Chap. VI.] GRAVITATION AND INERTION. 77 



just as certainly tend to carry the moon eastwards as 

 to carry the tide eastwards. 



The earth's gravitation, then, is constantly tend- 

 ing to draw the moon to the earth, and to carry the 

 moon eastwards with the earth's axial rotation. But 

 the moon is not drawn to the earth ; and, as regards 

 the earth's axial rotation, the moon is constantly 

 lagging behind, or falling westwards. 



The force by virtue of which the moon resists the 

 motions which the earth's attraction tends to give it 

 obviously accords with that which we have defined 

 as its innate property of vis -inertia), by virtue of 

 which it tends to maintain itself in a state of rest. 

 And this vis-inerti^e we have shown to be universal 

 gravitation. 



The earth's gravitation tends to draw the moon to 

 the earth, and the moon's gravitation tends to draw 

 the earth to the moon, but by opposing forces of 

 astral gravitation they are held in equilibrium. By 

 some force (with whose nature we are not here con- 

 cerned) the earth is caused to rotate eastwards on its 

 axis, and as it rotates, its gravitation tends to carry 

 the moon eastwards with that same motion ; but the 

 moon's vis-inertise resists this motion in precisely the 

 same manner as that in which we have described the 

 current-creating action of vis-inertiae in the ocean, for 

 the forces in play are precisely the same as those 

 acting on any given particles of water. The earth 

 tends to draw the moon to the position which the 



