Finally, as in all bodies re-action is equal to action, it is certain that the earth gravitates also towards the moon; and that the sun gravitates towards both.  That every one of the satellites of Saturn gravitates towards the other four, and the other four towards it; all five towards Saturn, and Saturn towards all.  That it is the same with regard to Jupiter; and that all these globes are attracted by the sun, which is reciprocally attracted by them.

This power of gravitation acts proportionably to the quantity of matter in bodies, a truth which Sir Isaac has demonstrated by experiments.  This new discovery has been of use to show that the sun (the centre of the planetary system) attracts them all in a direct ratio of their quantity of matter combined with their nearness.  From hence Sir Isaac, rising by degrees to discoveries which seemed not to be formed for the human mind, is bold enough to compute the quantity of matter contained in the sun and in every planet; and in this manner shows, from the simple laws of mechanics, that every celestial globe ought necessarily to be where it is placed.

His bare principle of the laws of gravitation accounts for all the apparent inequalities in the course of the celestial globes.  The variations of the moon are a necessary consequence of those laws.  Moreover, the reason is evidently seen why the nodes of the moon perform their revolutions in nineteen years, and those of the earth in about twenty-six thousand.  The several appearances observed in the tides are also a very simple effect of this attraction.  The proximity of the moon, when at the full, and when it is new, and its distance in the quadratures or quarters, combined with the action of the sun, exhibit a sensible reason why the ocean swells and sinks.

After having shown by his sublime theory the course and inequalities of the planets, he subjects comets to the same law.  The orbit of these fires (unknown for so great a series of years), which was the terror of mankind and the rock against which philosophy split, placed by Aristotle below the moon, and sent back by Descartes above the sphere of Saturn, is at last placed in its proper seat by Sir Isaac Newton.

He proves that comets are solid bodies which move in the sphere of the sun’s activity, and that they describe an ellipsis so very eccentric, and so near to parabolas, that certain comets must take up above five hundred years in their revolution.

The learned Dr. Halley is of opinion that the comet seen in 1680 is the same which appeared in Julius Caesar’s time.  This shows more than any other that comets are hard, opaque bodies; for it descended so near to the sun, as to come within a sixth part of the diameter of this planet from it, and consequently might have contracted a degree of heat two thousand times stronger than that of red-hot iron; and would have been soon dispersed in vapour, had it not been a firm, dense body.  The guessing the course