+he bears practically the same relation to the nineteenth-century
+discoverers of ether that the Norseman bears to Columbus.
+
+The true Columbus of the ether was Thomas Young. His discovery was
+consummated in the early days of the nineteenth century, when he brought
+forward the first, conclusive proofs of the undulatory theory of light.
+To say that light consists of undulations is to postulate something that
+undulates; and this something could not be air, for air exists only in
+infinitesimal quantity, if at all, in the interstellar spaces, through
+which light freely penetrates. But if not air, what then? Why, clearly,
+something more intangible than air; something supersensible, evading all
+direct efforts to detect it, yet existing everywhere in seemingly
+vacant space, and also interpenetrating the substance of all transparent
+liquids and solids, if not, indeed, of all tangible substances. This
+intangible something Young rechristened the Luminiferous Ether.
+
+In the early days of his discovery Young thought of the undulations
+which produce light and radiant heat as being longitudinal--a forward
+and backward pulsation, corresponding to the pulsations of sound--and as
+such pulsations can be transmitted by a fluid medium with the properties
+of ordinary fluids, he was justified in thinking of the ether as being
+like a fluid in its properties, except for its extreme intangibility.
+But about 1818 the experiments of Fresnel and Arago with polarization
+of light made it seem very doubtful whether the theory of longitudinal
+vibrations is sufficient, and it was suggested by Young, and
+independently conceived and demonstrated by Fresnel, that the
+luminiferous undulations are not longitudinal, but transverse; and all
+the more recent experiments have tended to confirm this view. But it
+happens that ordinary fluids--gases and liquids--cannot transmit lateral
+vibrations; only rigid bodies are capable of such a vibration. So
+it became necessary to assume that the luminiferous ether is a body
+possessing elastic rigidity--a familiar property of tangible solids, but
+one quite unknown among fluids.
+
+The idea of transverse vibrations carried with it another puzzle. Why
+does not the ether, when set aquiver with the vibration which gives us
+the sensation we call light, have produced in its substance subordinate
+quivers, setting out at right angles from the path of the original
+quiver? Such perpendicular vibrations seem not to exist, else we might
+see around a corner; how explain their absence? The physicist could
+think of but one way: they must assume that the ether is incompressible.
+It must fill all space--at any rate, all space with which human
+knowledge deals--perfectly full.
+
+These properties of the ether, incompressibility and elastic rigidity,
+are quite conceivable by themselves; but difficulties of thought appear
+when we reflect upon another quality which the ether clearly
+must possess--namely, frictionlessness. By hypothesis this rigid,
+incompressible body pervades all space, imbedding every particle of
+tangible matter; yet it seems not to retard the movements of this matter
+in the slightest degree. This is undoubtedly the most difficult to
+comprehend of the alleged properties of the ether. The physicist
+explains it as due to the perfect elasticity of the ether, in virtue
+of which it closes in behind a moving particle with a push exactly
+counterbalancing the stress required to penetrate it in front.
+
+To a person unaccustomed to think of seemingly solid matter as really
+composed of particles relatively wide apart, it is hard to understand
+the claim that ether penetrates the substance of solids--of glass,
+for example--and, to use Young's expression, which we have previously
+quoted, moves among them as freely as the wind moves through a grove
+of trees. This thought, however, presents few difficulties to the mind
+accustomed to philosophical speculation. But the question early arose
+in the mind of Fresnel whether the ether is not considerably affected by
+contact with the particles of solids. Some of his experiments led him to
+believe that a portion of the ether which penetrates among the molecules
+of tangible matter is held captive, so to speak, and made to move along
+with these particles. He spoke of such portions of the ether as "bound"
+ether, in contradistinction to the great mass of "free" ether. Half a
+century after Fresnel's death, when the ether hypothesis had become
+an accepted tenet of science, experiments were undertaken by Fizeau
+in France, and by Clerk-Maxwell in England, to ascertain whether any
+portion of ether is really thus bound to particles of matter; but the
+results of the experiments were negative, and the question is still
+undetermined.
+
+While the undulatory theory of light was still fighting its way, another
+kind of evidence favoring the existence of an ether was put forward by
+Michael Faraday, who, in the course of his experiments in electrical and
+magnetic induction, was led more and more to perceive definite lines or
+channels of force in the medium subject to electro-magnetic influence.
+Faraday's mind, like that of Newton and many other philosophers,
+rejected the idea of action at a distance, and he felt convinced that
+the phenomena of magnetism and of electric induction told strongly for
+the existence of an invisible plenum everywhere in space, which might
+very probably be the same plenum that carries the undulations of light
+and radiant heat.
