PHYSICAL AND LITERARY. S9 



defcrlbes an ellpfis, in one of the foci of 

 which the fun is placed. 



In the defcent of heavy bodies towards 

 the centre of the earth, the force of gra- 

 vity is fuppofed to be invariable. For, 

 though this force leffens by diftance, yet 

 the diftance of any two points, from 

 which we have accefs to drop a body, is 

 fo inconfiderable in refped of the diftance 

 of either from the centre, that ic is left 

 out in all calculations as imperceptible. 

 Hence, the following queftion is readily 

 fuggefted: How comes it, that the pow- 

 er of gravity, which in this cafe is fup- 

 pofed invariable, has the effed of accele- 

 rating the motion of a falling body ? It 

 fiiould be thought, that the power of gra- 

 vity, fuppofmg it invariable, muft pro- 

 duce an equable motion downwards,with- 

 out acceleration ; juft as the vis in/tta 

 does, laying afide gravity, in whatever 

 diredon the body is thrown. For, m ge- 

 neral, as effeas muft always correfpond 

 with their caufes, every force which is 

 uniformly exerted without diminution or 

 augmentation, muft produce an equable 



motion. 



