MR JOHN AITKEN ON DEW. 



45 



limited on this point, and owing to a simple wetting not being so conspicuous 

 as hoar-frost, it is possible trees may occasionally get wet in summer with clew 

 without its being observed. It however seems probable that it will be of much 

 more frequent occurrence in winter than in summer, owing to the much longer 

 absence of the sun during winter nights. 



Radiation from Snow."" 



In a previous part of this paper reference has been made to the radiating 

 and absorbing powers of snow. In the experiment detailed, comparative 

 readings are given of the temperature of snow and of a black surface exposed 

 in shade on a bright day. The temperatures taken under the conditions then 

 existing showed the snow to be much colder than the black surface. This 

 conclusion has since been confirmed by a number of readings with the radia- 

 tion thermometer under different conditions of climate. Of these observations 

 it will only be necessary to give those taken on two days. The following 

 temperatures were taken on the 19th January. In these tables the contents 

 of the columns are arranged as in previous one. 



Hour. 



Air. 



Black. 



Snow. 



Difference. 



10.0 A.M. 

 2.0 P.M. 



2.30 „ 

 3.30 „ 



20°-2 

 30°-0 

 30°1 

 30°-2 



16°-2 

 30°-0 

 30°-2 

 30°-2 



12°-0 

 26°-0 

 26°-4 

 28°-0 



-4°-2 

 -4°-0 

 -3°-8 

 — 2°-2 



In the morning the sky was clear, but by 2 p.m. it became overcast, with a 

 thin uniform covering of clouds ; and at 3.30, it was beginning to snow. The 

 next readings were taken on the 5th February. 



Hour. 



Air. 



Black. 



Snow. 



Difference. 



10.0 A.M. 



23°-0 



28°-8 



25°-0 



-3°-8 



12.0 P.M. 



25°-5 



31°-1 



26°-l 



-5°-0 



2.0 „ 



30°-0 



34°-8 



30°-0 



-4°-8 



3.30 „ 



31°-0 



34°-5 



30°-5 



-4°-0 



5.15 „ 



31°-5 



29°-0 



29°-0 



-o°-o 



On this occasion also the sky was overcast. In these two tables exactly 

 the same result is recorded, the black on both occasions being again much 

 warmer than the snow. These two tables are given, as they were taken under 

 quite different conditions. When the temperatures given in the first table 



*Read March 1, 1886. 



