Cn.vr. VIJ.] GRAVITATION AND INERTION. 89 



In the former case it is the motion of the stone which 

 tends to carry it out of the circle in which it is re- 

 volved : whereas in the case of the moon, as we 

 have shown that the motion is caused by the earth's 

 gravitation, it cannot tend to carry the moon onwards 

 at a tangent from its orbit, nor indeed can it tend to 

 carry it out of its orbit at all. The root of the differ- 

 ence is this : the force by which the stone is set in 

 motion is an illustration of the direct action of the 

 forces ordained to control matter ; whereas, in the 

 case of the moon, the motion is caused by gravitation, 

 an attribute of vis-inertise — the mode, in fact, in which 

 vis -inertias manifests its resistance to the motive force. 

 The motion of the moon in its orbit may rather 

 be compared to that of a cork floating in a basin of 

 water, in which, by rotating any object rapidly in 

 the centre of the basin, the whole mass of water is 

 caused to rotate, carrying the cork along with it. 

 The motion of the moon and that of the cork are 

 analogous, inasmuch as both are caused by gravita- 

 tion, this gravitation being an effect resulting from 

 the resistance which the vis-inertise of matter pre- 

 sents to the action of the force by which the object 

 in the centre of the basin is made to rotate. This is 

 so because, in consequence of its property of vis- 

 inertias, the water in the basin tends to maintain itself 

 in a state of rest ; and therefore, resisting any force 

 which may tend to set it in motion, it endeavours 

 to maintain its position and that of its parts in 



