Chapter VI: Part 6

And now I may be allowed to give in brief my view of the circulation of the blood, and to propose it for general adoption.

_The Conclusion_

Since all things, both argument and ocular demonstration, show that the blood passes through the lungs and heart by the action of the ventricles; and is sent for distribution to all parts of the body, where it makes its way into the veins and pores of the flesh; and then flows by the veins from the circumference on every side to the centre, from the lesser to the greater veins; and is by them finally discharged into the _vena cava_ and right auricle of the heart, and this in such a quantity or in such a flux and reflux, thither by the arteries, hither by the veins, as cannot possibly be supplied by the ingesta, and is much greater than can be required for mere purposes of nutrition; therefore, it is absolutely necessary to conclude that the blood in the animal body is impelled in a circle and is in a state of ceaseless motion; and that this is the act, or function, which the heart performs by means of its pulse, and that it is the sole and only end of the motion and contraction of the heart. For it would be very difficult to explain in any other way to what purpose all is constructed and arranged as we have seen it to be.

SIR JOHN HERSCHEL

Outlines of Astronomy

Sir John Frederick William Herschel, only child--and, as an
astronomer, almost the only rival--of Sir William Herschel, was
born at Slough, in Ireland, on March 7, 1792. At first privately
educated, in 1813 he graduated from St. John's College, Cambridge,
as senior wrangler and first Smith's prizeman. He chose the law as
his profession; but in 1816 reported that, under his father's
direction, he was going "to take up stargazing." He then began a
re-examination of his father's double stars. In 1825 he wrote that
he was going to take nebulæ under his especial charge. He embarked
in 1833 with his family for the Cape; and his work at Feldhausen,
six miles from Cape Town, marked the beginning of southern sidereal
astronomy. The result of his four years' work there was published
in 1847. From 1855 he devoted himself at Collingwood to the
collection and revival of his father's and his own labours. His
"Outlines of Astronomy," published in 1849, and founded on an
earlier "Treatise on Astronomy" of 1833, was an outstanding
success. Herschel's long and happy life, every day of which added
its share to his scientific services, came to an end on May 11,
1871.

_I.--The Wonders of the Milky Way_

There is no science which draws more largely than does astronomy on that intellectual liberality which is ready to adopt whatever is demonstrated or concede whatever is rendered highly probable, however new and uncommon the points of view may be in which objects the most familiar may thereby become placed. Almost all its conclusions stand in open and striking contradiction with those of superficial and vulgar observation, and with what appears to everyone the most positive evidence of his senses.

There is hardly anything which sets in a stronger light the inherent power of truth over the mind of man, when opposed by no motives of interest or passion, than the perfect readiness with which all its conclusions are assented to as soon as their evidence is clearly apprehended, and the tenacious hold they acquire over our belief when once admitted.

If the comparison of the apparent magnitude of the stars with their number leads to no immediately obvious conclusion, it is otherwise when we view them in connection with their local distribution over the heavens. If indeed we confine ourselves to the three or four brightest classes, we shall find them distributed with a considerable approach to impartiality over the sphere; a marked preference, however, being observable, especially in the southern hemisphere, to a zone or belt passing through _epsilon_ Orionis and _alpha_ Crucis. But if we take in the whole amount visible to the naked eye we shall perceive a great increase of numbers as we approach the borders of the Milky Way. And when we come to telescopic magnitudes we find them crowded beyond imagination along the extent of that circle and of the branches which it sends off from it; so that, in fact, its whole light is composed of nothing but stars of every magnitude from such as are visible to the naked eye down to the smallest points of light perceptible with the best telescopes.

These phenomena agree with the supposition that the stars of our firmament, instead of being scattered indifferently in all directions through space, form a stratum of which the thickness is small in comparison with its length and breadth; and in which the earth occupies a place somewhere about the middle of its thickness and near the point where it subdivides into two principal laminæ inclined at a small angle to each other. For it is certain that to an eye so situated the apparent density of the stars, supposing them pretty equally scattered through the space they occupy, would be least in the direction of the visual ray perpendicular to the lamina, and greatest in that of its breadth; increasing rapidly in passing from one to the other direction, just as we see a slight haze in the atmosphere thickening into a decided fog-bank near the horizon by the rapid increase of the mere length of the visual ray.

Such is the view of the construction of the starry firmament taken by Sir William Herschel, whose powerful telescopes first effected a complete analysis of this wonderful zone, and demonstrated the fact of its entirely consisting of stars.

So crowded are they in some parts of it that by counting the stars in a single field of his telescope he was led to conclude that 50,000 had passed under his review in a zone two degrees in breadth during a single hour's observation. The immense distances at which the remoter regions must be situated will sufficiently account for the vast predominance of small magnitudes which are observed in it.

The process of gauging the heavens was devised by Sir William Herschel for this purpose. It consisted simply in counting the stars of all magnitudes which occur in single fields of view, of fifteen minutes in diameter, visible through a reflecting telescope of 18 inches aperture, and 20 feet focal length, with a magnifying power of 180 degrees, the points of observation being very numerous and taken indiscriminately in every part of the surface of the sphere visible in our latitudes.

