XX.] METHOD OF VARIATIONS. 441 



menter, therefore, to obtain a regular and governable 

 supply of the force which he is investigating. To de- 

 termine correctly the efficiency of windmills, when the 

 natural winds were constantly varying in force, would be 

 exceedingly difficult. Smeaton, therefore, in his experi- 

 ments on the subject, created a uniform -wind of the 

 required force by moving his models against the air on the 

 extremity of a revolving arm.^ The velocity of the wind 

 could thus be rendered greater or less, it could be main- 

 tained uniform for any length of time, and its amount 

 could be exactly ascertained. In determining the laws of 

 the chemical action of light it would be out of the question 

 to employ the rays of the sun, which vary in intensity with 

 the clearness of the atmosphere, and with every passing 

 cloud. One great difficulty in pliotometry and the investi- 

 gation of the chemical action of light consists in obtaining 

 a uniform and governable source of light rays.^ 



Pizeau'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 advantage that various media could be sub- 

 stituted for air. and tlie temperature, density, and other 

 conditions of the experiment could be accurately governed 

 and measured. 



Measurement of the Variable. 



There is little use in obtaining exact measurements of 

 an effect unless we can also exactly measure its conditions. 

 _ It is absurd to measure the electrical resistance of a 

 piece of metal, its elasticity, tenacity, density, or other 

 physical qualities, if these vary, not only with the minute 

 impurities of the metal, but also with its physical con- 

 dition. If the same bar changes its properties by being 



• Philosophical Transactions, vol li. p. 138; abridgment, vol xL 

 P- 355- 



2 8(;f" Bunsen and Roscoe's researches, in Philosophical Transaction!, 

 (1859), vol. cxlix. p. 880, &c., where tliey dcscriljc a constant llanic of 

 carbon monoxide yas. 