+
+Then, about the middle of the century, came that final revolution of
+thought regarding the nature of energy which we have already outlined in
+the preceding chapter, and with that the case for ether was considered
+to be fully established. The idea that energy is merely a "mode
+of motion" (to adopt Tyndall's familiar phrase), combined with the
+universal rejection of the notion of action at a distance, made the
+acceptance of a plenum throughout space a necessity of thought--so, at
+any rate, it has seemed to most physicists of recent decades. The proof
+that all known forms of radiant energy move through space at the same
+rate of speed is regarded as practically a demonstration that but one
+plenum--one ether--is concerned in their transmission. It has, indeed,
+been tentatively suggested, by Professor J. Oliver Lodge, that there may
+be two ethers, representing the two opposite kinds of electricity, but
+even the author of this hypothesis would hardly claim for it a high
+degree of probability.
+
+The most recent speculations regarding the properties of the ether have
+departed but little from the early ideas of Young and Fresnel. It is
+assumed on all sides that the ether is a continuous, incompressible
+body, possessing rigidity and elasticity. Lord Kelvin has even
+calculated the probable density of this ether, and its coefficient of
+rigidity. As might be supposed, it is all but infinitely tenuous as
+compared with any tangible solid, and its rigidity is but infinitesimal
+as compared with that of steel. In a word, it combines properties of
+tangible matter in a way not known in any tangible substance. Therefore
+we cannot possibly conceive its true condition correctly. The nearest
+approximation, according to Lord Kelvin, is furnished by a mould of
+transparent jelly. It is a crude, inaccurate analogy, of course, the
+density and resistance of jelly in particular being utterly different
+from those of the ether; but the quivers that run through the jelly when
+it is shaken, and the elastic tension under which it is placed when its
+mass is twisted about, furnish some analogy to the quivers and strains
+in the ether, which are held to constitute radiant energy, magnetism,
+and electricity.
+
+The great physicists of the day being at one regarding the existence of
+this all-pervading ether, it would be a manifest presumption for any one
+standing without the pale to challenge so firmly rooted a belief. And,
+indeed, in any event, there seems little ground on which to base such
+a challenge. Yet it may not be altogether amiss to reflect that the
+physicist of to-day is no more certain of his ether than was his
+predecessor of the eighteenth century of the existence of certain
+alleged substances which he called phlogiston, caloric, corpuscles of
+light, and magnetic and electric fluids. It would be but the repetition
+of history should it chance that before the close of another century the
+ether should have taken its place along with these discarded creations
+of the scientific imagination of earlier generations. The philosopher of
+to-day feels very sure that an ether exists; but when he says there is
+"no doubt" of its existence he speaks incautiously, and steps beyond the
+bounds of demonstration. He does not KNOW that action cannot take place
+at a distance; he does not KNOW that empty space itself may not perform
+the functions which he ascribes to his space-filling ether.
+
+Meantime, however, the ether, be it substance or be it only dream-stuff,
+is serving an admirable purpose in furnishing a fulcrum for modern
+physics. Not alone to the student of energy has it proved invaluable,
+but to the student of matter itself as well. Out of its hypothetical
+mistiness has been reared the most tenable theory of the constitution of
+ponderable matter which has yet been suggested--or, at any rate, the
+one that will stand as the definitive nineteenth-century guess at
+this "riddle of the ages." I mean, of course, the vortex theory of
+atoms--that profound and fascinating doctrine which suggests that
+matter, in all its multiform phases, is neither more nor less than ether
+in motion.
+
+The author of this wonderful conception is Lord Kelvin. The idea was
+born in his mind of a happy union of mathematical calculations with
+concrete experiments. The mathematical calculations were largely the
+work of Hermann von Helmholtz, who, about the year 1858, had undertaken
+to solve some unique problems in vortex motions. Helmholtz found that
+a vortex whirl, once established in a frictionless medium, must go on,
+theoretically, unchanged forever. In a limited medium such a whirl may
+be V-shaped, with its ends at the surface of the medium. We may imitate
+such a vortex by drawing the bowl of a spoon quickly through a cup
+of water. But in a limitless medium the vortex whirl must always be
+a closed ring, which may take the simple form of a hoop or circle, or
+which may be indefinitely contorted, looped, or, so to speak, knotted.
+Whether simple or contorted, this endless chain of whirling matter (the
+particles revolving about the axis of the loop as the particles of a
+string revolve when the string is rolled between the fingers) must, in
+a frictionless medium, retain its form and whirl on with undiminished
+speed forever.
