1836.] of the Wet-bulb Hygrometer. 411 



Before proceeding to detail the experiments directed to the eluci- 

 dation of this point, it may be as well, as we have already become 

 acquainted with the theoretical expressions of other authors for the 

 maximum, to see how they also bear upon the intermediate depressions. 



The formula of M. Gay Lussac makes no provision for aught 

 but the maximum depression ; but the omission may be readily sup- 

 plied on the same simple principle as has been adopted by Professor 

 Apjohn ; namely, by the addition of — /", the aqueous tension at the 

 dew-point, to/' : thus, by the latter author, in all cases d = 87 /' — /". 



At first sight, this would seem a simple arithmetical ratio, like Dr. 

 Hudson's, but inasmuch as the tensions (f) are themselves in geome- 

 trical ratio to the temperature ft), the same parabolic curvature will 

 extend to the centesimal depressions ; or/' — /" will follow some low 

 power of t — - V. 



Captain Herbert's rule has the same happy introduction of /". 

 We have therefore but two theoretical enunciations to put to the test 

 of comparison with experiment : for which purpose I will now bring 

 forward such evidence as I have accumulated. In this branch of inquiry 

 materials are so numerous in my registers kept at Benares and Calcutta, 

 that it becomes expedient to gather selected data into groupes adapted 

 to elucidate various points of the hygrometric scale. Moreover, as unity, 

 or the maximum depression, varies in amount at each temperature, all 

 observations must be brought to common centesimal terms before they 

 can be compared in the manner which is best adapted to give a quick 

 perception of the relation of such phenomena ; namely, by a diagram, as 

 in PI. XXII. First, then, to enumerate the data afforded by method 5, or 

 comparison with the dew-point, of which, in addition to my Benares ob- 

 servations, I have profited by the presence of an American ice-house on 

 the banks of the Hughli to collect an accurate series made thrice per 

 diem in the hottest period of our Calcutta year. 



Tab. IV. — Comparison of intermediate Depressions with aqueous tensions, 

 ascertained by the dew-point method, at Benares. 



Number of observa- Temp, 

 tions agreeing close- of air. 

 ly in their respec- 

 tive particulars. t 







Wet- 

 bulb. 



*' 







Dew- 

 point. 



t" 







Centesimal 

 tension. 



/"+/ 



Depres 

 sion. 

 t — t' 



or, d 



• Comple- 

 ment cent. 



depn. 



D—d 



D 



89 



Tabular cente- ,; 

 sim. tension g 

 deduced. »- 



7 Obs. mean, 



85.0 



81.5 



79.4 



.83 



3.5 



.84 



+01 



12 Obs. ditto, 



87.5 



81.8 



78.7 



.76 



5.7 



83 



.76 







12 Obs. ditto, 



90.0 



80.5 



75.7 



.65 



9.5 



74 



.64 



—01 



6 Obs. ditto, 



94.0 



81.0 



73.2 



.52 



13.0 



66 



.54 



-f02 



6 Obs. ditto, 



92.5 



75.5 



64.5 



.41 



17.0 



55 



.40 



+01 



13 Obs. ditto, 



88.2 



67.3 



42.9 



.23 



20.9 



43 



.26 



+ 03 



8 Obs. ditto, 



92.6 



68.3 



36.4 



.16 



24.3 



35 



.20 



+04 



3 G 



2 

















