METHOD OF VARIATIONS. 53 



Fizeau's method of measuring the velocity of light 

 enabled him to appreciate the time occupied by light in 

 travelling through a distance of eight or nine thousand 

 metres. But the revolving mirror of Wheatstone sub- 

 sequently enabled Foucault and Fizeau to measure the 

 velocity in a space of four metres. In this latter method 

 there was the obvious advantage that various media could 

 be substituted for air, and the temperature, density, and 

 other conditions of the experiment accurately governed or 

 defined. 



Measurement of the Variable. 



There is little use in obtaining exact measurements of 

 an effect unless we can also exactly measure the conditions 

 with which the effect is to be connected. It is absurd to 

 measure the electrical resistance of a piece of metal, its 

 elasticity, tenacity, density, or other physical qualities, if 

 these vary in degree, not only with the minute and almost 

 inappreciable impurities of the metal, but also with its 

 physical condition. If the same bar changes its properties 

 by being heated and cooled, and we cannot exactly define 

 the state in which it is at any moment, our care in 

 measuring will be wasted, because it can lead to no law. 

 It is of little use to determine very exactly the electric 

 conductibility of carbon, which as graphite or gas carbon 

 conducts like a metal, as diamond is almost a non-con- 

 ductor, and in several other forms possesses variable and 

 intermediate powers of conduction. It will be of use only 

 for immediate practical applications. Before measuring 

 these we ought to have something to measure of which 

 the conditions are capable of exact definition, and to 

 which at a future time we or others can recur. Similarly 

 the accuracy of our measurement need not much surpass 

 the accuracy with which we can define the conditions of 

 the object treated. 



