114 



attending it, will injure plants to a far greater extent 

 than a temperature many degrees lower, with a still 

 atmosphere ; but it is aided by the operation of an- 

 other law of heat, viz., that aeriform bodies convey it 

 from a cooling body, as a wall or a tree, by an actual 

 change in the situation of their own particles. That 

 portion of the air which is nearest to the cooling body 

 is expanded, and becoming specifically lighter, ascends, 

 and is replaced by a colder portion. This, in its turn, 

 becomes heated and dilated, and gives place to an- 

 other colder portion. And thus the process goes on, 

 until the cooling body is reduced to the same tempe- 

 rature as the air. 



In a still atmosphere, this goes on slowly ; the air 

 in contact with the wall and tree rises very gradually 

 as it imbibes wwmth from them ; but if there be a 

 brisk wind, a constant current of air at the lowest 

 temperature then occurring, is brought in constant 

 contact with them, and the cooling is rapid, in accord- 

 ance with the law of equilibrium just noticed. A 

 shelter of netting, or even the sprays of evergreens, 

 are of the greatest service in preventing the sweeping 

 contact of cold air at such times. Snow is good 

 shelter ; it prevents heat radiating from plants ; pro- 

 tects them from the chilling blasts ; and is one of the 

 worst conductors of heat. We have never known the 

 surface of the earth, below a covering of snow, colder 

 than 32 degs., even when the temperature of the air 

 above has been 28 degs. 



