CnAP. IV.] GRAVITATION AND INERTION. 59 



in the opposite direction to that of the combined 

 action of all other forces of gravitation. Thus, as 

 the earth moves, tending to carry the particles in 

 question with it, the action of the foreign force of 

 gravitation necessarily tends to draw the particles in 

 exactly the opposite direction to that in which the 

 motion of the earth tends to carry them. 



This latter is precisely the action of vis-inertite 

 which we have described in the preceding Book ; 

 for those particles of the fluid on the surface of the 

 deep and central parts of the ocean, being the least 

 closely held to the surface of the earth, are most 

 free to move in the direction in which the foreign 

 force of gravitation tends to draw them ; and those 

 particles which, in consequence of their position, are 

 more closely held to the earth's surface, and there- 

 fore offer more resistance to the action of the foreign 

 force of gravitation, being under the dominion of 

 the earth's power of gravitation, are by that power 

 drawn into the positions vacated by those set in 

 motion (in relation to the surface of the earth) by 

 the foreign force of gravitation : and thus, as long 

 as the motion which causes this opposing action of 

 gravitation lasts, a constant circulation is effected. 

 The opposmg forces are — the earth's power of gravi- 

 tation acting in one direction, and the combined 

 action of all other powers of gravitation acting in the 

 opposite direction. 



Thus, the currents through the deep and central 



