270 INDUCTION. 



plex cases, still when the law, got at otherwise, is found to be In 

 accordance with the result of a complex case, that case becomes a 

 new experiment on the law, and helps to confirm what it did not 

 assist us to discover. It is a new trial of the principle in a different 

 set of circumstances ; and occasionally serves to eliminate some cir- 

 cumstance not previously excluded, and to effect the exclusion of 

 which, might require an experiment impossible to be executed. This 

 was strikingly conspicuous in the example formei'ly quoted, in which 

 the difference between the observed and the calculated velocity of 

 sound was ascertained to result from the heat extricated by the con- 

 densation which takes place in each sonorous vibration. This was a 

 trial, in new circumstances, of the law of the development of heat by 

 compression ; and it certainly added materially to the proof of the uni- 

 versality of that law. Accordingly any law of nature is deemed to 

 have gained in point of certainty, by being, found to explain some 

 complex case which had not previously been thought of in connexion 

 with it ; and this indeed is a consideration to which it is the habit of 

 scientific men to attach rather too much value than too little. 



To the Dedvictive Method, thus characterized in its three constituent 

 parts. Induction, Ratiocination, and Verification, the human mind is 

 indebted for its most glorious triumphs in the investigation of nature.. 

 To it we owe all the theories by which vast and complicated phenomena 

 are embraced under a iew simple laws, which, considered as the laws, 

 of those gz-eat phenomena, could never have been detected by their 

 direct study. We may form some conception of what the method has 

 done for us, from the case of the celestial motions ; one of the simplest 

 among the greater instances of the Composition of Causes, since (ex- 

 cept in a few cases not of primary importance) each of the heavenly 

 bodies may be considered, without material inaccuracy, to be never at 

 one time influenced by the attraction of more than two bodies, the 

 sun and one other planet or satellite, making, with the reaction of the 

 body itself, and the tangential force, only four different agents on the 

 concurrence of which the motion's of that body depend ; a much smaller 

 number, no doubt, than that by which any other of the gi'eat phenom- 

 ena of nature are determined or modified. Yet how could we ever 

 ha,ve ascertained the combination of forces upon which the motions of 

 the earth and planets are dependent, by merely comparing the orbits, 

 or velocities, of different planets, or the different velocities or positions 

 of the same planet ? Notwithstanding the regularity which manifests 

 itself in those motions, in a degree so rare among the effects of a con- 

 currence of causes ; although the periodical recurrence of exactly the 

 same effect, affords positive proof that all the combinations of causes 

 which occin- at all, recur periodically ; we -should never have known 

 what the causes were, if the existence of agencies precisely similar on 

 our own earth had not, fortunately, brought the causes themselves 

 within the reach of experimentation under simple circumstances. As 

 we shall have occasion to analyze, further on, this great example of 

 the Method of Deduction, we shall not occupy any time with it here, 

 but shall proceed to that secondary application of the Deductive 

 Method, the result of which is not to prove laws of phenomena but 

 to explain them. 