+
+While these theoretical calculations of Helmholtz were fresh in his
+mind, Lord Kelvin (then Sir William Thomson) was shown by Professor
+P. G. Tait, of Edinburgh, an apparatus constructed for the purpose
+of creating vortex rings in air. The apparatus, which any one may
+duplicate, consisted simply of a box with a hole bored in one side, and
+a piece of canvas stretched across the opposite side in lieu of boards.
+Fumes of chloride of ammonia are generated within the box, merely to
+render the air visible. By tapping with the band on the canvas side
+of the box, vortex rings of the clouded air are driven out, precisely
+similar in appearance to those smoke-rings which some expert
+tobacco-smokers can produce by tapping on their cheeks, or to those
+larger ones which we sometimes see blown out from the funnel of a
+locomotive.
+
+The advantage of Professor Tait's apparatus is its manageableness and
+the certainty with which the desired result can be produced. Before Lord
+Kelvin's interested observation it threw out rings of various sizes,
+which moved straight across the room at varying rates of speed,
+according to the initial impulse, and which behaved very strangely when
+coming in contact with one another. If, for example, a rapidly moving
+ring overtook another moving in the same path, the one in advance seemed
+to pause, and to spread out its periphery like an elastic band, while
+the pursuer seemed to contract, till it actually slid through the
+orifice of the other, after which each ring resumed its original size,
+and continued its course as if nothing had happened. When, on the other
+hand, two rings moving in slightly different directions came near each
+other, they seemed to have an attraction for each other; yet if they
+impinged, they bounded away, quivering like elastic solids. If an effort
+were made to grasp or to cut one of these rings, the subtle thing shrank
+from the contact, and slipped away as if it were alive.
+
+And all the while the body which thus conducted itself consisted simply
+of a whirl in the air, made visible, but not otherwise influenced, by
+smoky fumes. Presently the friction of the surrounding air wore the
+ring away, and it faded into the general atmosphere--often, however, not
+until it had persisted for many seconds, and passed clear across a large
+room. Clearly, if there were no friction, the ring's inertia must make
+it a permanent structure. Only the frictionless medium was lacking to
+fulfil all the conditions of Helmholtz's indestructible vortices. And
+at once Lord Kelvin bethought him of the frictionless medium which
+physicists had now begun to accept--the all-pervading ether. What
+if vortex rings were started in this ether, must they not have the
+properties which the vortex rings in air had exhibited--inertia,
+attraction, elasticity? And are not these the properties of ordinary
+tangible matter? Is it not probable, then, that what we call matter
+consists merely of aggregations of infinitesimal vortex rings in the
+ether?
+
+Thus the vortex theory of atoms took form in Lord Kelvin's mind, and its
+expression gave the world what many philosophers of our time regard as
+the most plausible conception of the constitution of matter hitherto
+formulated. It is only a theory, to be sure; its author would be the
+last person to claim finality for it. "It is only a dream," Lord Kelvin
+said to me, in referring to it not long ago. But it has a basis in
+mathematical calculation and in analogical experiment such as no other
+theory of matter can lay claim to, and it has a unifying or monistic
+tendency that makes it, for the philosophical mind, little less than
+fascinating. True or false, it is the definitive theory of matter of the
+twentieth century.
+
+Quite aside from the question of the exact constitution of the ultimate
+particles of matter, questions as to the distribution of such particles,
+their mutual relations, properties, and actions, came in for a full
+share of attention during the nineteenth century, though the foundations
+for the modern speculations were furnished in a previous epoch. The most
+popular eighteenth-century speculation as to the ultimate constitution
+of matter was that of the learned Italian priest, Roger Joseph
+Boscovich, published in 1758, in his Theoria Philosophiae Naturalis.
+"In this theory," according to an early commentator, "the whole mass of
+which the bodies of the universe are composed is supposed to consist
+of an exceedingly great yet finite number of simple, indivisible,
+inextended atoms. These atoms are endued by the Creator with REPULSIVE
+and ATTRACTIVE forces, which vary according to the distance. At very
+small distances the particles of matter repel each other; and this
+repulsive force increases beyond all limits as the distances are
+diminished, and will consequently forever prevent actual contact. When
+the particles of matter are removed to sensible distances, the repulsive
+is exchanged for an attractive force, which decreases in inverse ratio
+with the squares of the distances, and extends beyond the spheres of the
+most remote comets."
