18 H. MOHN. METEOROLOGY. [NORW. POL. EXP. 



was never noted until this was reached. The hygrometer in the screen 

 served as a check on the other, when the latter was set out after being cleaned 

 or verified. 



The comparisons made between the psychrometer and the hygrometer 

 give a mean error of comparison of from 1 to 2 %. For the winter, the error 

 of a single adopted value of the relative humidity is estimated to be from 3 % 

 to 5%. 



The vapour-tension (or absolute humidity) has been computed 



(1) from the psychrometer-observations and JELINEK'S Tables, when the 

 temperature of the wet-bulb thermometer was above 10; from to 

 10 by EKHOLM'S rule. 



(2) from the corrected readings of the hair-hygrometer. 



If 6m be the maximum tension of vapour at the temperature of the air, 

 e the vapour-tension sought, and 

 * the relative humidity, 



we have e = e m X 



The values for e m were taken from JELINEK'S Tables I a, Spannungstafel. 

 For temperatures from 40 to 55 the following auxiliary table was 

 computed by the formula 1 



where A = 4'57 mm, ft = 0'03134, a = 0-003667, and t = temperature 

 of the air. 



Temp, of Air. Max. Tension 



40 0-15 mm. 



45 0-08 



50 0-05 



55 0-02 



Vapour-tensions below 0'05 mm. are given in the tables as O'O mm. 

 The values given for the relative humidity may be uncertain to the 



extent of 5 %. The influence of such an error upon the deduced value of 

 the vapour-tension is trifling, and practically of no importance. The follow- 



1 0. J. BROCH. Tension de la vapeur d'eau p. A. 30. 



