Chap. Vr. ANTIENT METAPHYSICS. 413 



we can eflimate the velocity of the Planetary Motion, we nuift know 

 perfedly the nature of that Motion *. 



But Velocity, which, I fliy, is the meafure of the Force by wliich 

 any Body is moved, requires fome meafure or flandard iilelf : I-'or 

 there is nothing abfolute in Motion ; it is only relative, and relative 

 to two things j one, the fpace which the Body goes through, and the 

 other, the time which it requires for that Motion. The Moving 

 Force, therefore, or Velocity (for thefe two terms may be confidered 

 as fynonimous, though the one be the Caiifc^ and the other the 

 EffcSlJ, is as the Space through which the Body is moved, compared 

 with the Time of the Motion ; fo that, when we find that another 

 Body Is moved through the fame Space in a fhorter or longer time, 

 we fay that the Velocity of that other Body is greater or lefs. 



If the Space gone through were a Straight Line, of which we 

 knew the length and likewife the time of the Motion, and if the 

 Motion were equable, we fhould fay that the Moving Force, in fuch 

 a cafe, was a Force which carried the Body along a line of fuch a 

 length in fuch a time. But, fuppofe the Motion not equable, and 

 not in a ftraight line but in a curve, which is the cafe of the Plane- 

 tary Motion, by what rule or ftandard can we eflimate the Moving 

 Force of fuch a Motion ? 



Although the Motion In a ftraight line be the moft natural mea- 

 fure of the Moving Force or Velocity of any Motion, yet, if the 

 Motion were in a regular curvillneal figure, fuch as a circle of a 

 certain diameter, or an ellipfis of certain dimenfions, and if it were 



equable, 



* It is to be obferved, that this rule, that the Moving Power of the Body is to be 

 eftimated by the Velocity of the Motion, will apply only to the fame Body, or to two 

 Bodies of equal mafs ; for, if the mafles be different, tliough the Velocity of the Bo- 

 dies may be the fame, the Power which moves the greater mafs muft nccciliirily be 

 greater. 



