Psychro'inetric Fonnula. 203 



From these the conclusion is draAvn that the condensation 

 varies with the material, or that the material exerts a dif- 

 ferential hygroscopic effect. Assuming the air contained in the 

 vessel to be saturated, and adopting a value for the constant A 

 given by Regnault for a closed space, the values of rj to be 

 used in the formula are calculated — viz, : 



Linen - - . - i.OOO 



Silk - - _ . 0.967 



Wool - - - *- 0.947 



Cotton - - - - 0.944 



From other considerations the value of rj for linen is 



regarded as more probably 0.9737 ; in that case the air cannot 



have been saturated, and the list becomes — 



Linen - - - - 0.9737 

 Silk - - - - 0.940 



Wool - - - . 0.921 



Cotton - - - - 0.918 

 Now it is at once evident that these observations are much 

 too few to serve as a basis for any theory. It seems certain 

 that they were not reliable to the hundredth of a degree, and 

 it is only in the case of cotton and linen that any definite 

 difference appears. (Special attention was paid to these two 

 materials, which are the ones most commonly employed in 

 meteorological work.) It is not, however, our intention to 

 criticise these results themselves, but the conclusions w^hich 

 Ekholm, starting from them, reaches for the psychrometer in 

 general. At such low temperatures as this the vapour- 

 pressures are very small, and the instrument consequently 

 insensitive ; so we must agree with Ekholm's own remark that 

 systematic observations in cold countries with thermometers 

 graduated in hundredths of a degree are needed before the 

 question of the behaviour of the psychrometer below zero 

 can be satisfactorily discussed and settled. 



The modified formula thus obtained is applied by its author 

 to the observations of Regnault^ and Svensson'-* for tempera- 



1. Regnault, '-Ktudes sui- rhyyronietrie," Ann. Chiui. Phys., ISoS, 3 ser. vol. 37, 

 •2. Sveiissoii, " ITiitersuchuni-eri des Assinamischen P.svchronieters," Meteor. Zeitschr., 



1896, p. 201; Bihaiiy till Konol Sveii.-ka Vet.-Akad. Stockholm. 21, 1896. 



"Zur Keimtiiis des ventiliti-teii P-syehroiueters-," .\kad. Abhaiidluii.tf, Stockholm, 189^. 



The observations discussed are in the sei^ond paper. 



