The World's Greatest Books - Volume 15 Part 12

_The Conclusion_

Since all things, both argument and ocular demonstration, show that the blood pa.s.ses 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 circ.u.mference 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 quant.i.ty 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 nebulae 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 pa.s.sion, than the perfect readiness with which all its conclusions are a.s.sented 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 cla.s.ses, 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 pa.s.sing 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 s.p.a.ce, 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 princ.i.p.al laminae 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 s.p.a.ce 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 pa.s.sing 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 a.n.a.lysis 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 pa.s.sed 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 lat.i.tudes.

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 s.p.a.ce_

When examined with powerful telescopes, the const.i.tution 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 cl.u.s.tering 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 a.s.sociated 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 mult.i.tudes 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 mult.i.tude and excessive crowding of the smallest visible magnitudes, and of glare produced by the aggregate light of mult.i.tudes 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 s.p.a.ce-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 s.p.a.ces, 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 s.p.a.ce-penetrating power of telescopes far within that which distance alone, apart from such obscuration, would a.s.sign.

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 circ.u.mference our view is limited by this sort of cosmical veil, which extinguishes the smaller magnitudes, cuts off the nebulous light of distant ma.s.ses, 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 ma.s.ses and clouds of stars, which the telescope at last refuses to a.n.a.lyse.

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 s.p.a.ce, 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 l.u.s.tre, 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 a.s.signed 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 l.u.s.tre, 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 Ca.s.siopeia; 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 surpa.s.sed 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 const.i.tuting which may be called, to distinguish them from double stars, which are only optically double, binary stars.

That which since then has been most a.s.siduously 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 s.p.a.ce 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 pa.s.sage 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 Nebulae_

It is to Sir William Herschel that we owe the most complete a.n.a.lysis of the great variety of those objects which are generally cla.s.sed as nebulae. 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 nebulae can, of course, only be considered as cl.u.s.ters 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 cl.u.s.ters 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 phosph.o.r.escent 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.

a.s.suming 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 cl.u.s.ter of stars.

Among the mult.i.tude of nebulae 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 princ.i.p.alities of Bayreuth and Ans.p.a.ch 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 princ.i.p.al 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 t.i.tle 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 a.s.semblage of empirical specialities. Science only begins for man from the moment when his mind lays hold of matter--when he tries to subject the ma.s.s acc.u.mulated by experience to rational combinations.