64 ESSAYS And OBSERVATIONS 



that in motion, the adlion of gravity is 

 not to be confidered as one adlion exerted 

 through a length of time, but as a num- 

 ber of different av5lions exerted incefFant- 

 ly. For, if the gravity of a body in mo^ 

 tion continues not the fame any two fuc- 

 ccflive moments, but is continually vary- 

 ing, the a(5lion mufl vary with the pow- 

 er ; and confequently is not one, but a 

 number of different adions. Gravity in 

 a body at reft, is, like the I'is in/ita, oni 

 invaried power, which produces one in- 

 varied aclion exerted through a length of 

 time. But, when once the body is put 

 in motion, we muft neceffarily admit, 

 firft, that gravity is exerted by repeated 

 impulfes ; and next, that thefe impulfes, 

 continually turning ftronger, form an in- 

 creafing feries, holding an exadl propor- 

 tion with the growing velocity, of which 

 thefe impulfes are the caufe. 



And this leads us readily to conceive 

 the operation of gravity in the afcent and 

 defcent of bodies. A flone, in its perpen- 

 dicular motion, is carried upwards by its 

 vis iiifita cov.nter-a^^ed by gravity: In its 



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