94 THE OCEAN. [Book III. 



As the ratio iii which the force of astral gravita- 

 tion increases or decreases is dependent on that of the 

 revolving force ; and as the latter is inversely as the 

 cube of the distance, whereas the direct force is 

 inversely as the square of the distance from the earth, 

 therefore, however great the revolving force may be 

 at the surface of the earth, the opposing force of astral 

 gravitation which carries the moon backwards from 

 the position to which the revolving force tends to 

 carry it must at some certain distance become equal 

 to the direct force of the earth's gravitation ; and 

 there the moon would be revolved in equilibrium 

 between the force of astral gravitation tending to 

 draw it out of its orbit from the earth and that of 

 the earth's gravitation tending to draw it to the 

 earth. 



In the case of the stone, the motive force tends to 

 carry it off at a tangent to the circle in which it is 

 revolved in the direction of its motion, in spite of the 

 resistance of vis-inertise, which tends to bring it to a 

 state of rest : whereas the moon is held in equili- 

 brium by vis-inertia3, or universal gravitation, whilst 

 the force of astral gravitation tends to carry it from 

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

 to the earth ; and its motion is constantly restoring 

 the equilibrium of gravitation towards it as fast as that 

 equilibrium is disturbed by the action of the force 

 which causes the earth to rotate. 



The motion of the stone disturbs the equilibrium 



