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THE known quantity of matter bears a very small proportion to the immensity of space. Large as the bodies are, the distances which separate them are immeasurably greater; but as design is manifest in every part of creation, it is probable that, if the various systems in the universe had been nearer to one another, their mutual disturbances would have been inconsistent with the harmony and stability of the whole. It is clear that space is not pervaded by atmospheric air, since its resistance would, long ere this, have destroyed the velocity of the planets; neither can we affirm it to be a void, since it is replete with ether, and traversed in all directions by light, heat, gravitation, and possibly by influences whereof we can form no idea.
Whatever the laws may be that obtain in the more distant regions of creation, we are assured that one alone regulates the motions not only of our own system, but also the binary systems of the fixed stars; and as general laws form the ultimate object of philosophical research, we cannot conclude these remarks without considering the nature of gravitation -that extraordinary power whose effects we have been endeavouring to trace through some of their mazes. It was at one time imagined that the
acceleration in the moon's mean motion was occasioned by the successive transmission of the gravitating force; but it has been proved that, in order to produce this effect, its velocity must be about fifty millions of times greater than that of light, which flies at the rate of 200000 miles in a second its action, even at the distance of the sun, may therefore be regarded as instantaneous; yet so remote are the nearest of the fixed stars, that it may be doubted whether the sun has any sensible influence on them.
The curves in which the celestial bodies move by the force of gravitation are only lines of the second order; the attraction of spheroids, according to any other law of force than that of gravitation, would be much more complicated; and as it is easy to prove that matter might have been moved according to an infinite variety of laws, it may be concluded that gravitation must have been selected by Divine Wisdom out of an infinity of others, as being the most simple, and that which gives the greatest stability to the celestial motions.
It is a singular result of the simplicity of the laws of nature, which admit only of the observation and comparison of ratios, that the gravitation and theory of the motions of the celestial bodies are independent of their absolute magnitudes and
distances; consequently, if all the bodies of the solar system, their mutual distances, and their velocities, were to diminish proportionally, they would describe curves in all respects similar to those in which they now move; and the system might be successively reduced to the smallest sensible dimensions, and still exhibit the same appearances. We learn by experience that a very different law of attraction prevails when the particles of matter are placed within inappreciable distances from each other, as in chemical and capillary attraction and the attraction of cohesion: whether it be a modification of gravity, or that some new and unknown power comes into action, does not appear; but as a change in the law of the force takes place at one end of the scale, it is possible that gravitation may not remain the same throughout every part of space. Perhaps the day may come when even gravitation, no longer regarded as an ultimate principle, may be resolved into a yet more general cause, embracing every law that regulates the material world.
The action of the gravitating force is not impeded by the intervention even of the densest substances. If the attraction of the sun for the centre of the earth, and of the hemisphere diametrically opposite to him, were diminished by a difficulty in penetrating the interposed matter, the
tides would be more obviously affected. Its attraction is the same also, whatever the substances of the celestial bodies may be; for if the action of the sun upon the earth differed by a millionth part from his action upon the moon, the difference would occasion a periodical variation in the moon's parallax whose maximum would be the of a second, and also a variation in her longitude amounting to several seconds, a supposition proved to be impossible, by the agreement of theory with observation. Thus all matter is pervious to gravitation, and is equally attracted by it.
As far as human knowledge extends, the intensity of gravitation has never varied within the limits of the solar system; nor does even analogy lead us to expect that it should; on the contrary, there is every reason to be assured that the great laws of the universe are immutable, like their Author. Not only the sun and planets, but the minutest particles, in all the varieties of their attractions and repulsions, nay, even the imponderable mat. ter of the electric, galvanic, or magnetic fluid,— are all obedient to permanent laws, though we may not be able in every case to resolve their phenomena into general principles. Nor can we suppose the structure of the globe alone to be exempt from the universal fiat, though ages may pass before the changes it has undergone, or that
are now in progress, can be referred to existing causes with the same certainty with which the motions of the planets, and all their periodic and secular variations, are referable to the law of gravitation. The traces of extreme antiquity perpetually occurring to the geologist give that information as to the origin of things in vain looked for in the other parts of the universe. They date the beginning of time with regard to our system; since there is ground to believe that the formation of the earth was contemporaneous with that of the rest of the planets; but they show that creation is the work of Him with whom' a thousand years are as one day, and one day as a thousand years.'
It thus appears that the theory of dynamics, founded upon terrestrial phenomena, is indispensable for acquiring a knowledge of the revolutions of the celestial bodies and their reciprocal influences. The motions of the satellites are affected by the forms of their primaries, and the figures of the planets themselves depend upon their rotations. The symmetry of their internal structure proves the stability of these rotatory motions, and the immutability of the length of the day, which furnishes an invariable standard of time; and the actual size of the terrestrial spheroid affords the means of ascertaining the dimensions