102 FRUIT TREES AND THEIR ENEMIES 



become appreciably cooler than the surrounding air, 

 so that it is the temperature of the air which must 

 alone determine whether the blossom is injured or 

 not : for the same reason, the effect of this cold air is 

 probably not increased by the presence of the liquid 

 or solid water particles in it, for, although these make 

 it a better conductor of heat, the blossoms would 

 attain the temperature of the air almost immediately, 

 whether this were dry or moist. We must not be 

 misled by the fact that mist-laden air feels colder to 

 ourselves ; for this is simply due to our bodies being 

 at a much higher temperature than the air ; in which 

 case the coldness of the air is more appreciable, 

 according as it is in a better condition for conducting 

 away our heat. 



The loss of heat by radiation is much slower when 

 the air is moist than when dry, and, moreover, 

 much more heat must be lost before the temperature 

 of this moist air is reduced by any given amount. 

 Thus, to cool a mass of dry air one acre in extent 

 and twenty feet thick, from 37' F. down to 27' F., 

 requires the abstraction from it of nearly four times 

 as much heat when the air is saturated with (invisible) 

 moisture to start with, as when it is quite dry. 

 Hence the saturation of the air with moisture is 

 one of the best ways of preventing damage by frost, 

 and experiments have shown that distributing fine 

 jets of water, preferably warm water, over the ground 

 and trees, gives better results than any other methods. 



Placing a screen above the trees arrests the radia- 

 tion, and the thinnest gossamer is as effective for 

 the purpose as is the thickest material : a covering 



