MB. W. N. SHAW ON HYGROMKTRIC METHODS. 137 



P the whole pressure of the air. 



S is the specific heat of the air. 



L is the latent heat of the vajKnir at the temperature 6. 



<r is the specific gravity of the aqueous vapour referred to air. 



K is the coefficient of thermal conductivity of the air. 



D the coefficient of the diffusion of the vapour. 



R is the coefficient of radiation per unit of surface of the bulb. 



A is the area of the bulb. 



C is identical with the electrostatic capacity of the bulb, and is there- 

 fore equal to the radius for a spherical bulb. 



p is the mass per unit of volume of air. 



# the temperature of the air undisturbed by the wet bulb, i.e., the 

 dry- bulb temperature. 



Q l is the wet-bulb temperature. 



This equation, which has also been obtained by STEFAN, for a spherical bulb, differs 

 from the convection formula, which is p = p l PS (0 0^/Lcr, only by the factor 

 in the last term. The factor K/D is said, by MAXWELL, to be less than unity, and 

 probably about 77. STEFAN, in the ' Zeitschrift der Oesterreich. Gesell. fiir Meteoro- 

 logie,' vol. 16, 1881, assigns the value unity to K/D. He, moreover, deduces the 

 special case of the formula for a rotational ellipsoidal bulb. 



It appears from the formula that for spherical bulbs the effect of radiation is 

 proportional to the diameter of the bulb. Further, if we take account of convection 

 currents, their effect would practically be to increase R and D, keeping their ratio 

 constant, so that the relative importance of the radiation term would be diminished 

 by the effect of a current of air. 



The application of this formula is further considered by PERNTER,* who points out 

 that the usual assumption that the effect of radiation is negligible leads to serious 

 error for bulbs of considerable magnitude. Substituting in the formula, he finds that 

 for still air the radiation effect is equal to the conduction and diffusion effect if the 

 radius of the bulb (supposed spherical) is equal to 0*57 cm. With a convective 

 current the effect of radiation would diminish, so that for a current of given velocity 

 the formula might be written 



where A is a constant and probably equal to O'OOOGSO, a is constant for a given 

 velocity of the air, and for a given pressure. Its value, as deduced from a considera- 

 tion of observations by BLANFORD, ANQOT, CHISTONI, and the author of the paper, is 

 3'0 P/7GO. Moreover, PERNTER considers a certain further correction in the following 



* ' Sitzungsberichte der Wiener Akademie,' vol. 87, 1883. 



Ml MVi-I.X XXVIII. A. T 