On a comparison of many hundred such "gauges," or local enumerations, it appears that the density of starlight (or the number of stars existing on an average of several such enumerations in any one immediate neighbourhood) is least in the pole of the Galactic circle [_i.e._, the great circle to which the course of the Milky Way most nearly conforms: _gala_ = milk], and increases on all sides down to the Milky Way itself, where it attains its maximum. The progressive rate of increase in proceeding from the pole is at first slow, but becomes more and more rapid as we approach the plane of that circle, according to a law from which it appears that the mean density of the stars in the galactic circle exceeds, in a ratio of very nearly 30 to 1, that in its pole, and in a proportion of more than 4 to 1 that in a direction 15 degrees inclined to its plane.

As we ascend from the galactic plane we perceive that the density decreases with great rapidity. So far we can perceive no flaw in this reasoning if only it be granted (1) that the level planes are continuous and of equal density throughout; and (2) that an absolute and definite limit is set to telescopic vision, beyond which, if stars exist, they elude our sight, and are to us as if they existed not. It would appear that, with an almost exactly similar law of apparent density in the two hemispheres, the southern were somewhat richer in stars than the northern, which may arise from our situation not being precisely in the middle of its thickness, but somewhat nearer to its northern surface.

_II.--Penetrating Infinite Space_

When examined with powerful telescopes, the constitution of this wonderful zone is found to be no less various than its aspect to the naked eye is irregular. In some regions the stars of which it is composed are scattered with remarkable uniformity over immense tracts, while in others the irregularity of their distribution is quite as striking, exhibiting a rapid succession of closely clustering rich patches separated by comparatively poor intervals, and indeed in some instances absolutely dark and _completely_ void of any star even of the smallest telescopic magnitude. In some places not more than 40 or 50 stars on an average occur in a "gauge" field of 15 minutes, while in others a similar average gives a result of 400 or 500.

Nor is less variety observable in the character of its different regions in respect of the magnitude of the stars they exhibit, and the proportional numbers of the larger and smaller magnitudes associated together, than in respect of their aggregate numbers. In some, for instance, extremely minute stars, though never altogether wanting, occur in numbers so moderate as to lead us irresistibly to the conclusion that in these regions we are _fairly through_ the starry stratum, since it is impossible otherwise (supposing their light not intercepted) that the numbers of the smaller magnitudes should not go on increasing _ad infinitum_.

In such cases, moreover, the ground of the heavens, as seen between the stars, is for the most part perfectly dark, which again would not be the case if innumerable multitudes of stars, too minute to be individually discernible, existed beyond. In other regions we are presented with the phenomenon of an almost uniform degree of brightness of the individual stars, accompanied with a very even distribution of them over the ground of the heavens, both the larger and smaller magnitudes being strikingly deficient. In such cases it is equally impossible not to perceive that we are looking through a sheet of stars nearly of a size and of no great thickness compared with the distance which separates them from us. Were it otherwise we should be driven to suppose the more distant stars were uniformly the larger, so as to compensate by their intrinsic brightness for their greater distance, a supposition contrary to all probability.

In others again, and that not infrequently, we are presented with a double phenomenon of the same kind--_viz._, a tissue, as it were, of large stars spread over another of very small ones, the intermediate magnitudes being wanting, and the conclusion here seems equally evident that in such cases we look through two sidereal sheets separated by a starless interval.

Throughout by far the larger portion of the extent of the Milky Way in both hemispheres the general blackness of the ground of the heavens on which its stars are projected, and the absence of that innumerable multitude and excessive crowding of the smallest visible magnitudes, and of glare produced by the aggregate light of multitudes too small to affect the eye singly, which the contrary supposition would appear to necessitate, must, we think, be considered unequivocal indications that its dimensions, _in directions where those conditions obtain_, are not only not infinite, but that the space-penetrating power of our telescopes suffices fairly to pierce through and beyond it.

It is but right, however, to warn our readers that this conclusion has been controverted, and that by an authority not lightly to be put aside, on the ground of certain views taken by Olbers as to a defect of perfect transparency in the celestial spaces, in virtue of which the light of the more distant stars is enfeebled more than in proportion to their distance. The extinction of light thus originating proceeding in geometrical ratio, while the distance increases in arithmetical, a limit, it is argued, is placed to the space-penetrating power of telescopes far within that which distance alone, apart from such obscuration, would assign.

It must suffice here to observe that the objection alluded to, if applicable to any, is equally so to every part of the galaxy. We are not at liberty to argue that at one part of its circumference our view is limited by this sort of cosmical veil, which extinguishes the smaller magnitudes, cuts off the nebulous light of distant masses, and closes our view in impenetrable darkness; while at another we are compelled, by the clearest evidence telescopes can afford, to believe that star-strewn vistas _lie open_, exhausting their powers and stretching out beyond their utmost reach, as is proved by that very phenomenon which the existence of such a veil would render impossible--_viz._, infinite increase of number and diminution of magnitude, terminating in complete irresolvable nebulosity.