+
+This conception of the atom as a mere centre of force was hardly such
+as could satisfy any mind other than the metaphysical. No one made a
+conspicuous attempt to improve upon the idea, however, till just at the
+close of the century, when Humphry Davy was led, in the course of
+his studies of heat, to speculate as to the changes that occur in the
+intimate substance of matter under altered conditions of temperature.
+Davy, as we have seen, regarded heat as a manifestation of motion among
+the particles of matter. As all bodies with which we come in contact
+have some temperature, Davy inferred that the intimate particles of
+every substance must be perpetually in a state of vibration. Such
+vibrations, he believed, produced the "repulsive force" which (in common
+with Boscovich) he admitted as holding the particles of matter at a
+distance from one another. To heat a substance means merely to increase
+the rate of vibration of its particles; thus also, plainly, increasing
+the repulsive forces and expanding the bulk of the mass as a whole. If
+the degree of heat applied be sufficient, the repulsive force may become
+strong enough quite to overcome the attractive force, and the particles
+will separate and tend to fly away from one another, the solid then
+becoming a gas.
+
+Not much attention was paid to these very suggestive ideas of Davy,
+because they were founded on the idea that heat is merely a motion,
+which the scientific world then repudiated; but half a century later,
+when the new theories of energy had made their way, there came a revival
+of practically the same ideas of the particles of matter (molecules they
+were now called) which Davy had advocated. Then it was that Clausius in
+Germany and Clerk-Maxwell in England took up the investigation of
+what came to be known as the kinetic theory of gases--the now familiar
+conception that all the phenomena of gases are due to the helter-skelter
+flight of the showers of widely separated molecules of which they are
+composed. The specific idea that the pressure or "spring" of gases is
+due to such molecular impacts was due to Daniel Bournelli, who advanced
+it early in the eighteenth century. The idea, then little noticed, had
+been revived about a century later by William Herapath, and again with
+some success by J. J. Waterston, of Bombay, about 1846; but it gained
+no distinct footing until taken in hand by Clausius in 1857 and by
+Clerk-Maxwell in 1859.
+
+The considerations that led Clerk-Maxwell to take up the computations
+may be stated in his own words, as formulated in a paper "On the Motions
+and Collisions of Perfectly Elastic Spheres."
+
+"So many of the properties of matter, especially when in the gaseous
+form," he says, "can be deduced from the hypothesis that their minute
+parts are in rapid motion, the velocity increasing with the temperature,
+that the precise nature of this motion becomes a subject of rational
+curiosity. Daniel Bournelli, Herapath, Joule, Kronig, Clausius, etc.,
+have shown that the relations between pressure, temperature, and density
+in a perfect gas can be explained by supposing the particles to move
+with uniform velocities in straight lines, striking against the sides of
+the containing vessel and thus producing pressure. It is not necessary
+to suppose each particle to travel to any great distance in the same
+straight line; for the effect in producing pressure will be the same
+if the particles strike against each other; so that the straight line
+described may be very short. M. Clausius has determined the mean length
+of path in terms of the average of the particles, and the distance
+between the centres of two particles when the collision takes place. We
+have at present no means of ascertaining either of these distances;
+but certain phenomena, such as the internal friction of gases, the
+conduction of heat through a gas, and the diffusion of one gas through
+another, seem to indicate the possibility of determining accurately the
+mean length of path which a particle describes between two successive
+collisions. In order to lay the foundation of such investigations on
+strict mechanical principles, I shall demonstrate the laws of motion
+of an indefinite number of small, hard, and perfectly elastic spheres
+acting on one another only during impact. If the properties of such a
+system of bodies are found to correspond to those of gases, an important
+physical analogy will be established, which may lead to more accurate
+knowledge of the properties of matter. If experiments on gases are
+inconsistent with the hypothesis of these propositions, then our theory,
+though consistent with itself, is proved to be incapable of explaining
+the phenomena of gases. In either case it is necessary to follow out
+these consequences of the hypothesis.
+
+"Instead of saying that the particles are hard, spherical, and elastic,
+we may, if we please, say the particles are centres of force, of which
+the action is insensible except at a certain very small distance, when
+it suddenly appears as a repulsive force of very great intensity. It is
+evident that either assumption will lead to the same results. For the
+sake of avoiding the repetition of a long phrase about these repulsive
+bodies, I shall proceed upon the assumption of perfectly elastic
+spherical bodies. If we suppose those aggregate molecules which move
+together to have a bounding surface which is not spherical, then the
+rotatory motion of the system will close up a certain proportion of the
+whole vis viva, as has been shown by Clausius, and in this way we may
+account for the value of the specific heat being greater than on the
+more simple hypothesis."(1)
+
+
+The elaborate investigations of Clerk-Maxwell served not merely to
+substantiate the doctrine, but threw a flood of light upon the entire
+subject of molecular dynamics. Soon the physicists came to feel as
+certain of the existence of these showers of flying molecules making up
+a gas as if they could actually see and watch their individual actions.
