the duration of the system by views similar to those which she appears to us so admirably to follow upon the earth, to preserve the individual and insure the perpetuity of the species.

Let us now look beyond the solar system. Innumerable suns, which may be the foci of as many planetary systems, are spread out in the immensity of space, and at such a distance from the earth, that the entire diameter of it, seen from their centre, is insensible. Many stars experience both in their colour and splendour, periodical variations, very remarkable; there are some which have appeared all at once, and disappeared after having for some time spread a brilliant light. What prodigious change must have operated on the surface of these great bodies, to be thus sensible at the distance which separates them from ús, and how much they must exceed those which we observe on the surface of the sun? All these bodies which are become invisible, remain in the same place where they were observed, since there was no change during the time of their appearance; there exist then in space obscure bodies as considerable, and perhaps as numerous as the stars. A luminous star, of the same density as the earth, and whose diameter should be two hundred and fifty times larger than that of the sun, would not, in consequence of its attraction, allow any of its rays to arrive at us; it is therefore possible that the largest luminous bodies in the universe, may, through this cause, be invisible. A star, which, without being of this magnitude, should yet, considerably surpass the sun, would perceptibly weaken the velocity of its light, and thus augment the extent of its aberration. This difference in the aberration of stars and their situation, observed at the moment of their transient splendour, the determination of all the changeable stars, and the periodical variations of their light; in a word, the motions peculiar to all those great bodies, which, influenced by their mutual attraction, and probably by their primitive impulses, describe immense orbits, should, relatively to the stars, be the principal objects of future astronomy.

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It appears that these stars, far from being disseminated at distances nearly equal in space, are united in various groups, each consisting of many millions of stars. Our sun, and the most brilliant stars, probably make part of one of these groups, which, seen from the point where we are, seems to encircle the heavens, and forms the milky way. The great number of stars which are seen at once in

the field of a large telescope, directed towards this way, proves its immense depth, which surpasses a thousand times the distance of Sirius from the earth; as it recedes, it terminates, by presenting the appearance of a white and continued light of small diameter; for then, the irradiation which exists even in the most powerful telescopes, covers and obscures the intervals between the stars. It is then probable, that those nebulæ, without distinct stars, are groups of stars seen from a distance, and which, if approached, would present appearances similar to the milky way.

The relative distances of the stars which form each group, are at least a hundred thousand times greater than the distance of the sun from the earth. Thus, we may judge of the prodigious extent of these groups, by the number of stars which are perceived in the milky way if we afterwards reflect on the small extent and infinite number of nebula which are separated from one another by an interval incomparably greater than the relative distance of the stars of which they are formed; the imagination, lost in the immensity of the universe, will have difficulty to conceive its bounds.

From these considerations, founded on telescopic observations, it follows, that nebulæ, which appear so well defined, that their centres can be precisely determined, are, with regard to us, the celestial objects most fixed, and those to which it is best to refer the situation of all the stars. It follows then, that the motions of the bodies of our solar system are very complicated. The moon describes an orbit nearly circular around the earth; but, seen from the sun, she describes a series of epicycloids, of which the centres are on the circumference of the terrestrial orbit. In like manner, the earth describes a series of epicycloids, of which the centres are on the arch which the sun describes around the centre of gravity of our nebulæ ; finally, the sun himself describes a series of epicycloids, of which the centres are on the arch described by the centre of gra. vity of our nebulæ around that of the universe. Astronomy has already made one great step in making us acquainted with the motion of the earth, and the series of epicycles which the moon and the satellites describe upon the orbits of the planets. It remains to deterinine the orbit of the sun, and the centre of gravity of its nebulæ ; but, if ages were necessary to become acquainted with the motions of the planetary system, what a prodigious duration of time will it require to determine the motions of the sun and stars? Observa

tion begins to render them perceptible; an attempt has been made to explain them by a change of position in the sun, indicated by its rotatory motion. Many observations are sufficiently well explained, by supposing the solar system carried towards the constellation Hercules. Other observations seem to prove, that these apparent motions of the stars are a combination of their real motion with that of the sun. Upon this subject, time will discover curious and important facts.

There still remains numerous discoveries to be made in our own system. The planet Uranus and its satellites, but lately known to us, leave room to suspect the existence of other planets, hitherto unobserved. We cannot yet determine the rotatory motion, or the flattening of many of the planets, and the greatest part of their satellites. We know not, with sufficient precision, the density of all these bodies. The theory of their motions is a series of approximations, whose convergence depends, at the same time, on the perfection of our instruments, and the progress of analysis, and which must, by these means, daily acquire new degrees of correctness. By accurate and repeated measurement, the inequalities in the figure of the earth, and the variation of weight on its surface, will be determined. The return of comets, already observed, new comets which will appear, the appearance of those, which, moving in hyperbolic orbits, can wander from system to system, the disturbance all those stars experience, and which, at the approach of a large planet, may entirely change their orbits, as is conjectured happened by the action of Jupiter on the comet of 1770; the accidents, that the proximity, and even the shock of these bodies, may occasion in the planets, and in the satellites; in a word, the changes which the motions of the solar system experience with respect to the stars; such are the principal objects which the system presents to astronomical researches, and future geometricians.