Such is, in effect, the spectacle afforded by a very large portion of the Milky Way in that interesting region near its point of bifurcation in Scorpio, where, through the hollows and deep recesses of its complicated structure, we behold what has all the appearance of a wide and indefinitely prolonged area strewed over with discontinuous masses and clouds of stars, which the telescope at last refuses to analyse. Whatever other conclusions we may draw, this must anyhow be regarded as the direction of the greatest linear extension of the ground-plan of the galaxy. And it would appear to follow also that in those regions where that zone is clearly resolved into stars well separated and _seen projected on a black ground_, and where, by consequence, it is certain, if the foregoing views be correct, that we look out beyond them into space, the smallest visible stars appear as such not by reason of excessive distance, but of inferiority of size or brightness.

_III.--Variable, Temporary and Binary Stars_

Wherever we can trace the law of periodicity we are strongly impressed with the idea of rotatory or orbitual motion. Among the stars are several which, though in no way distinguishable from others by any apparent change of place, nor by any difference of appearance in telescopes, yet undergo a more or less regular periodical increase and diminution of lustre, involving in one or two cases a complete extinction and revival. These are called periodic stars. The longest known, and one of the most remarkable, is the star _Omicron_ in the constellation Cetus (sometimes called Mira Ceti), which was first noticed as variable by Fabricius in 1596. It appears about twelve times in eleven years, remains at its greatest brightness about a fortnight, being then on some occasions equal to a large star of the second magnitude, decreases during about three months, till it becomes completely invisible to the naked eye, in which state it remains about five months, and continues increasing during the remainder of its period. Such is the general course of its phases. But the mean period above assigned would appear to be subject to a cyclical fluctuation embracing eighty-eight such periods, and having the effect of gradually lengthening and shortening alternately those intervals to the extent of twenty-five days one way and the other. The irregularities in the degree of brightness attained at the maximum are also periodical.

Such irregularities prepare us for other phenomena of stellar variation which have hitherto been reduced to no law of periodicity--the phenomena of temporary stars which have appeared from time to time in different parts of the heavens blazing forth with extraordinary lustre, and after remaining awhile, apparently immovable, have died away and left no trace. In the years 945, 1264, and 1572 brilliant stars appeared in the region of the heavens between Cepheus and Cassiopeia; and we may suspect them, with Goodricke, to be one and the same star with a period of 312, or perhaps 156 years. The appearance of the star of 1572 was so sudden that Tycho Brahe, a celebrated Dutch astronomer, returning one evening from his laboratory to his dwellinghouse, was surprised to find a group of country people gazing at a star which he was sure did not exist half an hour before. This was the star in question. It was then as bright as Sirius, and continued to increase till it surpassed Jupiter when brightest, and was visible at midday. It began to diminish in December of the same year, and in March 1574 had entirely disappeared.

In 1803 it was announced by Sir William Herschel that there exist sidereal systems composed of two stars revolving about each other in regular orbits, and constituting which may be called, to distinguish them from double stars, which are only optically double, binary stars. That which since then has been most assiduously watched, and has offered phenomena of the greatest interest, is _gamma Virginis_. It is a star of the vulgar third magnitude, and its component individuals are very nearly equal, and, as it would seem, in some slight degree variable. It has been known to consist of two stars since the beginning of the eighteenth century, the distance being then between six and seven seconds, so that any tolerably good telescope would resolve it. When observed by Herschel in 1780 it was 5.66 seconds, and continued to decrease gradually and regularly, till at length, in 1836, the two stars had approached so closely as to appear perfectly round and single under the highest magnifying power which could be applied to most excellent instruments--the great refractor of Pulkowa alone, with a magnifying power of a thousand, continuing to indicate, by the wedge-shaped form of the disc of the star, its composite nature.

By estimating the ratio of its length to its breadth, and measuring the former, M. Struve concludes that at this epoch the distance of the two stars, centre from centre, might be stated at .22 seconds. From that time the star again opened, and is now again a perfectly easily separable star. This very remarkable diminution, and subsequent increase, of distance has been accompanied by a corresponding and equally remarkable increase and subsequent diminution of relative angular motion. Thus in 1783 the apparent angular motion hardly amounted to half a degree per annum; while in 1830 it had decreased to 5 degrees, in 1834 to 20 degrees, in 1835 to 40 degrees, and about the middle of 1836 to upwards of 70 degrees per annum, or at the rate of a degree in five days.

This is in entire conformity with the principles of dynamics, which establish a necessary connection between the angular velocity and the distance, as well in the apparent as in the real orbit of one body revolving about another under the influence of mutual attraction; the former varying inversely as the square of the latter, in both orbits, whatever be the curve described and whatever the law of the attractive force.

It is not with the revolutions of bodies of a planetary or cometary nature round a solar centre that we are concerned; it is that of sun round sun--each perhaps, at least in some binary systems, where the individuals are very remote and their period of revolution very long, accompanied by its train of planets and their satellites, closely shrouded from our view by the splendour of their respective suns, and crowded into a space bearing hardly a greater proportion to the enormous interval which separates them than the distances of the satellites of our planets from their primaries bear to their distances from the sun itself.

A less distinctly characterised subordination would be incompatible with the stability of their systems and with the planetary nature of their orbits. Unless close under the protecting wing of their immediate superior, the sweep of their other sun, in its perihelion passage round their own, might carry them off or whirl them into orbits utterly incompatible with conditions necessary for the existence of their inhabitants.