+Through study of the viscosity of gases--that is to say, of the degree
+of frictional opposition they show to an object moving through them
+or to another current of gas--an idea was gained, with the aid of
+mathematics, of the rate of speed at which the particles of the gas are
+moving, and the number of collisions which each particle must experience
+in a given time, and of the length of the average free path traversed
+by the molecule between collisions, These measurements were confirmed
+by study of the rate of diffusion at which different gases mix together,
+and also by the rate of diffusion of heat through a gas, both these
+phenomena being chiefly due to the helter-skelter flight of the
+molecules.
+
+It is sufficiently astonishing to be told that such measurements as
+these have been made at all, but the astonishment grows when one hears
+the results. It appears from Clerk-Maxwell's calculations that the mean
+free path, or distance traversed by the molecules between collisions in
+ordinary air, is about one-half-millionth of an inch; while the speed of
+the molecules is such that each one experiences about eight billions
+of collisions per second! It would be hard, perhaps, to cite an
+illustration showing the refinements of modern physics better than
+this; unless, indeed, one other result that followed directly from these
+calculations be considered such--the feat, namely, of measuring the size
+of the molecules themselves. Clausius was the first to point out how
+this might be done from a knowledge of the length of free path; and the
+calculations were made by Loschmidt in Germany and by Lord Kelvin in
+England, independently.
+
+The work is purely mathematical, of course, but the results are regarded
+as unassailable; indeed, Lord Kelvin speaks of them as being absolutely
+demonstrative within certain limits of accuracy. This does not mean,
+however, that they show the exact dimensions of the molecule; it means
+an estimate of the limits of size within which the actual size of the
+molecule may lie. These limits, Lord Kelvin estimates, are about
+the one-ten-millionth of a centimetre for the maximum, and the
+one-one-hundred-millionth of a centimetre for the minimum. Such figures
+convey no particular meaning to our blunt senses, but Lord Kelvin has
+given a tangible illustration that aids the imagination to at least a
+vague comprehension of the unthinkable smallness of the molecule. He
+estimates that if a ball, say of water or glass, about "as large as
+a football, were to be magnified up to the size of the earth, each
+constituent molecule being magnified in the same proportion, the
+magnified structure would be more coarse-grained than a heap of shot,
+but probably less coarse-grained than a heap of footballs."
+
+Several other methods have been employed to estimate the size of
+molecules. One of these is based upon the phenomena of contact
+electricity; another upon the wave-theory of light; and another upon
+capillary attraction, as shown in the tense film of a soap-bubble! No
+one of these methods gives results more definite than that due to the
+kinetic theory of gases, just outlined; but the important thing is that
+the results obtained by these different methods (all of them due to Lord
+Kelvin) agree with one another in fixing the dimensions of the molecule
+at somewhere about the limits already mentioned. We may feel very sure
+indeed, therefore, that the molecules of matter are not the unextended,
+formless points which Boscovich and his followers of the eighteenth
+century thought them. But all this, it must be borne in mind, refers
+to the molecule, not to the ultimate particle of matter, about which we
+shall have more to say in another connection. Curiously enough, we shall
+find that the latest theories as to the final term of the series are
+not so very far afield from the dreamings of the eighteenth-century
+philosophers; the electron of J. J. Thompson shows many points of
+resemblance to the formless centre of Boscovich.
+
+Whatever the exact form of the molecule, its outline is subject to
+incessant variation; for nothing in molecular science is regarded as
+more firmly established than that the molecule, under all ordinary
+circumstances, is in a state of intense but variable vibration. The
+entire energy of a molecule of gas, for example, is not measured by its
+momentum, but by this plus its energy of vibration and rotation, due
+to the collisions already referred to. Clausius has even estimated
+the relative importance of these two quantities, showing that the
+translational motion of a molecule of gas accounts for only three-fifths
+of its kinetic energy. The total energy of the molecule (which we call
+"heat") includes also another factor--namely, potential energy, or
+energy of position, due to the work that has been done on expanding,
+in overcoming external pressure, and internal attraction between the
+molecules themselves. This potential energy (which will be recovered
+when the gas contracts) is the "latent heat" of Black, which so long
+puzzled the philosophers. It is latent in the same sense that the energy
+of a ball thrown into the air is latent at the moment when the ball
+poises at its greatest height before beginning to fall.
