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To admit

of the integrant particles? this hypothesis we must suppose much more space empty than full in all bodies, so that the density of their particles must be incomparably greater than the mean density of their whole volume. A spherical particle of one hundred thousandth of a foot in diameter, should have a density at least ten thousand millions of times greater than the mean density of the Earth, to exert at its surface an attraction equal to the terrestial gravity. But the attrac tive forces of bodies greatly surpass this gravity, since they inflect light, whose direction is not sensibly changed by the attraction of the Earth; the density of these particles therefore should be to that of bodies in a ratio, which the imagination would fear to admit, if their affinities depended on the law of universal gravitation. The ratio of the intervals, which separate the particles of bodies, to their respective dimensions, would be of the same order, as relatively to stars which form a nebula, which in this point of view may be consi

The

dered as a great luminous body. There is no reason, however, which absolutely forbids us to consider all bodies in this manner. Many phenomena are favourable to the supposition, particularly the extreme facility with which light penetrates diaphonous bodies in all directions. affinities would then depend on the form of the integrant particles, and we might then by the variety of these forms, explain all the variety of attractive forces, and reduce to one general law all the phenomena of astronomy and natural philosophy.

But the impossibility of ascertaining these figures, renders this investigation useless to the advancement of science. Some geometricians, to account for these affinities, have added to the laws of attraction, inversely as the squares of the distance, new terms which are insensible at small distances, but these terms would be the expressions of as many different forces, and besides being complicated with the figures of the particles, they would only complicate the explanation of the phenomena.

Amidst these uncertainties the wisest plan seems to be, to endeavour to determine by numberless experiments the laws of affinities, and to effect this, the most simple method appears to be, by comparing these forces with the repulsive force of heat, which may itself be compared with that of gravity. Some experiments already made with this view, afford us reason to hope that one day these laws will be perfectly known, and that then, by the application of analysis, the philosophy of terrestrial bodies may be brought to the same degree of perfection, which the discovery of universal gravitation has procured to astronomy.

THE

SYSTEM OF THE WORLD.

BOOK V.

Of the History of Astronomy.

THE order in which I have treated the principal results of the system of the world, is not that which the human intellect has followed in the investigation. Its progress has been embarrassed and uncertain. Frequently mankind have not arrived at the true cause of these phenomena, till all the hypotheses which imagination could suggest have been exhausted; and the truths that have been discovered, have almost always been combined with errors, which time and observation only have separated. I shall comprise in a small compass, an outline of these attempts

We shall see that

and their success. astronomy remained during a great many ages in its infancy, that it increased and flourished in the school of Alexandria, became then stationary till the time of the Arabs, who improved it by their observations, and lastly that it is within the three last centuries, it has rapidly risen to that state of perfection, in which we behold it at the present day.

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