80 Daniell oji the Correction 



or^dissolved in it so as to form part of the same homogeneous 

 and invisible fluid, appears to have a powerful effect to increase 

 the elasticity of the air and its expansion, for every additional 

 degree of heat which it receives. Though the judicious and 

 accurate experiments of General Roy have ascertained this 

 effect of humidity, and have even gone far to determine the 

 law of its operation, yet, for want of a measure of the quantity 

 of it contained at any given time in the air, it is impossible 

 to make any application of this knowledge to the object under 

 our consideration." 



Lastly, Mr. Leslie, in an article upon Barometrical Measure- 

 ments in the Supplement to the Encyclopedia Britannica, con- 

 cludes his detail of corrections with the same acknowledgment : 

 " The humidity of the air also materially affects its elasticity, 

 and the hygrometer should therefore be conjoined with the 

 thermometer in correcting barometrical observations. But 

 nothing satisfactory has yet been done with regard to that 

 subject. The ordinary hygrometers, or rather hygroscopes, 

 are mere toys, and their application to science is altogether 

 hypothetical." 



Impressed with the importance of the object so clearly pointed 

 out by a succession of the most able philosophers, I had no 

 sooner succeeded in constructing an instrument which, upon un- 

 erring principles would shew the quantity of vapour contained 

 in the air at any given time, than I turned my attention to render 

 it available to the desired purpose. My first ideas and specu- 

 lations upon the subject were, however, very incorrect. In com- 

 mon with some others, who had not sufficiently considered the 

 subject, I imagined that the elastic force of the vapour was added 

 to that of the permanent gases without producing any change in 

 the latter, leaving the effects of expansion entirely out of conside- 

 ration. But, without entering into details of difficulties which 

 would only tend unnecessarily to complicate the subject, I shall 

 now endeavour to explain a method of observation and calcula- 

 tion, which, I trust, will be found fully and strictly to solve the 

 important problem of the effects of atmospheric vapour upon 

 barometrical mensuration. 



