62 THE OCEAJS". [Book III. 



relation to the conflicting forces, is just as certainly 

 the direct action of those other powers of gravitation, 

 tending to draw the ocean towards the position in 

 which the combined action of their gravitation tends 

 to place it. 



Therefore, as, when the earth's gravitation is 

 considered in opposition to all other powers of 

 gravitation, the water drawn westwards through 

 the equatorial regions by the latter is replaced by 

 an equal volume simultaneously drawn eastwards, 

 through the temperate zones, by the former power, 

 so also, when the sun's gravitation draws a volume 

 of water to the side of the earth turned towards the 

 sun, the joint action of all other forces of gravitation 

 draws an equal volume of water in the opposite 

 direction, thus preserving the equilibrium of the 

 ocean in relation to the conflicting forces. 



In the consideration of the action of gravitation 

 in causing the tides of the ocean, we may, therefore, 

 consider the action of the sun and of the moon, each 

 singly ; and that of all the other powers of gravita- 

 tion jointly, under the title of astral gi^avitation, 

 which is suflEiciently appropriate, though the gravita- 

 tion of the planets is included in it. 



This force of astral gravitation may be defined as 

 the combined action of the gravitation of the universe, 

 exceptmg the force whose action it opposes. And 

 vis-inertia3 is universal gravitation. 



Let us now reconsider the nature of the forces 



