Psychrometric Formtda. 219 



the limits of the probable range set by the other formula. The 

 probable errors of these values are 0.0077 and 0.0000341. It 

 is needless to calculate values of x and r from this formula ; 

 they could not show any perceptible difference from the 

 others. 



Conclusions. 



Our observations show clearly that the reading of a wet-bulb 

 thermometer is not dependent in any way on the nature of the 

 material with which the bulb is covered, provided at any rate 

 that the material be one of those generally recognised as suit- 

 able. (The only other work which deals with this question at 

 all, so far as we know, is that of Edelmann\ who examined 

 only various cotton-stuffs, and under the very special conditions 

 of perfectly dried air. The different materials were not com- 

 pared directly, but the mean values of the constant obtained by 

 observations with each differed slightly ; since, however, the 

 difference was not as much as the variations in any one set no 

 definite conclusion can be drawn from such experiments). This 

 proves that no hygroscopic action occurs at the temperatures 

 and under the conditions of these experiments, and they are 

 the ones of largest practical importance at present. 



Moreover, the result shows that in the great majority of 

 cases there is no such diminution of / as has been shown to be 

 possible, apart from hygroscopic action. The variations of A 

 are purely accidental, and not systematic. The conclusion is 

 abundantly justified that under proper working conditions the 

 constant r/ is unity, as in the ordinary theory, and this inde- 

 pendently of the kind of material used to coat the wet-bulb 

 thermometer. Conversely we may gather that the ordinary 

 theory of the psychrometer is essentially correct, and that the 

 conditions under which this work was done are eminently suit- 

 able to the proper use of it as a meteorological instrument. 



Further observations on the same lines are being carried 

 on, and we hope to publish next year a supplement to this paper 

 containing the results of a more extended experience. With 

 the advent of warmer weather, observations under conditions of 



1 Edelnuuui, Meteor. Zeitschr., 18%, p. o'25. 



