54 ESSAYS anv OBSERVATIONS 
which is gradually diminithed or augmented, 
muft proceed from a varied caufe. 
‘Tuts difficulty is endeavoured to be ex- 
plained in the following manner. Gravity, 
it is faid, has a remarkable property, ‘* That 
“it acts with the fame force upon a body 
‘¢ that is already in motion, as upon a body 
« that is at reft, fo as to produce equal acce- 
«< Jerations in falling bodies in equal times *.” 
But this manner of conceiving the effect of 
gravity, whatever obfcurity it may occafion, 
tends not to remove the difficulty. One thing 
is extremely clear, that we have no means 
afforded us, to eftimate the force of any 
power, other than the effects produced by it ; 
which holds in general, whether the power 
be internal or external. From the uniformity 
of the effects, we conclude the vzs znfita to 
be an uniform power ; and we have the fame 
foundation, from its varying effects, to con- 
clude gravity to be a varying power. In | 
fhort, when acceleration is produced by an 
external caufe, it will be evident to every 
one, that the force of the impulfive caufe, 
muft be continually increafing, to produce 
fuch 
* M‘Laurin’s account of Newton's philofophy, p. 248. 
