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at the earth's surface, or at the distance of one semi-diameter from its centre, is known from experiment to fall through 16 feet in the first second of its descent; and since the spaces described at different distances from the centre, are reciprocally as the squares of those distances, it may be readily computed what space a body would fall through in one second, when placed at the distance of the sun. And since the diameter of the earth's orbit is known, we can likewise easily ascertain how much this body is deflected from its tangent in one second, by the attractive power of the sun; or, which is the same, what space a body would descend through in one second towards the sun, when placed at the mean distance of the sun from it.

And we

have before seen what space would be described by a body descending towards the earth in the same time, and placed at the same distance; therefore, since the spaces are as the attractive powers, and these latter are as the masses of the attracting bodies, we have at once, by comparing the spaces, the relative proportion of the masses of the earth and sun; and then again dividing their relative masses by their absolute magnitudes, we obtain their proportional densities.

From this computation it will appear that the density of the earth is to the density of the sun, as 4 is to 1; and as the density of the earth is known from other experiments to be to the density of water, as 5 to 1; it follows, that the density of the sun is to that of common water as 13 to 1. We cannot, however, proceed in the same manner with the

other planets, because we have no means of ascertaining their respective attractive powers at their surfaces; on which account we must have recourse to their satellites, by comparing the deflection of each of them from its tangent with their respective distances from their primaries. For example, in order to find the relative densities of the Earth and Jupiter, we must first estimate how much the moon is deflected from its tangent in one second by the attractive power of the earth, and how much it would be deflected in the same time if it were placed at the same distance from the centre of the earth, as any one of Jupiter's satellites is from the centre of that planet; which distances are all known from their periodic times being given, by the second law of Kepler.

By this means, we shall have the absolute spaces described by two bodies in the same time towards the Earth and Jupiter; and these spaces, as we have before seen, being as the attractive power of the two bodies, and the attractive powers as their masses, it follows, that by comparing as above, the spaces described, we shall obtain the ratio of the masses; the division of which by their absolute magnitudes, will give us their proportional densities; from which computation it appears, that the density of Jupiter is to that of the

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earth as to 1, being a little less than the density of the sun, and a little more than that of sea

water.

It is likewise obvious, that the same method may be employed for determining the density of Saturn

and Uranus; but those planets which have no satellites, cannot be submitted to the same calculation; nor can I render the method that is made use of in these cases intelligible to you, as it requires a knowledge of some of the higher branches of mathematics; you must therefore, for the present, rest contented with the information you have acquired on this subject, which is undoubtedly as important and interesting, as any that falls under the contemplation of the human mind.

LETTER XIX.

OF THE MOTION, REFRACTION, AND ABERRATION OF LIGHT.

HAVING measured the globe of the earth, and determined the distances of the sun, moon, and planets, let us now consider the phænomena of light; a subject of no less importance than the former, and equally deserving your attention. It is in this branch of philosophy that the genius of Newton shines with uncommon lustre; and were I allowed to follow him through all his optical experiments and enquiries, I could present you with some of the most astonishing instances of human sagacity that the history of man affords. But as the nature of my plan admits not of such extensive digressions, I shall confine myself to those discoveries which are connected with astronomical observations, and leave the rest for your future

consideration.

Various opinions have been entertained concerning the nature of light. The Greeks considered it as an accident, or property, resulting from the first principles of things; and Descartes defines it to be a globulous matter, diffused through the universe; which being impelled by the sun, strikes upon our eyes, in the same man ner as a staff that is pushed at one end presses in the same instant at the other. Moses makes light to have been the first of created things; and Mil

ton, in one of the noblest invocations that poetry can boast, thus expresses the same sentiment.

"Hail, holy light, offspring of Heav'n first-born,

Or of th' Eternal co-eternal beam,

May I express thee unblam'd? since God is light,
And never but in unapproached light
Dwelt from eternity, dwelt then in thee,
Bright effluence of bright essence increate.
Or hear'st thou rather, pure ethereal stream,
Whose fountain who shall tell? before the sun,
Before the heav'ns thou wert, and, at the voice
Of God, as with a mantle didst invest

The rising world of waters dark and deep,
Won from the void and formless infinite."

The sacred author places the formation of light four days before that of the sun; and in this he appears to have been followed by all the philosophers of antiquity. It was, in those times, the general opinion, that the sun was not the source of light, but that he served to impel and spread it through space.

"Of all celestial bodies first the sun

A mighty sphere he fram'd, unlightsome first,
Though of ethereal mould: then form'd the moon
Globose, and every magnitude of stars,

And sow'd with stars the heav'n thick as a field.
First in his east the glorious lamp was seen,

Regent of day, and all th' horizon round

Invested with bright rays, jocund to run

His longitude through heav'n's high road; the gray
Dawn, and the Pleiades before him danc'd,

Shedding sweet influence."

MILTON.

This is the language of poetry, sublime and

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