VOL. XXIII.] PHILOSOPHICAL TRANSACTIONS. 11 



that the ancient philosophers, besides their knowledge of the true system of 

 the universe, were not unacquainted with the real and physical causes of the 

 planets motions, and that universal law of nature from which they all spring, 

 viz. that matter attracts matter in a reciprocal proportion of the square of the 

 distance. 



In the first book the author treats of the order, motions, and periods of the 

 primary planets, and the chief appearances that arise from thence : as their di- 

 rections, stations, retrogradations, &c. the different phases of the inferior, like 

 to those of the moon, the eclipses of both sun and moon; and then he proves 

 that the primary planets turn round the sun; and that the direction of their 

 gravity is not towards the earth, but towards the sun, and that it is by that 

 force of gravity, that they are kept from going out in right lines, and made to 

 turn round in orbits; that this force of gravity decreases in each of them in the 

 same proportion, that the square of the distance from the sun or centre in- 

 creases; as likewise, that all the secondary planets, in moving round their 

 primary, are kept in their orbits by the force of gravity, whose direction is 

 towards the centres of their respective primaries; and that the accelerating force 

 of this gravity is in a reciprocal proportion of the square of the distance ; and 

 that in the moon it is the very same with the gravity of bodies towards our 

 earth. Now, because we know by observations, that the planets move not in 

 circles, but in ellipses, in one of whose foci the sun is placed, there must ne- 

 cessarily be a particular law of gravity acting towards the sun, to make the 

 planet move in such curves. This he inquires into, and finds it to be the 

 very same with what he had before discovered, by comparing the distances of 

 the planets from the sun with their periodical times. 



Afterwards, in the 7 th section, he proves, that this law of attraction is uni- 

 versal in all matter to matter, and that it is the very same with gravity ; and 

 that the sun and planets mutually gravitating towards one another, must neces- 

 sarily turn round their common centre of gravity, which by reason of the vast 

 magnitude of the sun, in respect of the rest, cannot be far from the sun*s own 

 centre. Now, since by this law of attraction, the moon must not only be 

 drawn towards the earth, but also towards the sun, and that differently, ac- 

 cording as its distance and position varies; he from thence, and the inclination 

 of its orbit to the plane of the ecliptic, gives the reason why the nodes move 

 backwards; and on the same account, there being as it were a redundant ring 

 of matter about the earth's equator, he shows how the equinoctial points must 

 necessarily constantly go back a little, and make a precession of the equinox. 



Having thus settled the true causes of the celestial motions, he next inquires 

 into the various opinions of philosophers on that subject, and examines the 



c 2 



