EXPLANATION OF LAWS. 275 



trial gravity and the central force of the solar system were brought 

 together under the general law of gravitation. It had been proved 

 antecedently that the earth and the other planel^ tended to the sun; 

 and it had been known from the earliest times that all tcnestrial bodies 

 tend towards the earth. These were similar phenomena; and to enable 

 tliem both to be subsumed under one law, it was only necessary to 

 proTe 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 wth tcnestrial objects not only in tending 

 to a centre, but in the fact that lliis centre \v~as 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 calcu- 

 lation, that if the moon were as near to the eiuth 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 weiglit. Hence, the inference was irre- 

 sistible, 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 tk^ tendency of teirestnal objects to the earth, being not only 

 similar in qdality, but, when under the same circumstances, identical 

 in quantity, are cases of one and the same law of causation. 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 

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

 terrestrial gravity, were recognized as identicg,l, or, in other words, 

 were subsumed under one general law, that of gravitation. 



In a similar manner, the laws of magnetic phenomena have recently 

 been subsumed under Ivuown , laws of ele'ctricity. 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 con-ect 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 sots of experi- 

 ments or observations, conducted at dittbrent times aiid by different 

 people. One part of the law is first ascertained, afterwards another 

 part: one set of observations-teaches us tliat 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 con- 

 ditions much more j^eneral, or even universally. The general law, in 

 this case, is literally the sum of all the partial ones ; it is the recog- 

 nition of the same sequence in different sets of instances ; and rfiay, in 

 fact, be regarded as- merely one step in the process of elimination. 

 That tendency of bodies towards one another, which .we now call 

 gravity, Jiad at first been 6bsexved only upon the earth's surface, where 

 it manifested itself only *as a tendency of all bodies towards the earth, 

 and might, therefore, be asctibed to a peculiar property of the earth 

 itself: one of the circumstances, namely, tlie proximity of the earth, 

 had not been eliminated. To eliminate this circumstance required a 

 fresh set of instances in other parts of the universe : these we couli 

 not ourselves create ; and though nature had ci'cated them for us, we 

 were placed in very unfavorable circumstances for obsennng them. 

 To make these observations, fell naturally to the lot of a different set of 

 persons from those who studied terrestrial phenomena, and had, in- 



