EXPLANATION OF LAWS. 



309 



dent 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 ex- 

 ample of this operation was when 

 terrestrial gravity and the central 

 force of the solar system were brought 

 together under the general law of 

 gravitation. It had been proved an- 

 tecedently that the earth and the 

 other planets tend to the sun ; and it 

 had been known from the earliest 

 times that terrestrial bodies tend to- 

 wards 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 cal- 

 culation, 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 sus- 

 pended, 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 vir- 

 tue of its weight, and that the two 

 phenomena, the tendency of the moon 

 to the earth and the tendency of ter- 

 restrial objects to the earth, being 

 not only similar in quality, but, when 

 in the same circumstances, identical 

 in quantity, are cases of one and the 

 same law of causation. But the ten- 

 dency of the moon to the earth, and 

 the tendency of the earth and planets 

 to the sun, were already known to be 

 of the same law of causation : 



and thus the law of all these ten- 

 dencies and the law of terrestrial 

 gravity were recognised as identical, 

 and were subsumed under one general 

 law, that of gravitation. 



In a similar manner, the laws of 

 magnetic phenomena have more re- 

 cently been subsumed under known 

 laws of electricity. It is thus that 

 the most general laws of nature are 

 usually arrived at : we mount to them 

 by successive steps. For, to arrive 

 by correct induction at laws which 

 hold under such an immense variety 

 of circumstances, laws so general as 

 to be independent of any varieties of 

 space or time which we are able to 

 observe, requires for the most part 

 many distinct sets of experiments or 

 observations, conducted at different 

 times and by different people. One 

 part of the law is first ascertained, 

 afterwards another part : one set of 

 observations teaches us that the law 

 holds good under some conditions, 

 another that it holds good under 

 other conditions, by combining which 

 observations we find that it holds 

 good under conditions much more 

 general, or even universally. The 

 general law, in this case, is literally 

 the sum of all the partial ones ; it 

 is a recognition of the same sequence 

 in different sets of instances, and 

 may, in fact, be regarded as merely 

 one step in the process of elimination. 

 The tendency of bodies towards one 

 another, which we now call gravity, 

 had at first been observed only on the 

 earth's surface, where it manifested 

 itself only as a tendency of all bodies 

 towards the earth, and might, there- 

 fore, be ascribed to a peculiar pro- 

 perty of the earth itself : one of the 

 circumstances, namely, the proximity 

 of the earth, had not been eliminated. 

 To eliminate this circumstance re- 

 quired a fresh set of instances in 

 other parts of the universe : these 

 we could not ourselves create ; and 

 though nature had created them for 

 us, we were placed in very unfavour- 

 able circumstances for observing them. 

 To make these observations fell natur- 



