206 THE PRINCIPLES OF SCIENCE. 



70 elements which mode of determination is most accurate. 

 Modern chemistry presents us with an almost infinitely 

 extensive web of numerical ratios developed out of a com- 

 paratively few fundamental ratios. 



In hygrometry we are presented with a choice among 

 at least four modes of measuring the quantity of aqueous 

 vapour contained in a given bulk of air. We can extract 

 the vapour by absorption in sulphuric acid, and directly 

 weigh its amount ; we can place the air in a barometer 

 tube and observe how much the absorption of the vapour 

 alters the elastic force of the air ; we can observe the dew 

 point of the air, or the temperature at which the vapour 

 becomes saturated ; or, lastly, we can insert a dry and wet 

 bulb thermometer and observe the temperature of an 

 evaporating surface. Now the results of each such mode 

 can be connected by well-established theory with those 

 of the other modes, and we can select for each experiment 

 that mode which is either most accurate or most conve- 

 nient. The chemical method of direct measurement is 

 probably capable of the greatest accuracy, but is trouble- 

 some ; the dry and wet bulb thermometer is sufficiently 

 exact for meteorological purposes. 



Agreement of Distinct Modes of Measurement. 



Many illustrations might be given of the accordance 

 which has been found to exist in some cases between the 

 results of entirely different methods of arriving at the 

 measurement of a physical quantity. While such accord- 

 ance must, in the absence of any information to the contrary, 

 be regarded as the best possible proof of the approximate 

 correctness of the mean result, yet instances have occurred 

 to show that we can never take too much trouble in con- 

 firming experimental results of great importance. Even 

 when three or more distinct methods have given nearly 



