22 



Mr. J. Tyndall. 



[Feb. 8, 



During the first and last of these observations the sky was entirely 

 overcast, during the other two a few stars were dimly visible. 



On November 13th, 25th, and 26th, observations were also made, 

 but they presented nothing remarkable. 



It was otherwise, however, on December 10th. On the morning of 

 that day the temperature was very low, snow a foot deep covered the 

 heather, while there was a very light movement of the air from the 

 north-east. Assuming aqueous vapour to play the part that I have 

 ascribed to it, the conditions were exactly such as would entitle us on a 

 priori grounds to expect a considerable waste of the earth's heat. At 

 8.5 a.m. the thermometers were placed in position, having left the hut 

 at a common temperature of 35°. The cotton wool on which the 

 surface thermometer was laid was of the same temperature. A single 

 minute's expo-sure sufficed to establish a difference of 5° between the 

 two thermometers. The following observations were then made : — 



8.10 a.m Air 29° Wool 16° 



8-15 „ „ 29 „ 12 



Thus, in ten minutes, a difference of no less thau 17° had established 

 itself between the two thermometers. 



Up to this time the sun was invisible : a dense dark cloud, resting 

 on the opposite ridge of Blackdown, virtually retarded his rising. 



8.20 a.m Air 27° Wool 12° 



8.30 „ ........ „ 26 „ 11 



8.40 „ „ 26 „ 10 



8.45 „ „ 27 „ 11 



8.50 „ „ 29 „ 11 



During the last two observations, the newly risen sun shone upon 

 the air thermometer. As the day advanced the difference between air 

 and wool became gradually less. From 18° at 8.50 a.m., it had sunk 

 at 9.25 to 15°, at 9.50 to 13°, while at 10.25, the sun being unclouded 

 at the time, the difference was 11° ; the air at that hour being 31° 

 and the wool 20°. 



In the celebrated experiments of Patrick Wilson, the greatest 

 difference observed between a surface of snow and the air 2 feet 

 above the snow, was 16° ; while the greatest difference noticed by 

 Wells during his long continued observations, fell short of this 

 amount. Had Wilson employed swandown or cotton wool, and had 

 he placed his thermometer 4 feet instead of 2 feet above the surface, his 

 difference would probably have surpassed mine, for his temperatures 

 were much lower than those observed by me. There is, however, 

 considerable similarity in the conditions under which we operated. 

 Snow in both cases was on the ground, and with him, there was a 

 light movement of the air from the east, while with me the motion 



