88 



THE OCEAN. 



[Book III. 



The force of universal gravitation in fact opposes 

 tlie motion of the stone in the sling just in the same 

 manner as that of astral gravitation opposes the 

 motion of the moon ; but there is this radical differ- 



FiG. 19. 



ence between the case of the stone revolved in the 

 sling and that of the moon revolving round the earth. 



issue is as to how those effects are caused ; and I maintain that 

 vis-inertife holds the planets in equilibrium, the centripetal force 

 of the sun's gravitation being a part of the action of the planet's 

 vis-inertise just as much as the centrifugal force. The error lies 

 in the cause to which the tangental effort is attributed; and I 

 have endeavoured to show that that effort is not caused by the 

 vis-inertije of the planet tending to carry it onwards along the 

 tangent, but by astral gravitation (a part of the action of vis-inertise) 

 retarding it, and tending to draw it backwards farther and farther 

 from the successive positions to which the revolving force carries 

 it along its orbit. 



Vis-inertise is the combined action of universal gravitation, 

 which actually does keep the planet on the line of its orbit ; 

 opposing any tendency from, as much as towards, the sun. — The 

 Elements, Preface, Yol. I. London, 1866. 



