I 



[ '65 ] 



To prove ihis I need only refer to Euler's Memoir on the 

 Origin of Forces among thofe of the Academy of Berlin for 1750, 

 and the 74th of his Letters to a German Princcfs. He there fhews 

 that, from a knowledge of the impenetrability of bodies, a ftranger 

 to the communication of motion muft infer that a ball in motion 

 cannot pafs through a ball at reft; therefore a change of fome kind 

 muft happen. And from another principle flili more general, he 

 muft infer that the change produced is the fmalleft poffible ; a 

 principle from which Maupertuis has deduced all the laws of 

 motion. This principle refts on this evident ground ; that no 

 change is ever greater than the exigency of the circumftances in 

 which it happens requires; for if it exceeded that exigency, it 

 would, as to fuch excefs, be an effed without a caufe, which we 

 have already fliewn to be impoflible. It alfo excludes the other 

 vague fuppofitions of Mr. Hume, as Maupertuis fhews at large in- 

 the Memoirs of Berlin for 1746. To be oppofed by fuch high 

 authorities is already a ftrong prejudice againft him. 



In chymiftry inftances of inferences of this fort are fo numerous 

 that it would be endlefs to recite them. It is indeed in deducing 

 properties before unknown from thofe already known, that chy- 

 vn\c'd\ fagacity chiefly confifts. The properties inferred were affur- 

 edly conneded with thofe already known, as without fuch con- 

 nexion there could be no inference ; juft as every mathematical 

 propofuion is conneded with the propofitions and axioms that' 



ferve 



