On the Hi/gro?i>c/er b^j Evaporation. 83 



it would be easy to guard against, were tlie eftect well ascer- 

 tained by exjierinient. 



Another disturbing cause it seems difficult entirely to ob- 

 viate. For the stem of the thermometer, having constantly 

 the general temperature ot" the atmosphere, will communicate 

 some of its heat to the colder bulb ; and although the air which 

 touches the bulb may in this case be fully saturated with hu- 

 midity, yet the heat extricated from it in cooling will be less 

 than the latent heat of the vapour diffused in it : whereas the 

 success of the experiment requires an exact equality between 

 these two quantities of heat. 



In this ex2:)eriment it is presumed that all the air that comes 

 in contact with the wet bulb is continually saturated with va- 

 pour, and likewise that all the heat necessary for feeding the 

 evaporation is furnished by the same air. If the air be dis- 

 placed from the humid surface williout being saturated with 

 moisture ; or if the heat that enters in a latent form into the 

 constitution of the rising vapour, be derived from any other 

 source than the air that comes successively into contact with 

 the wet bulb ; in both these cases the experiment will fail as 

 a means of discovering the state of the atmosphere in regard 

 to moisture. The effects of the disturbing causes that have 

 been enumerated, do not appear to be so formidable as to en- 

 danger the success of the experiment to a degree of accuracy 

 sufficient for most practical purposes. 



3. If we now suppose that the temperature of the thermo- 

 meter is stationary, and that every portion of air becomes sa- 

 turated with moisture by its contact with the wet bulb, and 

 likewise that it deposits just as nuicii of its heat as is con- 

 tained in a latent form in the vapour it absorbs ; we are led 

 to this question ; 



Piioii. A given quantity of air, as a cubic foot, under a 

 given pressure, having a given temperature, and containing 

 vapour of some unknown tension, being sup})oscd to liave its 

 temperatiu-e depressed by any cause, and likewise to luive the 

 vapour due to all the lieat extricated in cooling continually 

 diffused in it, till at length it becomes saturated witli moisture 

 at sfune known tempeiature ; it is recjuired to find ihe un- 

 known tension of the aqueous vapour contained in the air. 



By the vapour due to a certain (|uant!ly of heat is meant, 

 the vajwur which contains the ))ropo::ed heat in a late nt lonn ; 

 and it is supposed that the same Cjuanlity of heat is absorbed 

 in the formation of vapour at all temperatures. 



To solve this problem, let b denote thf height of tlie baro- 

 meter, T the temperature estimiiled (»n tlie centigrade scale, 



L 2 and 



