from the Indications of the Wet-bulb Hygrometer. 259 



(moisture of saturation at 61): '01117382 = density of va- 

 pour of saturation at 52 o, 408, the dew-point of the air in ques- 

 tion. 



To inquire into what the actual values (in degrees of Fah- 

 renheit) may be of these proportional numbers (derived from 

 formula B) we may adopt three different courses. We may 

 calculate them, lmo, From assumptions as to the capacity of 

 air for heat, &c. &c. ; 2do, From actual observation of the 

 values of the numbers in the table (in degrees of the thermo- 

 meter) when dry air makes hygrometer stand at any tempera- 

 ture; 3tio, By conjoint observations of the thermometer, wet- 

 bulb, and dew-point of the air which has produced this re- 

 duction in temperature of hygrometer. 



Thus, lmo, Suppose that De la Roche's estimate of capa- 

 city of air for heat is correct, or that capacity (by weight) of 

 water: capacity (by weight) of air : : 1000 : 267. Now, latent 

 heat of vapour of saturation at 212° = 956 ; hence it should 

 raise temperature of same weight of air 3580 o, 5 (for 267 : 1000 

 :: 956: 3580*5). Hence also {vice versa) supposing capacity 

 of air 'not to vary with the temperature, the latent heat re- 

 quired by the " vapour of saturation" at 212°, ought, in its va- 

 porization, to reduce temperature of dry air (in equal weights) 

 through 3580°*5 (down to 212°). But supposing air to carry 

 off its saturating quantity of moisture, the weight of air will 

 be to weight of vapour : : 1000 : 625 ; v the reduction of tem- 

 perature should be less in that proportion which would give 



= J 2237° for the actual reduction on these 



1000 / 



suppositions. I shall not, however, dwell further on this 

 method, nor the corrections it would require, as I place no 

 reliance on the truth of the requisite assumptions # . 



2do. Since (B.) gives the quantities of heat which dry air 

 must lose when (in sinking to the temperature of hygrometer) 

 it has become saturated with moisture, it follows that if we 

 know the number of degrees through which this air falls at 

 any one temperature of hygrometer, we can find (by pro- 



* If Dr. Apjohn's assumed data should be admitted, his formula cor- 

 rected would become 



1J79-* * 3 ^??? x / = weight of moisture of saturation (at /) 



448+7x(-0223p+-3312) 

 of that number of cubic inches of air which in falling one degree vapourize 

 ene grain of water ; therefore this weight in grains is the same as the num- 

 ber of degrees dry air must fall when hygrometer stands at this tempera- 

 ture in it, i. e. it is the number V in my formula, and (if correct) might be 

 used in the way I have pointed out to infer the dew-point. 



2L2 