+
+It thus appears that a variety of motions, real and potential, enter
+into the production of the condition we term heat. It is, however,
+chiefly the translational motion which is measurable as temperature;
+and this, too, which most obviously determines the physical state of the
+substance that the molecules collectively compose--whether, that is to
+say, it shall appear to our blunt perceptions as a gas, a liquid, or a
+solid. In the gaseous state, as we have seen, the translational motion
+of the molecules is relatively enormous, the molecules being widely
+separated. It does not follow, as we formerly supposed, that this
+is evidence of a repulsive power acting between the molecules. The
+physicists of to-day, headed by Lord Kelvin, decline to recognize any
+such power. They hold that the molecules of a gas fly in straight lines
+by virtue of their inertia, quite independently of one another, except
+at times of collision, from which they rebound by virtue of their
+elasticity; or on an approach to collision, in which latter case, coming
+within the range of mutual attraction, two molecules may circle about
+each other, as a comet circles about the sun, then rush apart again, as
+the comet rushes from the sun.
+
+It is obvious that the length of the mean free path of the molecules
+of a gas may be increased indefinitely by decreasing the number of the
+molecules themselves in a circumscribed space. It has been shown by
+Professors Tait and Dewar that a vacuum may be produced artificially of
+such a degree of rarefaction that the mean free path of the remaining
+molecules is measurable in inches. The calculation is based on
+experiments made with the radiometer of Professor Crookes, an instrument
+which in itself is held to demonstrate the truth of the kinetic theory
+of gases. Such an attenuated gas as this is considered by Professor
+Crookes as constituting a fourth state of matter, which he terms
+ultra-gaseous.
+
+If, on the other hand, a gas is subjected to pressure, its molecules are
+crowded closer together, and the length of their mean free path is thus
+lessened. Ultimately, the pressure being sufficient, the molecules are
+practically in continuous contact. Meantime the enormously increased
+number of collisions has set the molecules more and more actively
+vibrating, and the temperature of the gas has increased, as, indeed,
+necessarily results in accordance with the law of the conservation
+of energy. No amount of pressure, therefore, can suffice by itself to
+reduce the gas to a liquid state. It is believed that even at the centre
+of the sun, where the pressure is almost inconceivably great, all matter
+is to be regarded as really gaseous, though the molecules must be so
+packed together that the consistency is probably more like that of a
+solid.
+
+If, however, coincidently with the application of pressure, opportunity
+be given for the excess of heat to be dissipated to a colder surrounding
+medium, the molecules, giving off their excess of energy, become
+relatively quiescent, and at a certain stage the gas becomes a liquid.
+The exact point at which this transformation occurs, however, differs
+enormously for different substances. In the case of water, for
+example, it is a temperature more than four hundred degrees above zero,
+centigrade; while for atmospheric air it is one hundred and ninety-four
+degrees centigrade below zero, or more than a hundred and fifty degrees
+below the point at which mercury freezes.
+
+Be it high or low, the temperature above which any substance is always
+a gas, regardless of pressure, is called the critical temperature, or
+absolute boiling-point, of that substance. It does not follow, however,
+that below this point the substance is necessarily a liquid. This is a
+matter that will be determined by external conditions of pressure. Even
+far below the critical temperature the molecules have an enormous degree
+of activity, and tend to fly asunder, maintaining what appears to be
+a gaseous, but what technically is called a vaporous, condition--the
+distinction being that pressure alone suffices to reduce the vapor to
+the liquid state. Thus water may change from the gaseous to the liquid
+state at four hundred degrees above zero, but under conditions of
+ordinary atmospheric pressure it does not do so until the temperature
+is lowered three hundred degrees further. Below four hundred degrees,
+however, it is technically a vapor, not a gas; but the sole difference,
+it will be understood, is in the degree of molecular activity.
+
+It thus appeared that the prevalence of water in a vaporous and liquid
+rather than in a "permanently" gaseous condition here on the globe is a
+mere incident of telluric evolution. Equally incidental is the fact that
+the air we breathe is "permanently" gaseous and not liquid or solid,
+as it might be were the earth's surface temperature to be lowered to a
+degree which, in the larger view, may be regarded as trifling. Between
+the atmospheric temperature in tropical and in arctic regions there is
+often a variation of more than one hundred degrees; were the temperature
+reduced another hundred, the point would be reached at which oxygen
+gas becomes a vapor, and under increased pressure would be a liquid.
+Thirty-seven degrees more would bring us to the critical temperature of
+nitrogen.
