METHOD OF VARIATIONS. 55 



Maintenance of Similar Conditions. 



Our ultimate object in induction must be to obtain the 

 complete relation between the conditions and the effeci, 

 but this relation will generally be so complex that we can 

 only attack it in detail. We must, as far as possible, con 

 fine the variation to one condition at a time, and establish 

 a separate relation between each condition and the effect. 

 This will be at any rate the first step in approximating to 

 the complete law, and it will be a subsequent question 

 how far the simultaneous variation of several conditions 

 modifies their separate actions. In many of the most im- 

 portant experiments, indeed, it is only one condition which 

 we wish to study, and the others are merely interfering 

 forces which we would gladly avoid if possible. One of 

 the conditions of the motion of a pendulum is the resist- 

 ance of the air, or other medium in which it swings ; but 

 when Newton was desirous of proving the equal gravita- 

 tion of all substances, he had no interest in so entirely 

 different a force as the effect of the air. His object was 

 then to observe a single force only, and so it is in a great 

 many other experiments. Accordingly one of the most 

 important methods of investigation consists in maintaining 

 all the conditions of like magnitude except that which is 

 to be studied. As that admirable experimental philosopher, 

 Gilbert, expressed it f , * There is always need of similar 

 preparation, of similar figure, and of equal magnitude, for 

 in dissimilar and unequal circumstances the experiment is 

 doubtful/ 



In Newton's decisive experiment similar conditions were 

 provided for, with the usual simplicity which characterizes 

 the highest art. The pendulums of which the oscillations 

 were compared consisted of exactly equal boxes of wood, 

 hanging by equal threads, and filled with different sub- 

 f Gilbert, * De Magnate,' p. 109 



