Chap. VII.] GRAVITATION AND INERTION. 93 



of astral gravitation. Thus the force of astral gravi- 

 tation, opposing that of the earth's gravitation, is 

 constantly endeavouring to draw the moon out of its 

 orbit, and the earth's gravitation revolves it in the 

 orbit in which it is held by those opposing forces. 



The foregoing shows that it is an error to sup- 

 pose the orbital motion of the moon to be analogous 

 to that of a stone swung round in a sling. Because : 

 it is the motion of the stone which tends to carry it 

 out of its orbit ; that motion tends to carry it on- 

 wards at a tangent to its orbit in the direction in 

 which it is revolved ; and the motion is caused by a 

 force which moves the stone in spite of the resistance 

 of vis-inertia3. Whereas : it is the force which op- 

 poses the motion of the moon that tends to carry it out 

 of its orbit ; that force tends to draw it backwards 

 in the opposite direction to that in which it is re- 

 volved ; and the motion of the moon is caused by 

 gravitation, which is the effort of vis-inerti^e to bring 

 matter to a state of rest. For the motion of the 

 moon is its effort or gravitation towards the nearest 

 part of the surface of the earth which is constantly 

 rolling from it. An increase of the velocity with 

 which that surface moves would increase the distance 

 to which the moon is drawn from it ; because the 

 force of astral gravitation which prevents the moon 

 from keeping pace with the surface increases as the 

 square of the velocity of its motion. 



To make this clearer we may recapitulate : 



