some Experiments in Climatological Physiology, 331 



2. Computation of Dryness. 



To calculate the dryness we may proceed as follows : — 

 If 5 denote the specific heat of air at constant pressure, 

 = 0*2374 (Regnault) ; I the latent heat of water at tem- 

 perature t, =607— -0*708 £ (Olausius) ; r the relative density 

 of aqueous vapour as compared with air =0*6221 about ; 

 P the barometric pressure, the units being metric, then, ac- 

 cording to the Ivory- August- Apjohn formula, the pressure /' 

 of aqueous vapour in the air is 



/'=/-^(T-0, (1) 



/ being the saturation-pressure at temperature t, and f, /, 

 and P being measured in the same units. Let F denote the 

 saturation-pressure at temperature T, viz. that of the dry-bulb 

 thermometer, as / denotes the saturation-pressure at the 

 temperature t of the wet-bulb thermometer ; then the relative 

 humidity is measured by the ratio 



r f-~iV-» f . q 

 ^ = f = — — jr- -= jfS sa y> • • ( 2 ) 



where q is a tabular value of sP(T — t)jrl. It is found 

 that with moderate velocities of motion of the wet-bulb 

 thermometer the value of q is as follows : 



q = 0-000660 P(T-0(l+^), ... (3) 



which has been tabulated *. 



Since the relative dryness tj is zero for saturated air, and 

 unity for perfectly dry air, we may adopt the definition, 

 relative humidity plus dryness equals unity. Hence 



, = 1-^=1-^ (4) 



Thus for practical working F and f may be taken from 

 Table 43, p. 142, and q from Table 44, p. 143, of the third 



* An important article on " The Pressure of Saturated Vapour from 

 Water and Ice as measured by different authorities " is given by 

 Prof. C. F. Marvin in ■ Monthly Weather Review/ Jan. 1909, pp. 3-9. 

 See also the Bibliography, ibid. p. 9. 



