ON DEW, &c. 175 



13, and that of swandown, lying on the snow, 

 8. At 9h. the air was 23J, . snow 17, and 

 swandown 15. The sky being now, in great 

 measure, covered with high thin clouds, my 

 experiments ceased. At lO^h. the sky was 

 very bright, and the atmosphere very calm ; but 

 it was not then convenient to me to renew my 

 observations. Had I repeated them at that 

 time, I should probably have found a difference, 

 between the temperature of the swandown and 

 air, several degrees more considerable than 

 the one of 14^, which had already occurred on 

 this evening, and consequently greater than the 

 greatest observed by Mr. Wilson, between the 

 temperatures of snow and of the atmosphere, 

 which was one of 16. 



The next favourable evening was that of the 

 21st. Much snow having in the meanwhile 

 fallen, its depth was now more than a foot. 

 The thermometers were observed 5 times be- 

 tween 4 h. 15m. and 4 h. 55 m. At 4 of those 

 times, the swandown was 13, and at one of 

 them 13^, colder than the air, the heat of 

 which at the 4 first observations was 26, and 

 at the last 25^-. The temperature of the sur- 

 face of the snow, during the whole period of 

 observation, was 17, and consequently 4 times 

 it was 4, and once 5, less cold, than that of 

 the swandown. The atmosphere was altogether 



