CHAP. XXIV.

ON THE FUTURE PROGRESS OF ASTRONOMY.

FROM

ROM the preceding observations, it has sufficiently appeared that the immense globe of the sun, the focus of all the motions of the heavenly bodies, revolves upon its axis in twenty-five days and a half. Its surface is covered with an ocean of luminous matter, whose active effervescence forms variable spots, often very numerous, and sometimes larger than the earth. Above this ocean exists an immense atmosphere, in which the planets, with their satellites, move, in orbits nearly circular, and in planes little inclined to the ecliptic. Innumerable comets, after having approached the sun, remove to distances, which evince that his empire extends beyond the known limits of the planetary system. This luminary not only acts by its attraction upon all these globes, and compels them to move around him, but imparts to them both light and heat; his benign influence gives birth to the animals and plants which cover the surface of the earth, and analogy induces us to believe, that it produces similar effects on the planets; for, it is not natural to suppose that matter, of which we see the fecundity develop itself in such various ways, should be sterile upon a planet so large as Jupiter, which, like the earth, has its days, its nights, and his years, and on which observation discovers changes that indicate very active forces. Man, formed for the temperature which he enjoys upon the earth, could not, according to all appearance, live upon the other planets; but ought there not to be a diversity of organization suited to the various temperatures of the globes of this universe? If the difference of elements and climates alone, causes such variety in the productions of the earth, how infinitely diversified must be the productions of the planets and their satellites? The most active imagination cannot form any just idea of them, but still their existence is extremely probable.

However arbitrary the system of the planets may be, there exists between them some very remarkable relations, which may throw

light on their origin; considering them with attention, we are astonished to see all the planets move round the Sun from west to east, and nearly in the same plane all the satellites moving round their respective planets in the same direction, and nearly in the same plane with the planets. Lastly, the sun, the planets, and those satellites in which a motion of rotation has been observed, turn on their own axis, in the same direction, and nearly in the same plane as their motion of projection.

A phenomenon so extraordinary, is not the effect of chance; it indicates an universal cause, which has determined all these motions. To approximate somewhat to the probable explanation of this cause, we should observe that the planetary system, such as we now consider it, is composed of seven planets, and fourteen satellites. We have observed the rotation of the sun, of five planets, of the moon, of Saturn's ring, and of his farthest satellite; these motions with those of revolution, form together thirty direct movements, in the same direction. If we conceive the plane of any direct motion whatever, coinciding at first with that of the ecliptic, afterwards inclining itself towards this last plane, and passing over all the degrees of inclination, from zero to half the circumference; it is clear that the motion will be direct in all its inferior inclinations to a hundred degrees, and that it will be retrograde in its inclination beyond that; so that, by the change of inclination alone, the direct and retrograde motions of the solar system, can be represented. Beheld in this point of view, we may reckon twenty-nine motions, of which the planes are inclined to that of the earth, at most 4th of the circumference; but, supposing their inclinations had been the effect of chance, they would have extended to half the circumference, and the probability that one of them would have exceeded the quarter, would be 1-, or 136870311. It is then extremely probable, that the direction of the planetary motion is not the effect of chance, and this becomes still more probable, if we consider that the incli nation of the greatest number of these motions to the ecliptic, is very small, and much less than a quarter of the circumference.

Another phenomenon of the solar system equally remarkable, is the small eccentricity of the orbits of the planets and their satellites, while those of comets are much extended. The orbits of the system offer no intermediate shades between a great and small excentricity. We are here again compelled to acknowledge the effect

of a regular cause; chance alone could not have given a form nearly circular, to the orbits of all the planets. This cause then must also have influenced the great eccentricity of the orbits of comets, and, what is very extraordinary, without having any influence on the direction of their motion; for, in observing the orbits of retrograde comets, as being inclined more than 100° to the ecliptic, we find that the mean inclination of the orbits of all the observed comets, approaches near to 100°, which would be the case if the bodies had been projected at random.

Thus, to investigate the cause of the primitive motions of the planets, we have given the five following phænomena: 1st, The motions of planets in the same direction, and nearly in the same plane. 2d, The motion of their satellites in the same direction, and nearly in the same plane with those of the planets. 3d, The motion of rotation of these different bodies, and of the sun in the same direction as their motion of projection, and in planes but little different. 4th, The small eccentricity of the orbits of the planets, and of their satellites. 5th, The great eccentricity of the orbits of comets, although their inclinations may have been left to chance.

