556 INDUCTION. 



under the general law of gravitation. It had been 

 proved antecedently that the earth and the other 

 planets tended to the sun ; and it had been known 

 from the earliest times that all terrestrial bodies tend 

 towards the earth. These were similar phenomena ; 

 and to enable them both to be subsumed under one 

 law., it was only necessary to prove that, as the effects 

 were similar in quality, so also they, as to quantity, 

 conform to the same rules. This was first shown to 

 be true of the moon, which agreed with terrestrial 

 objects not only in tending to a centre, but in the 

 fact that this centre was the earth. The tendency of 

 the moon to the earth was already known to vary as 

 the inverse square of the distance ; and it was deduced 

 from this, by direct calculation, that if the moon were 

 as near to the earth as terrestrial objects are, and the 

 tangential force were suspended, the moon would fall 

 towards the earth through exactly as many feet in a 

 second as those objects do by virtue of their weight. 

 Hence, the inference was irresistible, that the moon 

 also tends to the earth by virtue of its weight : and 

 that the two phenomena, the tendency of the moon 

 to the earth and the tendency of terrestrial objects to 

 the earth, being not only similar in quality, but, 

 when under the same circumstances, identical in 

 quantity, are cases of one and the same law of causa- 

 tion. But the tendency of the moon to the earth and 

 the tendency of the earth and planets to the sun, were 

 already known to be cases of the same law of causa- 

 tion : and thus the law of all these tendencies, and 

 the law of terrestrial gravity, were recognised as iden- 

 tical, or, in other words, were subsumed under one 

 general law, that of gravitation. 



In a similar manner, the laws of magnetic pheno- 



