234 GRAVITY AND LEVITY 



respectively. This consideration, therefore, 

 without entering into any further examination 

 of this hypothesis, is sufficient to show that it 

 cannot be admitted. 



" The gravitation of a planet towards the sun 

 varies inversely* as the square of its distance 

 from the sun ; that is, whatever be the magni- 

 tude, or density, of the planet, ite acceleration 

 towards the sun varies in that ratio. Whatever 

 cause, therefore, is assigned, it must satisfy 

 this law of variation. 



" If we suppose material particles to emanate 

 from the sun, and ct upon the surface of the 

 planet, or to pervade the body and act upon 

 the whole of it, the tendency of such action, from 

 the known mechanical operations of bodies 

 upon each other, must be to drive the planet 

 from the sun. Such an emanation, therefore, 

 cannot be admitted as the cause of gravita- 

 tion. We will, therefore, next consider Sir I. 

 NEWTON'S hypothesis. 



" If the sun and planets act upon each other, 

 it must be by some intermediate substance 

 which is invisible; this substance we must, 

 therefore, suppose to be an elastic fluid ; and 

 upon such a cause, Sir I. Newton attempts to 

 account for gravitation. In his second adver- 

 tisement to the second edition of his Optics, he 

 says, " To show that I do not take gravity for 

 an essential property of bodies, I have added 