Buffon is the only one whom I have known, who, since the discovery of the true system of the world, has endeavoursd to investigate the origin of the planets, and of their satellites. He supposes that a comet, in falling from the sun, may have driven off a torrent of matter, which united itself at a distance, into various globes, greater or smaller, and more or less distant from this luminary. These globes are the planets and satellites, which, by their cooling, are become opaque and solid.

This hypothesis accounts for the first of the five preceding phæ nomena; for, it is clear that all bodies thus formed, must move nearly in the plane which passes through the centre of the sun, and in the direction of the torrent of matter which produces them. The four other phænomena appears to me inexplicable by his theory. In fact, the absolute motion of the particles of a planet would then be in the same direction of the motion of its centre of gravity; but it does not follow that the rotation of the planet would be in the same direction. Thus, the earth may turn from west to east, and yet the absolute direction of each of its particles may be from east to west. What I say of the rotatory motion of the planets, is equally applicable to the motion of their satellites in their orbits, of which

the direction in the hypothesis he adopts, is not necessarily the same with the projectile motion of the planets.

The small eccentricity in the motion of the planetary orbits, is not only very difficult to explain on this hypothesis, but the phenomenon contradicts it. We know by the theory of central forces, that if a body moving in an orbit round the sun, touched the surface of this luminary, it would uniformly return to it at the completion of each revolution; from whence it follows, that if the planets had originally been detached from the Sun, they would have touched it at every revolution, and their orbits, far from being circular, would be very eccentric. It is true, that a torrent of matter, sent off from the sun, cannot correctly be compared to a globe which touches its surface. The impulse which the particles of this torrent receive from one another, and the reciprocal attraction exercised among them, may change the direction of their motion, and increase their perihelion distances; but their orbits would uniformly become very eccentric, or at least it must be a very extraordinary chance that would give them eccentricities so small as those of the planets. In a word, we do not see, in this hypothesis of Buffon, why the orbits of about eighty comets, already observed, are all very elliptical. This hypothesis, then, is far from accounting for the preceding phænomena. Let us see if it is possible to arrive at their true cause.

Whatever be the nature of this cause, since it has produced or directed the motion of the planets and their satellites, it must have embraced all these bodies; and considering the prodigious distance which separates them, it can only be a fluid of immense extent. To have given in the same direction, a motion nearly circular round the sun, this fluid must have surrounded the luminary like an atmosphere. This view, therefore, of planetary motion, leads us to think, that in consequence of excessive heat, the atmosphere of the sun originally extended beyond the orbits of all the planets, and that it has gradually contracted itself to its present limits, which may have taken place from causes similar to those which made the famous star that suddenly appeared in 1572, in the constellation Cassiopeia, shine with the most brilliant splendour during many months.

The great eccentricity of the orbits of comets, leads to the same result; it evidently indicates the disappearance of a great number

of orbits less eccentric, which intimates an atmosphere round the sun, extending beyond the perihelion of observable comets; and which, in destroying the motion of those which have traversed it in a duration of such extent, has re-united them to the sun. Thus, we see that there can at present only exist such comets as were beyond this limit at that period. And as we can observe only those which in their perihelion approach near the sun, their orbits must be very eccentric: but, at the same time, it is evident that their inclinations must present the same inequalities as if the bodies had been sent off at random, since the solar atmosphere has no influence over their motions. Thus, the long period of the revolu tions of comets, the great eccentricity of their orbits, and the variety of their inclinations, are very naturally explained by means of this atmosphere.

But how has it determined the motions of revolution and rotation of the planets? If these bodies had penetrated this fluid, its resistance would have caused them to fall into the sun. We may then 'conjecture, that they have been formed at the successive bounds of this atmosphere, by the condensation of zones, which it must have abandoned in the plane of its equator, and in becoming cold have condensed themselves towards the surface of this luminary. One may likewise conjecture, that the satellites have been formed in a similar way by the atmosphere of the planets. The five phænomena, explained above, naturally result from this hypothesis, to which the rings of Saturn add an additional degree of probability.

Whatever may have been the origin of this arrangement of the planetary system, which I offer with that distrust which every thing ought to inspire that is not the result of observation or calculation; it is certain that its elements are so arranged, that it must possess the greatest stability, if future observations do not disturb it. Through this cause alone, that the motions of planets and satellites are nearly circular, and impelled in the same direction, and in planes differing but little from each other, it arises that this system can only oscillate to a certain extent, from which its deviation must be extremely limited; the mean motions of rotation and revolution of these different bodies are uniform, and their mean distances to the foci of the principal forces which animate them, are uniform. It seems that nature has disposed every thing in the heavens to insure