_IV.--The Nebulæ_

It is to Sir William Herschel that we owe the most complete analysis of the great variety of those objects which are generally classed as nebulæ. The great power of his telescopes disclosed the existence of an immense number of these objects before unknown, and showed them to be distributed over the heavens not by any means uniformly, but with a marked preference to a certain district extending over the northern pole of the galactic circle. In this region, occupying about one-eighth of the surface of the sphere, one-third of the entire nebulous contents of the heavens are situated.

The resolvable nebulæ can, of course, only be considered as clusters either too remote, or consisting of stars intrinsically too faint, to affect us by their individual light, unless where two or three happen to be close enough to make a joint impression and give the idea of a point brighter than the rest. They are almost universally round or oval, their loose appendages and irregularities of form being, as it were, extinguished by the distance, and only the general figure of the condensed parts being discernible. It is under the appearance of objects of this character that all the greater globular clusters exhibit themselves in telescopes of insufficient optical power to show them well.

The first impression which Halley and other early discoverers of nebulous objects received from their peculiar aspect was that of a phosphorescent vapour (like the matter of a comet's tail), or a gaseous and, so to speak, elementary form of luminous sidereal matter. Admitting the existence of such a medium, Sir W. Herschel was led to speculate on its gradual subsidence and condensation, by the effect of its own gravity, into more or less regular spherical or spheroidal forms, denser (as they must in that case be) towards the centre.

Assuming that in the progress of this subsidence local centres of condensation subordinate to the general tendency would not be wanting, he conceived that in this way solid nuclei might arise whose local gravitation still further condensing, and so absorbing the nebulous matter each in its immediate neighbourhood, might ultimately become stars, and the whole nebula finally take on the state of a cluster of stars.

Among the multitude of nebulæ revealed by his telescope every stage of this process might be considered as displayed to our eyes, and in every modification of form to which the general principle might be conceived to apply. The more or less advanced state of a nebula towards its segregation into discrete stars, and of these stars themselves towards a denser state of aggregation round a central nucleus, would thus be in some sort an indication of age.

ALEXANDER VON HUMBOLDT

Cosmos, a Sketch of the Universe

Frederick Henry Alexander von Humboldt was born in Berlin on
September 14, 1769. In 1788 he made the acquaintance of George
Forster, one of Captain Cook's companions, and geological
excursions made with him were the occasion of his first
publications, a book on the nature of basalt. His work in the
administration of mines in the principalities of Bayreuth and
Anspach furnished materials for a treatise on fossil flora; and in
1827, when he was residing in Paris, he gave to the world his
"Voyage to the Equinoctial Regions of the New Continent," which
embodies the results of his investigations in South America. Two
years later he organised an expedition to Asiatic Russia, charging
himself with all the scientific observations. But his principal
interest lay in the accomplishment of that physical description of
the universe for which all his previous studies had been a
preparation, and which during the years 1845 to 1848 appeared under
the comprehensive title of "Cosmos, or Sketch of a Physical
Description of the Universe." Humboldt died on May 6, 1859.

_I.--The Physical Study of the World_

The natural world may be opposed to the intellectual, or nature to art taking the latter term in its higher sense as embracing the manifestations of the intellectual power of man; but these distinctions--which are indicated in most cultivated languages--must not be suffered to lead to such a separation of the domain of physics from that of the intellect as would reduce the physics of the universe to a mere assemblage of empirical specialities. Science only begins for man from the moment when his mind lays hold of matter--when he tries to subject the mass accumulated by experience to rational combinations.

Science is mind applied to nature. The external world only exists for us so far as we conceive it within ourselves, and as it shapes itself within us into the form of a contemplation of nature. As intelligence and language, thought and the signs of thought, are united by secret and indissoluble links, so, and almost without our being conscious of it, the external world and our ideas and feelings melt into each other. "External phenomena are translated," as Hegel expresses it in his "Philosophy of History," "in our internal representation of them." The objective world, thought by us, reflected in us, is subjected to the unchanging, necessary, and all-conditioning forms of our intellectual being.

The activity of the mind exerts itself on the elements furnished to it by the perceptions of the senses. Thus, in the youth of nations there manifests itself in the simplest intuition of natural facts, in the first efforts made to comprehend them, the germ of the philosophy of nature.

If the study of physical phenomena be regarded in its bearings not on the material wants of man, but on his general intellectual progress, its highest result is found in the knowledge of those mutual relations which link together the general forces of nature. It is the intuitive and intimate persuasion of the existence of these relations which at once enlarges and elevates our views and enhances our enjoyment. Such extended views are the growth of observation, of meditation, and of the spirit of the age, which is ever reflected in the operations of the human mind whatever may be their direction.

From the time when man, in interrogating nature, began to experiment or to produce phenomena under definite conditions, and to collect and record the fruits of his experience--so that investigation might no longer be restricted by the short limits of a single life--the philosophy of nature laid aside the vague and poetic forms with which she had at first been clothed, and has adopted a more severe character.

The history of science teaches us how inexact and incomplete observations have led, through false inductions, to that great number of erroneous physical views which have been perpetuated as popular prejudices among all classes of society. Thus, side by side with a solid and scientific knowledge of phenomena, there has been preserved a system of pretended results of observation, the more difficult to shake because it takes no account of any of the facts by which it is overturned.

