[Proc. Rot. Soc. Victokia, 25 (N.S.), Ft. I., 1912.] 



Ar'i'. XII. — The Pfij/chi'oinetric Fonmda. 



By G. SMKAI., B.Sc. 

 (Goveruuient Re-st-arcli Scholar in tlic University of Mulboiirne). 



[Read lllh July, 1912.] 



In a pai>er published last year by Dr. Love and myself^ we dis- 

 cussed a uioditication proposed by Ekhohn to be made in the formula 

 for the wet-and-dry-bulb hygrometer, wlileh would have important 

 consequences if confirmed. The formula so modified would be 



a- = 77/- AB (/-/'), 

 where x and f are respectively the actual vapour-pressure in the 

 atmosphere and the saturation vapour-pressure at the temperature t' 

 of the wet-bulb. A is the ordinary p^ychrometric constant, and r/ tlie 

 co-efficient, le.s« than unity, whose insertion Ekholm advises in order 

 to allow for diminution of vapour-pressure at the surface of the wet- 

 IniU) h\ a hygroscopic action of the material covering it. 



It was shown that, if there were an appreciable hygroscopic action 

 affecting the temperature of the wet-bulb, a perceptible difference 

 would be observed between several thermou)eters covered with dif- 

 ferent materials. The results recorded showed, however, that three 

 wet-bulb thermouieters, covered respectively with silk, linen and 

 cotton, agreed in their readings to within the limits of observation, 

 which were 0.05 C, the thermometers being divided into tenth- 

 degrees. In all, 63 sets of observations were given, each set compris- 

 ing a comparison of the tln-ee wet-bulbs with a Regnault condensa- 

 tion hygrometer. By the application of least squares to the niudi- 

 tied formula, the value of // was found to be 0.9974, which is unity 

 to the order of approximation possible in such experiments. Tlie 

 conclusions arrived at were that the proposed change was not justitied 

 by the evidence which Ekholm himself produced, and that a direct 

 test showed the supposed basis of it to be incorrect. 



These observations, taken in May and June, were all under con- 

 ditions of fairly high humidity, above 50 per cent, in every case, and 

 consequently low values of the dift'erence t - t'. In order to test the 

 formula under a more extended range of circumstances, the observa^ 

 tions were resumed, and continued until December. No change was 

 made in the conditions of exposure, nor in the method of procedure, 

 both of which were fully described in the former paper. The cover- 



1 Proc. Roy. .Soc. Victoria 24 (n.s.), Pt. II., lOll. 