+
+Nor is this a mere theoretical assumption; it is a determination of
+experimental science, quite independent of theory. The physicist in the
+laboratory has produced artificial conditions of temperature enabling
+him to change the state of the most persistent gases. Some fifty years
+since, when the kinetic theory was in its infancy, Faraday liquefied
+carbonic-acid gas, among others, and the experiments thus inaugurated
+have been extended by numerous more recent investigators, notably
+by Cailletet in Switzerland, by Pictet in France, and by Dr. Thomas.
+Andrews and Professor James Dewar in England. In the course of these
+experiments not only has air been liquefied, but hydrogen also, the most
+subtle of gases; and it has been made more and more apparent that gas
+and liquid are, as Andrews long ago asserted, "only distant stages of
+a long series of continuous physical changes." Of course, if the
+temperature be lowered still further, the liquid becomes a solid; and
+this change also has been effected in the case of some of the most
+"permanent" gases, including air.
+
+The degree of cold--that is, of absence of heat--thus produced is
+enormous, relatively to anything of which we have experience in nature
+here at the earth now, yet the molecules of solidified air, for
+example, are not absolutely quiescent. In other words, they still have
+a temperature, though so very low. But it is clearly conceivable that
+a stage might be reached at which the molecules became absolutely
+quiescent, as regards either translational or vibratory motion. Such a
+heatless condition has been approached, but as yet not quite
+attained, in laboratory experiments. It is called the absolute zero
+of temperature, and is estimated to be equivalent to two hundred and
+seventy-three degrees Centigrade below the freezing-point of water, or
+ordinary zero.
+
+A temperature (or absence of temperature) closely approximating this
+is believed to obtain in the ethereal ocean of interplanetary and
+interstellar space, which transmits, but is thought not to absorb,
+radiant energy. We here on the earth's surface are protected from
+exposure to this cold, which would deprive every organic thing of life
+almost instantaneously, solely by the thin blanket of atmosphere with
+which the globe is coated. It would seem as if this atmosphere,
+exposed to such a temperature at its surface, must there be incessantly
+liquefied, and thus fall back like rain to be dissolved into gas again
+while it still is many miles above the earth's surface. This may be the
+reason why its scurrying molecules have not long ago wandered off into
+space and left the world without protection.
+
+But whether or not such liquefaction of the air now occurs in our outer
+atmosphere, there can be no question as to what must occur in its entire
+depth were we permanently shut off from the heating influence of the
+sun, as the astronomers threaten that we may be in a future age.
+Each molecule, not alone of the atmosphere, but of the entire earth's
+substance, is kept aquiver by the energy which it receives, or has
+received, directly or indirectly, from the sun. Left to itself, each
+molecule would wear out its energy and fritter it off into the
+space about it, ultimately running completely down, as surely as any
+human-made machine whose power is not from time to time restored. If,
+then, it shall come to pass in some future age that the sun's rays
+fail us, the temperature of the globe must gradually sink towards the
+absolute zero. That is to say, the molecules of gas which now fly about
+at such inconceivable speed must drop helpless to the earth; liquids
+must in turn become solids; and solids themselves, their molecular
+quivers utterly stilled, may perhaps take on properties the nature of
+which we cannot surmise.
+
+Yet even then, according to the current hypothesis, the heatless
+molecule will still be a thing instinct with life. Its vortex whirl will
+still go on, uninfluenced by the dying-out of those subordinate quivers
+that produced the transitory effect which we call temperature. For those
+transitory thrills, though determining the physical state of matter as
+measured by our crude organs of sense, were no more than non-essential
+incidents; but the vortex whirl is the essence of matter itself. Some
+estimates as to the exact character of this intramolecular motion,
+together with recent theories as to the actual structure of the
+molecule, will claim our attention in a later volume. We shall also
+have occasion in another connection to make fuller inquiry as to the
+phenomena of low temperature.
+
+
+
+
+APPENDIX
+
+  REFERENCE-LIST
+
+  CHAPTER I
+
+  THE SUCCESSORS OF NEWTON IN ASTRONOMY (1) (p. 10). An Account of Several
+  Extraordinary Meteors or Lights in the Sky, by Dr. Edmund Halley. Phil.
+  Trans. of Royal Society of London, vol. XXIX, pp. 159-162. Read before
+  the Royal Society in the autumn of 1714. (2) (p. 13). Phil. Trans. of
+  Royal Society of London for 1748, vol. XLV., pp. 8, 9. From A Letter to
+  the Right Honorable George, Earl of Macclesfield, concerning an Apparent
+  Motion observed in some of the Fixed Stars, by James Bradley, D.D.,
+  Astronomer Royal and F.R.S.
