258 Dr. Hudson's Method for inferring the Dew-point 



Now, in the wet-bulb hygrometer, when liquid becomes 

 stationary, the temperature of wet ball gives the temperature 

 of vapour carried off, and also of the air which carries away 

 this moisture, otherwise the instrument could not remain sta- 

 tionary. Hence the heat lost by air in sinking from its pre- 

 vious temperature to that of wet ball, gives the latent heat of 

 the vapour formed. Now, as no increase of the velocity with 

 which air approaches ball makes any difference in the tem- 

 perature of hygrometer, I must conclude that the air thus 

 chilled in causing evaporation carries off all the vapour that 

 can exist in it at this reduced temperature ; and hence (if the 

 air be perfectly dry) that the depression of temperature of hy- 

 grometer gives the measure of the quantity of heat requisite 

 for the conversion " into vapour of all the moisture that can 

 exist in a given space at the temperature of the wet ball," or, 

 in fact, that, (in perfectly dry air at different temperatures, 

 the depressions of temperature of wet ball ought to be pro- 



(\\6S-—t x 22 /' \ 

 — — — — — ) for the tem- 

 perature of hygrometer in each case. 



If air be not dry, of course hygrometer will be cooled less 

 in proportion to the previous dampness of air. Thus if hygro- 

 meter falls in dry air from t + V to t 9 and in moist air from 



t + d to / ; then V: V—d : : the moisture of saturation at t : 



" the quantity of moisture existing in this air" or, in fact, to 

 the " moisture of saturation at the dew-point" Hence, if the 

 value (in degrees of the thermometer) of any one of the pro- 

 portional numbers representing the relative fall in dry air be 

 found, you can at once by proportion assign the depression 

 in dry air at any other temperature of the hygrometer ; and 

 consequently, from this, infer the dew-point from the differ- 

 ence of temperature of thermometer and wet ball by the pro- 

 portion V (the fall in dry air) : V — d (d being fall in moist 

 air) : : m'oisture of saturation at f (temperature of hygrome- 

 ter) : nV'oisture of saturation at dew-point. (See table at end.) 



Suppose (for example) that, when hygrometer marks 61° in 

 dry air, thermometer stands at 122 0, 124 (V being = 51°124. 

 Hence since the thermometric value of Q f at 61° = 51° # 124, 

 all the numbers in fourth column of table (derived from 

 formula B) should be increased in the proportion of 25*933 

 : 51*124, in order that they should represent the number of 

 degrees that dry air should cause the hygrometer to fall at the 

 various temperatures of the latter. Now, suppose hygrometer 

 marks 61° in moist air, while thermometer marks 100°, we 

 have (by proportion) V: V — d, i.e. 51-124 : 39 : : 0146475 



