550 



DISSERTATION ON 



they could be refolved. Thus, the motion of a body in a ftraight 

 line being refolvable into the motions of the two fides of a parallelo- 

 gram, of which it is the diagonal, though it be adually not fo com- 

 pounded, is governed by the laws of the two motions of which it 

 might have been compounded, and into which it is refolved ; for it is 

 in a ftraight line, as they are, and it is performed in the fame time. 

 In like manner, the motion of the planets being refolvable, as the mo- 

 tion of projeQiles on earth is, into the motion of projedion and gravi- 

 tation, though not actually fo compounded, is a motion of the fam.e 

 kind, and carried on by the fame laws. Upon this fimple principle, all 

 Sir Ifaac's demonftrations and calculations would have flood as firm 

 as upon the fuppofition that he has made, of the celeftial motions 

 being produced in the fame manner as the motion of projediles on 

 earth. 



There is a proportion which the Newtonians lay down as an 

 axiom, and upon which they feem to build their whole philofophy, 

 viz. * That, where there are the fame effeds, there muft be the fame 

 ' caufes *.' * Therefore,' lay they, ' the motions of projectiles on 



* earth, and of the celeftial bodies, being of the fame kind, muft be 



* produced by the fame caufes, that is, projedion and gravitation.' But 

 this propofition appears to me to be mifapplied ; for it only proves, 

 that, where there is a motion of the fame kind, it (hould produce the 

 fame cffeds ; but it does not at all prove that the two motions muft a- 

 rife from the fame caufe, and be produced by the operations of the fame 

 powers ; for two motions, if they be of the fame kind, will have the 

 fame effeds, and be governed by the fame laws, though they proceed 

 from caufes quite dift*erent, like a fimple motion in a ftraight line, 

 produced by one impelling force, and the fame motion, produced by 

 two impelling forces, ading in different diredions. 



From 

 • See page 267, 



