Preliminary Discourse on the Study of Natural Philosophy

by John F. W. Herschel

(307.) Since Hipparchus, at various periods in the history of astronomy, catalogues of stars have been formed, among which that of Ulugh Begh, comprising about 1000 stars, constructed in 1437, is remarkable as the production of a sovereign prince, working personally in conjunction with his astronomers; and that of Tycho Brahe, containing 777 stars, constructed in 1600, as having originated in a phenomenon similar to that which drew the attention of Hipparchus. In more recent times, astronomers provided with the finest instruments their respective eras could supply, and established in observatories, munificently endowed by the sovereigns and governments of different European nations, have vied and are still vying with each other, in extending the number of registered stars, and giving the utmost possible degree of accuracy to the determination of their places. Among these, it would be ungrateful not to claim especial notice for the superb series of observations which, under a succession of indefatigable and meritorious astronomers, has, for a very long period, continued to emanate from our own national observatory of Greenwich.

(308.) The distance of the fixed stars is so immense, that every attempt to assign a limit, within which it must fall, has hitherto failed. The enquiries of astronomers of all ages have been directed to ascertain this distance, by taking the dimensions of our own particular system of sun and planets, or of the earth itself, as the unit of a scale on which it might be measured. But although many have imagined that their observations afforded grounds for the decision of this interesting point, it has uniformly happened either that the phenomena on which they relied have proved to be referable to other causes not previously known, and which the superior accuracy of their researches has for the first time brought to light; or to errors arising from instrumental imperfections and unavoidable defects of the observations themselves.

(309.) The only indication we can expect to obtain of the actual distance of a star, would consist in an annual change in its apparent place corresponding to the motion of the earth round the sun, called its annual parallax, and which is nothing more than the measure of the apparent size of the earth’s orbit as seen from the star. Many observers have thought they have detected a measurable amount of this parallax; but as astronomical instruments have advanced in perfection, the quantity which they have successively assigned to it has been continually reduced within narrower and narrower limits, and has invariably been commensurate with the errors to which the instruments used might fairly be considered liable. The conclusion this strongly presses on us is, that it is really a quantity too small to admit of distinct measurement in the present state of our means for that purpose; and that, therefore, the distance of the stars must be a magnitude of such an order as the imagination almost shrinks from contemplating. But this increase in our scale of dimension calls for a corresponding enlargement of conception in all other respects. The same reasoning which places the stars at such immeasurable remoteness, exalts them at the same time into glorious bodies, similar to, and even far surpassing, our own sun, the centres perhaps of other planetary systems, or fulfilling purposes of which we can have no idea, from any analogy in what passes immediately around us.

(310.) The comparison of catalogues, published at different periods, has given occasion to many curious remarks, respecting changes both of place and brightness among the stars, to the discovery of variable ones which lose and recover their lustre periodically, and to that of the disappearance of several from the heavens so completely as to have left no vestige discernible even by powerful telescopes. In proportion as the construction of astronomical and optical instruments has gone on improving, our knowledge of the contents of the heavens has undergone a corresponding extension, and, at the same time, attained a degree of precision which could not have been anticipated in former ages. The places of all the principal stars in the northern hemisphere, and of a great many in the southern, are now known to a degree of nicety which must infallibly detect any real motions that may exist among them, and has in fact done so, in a great many instances, some of them very remarkable ones.

(311.) It is only since a comparatively recent date, however, that any great attention has been bestowed on the smaller stars, among which there can be no doubt of the most interesting and instructive phenomena being sooner or later brought to light. The minute examination of them with powerful telescopes, and with delicate instruments for the determination of their places, has, indeed, already produced immense catalogues and masses of observations, in which thousands of stars invisible to the naked eye are registered; and has led to the discovery of innumerable important and curious facts, and disclosed the existence of whole classes of celestial objects, of a nature so wonderful as to give room for unbounded speculation on the extent and construction of the universe.

(312.) Among these, perhaps the most remarkable are the revolving double stars, or stars which, to the naked eye or to inferior telescopes, appear single; but, if examined with high magnifying powers, are found to consist of two individuals placed almost close together, and which, when carefully watched, are (many of them) found to revolve in regular elliptic orbits about each other; and so far as we have yet been able to ascertain, to obey the same laws which regulate the planetary movements. There is nothing calculated to give a grander idea of the scale on which the sidereal heavens are constructed than these beautiful systems. When we see such magnificent bodies united in pairs, undoubtedly by the same bond of mutual gravitation which holds together our own system, and sweeping over their enormous orbits, in periods comprehending many centuries, we admit at once that they must be accomplishing ends in creation which will remain for ever unknown to man; and that we have here attained a point in science where the human intellect is compelled to acknowledge its weakness, and to feel that no conception the wildest imagination can form will bear the least comparison with the intrinsic greatness of the subject.

Geology.

(313.) The researches of physical astronomy are confessedly incompetent to carry us back to the origin of our system, or to a period when its state was, in any great essential, different from what it is at present. So far as the causes now in action go, and so far as our calculations will enable us to estimate their effects, we are equally unable to perceive in the general phenomena of the planetary system either the evidence of a beginning, or the prospect of an end. Geometers, as already stated, have demonstrated that, in the midst of all the fluctuations which can possibly take place in the elements of the orbits of the planets, by reason of their mutual attraction, the general balance of the parts of the system will always be preserved, and every departure from a mean state periodically compensated. But neither the researches of the physical astronomer, nor those of the geologist, give us any ground for regarding our system, or the globe we inhabit, as of eternal duration. On the contrary, there are circumstances in the physical constitution of our own planet which at least obscurely point to an origin and a formation, however remote, since it has been found that the figure of the earth is not globular but elliptical, and that its attraction is such as requires us to admit the interior to be more dense than the exterior, and the density to increase with some degree of regularity from the surface towards the centre, and that, in layers arranged elliptically round the centre, circumstances which could scarcely happen without some such successive deposition of materials as would enable pressure to be propagated with a certain degree of freedom from one part of the mass to another, even if we should hesitate to admit a state of primitive fluidity.

(314.) But from such indications nothing distinct can be concluded; and if we would speculate to any purpose on a former state of our globe and on the succession of events which from time to time may have changed the condition and form of its surface, we must confine our views within limits far more restricted, and to subjects much more within the reach of our capacity, than either the creation of the world or its assumption of its present figure. These, indeed, were favourite speculations with a race of geologists now extinct; but the science itself has undergone a total change of character, even within the last half century, and is brought, at length, effectually within the list of the inductive sciences. Geologists now no longer bewilder their imaginations with wild theories of the formation of the globe from chaos, or its passage through a series of hypothetical transformations, but rather aim at a careful and accurate examination of the records of its former state, which they find indelibly impressed on the great features of its actual surface, and to the evidences of former life and habitation which organised remains imbedded and preserved in its strata indisputably afford.

(315.) Records of this kind are neither few nor vague; and though the obsoleteness of their language when we endeavour to interpret it too minutely, may, and no doubt often does, lead to misapprehension, still its general meaning is, on the whole, unequivocal and satisfactory. Such records teach us, in terms too plain to be misunderstood, that the whole or nearly the whole of our present lands and continents were formerly at the bottom of the sea, where they received deposits of materials from the wearing and degradation of other lands not now existing, and furnished receptacles for the remains of marine animals and plants inhabiting the ocean above them, as well as for similar spoils of the land washed down into its bosom.