Chap. XXI.] THE HEAVENS. 227 



PROPOSITION II. 



TJieorem. — The rotation of a sphere tends to cause 

 surrounding bodies to revolve around it. 



Because, at any point outside the sphere, the 

 force of the gravitation of the nearest part is greater 

 than that of any other equal part ; and therefore the 

 gravitation of the body at the given point towards 

 the nearest part of the sphere will tend to cause it 

 to follow the motion of that part. 



Note. — By the corollary to Proposition I. the 

 velocity of a body moved by the gravitation of 

 another body in motion cannot be so great as that of 

 the latter ; and therefore, if the motion were in a 

 right line, the motive force would gradually be 

 decreased by the increasing distance. But as regards 

 the rotation of the sphere, though the body set in 

 motion cannot keep pace with the velocity of rota- 

 tion, its recession from one part brings it over an- 

 other part of the rotating surface ; and thus the 

 motive force becomes as constant and equable as the 

 rotation. 



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