524 INDUCTION. 



complex law is resolved. The law, in short, of each of the 

 concurrent causes remains the same, however their colloca- 

 tions may vary; hut the law of their joint effect varies with 

 every difference in the collocations. There needs no more 

 to show how much more general the elementary laws must 

 be, than any of the complex laws which are derived from 

 them. 



5. Besides the two modes which have been treated of, 

 there is a third mode in which laws are resolved into one 

 another; and in this it is self-evident that they are resolved 

 into laws more general than themselves. This third mode is 

 the subsumption (as it has been called) of one law under 

 another : or (what comes to the same thing) the gathering up 

 of several laws into one more general law which includes 

 them all. The most splendid example of this operation was 

 when terrestrial gravity and the central force of the solar 

 system were brought together under the general law of gravi- 

 tation. It had been proved antecedently that the earth and 

 the other planets tend to the sun ; and it had been known 

 from the earliest times that 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 towards 

 the earth being ascertained to vary as the inverse square of 

 the distance, it was deduced from this, by direct calculation, 

 that if the moon were as near to the earth as terrestrial objects 

 are, and the acquired force in the direction of the tangent were 

 suspended, the moon would fall towards the earth through ex- 

 actly 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 



