CHANCE, AND ITS ELIMINATION. 65 



these are so frequent, or so indeterminate, that we 

 cannot possibly exclude all of them from any experi- 

 ment, although 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 inva- 

 riable. The results of these different trials will natu- 

 rally 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 under as great a variety of 

 circumstances as possible) yield the same mean, pro- 

 vided only they be sufficiently numerous ; then that 

 mean, or average result, is the part, in each experi- 

 ment, 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 suffi- 

 ciency of the induction in this case is, when any 

 increase of the number of trials from which the 

 average is struck, does not materially alter the average. 



This kind of elimination, in which we do not 

 eliminate any one assignable cause, but the multitude 

 of floating unassignable ones, may be termed the Eli- 

 mination of Chance. We afford an example of it 

 when we repeat an experiment, in order, by taking the 

 mean of different results, to get rid of the effects of 

 the unavoidable errors of each individual experiment, 

 When there is no permanent cause such as would 



VOL. n. F 



