DIFFUSION. 637 



bodies that the flow of heat due to the difference of temperature is equal to 

 the latent heat of the vapour which leaves the bulb. 



The use of the wet bulb thermometer as a means of estimating the 

 humidity of the atmosphere was employed by Hutton* and Leslie f, but the 

 formula by which the dew-point is commonly deduced from the readings of the 

 wet and dry thermometers was first given by Dr ApjohnJ. 



Dr Apjohn assumes that, when the temperature of the wet bulb is sta- 

 tionary, the heat required to convert the water into vapour is given out by 

 portions of the surrounding air in cooling from the temperature of the atmo- 

 sphere to that of the wet bulb, and that the air thus cooled becomes saturated 

 with the vapour which it receives from the bulb. 



Let m be the mass of a portion of air at a distance from the wet bulb, 

 # its temperature, p, the pressure due to the aqueous vapour in it, and P the 

 whole pressure. 



If cr is the specific gravity of aqueous vapour (referred to air), then the 



mass of water in this portion of air is -p 



Let this portion of air communicate with the wet bulb till its temperature 

 sinks to l} that of the wet bulb, and the pressure of the aqueous vapour in 

 it rises to p 1} that corresponding to the temperature 0^ 



The quantity of vapour which has been communicated to the air is 



, x a-m 



(Pi-l>.)-p 



and if L is the latent heat of vapour at the temperature 0,, the quantity of 

 heat required to produce this vapour is 



crm T 

 -- L - 



According to Apjohn's theory, this heat is supplied by the mixed air and 

 vapour in cooling from to Q v 



If S is the specific heat of the air (which will not be sensibly different 

 from that of dry air), this quantity of heat is 



(0,-ejmS. 



* Playfair's "Life of Hutton," Edinburgh Transactions, Vol. v. p. 67, note. 

 t Eneyc. Brit., 8th ed. Vol. i. "Dissertation Fifth," p. 764. 

 J Trans. Royal Irish Academy, 1834. 



