that within the limited range of atmospherical pressure, 

 the evaporation from the moistened bulb of a thermo- 

 meter is invi-r^ily proportional to the height of the ba- 

 rometer. This conclusion mij;ht, indeed, have been 

 drawn from the (act, that the evaporation from the sur- 

 face of water is, in all cases, proportional to /', /'; 

 since the mechanical influence of pressure must be the 

 tame, whether resulting from the elasticity of air or va- 



H Y G R O M E T R Y. 



Hence we have by substitution, as before, 



585 



pour. ence our formula D = (p -- J (f t /"') 

 corrected for atmospherical pressure, become* 



It now only remains, therefore, to determine the co-effi- 

 cients p and ; and for this purpose twu equations are 



necessary. 



61. In one of the experiments which we made to 

 ascertain the necessary data, when the barometer 

 itood at 29.75, Leslie's hygrometer at 85", and DC 

 Luc's at 27.6, the temperature of the air wa 67.2, 

 that of the thermometers with moistened bulbs 52", and 

 the point of deposition 35".7. We thus have 



/, rr/er., =.66357 ........ {31. 



<> T = Pii7 = -22723 ........ $31. 



D = 67.* 52 = 15.2 by the covered thermometer*, 

 or D = 85 x ~ = 15.3 by Leslie's hygrometer { ?5. 



D=- - = 15*.25, using the mean. 



The values off, , f, and D being substituted in the 



general equation D = (p \ (/,_/'), we ob- 

 tain 



p ~ 



And p- 



= 54.66 

 15.25 



or p = 34.66 + 



In another experiment of the same kind, when the 

 barometer stood at SO.OS5, Leslie's hygrometer at 

 $8J, and De Luc's at 4*, the temperature of the arr 

 wa 56 .42, the temperature of the thermometers with 

 Moistened bulbs 48P.51, and the point of deposition 

 89*.5. From these data we have, 



f, =f**=. 46421 

 sjtJUWt 



D = 56.48 49 51=6.91, by the covered thermometers. 

 D= 38 3 X ~ = 6 89, by Leslie', hygrometer. 



Using the mean D = 



VOL. XL FART IL 



= 6 . 9 . 



Hygrome- 

 try. 



A nd /) = 35.34 + - 



But p = 34.66 + 



15.25 



Therefore p = 35.9, and r = 12.3. 



62. It appeared by the mean result of a very great Formulafor 

 number of experiments, performed with the utmost lhe forcc of 

 care, and in various states of the air with regard to ^l]" 1 /" 

 pressure, temperature, and humidity, that the value of i sllD g * n 

 p must be greater than 35.5, but less than 36.5. The the atmo. 

 value of r embraced a wider range, and varied from 5 sphere. 

 to 15; it is obvious however, from the nature of the 

 equation, that p ia the more important of these co-effi- 

 cients, and that the value of D can be but slightly af- 

 fected by any change in the value of r. We cannot err 

 greatly, therefore, if we take 36 for the value of p, and 



1 for that of r. The formula D = (p } (f, /') 



ft \ ft f 

 will thus become 



;-, 



Orf'-f 



- 



180 ID 



This formula will enable us to find, by means of the 

 Table in J 39, the absolute quantity of moisture con- 

 tained in the atmosphere, when the pressure of the ba- 

 rometer, the temperature of the air, and that of a ther- 

 mometer having its bulb covered with moistened paper, 

 are riven. Thus, let it be required to find the quanti- 

 ty of moisture contained in a cubic inch of air, when 

 the barometrical pressure is 29-35, the temperature of 

 the air 65, and the temperature of a thermometer with 

 a moistened bulb 51.5. 



f f T* P f ^T X 99-35 X 13.5 



J ~ Jt ~~ 180 JD ~ Ja 180 6.75 



= .61734 .381 16 = .83618 



And by $ 50, 



gntm. 



.61734 : .83618 :: .00395897 : .0015146 



Hence, in the supposed state of thing*, there is 

 .001 5 1-kjgr. of moisture in a cubic inch of air, which 

 would correspond to 35, as the point of deposition. The 

 point of deposition by actual observation was 35. 45 ; 

 and De Luc's hygrometer stood, at the time, at 27". 



63. If we substitute the value off tor F, in the for- 



mula g 7 = 

 obtain 





' " kid down in 



we 



*=' 



447.4 + t 

 4K 



Formula for 

 the quanti- 

 ty of moil- 

 ture in a 

 cubic inch 

 of air. 



