1836.] 



of the Wet-lulb Hygrometer. 



415 



at 81°. 2= 1.040 x .71 =.738; at 83°.8 = 1.128 X.63 = .711 (or .716 at 81°.2) 

 being at the two periods of the day, on an average, very nearly 

 equal ; though, relatively, the air is much drier in the afternoon. 



A similar comparison to that afforded by the above table would 

 have been published with my journals for 1825-6 in the Philosophical 

 Transactions for 1827, had the registers been allowed to stand as 

 they were ; but the columns of aqueous tension were struck out, 

 although from the elaborate care I had taken in valuing the degrees 

 of my hair hygrometer they were entitled to some reliance. It is, 

 however, not worth while to republish them, as the wet-bulb instru- 

 ment was then situated outside and the hair hygrometer inside the 

 house*, and the two columns are not strictly comparable. One little 

 table, however, deduced from four years' daily experiments at Benares, 

 which was also suppressed at home, I think likely to prove useful, while 

 it bears directly on the wet-bulb theory, and exemplifies the truth of 

 the assumption of its immediate dependence on/'. This table shews 

 the actual evaporation in depth per month, as measured by a small 

 evaporameter suspended in the open air, for the opposite extremes of 

 the year. The instrument is described in the fifteenth volume of the 

 Asiatic Researches. I have collected on the left hand the observed 

 quantities, and have now inserted on the right the theoretical num- 

 bers which should express the ratio of evaporation. The results are 

 even more satisfactory than could have been anticipated ; and lead to 

 the following very simple rule to find the amount of evaporation 

 roughly in inches per diem. " Multiply the aqueous tension at the 

 wet-bulb temperature by the observed depression in degrees, and 

 divide by 34." Omitting the latter operation, the product will express 

 in round terms the evaporation per month in the open air, or in a 

 moderate breeze. 



Tab. VI, 



Months. 



April 



and 



May, 



March, 



-Rate of Evaporation and simultaneous depression observed at Benares. 



Temp, 

 of air. 



Depres- Obsvd. Eva- 

 sion, poration 

 d per month 

 o inches. 

 19.1 13.9 

 21.3 11.9 

 18.1 14.7 

 20.7 15.1 



Ditto 



per 



diem 



inch. 



Means 79.4 



0.243 



c~- Depres- 



Calculated 

 a .a „• siuu ,*, daily eva. 

 g 1 -/! S tension, poration, 

 ""i-I dXf. *Xfz 

 34 



0.748 15.18 0.447 



0.599 8.16 



0.240 



* The Calcutta Oriental Magazine, 1827, contains the whole paper. 



