436 



lion (I), and writing in it the values just found for m' 

 and nii we arrive at the final equation 



/"=/'-^^^^^x^-^, (II) 



in which the force of vapour at the dew-point is expressed 

 in terms of the force of vapour at t' , and of the difference 

 of the temperatures of the wet and dry thermometers. 



" This formula is applicable for all values of ^' above 32^ ; 

 but when the stationary temperature of the wet thermometer 

 is lower than the freezing point, it will require modification. 



- grains of air, we have seen, in cooling through t — i' 



degrees, convert into vapour t — t' grains of moisture. But 

 if/' be less than 32°, a greater amount of air will be neces- 

 sary for accomplishing this, inasmuch as the heat evolved 

 has first to liquify ice, and then to convert the water jnto 

 vapour. The additional quantity is obviously represented 



135 

 by the fraction -; ; 135 being the caloric of liquidity 



of water, and 1179 — t' the latent heat of aqueous vapour at 

 t'. But this fraction, if we substitute S2^ for t'^ (which may 

 be always done without sensible error) is equal to 0-118. 



Hence for values of /'below 32°, - +0*118-= M18- 



a a a 



is, in grains, the weight of air which, in cooling through 



t — t' degrees, vaporizes t—t' grains of moisture. When 



this correction is applied, the final equation, applicable to 



observations in which the wet thermometer indicates lower 



temperatures than 32°, becomes 



/"=/'-'-^^^x^-^' (III) 



" Assuming, as before, the specific heat of air, a, to be 

 •267, the value assigned to it by Delaroche and Berard, 

 and taking for e the value it would have at 50°, upon the hy- 

 pothesis that 967° is the latent heat of vapour at 212°, and 



