— St 
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“PHYSICAL ann LITERARY, 59 
return downwards, gravity, being exerted 
without an antagonift, every new impulfe 
prodyces an acceleration of motion ; and, 
thefe impulfes being repeated inceffantly, the 
acceleration goes on, till the {tone touch the 
ground. 
Ir remains only to be obferved, that, as 
the force of gravity is accurately proportion- 
ed to the velocity, gravity muft produce the 
fame effect, whether its force be exerted in 
the direction of the motion, or contrary to 
it; and that the acceleration in the former 
cafe muft be equal to the retardation, in the 
latter, in any two points where the velocities 
are equal. Thus, upon the whole, the gra- 
dual retardation of upward motion, is equal 
to the gradual acceleration of downward mo- 
tion. The fame feries of numbers repre- 
fents both ; a progrets from the {malleft to 
the greateft number being fimilar to accele- 
ration ; the contrary progrefs, to retardation. 
And hence itis that thefe two contrary mo- 
tions are performed precifely in the fame 
time. | 
Ir is now eafy to account for the different 
velocities of a planet in the different points 
»of its orbit. The motion of a planet may 
“hi be 
