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diff --git a/1707.txt b/1707.txt new file mode 100644 index 0000000..b889751 --- /dev/null +++ b/1707.txt @@ -0,0 +1,8809 @@ +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. 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