CHAPTER XV. 



ANALYSIS OF QUANTITATIVE PHENOMENA. 



IN the two preceding chapters we have been engaged 

 in considering how a phenomenon may be accurately 

 measured and expressed. So delicate and complex an 

 operation is a measurement which pretends to any con- 

 siderable degree of exactness, that no small part of the 

 skill and patience of physicists is usually spent upon this 

 operation. Much of this difficulty arises from the fact that 

 it is scarcely ever possible to measure one simple pheno- 

 menon at a time. The ultimate object must be to discover 

 the mathematical equation or law connecting a quantitative 

 cause with its quantitative effect ; this purpose usually 

 involves, as we shall see, the varying of one condition at 

 a time, the other conditions being maintained constant. 

 The labours of the experimentalist would be comparatively 

 light if he could carry out this rule of varying one circum- 

 stance at a time. He would then obtain a series of cor- 

 responding values of the variable quantities concerned, 

 from which he might by proper hypothetical treatment 

 obtain the required law of connexion. But in reality 

 it is seldom possible to carry out this direction except 

 in an approximate manner. Before then we proceed to 

 the consideration of the actual process of quantitative 

 induction, it is necessary to review the several devices 

 by which the complication of effects can be disentangled. 

 Every phenomenon measured will usually be the sunij 

 difference or product of two or more different effects, 

 and these must be in some way analysed and separately 



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