PHYSICAL LITERARY. 53 
other planets, defcribe an clipfis, in one of 
the foci of which the fun is placed. 
In the defcent of heavy bodies towards 
the center of the earth, the force of gravity 
is fuppofed to be invariable. For, tho’ this 
force leflens by diftance, yet the diftance of 
“any two points, from which we have accefs 
- to drop’a body, is fo inconfiderable in refpect 
of the diftance of either from the center, 
' that it is left out in all calculations as imper- 
ceptible. Hence the following queftion is 
readily fuggefted, How comes it, that the 
power of gravity which in this cafe is fup- 
_pofed invariable, has the effect of accelerating 
the motion of a falling body? It fhould be 
_ thought, that the power of gravity, fuppo- 
fing it invariable, muft produce an equable 
motion downwards, without acceleration ; 
 juft as the vis infta does, laying afide gravi- 
ty, in whatever direction the body is thrown. 
_ For in general, as effects muft always corre- 
fpond with their caufes, every force which 
is uniformly exerted without diminution or 
- augmentation, muft produce an equable mo- 
: tion, without acceleration or retardation: 
J 
’ 
: 
and, on the other fide, every varied effect 
which 
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