9t» DISSERTATION SECOND. [part n. 



sion was obtained for the law of gravity, involving both the 

 conditions on which it must depend, the quantity of matter 

 in the gravitating bodies, and the distance at which the 

 bodies were placed. There could be no doubt that this 

 tendency was always mutual, as there appeared nowhere 

 any exception to the rule that action and reaction are 

 equal ; so that if a stone gravitated to the earth, the earth 

 gravitated equally to the stone -, that is to say, that the two 

 bodies tended to approach one another with velocities which 

 were inversely as their quantities of matter. 1 There ap- 

 peared to be no limit to the distance to which this action 

 reached ; it was a force that united all the parts of matter 

 to one another, and if it appeared to be particularly directed 

 to certain points, such as the centres of the sun or of the 

 planets, it was only on account of the quantity of matter 

 collected and distributed uniformly round those points, 

 through which, therefore, the force resulting from the com- 

 position of all those elements must pass either accurately or 

 nearly. 



A remarkable inference was deduced from this view of 

 the planetary motions, giving a deep insight into the con- 

 stitution of our system in a matter that seems the most re- 

 condite, and the furthest beyond the sphere which necessari- 

 ly circumscribes human knowledge. The quantity of mat- 

 ter, and even the density of the planets, was determined. 

 We have seen how Newton compared the intensity of gravi- 

 tation at the surface of the earth, with its intensity at the 

 moon, and by a computation somewhat similar, he compar- 



1 If M and M' are the masses of two spheres, and x the dis- 



- ... . M + M' . x , . 



tance ot their centres, ! is the accelerating force with 



ar 



which they tend to unite ; but the velocity of ; . >aoh of 



M 

 M will be — , and of M', -5. 



