PREFACE. 



SIR ISAAC NEWTON, in the preface to his 

 i Principia,' remarks that ' all the difficulty of 

 philosophy seems to consist in this from the 

 phenomena of motions to investigate the forces 

 of nature, and then from these forces to de- 

 monstrate the other phenomena/ And then, 

 after stating that on this principle he had, in 

 the work above mentioned, demonstrated the 

 motion of the heavenly bodies, he goes on to 

 say : ' I wish we could derive the rest of the 

 phenomena of nature by the same kind of 

 reasoning from mechanical principles; for I 

 am induced by many reasons to suspect that 

 they may all depend upon certain forces by 

 which the particles of bodies, by some causes 

 hitherto unknown, are either mutually im- 

 pelled towards each other, and cohere in re- 

 gular figures, or are repelled and recede from 



