336 INDUCTION. 



centre, but in the fact that this centre was the earth. The tendency of the 

 moon toward 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 toward 

 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 phenome- 

 na, 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 in 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 tend- 

 ency of the earth and planets to the sim, 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 identical, and were sub- 

 sumed under one general law, that of gravitation. 



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

 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 suc- 

 cessive steps. For, to arrive by correct induction at laws which hold un- 

 der sucli an immense variety of circumstances, laws so general as to be in- 

 dependent of any varieties of space or time which we are able to observe, 

 requires for the most part many distinct sets of experiments or observa- 

 tions, conducted at different times and by different people. One part of 

 the law is first ascertained, afterward 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 uni- 

 versally. 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 elimi- 

 nation. The tendency of bodies toward 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 toward the earth, and 

 might, therefore, be ascribed to a peculiar property of the earth itself : one of 

 the circumstances, namely, the proximity of the earth, had not been elimi- 

 nated. To eliminate this circumstance required 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 unfavorable 

 cii'cumstances for observing them. To make these observations, fell nat- 

 urally to the lot of a different set of persons from those who studied tei'- 

 restrial phenomena; and had, indeed, been a matter of great interest at 

 a time when the idea of explaining celestial facts by terrestrial laws was 

 looked upon as the confounding of an indefeasible distinction. When, 

 however, the celestial motions were accurately ascertained, and the deduc- 

 tive processes performed, from which it appeared that their laws and 

 those of terrestrial gravity corresponded, those celestial observations be- 

 came a set of instances which exactly eliminated the circumstance of prox- 

 imity to the earth ; and proved that in the original case, that of terrestrial 

 objects, it was not the earth, as such, that caused the motion or the press- 

 ure, but the circumstance common to that case with the celestial instances, 

 namely, the presence of some great body within certain limits of dis- 

 tance. 