This empiricism--melancholy inheritance of earlier times--invariably maintains whatever axioms it has laid down; it is arrogant, as is everything that is narrow-minded; while true physical philosophy, founded on science, doubts because it seeks to investigate thoroughly--distinguishes between that which is certain and that which is simply probable--and labours incessantly to bring its theories nearer to perfection by extending the circle of observation. This assemblage of incomplete dogmas bequeathed from one century to another, this system of physics made up of popular prejudices, is not only injurious because it perpetuates error with all the obstinacy of ill-observed facts, but also because it hinders the understanding from rising to the level of great views of nature.

Instead of seeking to discover the _mean_ state around which, in the midst of apparent independence and irregularity, the phenomena really and invariably oscillate, this false science delights in multiplying apparent exceptions to the dominion of fixed laws, and seeks, in organic forms and the phenomena of nature, other marvels than those presented by internal progressive development, and by regular order and succession. Ever disinclined to recognise in the present the analogy of the past, it is always disposed to believe the order of nature suspended by perturbations, of which it places the seat, as if by chance, sometimes in the interior of the earth, sometimes in the remote regions of space.

_II.--The Inductive Method_

The generalisation of laws which were first applied to smaller groups of phenomena advances by successive gradations, and their empire is extended, and their evidence strengthened, so long as the reasoning process is directed to really analogous phenomena. Empirical investigation begins by single perceptions, which are afterwards classed according to their analogy or dissimilarity. Observation is succeeded at a much later epoch by experiment, in which phenomena are made to arise under conditions previously determined on by the experimentalist, guided by preliminary hypotheses, or a more or less just intuition of the connection of natural objects and forces.

The results obtained by observation and experiment lead by the path of induction and analogy to the discovery of empirical laws, and these successive phases in the application of human intellect have marked different epochs in the life of nations. It has been by adhering closely to this inductive path that the great mass of facts has been accumulated which now forms the solid foundation of the natural sciences.

Two forms of abstraction govern the whole of this class of knowledge--_viz._, the determination of quantitative relations, according to number and magnitude; and relations of quality, embracing the specific properties of heterogeneous matter.

The first of these forms, more accessible to the exercise of thought, belongs to the domain of mathematics; the other, more difficult to seize, and apparently more mysterious, to that of chemistry. In order to submit phenomena to calculation, recourse is had to a hypothetical construction of matter by a combination of molecules and atoms whose number, form, position, and polarity determine, modify, and vary the phenomena.

We are yet very far from the time when a reasonable hope could be entertained of reducing all that is perceived by our senses to the unity of a single principle; but the partial solution of the problem--the tendency towards a general comprehension of the phenomena of the universe--does not the less continue to be the high and enduring aim of all natural investigation.

_III.--Distribution of Matter in Space_

A physical cosmography, or picture of the universe, should begin, not with the earth, but with the regions of space--the distribution of matter in the universe.

We see matter existing in space partly in the form of rotating and revolving spheroids, differing greatly in density and magnitude, and partly in the form of self-luminous vapour dispersed in shining nebulous spots or patches. The nebulæ present themselves to the eye in the form of round, or nebulous discs, of small apparent magnitude, either single or in pairs, which are sometimes connected by a thread of light; when their diameters are greater their forms vary--some are elongated, others have several branches, some are fan-shaped, some annular, the ring being well defined and the interior dark. They are supposed to be undergoing various and progressive changes of form, as condensation proceeds around one or more nuclei in conformity with the laws of gravitation. Between two and three thousand of such unresolvable nebulæ have already been counted, and their positions determined.

If we leave the consideration of the attenuated vaporous matter of the immeasurable regions of space, whether existing in a dispersed state as a cosmical ether without form or limits, or in the shape of nebulæ, and pass to those portions of the universe which are condensed into solid spheres or spheroids, we approach a class of phenomena exclusively designated as stars or as the sidereal universe. Here, too, we find different degrees of solidity or density in the agglomerated matter.

If we compare the regions of space to one of the island-studded seas of our planet, we may imagine we see matter distributed in groups, whether of unresolvable nebulæ of different ages condensed around one or more nuclei, or in clusters of stars, or in stars scattered singly. Our cluster of stars, or the island in space to which we belong, forms a lens-shaped, flattened, and everywhere detached stratum, whose major axis is estimated at seven or eight hundred, and its minor axis at a hundred and fifty times, the distance of Sirius. If we assume that the parallax of Sirius does not exceed that accurately determined for the brightest stars in Centaur (0.9128 sec.), it will follow that light traverses one distance of Sirius in three years, while nine years and a quarter are required for the transmission of the light of the star 61 Cygni, whose considerable proper motion might lead to the inference of great proximity.

Our cluster of stars is a disc of comparatively small thickness divided, at about a third its length, into two branches; we are supposed to be near this division, and nearer to the region of Sirius than to that of the constellation of the Eagle; almost in the middle of the starry stratum in the direction of its thickness.

