66 ESSAYS ann OBSERVATIONS 
conclufive. But it is not fo. Laying afide 
gravity, and the refiftance of the air, a body: 
thrown upwards with the fmalleft force, will 
move on i zifinitum. It is by the operation 
of gravity, and the refiftance of the air, that 
motion ceafes, when the body arrives toa cer- 
tain height. The retardation, therefore, of 
motion, in bodies thrown up with different 
velocities, laying afide the refiftance of the 
air, may be a meafure of the force of gravity, 
of which it is the effect; but can never be a 
mea{ure of the force with which the body is 
thrown up, of which it is not the effec. 
And, from the fact of a body’s arriving at 
four times the height with double the veloci- 
ty, toinfer, that the momentum, at its out- 
fetting, muft be as the {quare of the velocity, 
is really not more juft, than to infer, when one 
body is let drop from four times the height of 
another body, that it muft acquire four times 
the force of the other body, tho’ it acquire 
but double its velocity; which does not af- 
ford the (hadow of an argument. When a 
body is thrown up with a double velocity, 
and confequently with a double force, the 
reafon why it afcends four times its former 
height, is plainly this, that the counteraction 
of gravity, while it has a double force to 
{truggle 
