CnAP. IV.] GRAVITATION AND INERTION. 61 



gravitation maintains the equilibrium of the ocean 

 as a whole, by drawing an equal volume of water 

 eastwards through the temperate zones : but, in fact, 

 this latter movement, when considered in relation to 

 the joint action of all foreign powers of gravitation, 

 is just as truly a movement resulting from the vis- 

 inertia3 of the ocean. 



Yis-inertise and gravitation are, therefore, as far as 

 the arguments hitherto adduced are concerned, con- 

 vertible terms. Any effect termed an action of vis- 

 inertia in relation to any given power of gravitation, 

 is, in fact, the direct result of some other power of 

 gravitation ; and, inversely, any effect caused directly 

 by any given power of gravitation is an action of 

 vis-inertias in relation to some other power of gravi- 

 tation. 



The action of vis-inertias in the ocean and atmo- 

 sphere can, therefore, with no more reason be denied 

 than the action of gravitation towards foreign bodies. 

 To study the movements resulting from the action 

 of gravitation, or to study those resulting from the 

 action of vis-inertite, are therefore simply different 

 modes of studying the same phenomena. A move- 

 ment of the waters of the ocean caused by the direct 

 action of the sun's power of gravitation, tending to 

 draw the ocean towards it, is, therefore, an action of 

 vis-inertias in relation to the joint action of all other 

 powers of gravitation : and the counter-action of vis- 

 inertias, by which the ocean maintains its position in 