The place of our solar system and the form of the whole lens are inferred from a kind of scale--_i.e._, from the different number of stars seen in equal telescopic fields of view. The greater or less number of stars measures the relative depth of the stratum in different directions; giving in each case, like the marks on a sounding-line, the comparative length of visual ray required to reach the bottom; or, more properly, as above and below do not here apply, the outer limit of the sidereal stratum.

In the direction of the major axis, where the greater number of stars are placed behind each other, the remoter ones appear closely crowded together, and, as it were, united by a milky radiance, and present a zone or belt projected on the visible celestial vault. This narrow belt is divided into branches; and its beautiful, but not uniform brightness, is interrupted by some dark places. As seen by us on the apparent concave celestial sphere, it deviates only a few degrees from a great circle, we being near the middle of the entire starry cluster, and almost in the plane of the Milky Way. If out planetary system were far outside the cluster, the Milky Way would appear to telescopic vision as a ring, and at a still greater distance as a resolvable disc-shaped nebula.

_IV.--On Earth History_

The succession and relative age of different geological formations are traced partly by the order of superposition of sedimentary strata, of metamorphic beds, and of conglomerates, but most securely by the presence of organic remains and their diversities of structure. In the fossiliferous strata are inhumed the remains of the floras and faunas of past ages. As we descend from stratum to stratum to study the relations of superposition, we ascend in the order of time, and new worlds of animal and vegetable existence present themselves to the view.

In our ignorance of the laws under which new organic forms appear from time to time upon the surface of the globe, we employ the expression "new creations" when we desire to refer to the historical phenomena of the variations which have taken place at intervals in the animals and plants that have inhabited the basins of the primitive seas and the uplifted continents.

It has sometimes happened that extinct species have been preserved entire, even to the minutest details of their tissues and articulations. In the lower beds of the Secondary Period, the lias of Lyme Regis, a sepia has been found so wonderfully preserved that a part of the black fluid with which the animal was provided myriads of years ago to conceal itself from its enemies has actually served at the present time to draw its picture. In other cases such traces alone remain as the impression which the feet of animals have left on wet sand or mud over which they passed when alive, or the remains of their undigested food (coprolites).

The analytical study of the animal and vegetable kingdoms of the primitive world has given rise to two distinct branches of science; one purely morphological, which occupies itself in natural and physiological descriptions, and in the endeavour to fill up from extinct forms the chasms which present themselves in the series of existing species; the other branch, more especially geological considers the relations of the fossil remains to the superposition and relative age of the sedimentary beds in which they are found. The first long predominated; and the superficial manner which then prevailed of comparing fossil and existing species led to errors of which traces still remain in the strange denominations which were given to certain natural objects. Writers attempted to identify all extinct forms with living species, as, in the sixteenth century, the animals of the New World were confounded by false analogies with those of the Old.

In studying the relative age of fossils by the order of superposition of the strata in which they are found, important relations have been discovered between families and species (the latter always few in numbers) which have disappeared and those which are still living. All observations concur in showing that the fossil floras and faunas differ from the present animal and vegetable forms the more widely in proportion as the sedimentary beds to which they belong are lower, or more ancient.

Thus great variations have successively taken place in the general types of organic life, and these grand phenomena, which were first pointed out by Cuvier, offer numerical relations which Deshayes and Lyell have made the object of researches by which they have been conducted to important results, especially as regards the numerous and well-preserved fossils of the Tertiary formation. Agassiz, who has examined 1,700 species of fossil fishes, and who estimates at 8,000 the number of living species which have been described, or which are preserved in our collections, affirms that, with the exception of one small fossil fish peculiar to the argillaceous geodes of Greenland, he has never met in the Transition, Secondary, or Tertiary strata with any example of this class specifically identical with any living fish; and he adds the important remark that even in the lower Tertiary formations a third of the fossil fishes of the _calcaire grossier_ and of the London clay belong to extinct families.

We have seen that fishes, which are the oldest vertebrates, first appear in the Silurian strata, and are found in all the succeeding formations up to the birds of the Tertiary Period. Reptiles begin in like manner in the magnesian limestone, and if we now add that the first mammalia are met with in Oolite, the Stonefield slate; and that the first remains of birds have been found in the deposits of the cretaceous period, we shall have indicated the inferior limits, according to our present knowledge, of the four great divisions of the vertebrates.

In regard to invertebrate animals, we find corals and some shells associated in the oldest formations with very highly organised cephalopodes and crustaceans, so that widely different orders of this part of the animal kingdom appear intermingled; there are, nevertheless, many isolated groups belonging to the same order in which determinate laws are discoverable. Whole mountains are sometimes found to consist of a single species of fossil goniatites, trilobites, or nummulites.

Where different genera are intermingled, there often exists a systematic relation between the series of organic forms and the superposition of the formations; and it has been remarked that the association of certain families and species follows a regular law in the superimposed strata of which the whole constitutes one formation. It has been found that the waters in the most distant parts of the globe were inhabited at the same epochs by testaceous animals corresponding, at least in generic character, with European fossils.

