Effects of Radiation. 



Tlieim. in Air. On Grass. On Cotton-Wool. 



1845, 



26 



In all these instances of marked reduction of temperature, 

 in connection with radiation, the qualities of atmosphere, of 

 course, were such as are favourable to radiation, namely, a 

 clear and calm state of air, such as accompanies the finest 

 weather, and is characteristic of it. A clouded atmosphere, 

 as is well known, is incompatible with it ; — cloud or mist, 

 not only acting as a screen, — but also as a radiating surface, 

 returning to the earth a great pai-t of the heat which it re- 

 ceives. The difference of temperature here in December 

 1843, and of the same month in 1844, displays this in a very 

 striking manner. The one, the former, was singularly mild ; 

 spring flowers were then appearing ; the blackbird had be- 

 gun his song ; the thermometer was never below 32°, com- 



