COSMIC PHILOSOPHY 



of the particular logical method which you are 

 employing. Nevertheless it cannot but be of 

 advantage to you to know that you are using 

 the " method of concomitant variations," and to 

 understand on general principles the conditions 

 under which this method may be employed 

 and the precautions required in order to make 

 it valid. For want of such general knowledge 

 of method, even trained physicists not unfre- 

 quently make grave errors of inference, apply- 

 ing some powerful implement of research in 

 cases where interfering circumstances, not suf- 

 ficiently taken into account, render it powerless. 

 Thus the method just alluded to, of varying the 

 cause in order to observe and note the concomi- 

 tant variations of the effect, is a very powerful 

 instrument of induction ; but in order to use it 

 effectively, we need to bear in mind two things. 

 First, we need to know the quantitative relation 

 between the variation of the cause and that of 

 the effect ; and secondly, we need to know that 

 the intermixture of circumstances will not, after 

 a certain point, alter the order of the variations. 

 In the case of the pendulum, just cited, we know 

 both of these points. We know that the only 

 factors in the case are the momentum of the 

 pendulum, acting in concert with gravity, the 

 friction at the point of support, and the friction 

 and resistance of the atmosphere ; and as we 

 progressively diminish these latter retarding 



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