264 THE OCEAN. [Book X. 



the motive force, being independent of the sun, does 

 not decrease as the distance from it increases ; whereas 

 the power of the sun's gravitation to retain the planet 

 does constantly decrease : therefore a planet acted on 

 by such a force could not be retained within the solar 

 system. 



PROPOSITION XXIX. 



Theorefn. — Objects lying loosely on the surface of a 

 rotating sphere, of which the direct force of 

 gravitation is greater than the revolving force, 

 will rest upon it, but at the same time have 

 relative motions along the surface. 



Because the objects would be held to the surface 

 by the direct force of gravitation ; but on the surface, 

 if free to move, vis-inertias would determine their 

 positions between the conflicting forces of astral 

 gravitation and the revolving force of gravitation 

 (Proposition VII.), and so give them a relative 

 motion along the surface. 



Note. — Bricks, even loosely piled, are too firmly 

 held by the direct force of the earth's gravitation to 

 be sufficiently free to move ; but the drops of water 

 superimposed on each other in the ocean arc inces- 