Strata defined by their fossil contents, or by the fragments of other rocks which they include, form a geological horizon by which the geologist may recognise his position, and obtain safe conclusions in regard to the identity or relative antiquity of formations, the periodical repetition of certain strata--their parallelism--or their entire suppression. If we would thus comprehend in its greatest simplicity the general type of the sedentary formations, we find in proceeding successively from below upwards: (1) The Transition group, including the Silurian and Devonian (Old Red Sandstone) systems; (2) the Lower Trias, comprising mountain limestone, the coal measures, the lower new red sandstone, and the magnesian limestone; (3) the Upper Trias, composing the bunter, or variegated sandstone, the muschelkalk, and the Keuper sandstone; (4) the Oolitic, or Jurassic series, including Lias; (5) the Cretaceous series; (6) the Tertiary group, as represented in its three stages by the _calcaire grossier_ and other beds of the Paris basin, the lignites, or brown coal of Germany, and the sub-Apennine group of Italy.

To these succeed transported soils (_alluvium_), containing the gigantic bones of ancient mammalia, such as the mastodons, the dinotherium, and the megatheroid animals, among which is the mylodon of Owen, an animal upwards of eleven feet in length, allied to the sloth. Associated with these extinct species are found the fossil remains of animals still living: elephants, rhinoceroses, oxen, horses, and deer. Near Bogota, at an elevation of 8,200 French feet above the level of the sea, there is a field filled with the bones of mastodon (_Campo de Gigantes_), in which I have had careful excavations made. The bones found on the table-lands of Mexico belong to the true elephants of extinct species. The minor range of the Himalaya, the Sewalik hills, contain, besides numerous mastodons, the sivatherium and the gigantic land-tortoise (_Colossochelys_), more than twelve feet in length and six in height, as well as remains belonging to still existing species of elephants, rhinoceroses, and giraffes. It is worthy of notice that these fossils are found in a zone which enjoys the tropical climate supposed to have prevailed at the period of the mastodons.

_V.--The Permanence of Science_

It has sometimes been regarded as a discouraging consideration that, while works of literature being fast-rooted in the depths of human feeling, imagination and reason suffer little from the lapse of time, it is otherwise with works which treat of subjects dependent on the progress of experimental knowledge. The improvement of instruments, and the continued enlargement of the field of observation, render investigations into natural phenomena and physical laws liable to become antiquated, to lose their interest, and to cease to be read.

Let none who are deeply penetrated with a true and genuine love of nature, and with a lively appreciation of the true charm and dignity of the study of her laws, ever view with discouragement or regret that which is connected with the enlargement of the boundaries of our knowledge. Many and important portions of this knowledge, both as regards the phenomena of the celestial spaces and those belonging to our own planet, are already based on foundations too firm to be lightly shaken; although in other portions general laws will doubtless take the place of those which are more limited in their application, new forces will be discovered, and substances considered as simple will be decomposed, while others will become known.

JAMES HUTTON

The Theory of the Earth

James Hutton, the notable Scotch geologist, was born at Edinburgh
on June 3, 1726. In 1743 he was apprenticed to a Writer to the
Signet; but his apprenticeship was of short duration and in the
following year he began to study medicine at Edinburgh University,
and in 1749 graduated as an M.D. Later he determined to study
agriculture, and went, in 1752, to live with a Norfolk farmer to
learn practical farming. He did not devote himself entirely to
agriculture, but gave a considerable amount of his time to chemical
and geological researches. His geological researches culminated in
his great work, "The Theory of the Earth," published at Edinburgh
in 1795. In this work he propounds the theory that the present
continents have been formed at the bottom of the sea by the
precipitation of the detritus of former continents, and that the
precipitate had been hardened by heat and elevated above the sea by
the expansive power of heat. He died on March 26, 1797. Other works
are his "Theory of Rain," "Elements of Agriculture," "Natural
Philosophy," and "Nature of Coal."

_I.--Origin and Consolidation of the Land_

The solid surface of the earth is mainly composed of gravel, of calcareous, and argillaceous strata. Sand is separated by streams and currents, gravel is formed by the attrition of stones agitated in water, and argillaceous strata are deposited by water containing argillaceous material. Accordingly, the solid earth would seem to have been mainly produced by water, wind, and tides, and this theory is confirmed by the discovery that all the masses of marble and limestone are composed of the calcareous matter of marine bodies. All these materials were, in the first place, deposited at the bottom of the sea, and we have to consider, firstly, how they were consolidated; and secondly, how they came to be dry land, elevated above the sea.

It is plain that consolidation may have been effected either through the concretion of substances dissolved in water or through fusion by fire. Consolidation through the concretion of substances dissolved in the sea is unlikely, for, in the first place, there are strata, such as siliceous matter, which are insoluble, and which could not therefore have been in solution; and, in the second place, the appearance of the strata is contrary to this supposition. Consolidation was probably effected by heat and fusion. All the substances in the earth may be rendered fluid by heat, and all the appearances in the earth's crust are consistent with the consolidation and crystallisation of fused substances. Not only so, but we find rents and separations and veins in the strata, such as would naturally occur in strata consolidated by the cooling of fused masses, and other phenomena pointing to fusion by heat. We may conclude, then, that all the solid strata of the globe have been hardened from a state of fusion.

But how were these strata raised up from the bottom of the sea and transformed into dry land? Even as heat was the consolidating power, so heat was also probably the elevating power. The power of heat for the expansion of bodies is, as we know, unlimited, and the expansive power of heat was certainly competent to raise the strata above the sea. Heat was certainly competent, and if we examine the crust of the earth we find evidence that heat was used.

