Chap. VL ANTIENT METAPHYSICS. 415 



the Planet on in its Orbit, with which, as we have feen, this de- 

 fcent is neceflarily connected. 



That this may be the better underftood, let us take, for example, 

 the neareft planet to us, the Moon, of which our Earth is the centre. 

 As we know in what period of time the Moon dcfcribes her whole' 

 Orbit, fo we know alfo in what time fhe defcribes any given arc of 

 that Orbit. Suppofe, then, a Tangent drawn at one extremity of 

 this given arc ; we know alfo how much fhe has fallen below the Tan- 

 gent, and in what time, at the other extremity of this Arc, and if 

 the Arc be a fmall one, it is eafy to be fhewn, that if flic had fallen 

 diredtly from the Point of Contad, (he would have fallen juft as far 

 in the fame time*. Here, then, we have the Moon defcending from 

 the Tangent, in a flraight line of a certain length, in a certain time : 

 And, if the Motion in that flraight line were equable, without in- 

 creafe or diminution, it would be its own meafurc, and we fliould 

 fay, that the Moving Force or Velocity was fuch as carried the 

 Body through fuch a fpace in fuch a time, which, as I have faid, is 

 the only ftandard by which we can eftimate any Velocity. But we 

 are fure that this ftandard, when applied to the defcent of the Moon» 

 will not anfwer ; and, therefore, that her Motion in the Defcent is 

 not equable ; and, confequently, we muft feek for fome other Law 

 of this Motion. 



And here we muft leave the Moon, and have recourfe to a Motion 

 on Earth, with which we are much better acquainted ; for, as I 

 have obferved, nothing is more natural, than that we fhould endea- 

 vour to explain the Motions in the Heavens by thofe on Earth ■]". 



* Speaking phyfically, and not with geometrical Exa(ftners, the Truth is, that as 

 the Arc is lefiencd, the Diftance of the Extremity of the Arc from the Tangent, and 

 the direil Fall approximate to Equality, and are ultimately Equal. If the Arc be not 

 greater than that which the Moon traverfes in the Space of a Minute, the difference 

 of thefe two Qiiantities will appear quite infcnfiblc. 



•t P. 382. 



Nowj 