+
+  CHAPTER II
+
+  THE PROGRESS OF MODERN ASTRONOMY
+
+  (1) (p. 25). William Herschel, Phil. Trans. for 1783, vol. LXXIII. (2)
+  (p. 30). Kant's Cosmogony, ed. and trans. by W. Hartie, D.D., Glasgow,
+  900, pp. 74-81. (3) (p. 39). Exposition du systeme du monde (included in
+  oeuvres Completes), by M. le Marquis de Laplace, vol. VI., p. 498. (4)
+  (p. 48). From The Scientific Papers of J. Clerk-Maxwell, edited by W.
+  D. Nevin, M.A. (2 vols.), vol. I., pp. 372-374. This is a reprint of
+  Clerk-Maxwell's prize paper of 1859.
+
+  CHAPTER III
+
+  THE NEW SCIENCE OF PALEONTOLOGY
+
+  (1) (p. 81). Baron de Cuvier, Theory of the Earth, New York, 1818, p.
+  98. (2) (p. 88). Charles Lyell, Principles of Geology (4 vols.),
+  London, 1834. (p. 92). Ibid., vol. III., pp. 596-598. (4) (p. 100). Hugh
+  Falconer, in Paleontological Memoirs, vol. II., p. 596. (5) (p. 101).
+  Ibid., p. 598. (6) (p. 102). Ibid., p. 599. (7) (p. 111). Fossil Horses
+  in America (reprinted from American Naturalist, vol. VIII., May, 1874),
+  by O. C. Marsh, pp. 288, 289.
+
+  CHAPTER IV
+
+  THE ORIGIN AND DEVELOPMENT OF MODERN GEOLOGY
+
+  (1) (p. 123). James Hutton, from Transactions of the Royal Society of
+  Edinburgh, 1788, vol. I., p. 214. A paper on the "Theory of the Earth,"
+  read before the Society in 1781. (2) (p. 128). Ibid., p. 216. (3)
+  (p. 139). Consideration on Volcanoes, by G. Poulett Scrope, Esq., pp.
+  228-234. (4) (p. 153). L. Agassiz, Etudes sur les glaciers, Neufchatel,
+  1840, p. 240.
+
+  CHAPTER V
+
+  THE NEW SCIENCE OF METEOROLOGY
+
+  (1) (p. 182). Theory of Rain, by James Hutton, in Transactions of the
+  Royal Society of Edinburgh, 1788, vol. 1, pp. 53-56. (2) (p. 191). Essay
+  on Dew, by W. C. Wells, M.D., F.R.S., London, 1818, pp. 124 f.
+
+  CHAPTER VI
+
+  MODERN THEORIES OF HEAT AND LIGHT
+
+  (1) (p. 215). Essays Political, Economical, and Philosophical, by
+  Benjamin Thompson, Count of Rumford (2 vols.), Vol. II., pp. 470-493,
+  London; T. Cadell, Jr., and W. Davies, 1797. (2) (p. 220). Thomas Young,
+  Phil. Trans., 1802, p. 35. (3) (p. 223). Ibid., p. 36.
+
+  CHAPTER VII
+
+  THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM
+
+  (1) (p. 235). Davy's paper before Royal Institution, 1810. (2) (p. 238).
+  Hans Christian Oersted, Experiments with the Effects of the Electric
+  Current on the Magnetic Needle, 1815. (3) (p. 243). On the Induction
+  of Electric Currents, by Michael Faraday, F.R.S., Phil. Trans. of Royal
+  Society of London for 1832, pp. 126-128. (4) (p. 245). Explication of
+  Arago's Magnetic Phenomena, by Michael Faraday, F.R.S., Phil. Trans.
+  Royal Society of London for 1832, pp. 146-149.
+
+  CHAPTER VIII
+
+  THE CONSERVATION OF ENERGY
+
+  (1) (p. 267). The Forces of Inorganic Nature, a paper by Dr. Julius
+  Robert Mayer, Liebig's Annalen, 1842. (2) (p. 272). On the Calorific
+  Effects of Magneto-Electricity and the Mechanical Value of Heat, by J.
+  P. Joule, in Report of the British Association for the Advancement of
+  Science, vol. XII., p. 33.
+
+  CHAPTER IX
+
+  THE ETHER AND PONDERABLE MATTER
+
+  (1) (p. 297). James Clerk-Maxwell, Philosophical Magazine for January
+  and July, 1860.
+
+END OF VOL. III
+
+
+
+
+
+
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