Contemplated as one grand whole, astronomy is the most beautiful monument of the human mind; the noblest record of its intelligence. Seduced by the illusions of the senses, and of selflove, man considered himself for a long time as the centre of the motion of the celestial bodies, and his pride was justly punished by the vain terrors they inspired. The labour of many ages has at length withdrawn the veil which covered the system. Man appears, upon a small planet, almost imperceptible in the vast extent of the

solar system, itself only an insensible point in the immensity of space. The sublime results to which this discovery has led, may console him for the limited place assigned him in the universe. Let us carefully preserve, and even augment the number of these sublime discoveries, which form the delight of thinking beings.

They have rendered important services to navigation and astronomy; but their great benefit has been the having dissipated the alarms occasioned by extraordinary celestial phænomena, and destroyed the errors springing from the ignorance of our true relation with nature; errors so much the more fatal, as social order can only rest in the basis of these relations. TRUTH, JUSTICE; these are its immutable laws. Far from us be the dangerous maxim, that it is sometimes useful to mislead, to deceive, and enslave mankind, to insure their happiness. Cruel experience has at all times proved, that with impunity these sacred laws can never be infringed. [La Place, Systême du Monde.]

CHAP. XXV.

HISTORY OF SYSTEMATICAL PHSYSICS,

THOUGH We commonly give the appellation of systems to the dif ferent suppositions by which Ptolemy, Copernicus, and Tycho Brahe, have endeavoured to account for the course of the heavens, it is not what we now mean by general and systematical physics. We are upon a philosophy which undertakes to explain the profound construction of the whole universe. The project is noble: four or five celebrated philosophers have employed themselves in it; they have made numerous parties, and many disputes. The history of their pretensions may determine us in the choice of the best side, or in remaining entirely neuter.

Epicurus, reviving the ideas of Leucippus and Democritus, thought he very well comprehended that particles of matter different in form, having subsisted from all eternity, had, after a certain time, linked themselves to one another in the vacuum; that some pro

ceeding in straight lines, and others in curved, fell into different clusters, and formed bodies and spirits; that the free agency of man was, above all, the work of atoms which moved in a declining line; thus chance made the sun, peopled the earth, established the order which subsists in it *; and framed, out of one and the same paste, the world, and the intelligent being, which is the spectator of it; that we are not to imagine the sun was made to light us, or our eyes to

* Atheism does not appear to have extended so far as is here conceived: and it is due to the character of Epicurus, and is demanded of us by the candour of genuine philosophy, to observe, that the common opinion that this great supporter of the atomic sect supposed the visible universe to have been produced by chance, has no foundation in any part of his writings that have descended to our day, and is in many passages most peremptorily contradicted by them. "Whom," says Epicurus, in a letter to Menæceus, preserved in Diogenes Laertius, "do you believe to be more excellent than he who piously reveres the gods, who feels no dread of death, and rightly estimates the design of nature? Such a man does not, with the multitude, regard CHANCE as a God, for he knows that God can never act at random; nor as a contingent cause of events; nor does he conceive that from any such power flows the good or the evil that attempers the real happiness of man," So, in another passage of another letter," Believe, before all things, that God is an immortal and blessed being; as indeed common sense should teach us concerning God. Conceive nothing of him that is repugnant to blessedness and immortality, and admit every thing that is consistent with these perfections,"

In common, however, with Aristotle and most of the other philosophers of Greece, Epicurus conceived that perfect rest and quiet were essentially necessary to the perfect happiness of the great Creator; and he hence supposed that all the phænomena of the natural, and all the events of the moral world, are perpetually taking place without any direct interference or general providence of the Deity, since this, in his opinion, would be to produce a disturbance in the calm of his essential felicity; but at the same time that they do not take place at random, or by chance or accident, but under the control of a general system of laws established at the beginning, and on the formation of the world. To which opinion he expressly alludes in the following passage from another letter preserved by Diogenes Laertius: "Think not that the different motions and revolutions of the heavens; the rising, setting, eclipses, and other phænomena of the planets are produced by the immediate control, superintendance, or ministration of him who possesses all immortality and beatitude; it is from the immutable laws which they received at the beginning, in the creation of the universe, that they inflexibly fulfil their various circuits!

For a further account of this subject we refer the reader to Mr. Good's elaborate examination of it in his Life of Lucretius, prefixed to his translation of the NATURE OF THINGS, to which indeed we are indebted for these ex, tracts.-Editor.