66 ESSAYS AND OBSERVATIONS 



portioned to the velocity, gravity muft 

 produce the fame efFedl, whether its 

 force be exerted in the diredlion 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 arc 

 equal. Thus, upon the whole, the gra- 

 dual retardation of upward motion, is e- 

 qual lo the gradual acceleration of down- 

 ward motion. The fame feries of num- 

 bers reprefents both ; a progrefs from the 

 fmallell to the greateft number being fi- 

 milar to acceleration ; the contrary pro- 

 grefs, to retardation. And hence it is that 

 thefe two contrary motions are perform- 

 ed precifely in the fame time. 



It is now eafy to account for the dif- 

 ferent velocities of a planet in the diffe- 

 rent points of its orbit. The motion of 

 a planet may be juQly reprefented by per- 

 pendicular afcent and defcent. A planet 

 moving from the fun, relembles a (lone 

 moving from the earth upwards ; and, in 

 its return towards the fun, refembles the 

 fame ftone returning to the earth, after 



its 



