Chap. VII.] GRAVITATION AND INERTION. 85 



Now it must be observed that, in treating of the 

 action of vis-inertiae in causing currents in the ocean, 

 we have been driven to the conclusion that the moon 

 must be acted on by the same forces that impel any 

 given drop of water through its circulation. That is 

 to say, the revolving force of the earth's gravitation 

 is constantly dragging them eastwards, whilst their 

 own inherent force of inertion is constantly opposing 

 that motion and causing them to lag westwards. 



And then we have naturally inferred that the sun, 

 by a motion of rotation, must tend to carry the 

 planets round with it, just as the earth tends to carry 

 along the water and the moon. 



To test this we have calculated the relative force 

 of the sun's revolving action in the orbits of the 

 planets, considering that the nearest part of the 

 surface tends to carry them in one direction, whilst 

 the farthest part tends to carry them in the opposite 

 direction, and we have found that the actual relative 

 velocities with which the planets move round the sun 

 bear a fixed ratio to the relative amounts by which 

 the force of the gravitation of the nearest exceeds 

 that of the remotest part of the sun in their respec- 

 tive orbits. 



When a theory which explains the movements of 

 the ocean and atmosphere — as shown in Vis-inertice, 

 and farther on in this volume — is corroborated 

 like this by the motions of the heavenly bodies, 

 and wherever extended finds nothing in nature at 



