ANALYSIS OF QUANTITATIVE PHENOMENA. 391 



obtained a measure of the effect of atmospheric pressure 

 on the bulb, he opened the top of the thermometer tube 

 and admitted the air. The level of the water now sank 

 still more, partly from the pressure on the bulb being 

 now compensated, and partly from the compression of the 

 water by the atmospheric pressure. It is obvious that 

 the amount of the latter effect was approximately the 

 difference of the two observed depressions. 



Not uncommonly indeed thje actual phenomenon which 

 we wish to measure is considerably less than various 

 disturbing effects which enter into the question. Thus 

 the compressibility of mercury is considerably less than 

 the expansion of the vessels in which it is measured 

 under pressure, so that the attention of the experi- 

 mentalist has chiefly to be concentrated on the change 

 of magnitude of the vessels. Many astronomical phe- 

 nomena, such as the parallax or proper motions of the 

 fixed stars, are far less than the instrumental imper- 

 fections, and the other phenomena of precession, nutation, 

 aberration, &c. Even Flamsteed imagined he had dis- 

 covered the parallax of the pole star 6 , and time after 

 time astronomers mistook various other phenomena for 

 that minute motion which they were so desirous to 

 discover. 



Methods of Eliminating Error. 



In any particular experiment it is the object of the 

 experimentalist to measure a single effect only, and he 

 endeavours to obtain that effect free from any interfering 

 effects. If this cannot be, as it seldom or never can 

 really be, he makes the effect as considerable as possible 

 compared with the other effects, which he reduces to a 

 minimum, and treats as noxious errors. Those quantities, 



e Baily's ' Account of the Rev. John Flamsteed,' p. 58. 