If the strata cemented by the heat of fusion were created by the expansive power of heat acting from below, we should 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. And this is just what we do find. From horizontal, the strata are frequently found vertical; from continuous, broken, and separated in every possible direction; and from a plane, bent and doubled. The theory is confirmed by an examination of the veins and fissures of the earth which contain matter foreign to the strata they traverse, and evidently forced into them as a fluid under great pressure. Active volcanoes, and extinct volcanoes, and the marks everywhere of volcanic action likewise support the theory of expansion and elevation by heat. A volcano is not made on purpose to frighten superstitious people into fits of piety and devotion; it is to be considered as a spiracle of a subterranean furnace.

Such being the manner of the formation of the crust of the world, can we form any judgment of its duration and durability? If we could measure the rate of the attrition of the present continents, we might estimate the duration of the older continents whose attrition supplied the material for the present dry land. But as we cannot measure the wearing-away of the land, we can merely state generally, first, that the present dry land required an indefinitely long period for its formation; second, that the previous dry land which supplied material for its formation required equal time to make; third, that there is at present land forming at the bottom of the sea which in time will appear above the surface; fourth, that we find no vestige of a beginning, or of an end.

Granite has in its own nature no claim to originality, for it is found to vary greatly in its composition. But, further, it is certain that granite, or a species of the same kind of stone, is found stratified. It is the _granit feuilletée_ of M. de Sauffure, and, if I mistake not, is called _gneiss_ by the Germans. Granite being thus found stratified, the masses of this stone cannot be allowed to any right of priority over the schistus, its companion in Alpine countries.

Lack of stratification, then, cannot be considered a proof of primitive rock. Nor can lack of organized bodies, such as shells, in these rocks, be considered a proof; for the traces of organized bodies may be obliterated by the many subsequent operations of the mineral region. In any case, signs of organized bodies are sometimes found in "primitive" mountains.

Nor can metallic veins, found plentifully in "primitive" mountains, prove anything, for mineral veins are found in various strata.

We maintain that _all_ the land was produced from fused substances elevated from the bottom of the sea. But we do not hold that all parts of the earth have undergone exactly similar and simultaneous vicissitudes; and in respect to the changes which various parts of the land have undergone we may distinguish between primary and secondary strata. Nothing is more certain than that there have been several repeated operations of the mineralising power exerted upon the strata in particular places, and all those mineral operations tend to consolidation. It is quite possible that "primitive" masses which differ from the ordinary strata of the globe have been twice subjected to mineral operations, having been first consolidated and raised as land, and then submerged in order to be again fused and elevated.

_II.--The Nature of Mineral Coal_

Mineral, or fossil, coal is a species of stratum distinguished by its inflammable and combustible nature. We find that it differs in respect to its purity, and also in respect to its inflammability. As is well known, some coals have almost no earthy ash, some a great deal; and, again, some coals burn with much smoke and fire, while others burn like coke. Where, then, did coal come from, and how can we account for its different species?

A substance proper for the formation of coaly matter is found in vegetable bodies. But how did it become mixed with earthy matter?

Vegetable bodies may be resolved into bituminous or coaly matter either by means of fire or by means of water. Both may be used by nature in the formation of coal.

By the force of subterranean heat vegetable matter may have been charred at the bottom of the sea, and the oleaginous, bituminous, and fuliginous substances diffused through the sea as a result of the burning may have been deposited at the bottom of the sea as coal. Further, the bituminous matter from the smoke of vegetable substances burned on land would ultimately be deposited from the atmosphere and settle at the bottom of the sea.

Many of the rivers contain in solution an immense quantity of inflammable vegetable substance, and this is carried into the sea, and precipitated there.

From these two sources, then, the sea gets bituminous material, and this material, condensed and consolidated by compression and by heat, at the bottom of the sea, would form a black body of a most uniform structure, breaking with a polished surface, and burning with more or less smoke or flame in proportion as it be distilled less or more by subterranean heat. And such a body exactly represents our purest fossil coal, which gives the most heat and leaves the least ash.

In some cases the bituminous material in suspension in the sea would be mixed more or less with argillaceous, calcareous, and other earthy substances; and these being precipitated along with the bituminous matter would form layers of impure coal with a considerable amount of ash.

But there is still a third source of coal. Vegetable bodies macerated in water, and consolidated by compression, form a body almost indistinguishable from some species of coal, as is seen in peat compressed under a great load of earth; and there can be no doubt that coal sometimes originates in this way, for much fossil coal shows abundance of vegetable bodies in its composition.

There remains only to consider the change in the disposition of coal strata. Coal strata, which had been originally in a horizontal position, are now found sometimes standing erect, even perpendicular. This, also, is consistent with our theory of the earth. Indeed, there is not a substance in the mineral kingdom in which the action of subterranean heat is better shown. These strata are evidently a deposit of inflammable substances which all come originally from vegetable bodies. In this stage of their formation they must all contain volatile oleaginous constituents. But some coal strata contain no volatile constituents, and the disappearance of the volatile oleaginous substances must have been produced by distillation, proceeding perhaps under the restraining force of immense compression.