296 A P P E N D I X. CTnap. I. 



A third confequence is, that Motion once begun can only ceafe 

 in one way, that is, by the obftrudtion of the medium, whereas all 

 philofophers before Sir Ifaac Newton held that Motion ceafed in two 

 ways, either by the obftrudion of the medium, or by the ceafingof 

 the Moving Power to a£l. And, indeed, I think every man's daily 

 experience muft convince him this is the truth *. 



And, laftly, if the Motion be begun by impulfe, though the ve- 

 locity of it will be different, according to the greater or lefs violence 

 of the impulfe, the duration of the Motion will be the fame, that is, 

 for ever ; and, therefore, an eternal Motion may be produced by the 

 riighteft impulfe f* 



1 have heard it objeded to this double caufe which I aflign for 

 the ceafing of Motion, that one is fufficient, namely, the obftacle 

 in the way of the Body ih Motion, and that Nature is fo frugal that 

 file never employs two caufes to produce the fame effed, when one 

 will do the bufmefs. — To which I anfwer, that Nature never em- 

 ploys two caufes at the fame time to produce the fame effed, if one 

 will do the bufmefs ; but, at different times and in different cir- 

 cumftances, different caufes will produce the fame effed. Thus, to 

 give an example from the very thing we are fpeaking of, viz. Motion, 

 and a Motion moft perfedly fimple, I mean Motion in a ftraight 

 line, for which, if for any thing, one caufe may be thought fuffi- 

 cient : This Motion may, according to circumftances, be produced 

 by three different caufes ; by the dired impulfe of one Body, the 

 lateral impulfe of two, or, laftly, by Mind. We are not, therefore, 

 to wonder that the ceffation of Motion may be the effed of 

 two different caufes. In vacuo, it ceafes only in one of the 



ways. 





* Vol. i. p. 541. 



t Vol. i. p. 540, Vol. ii. p. 348- 



