1894.] Atmospheric Temperature, especially during Fohn. 115 



other. Considering the temperatures at a height of 1 m. there is 

 a difference of 6'5 between the, land and the ice. The difference of 

 vapour tension, 0'2 mm., is insignificant, and shows that substantially 

 the air is the same. The dew point in both cases is several degrees 

 below 0, so that, on coming in contact with the ice, there would be 

 evaporation from it. The evaporating power of the air may be repre- 

 sented by the difference between the tension of saturation and the 

 actual vapour tension. It is very great on land, being 10'75 mm. at 

 16'53 C., and it would rapidly evaporate water having that tempera- 

 tare. On coming in contact, however, with ice the air actually in 

 contact, which alone comes under consideration, is first cooled to C., 

 which reduces its saturation tension to 4'6 mm., and the difference is 

 only 1'4 mm. We see, however, that this has been sufficient to increase 

 the absolute humidity of the air in close proximity to the ice. At 



1 m. above the ice the air had an average temperature of 10 C. ; at 



2 cm. from the ice its temperature was as high as 6'65 C., and the 

 air in actual contact with the ice must have been at C. Many 

 observations have been made of the temperature of the air at different 

 heights above glaciers, and, as might be expected, considerable dif- 

 ferences have been observed ; but I am not aware that any observations 

 have been made on the air almost but not quite in contact with the 

 ice, as are those which have been made at 2 cm. from the ice. The 

 bulb was perfectly shaded from the sun but freely exposed to the 

 wind, it was also fully exposed to any cold radiations from the ice 

 There is, therefore, no doubt that 6'65 was the temperature of the 

 air passing the bulb of the thermometer. The vertical distribution 

 of temperature shown by these figures is remarkable. From 



height of 1 m. to within 2 cm. of the ice there is a gradient of 



'4 per metre, in the remaining 2 cm. there is a gradient at the rate 

 )f 33 per metre ; and, from various observations and considerations, 

 it is probable that the moderate gradient is continued to within a 



lillimetre of the ice, when it becomes precipitous. It is to be noted 

 that the absolute humidity, as shown by the vapour tension of the air, 

 las increased from 3'4 mm., at 1 m., to 4'1 mm., at 2 cm. ; showing 



lat ice is being evaporated and transferred from the glacier to the 

 itmosphere. The wind was blowing freshly down the glacier, and 

 its velocity was measured by noting the time which pieces of paper 

 allowed to drift took to reach the ice, and then pacing the distance. 



he mean velocity was found to be from 8 to 10 kiloms. per hour. 

 The observations made on the 21st and on the 22nd confirmed 

 those of the 19th. The same variability of the air temperature at the 



md stations was noticed. Between 12.55 and 1.6 P.M. the following 

 temperatures were observed by whirling : 16'2, 16'2, 16'0, 15'5, 

 L6-0, 15-5, 15-0, 14-2, 13'8, 14 0> 0, 13'5, 13'5. These are all good ob- 

 servations, and represent real variations of the temperature, or rather 



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