Chap. VTL] GRAVITATION AND INERTION. Of, 



of gravitation, whereas the motion of the moon main- 

 tains it.^ 



* Newton erred in assuming the centrifugal for-ce whicli op- 

 poses the centripetal force of solar gravitation, and holds the planets 

 at their mean distances from the sun, to result from an innate 

 tendency to move unifoimly forwards in a straight line : and he 

 doubly erred in assuming this asserted tendency to move uniformly 

 forwards in a straight line to result from innate vis inertise, by 

 virtue of which any body once set in motion tends to continue that 

 motion imiformly forwards in a straight line until other forces from 

 without stop it. For, first, I have shown that vis-inertise opposes 

 motion in everything, and that its own inherent property of vis- 

 inertise must tend to bring a body to rest, under any circumstances 

 whatever, just as much, and in the same manner, as the action of 

 any force from without. And, secondly, I have shown that, as 

 regards the motions of the planets in their orbits, the centrifugal 

 force which opposes the centripetal force of the bodies which com- 

 pose the solar system one towards another, and all towards their 

 common centre of gravity, is the force of astral gi-avitation oppos- 

 ing that of solar gravitation ; so that, in their courses, they are 

 borne smoothly along the lines of equilibrium lying between 

 opposing forces of gravitation. This subject is more fully discussed 

 in The, Neio Principles of Natural Philosophy. 



