56 INDUCTION. 



position tends to produce a constant increase of temperature ; 

 but with this effect of a constant cause, there are blended the 

 effects of many variable causes, winds, clouds, evaporation, 

 electric agencies and the like, so that the temperature of any 

 given day depends in part on these fleeting causes, and only in 

 part on the constant cause. If the effect of the constant cause 

 is always accompanied and disguised by effects of variable 

 causes, it is impossible to ascertain the law of the constant 

 cause in the ordinary manner, by separating it from all other 

 causes and observing it apart. Hence arises the necessity of 

 an additional rule of experimental inquiry. 



When the action of a cause A is liable to be interfered 

 with, not steadily by the same cause or causes, but by diffe- 

 rent causes at different times, and when these are so frequent, 

 or so indeterminate, that we cannot possibly exclude all of 

 them from any experiment, though we may vary them ; 

 our resource is, to endeavour to ascertain what is the effect 

 of all the variable causes taken together. In order to do 

 this, we make as many trials as possible, preserving A in- 

 variable. The results of these different trials will naturally 

 be different, since the indeterminate modifying causes are 

 different in each : if, then, we do not find these results to be 

 progressive, but, on the contrary, to oscillate about a certain 

 point, one experiment giving a result a little greater, another 

 a little less, one a result tending a little more in one direction, 

 another a little more in the contrary direction ; while the 

 average or middle point does not vary, but different sets of 

 experiments (taken in as great a variety of circumstances as 

 possible) yield the same mean, provided only they be suffi- 

 ciently numerous ; then that mean or average result, is the 

 part, in each experiment, which is due to the cause A, and 

 is the effect which would have been obtained if A could have 

 acted alone : the variable remainder is the effect of chance, 

 that is, of causes the coexistence of which with the cause A 

 was merely casual. The test of the sufficiency of the induc- 

 tion in this case is, when any increase of the number of trials 

 from which the average is struck, does not materially alter the 

 average. 



