122 H. MOHN. METEOROLOGY. [2nd. arc. exp. fram 



MOISTURE OF THE ATMOSPHERE. 



In summer and autumn the observations for determining the force 

 or tension of aqueous vapour in the atmosphere and the relative 

 humidity, were made by the psychrometer or dry and wet-bulb thermo- 

 meters suspended in the thermometer-screen. When the temperature 

 was above — 10° C, the force of vapour and the relative humidity were 

 computed by means of Jelinek's Psychrometer Tables, the readings of 

 the wet bulb having been corrected by Ekholm's rule for tempera- 

 tures below zero. 



The Expedition had two hair-hygrometers, system Koppe, made by 

 Pfister & Streit in Bern. They were suspended in the thermometer- 

 screen, and could be verified in saturated air which is produced by 

 putting the cover with a wet side over them. When the hygrometer 

 in use required cleaning, or did not seem to work well, it was taken 

 in, and the other instrument put out, after having been verified and cor- 

 rected in saturated air. The occasions noted in the journal when the 

 hygrometer was changed were: in 1899, March 25 and 31, May 18, 28, 

 June 22, 23; in 1900, January 16, April 28, October 4; in 1901, April 

 23, 24, June 1, September 2, 3, 28, October 20, November 22; in 1902, 

 January 7, 28, February 11, 12, 13, March 24. 



When the temperature of the air was below — 10°, the force of 

 vapour was computed by means of the psychrometer Tables from the 

 readings of the hair-hygrometer and those of the dry thermometer. 

 An error in the reading of the hygrometer has very little effect, in low 

 temperatures, upon the amount of the force of vapour. The num- 

 bers in the following Tables have been found from the psychrometer; 

 in 1899, June, July; in 1900, July, August; in 1901, July, August 

 (missing psychrometer-observations completed by hair-hygrometer obser- 

 vations corrected by the corrections found from the simultaneous obser- 

 vations with both) ; in 1902, June (to which the above remark applies), 

 July; and from the reading of the hair-hygrometer in all the other 

 months, corrected as far as possible. 



In the Tables, the force of vapour is given in millimetres (mm.) and 

 the relative humidity as percentages (p. c). The last row gives the 

 monthly means. 



