^20 PRINCIPLES OF GARDENING. [CH. VII. 



anotlier law of heat, viz. that aeriform bodies convey 

 it from a cooling hodj, as a wall or a tree, hy 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 another colder portion, and thus the process 

 goes on imtil the cooling body is reduced to the 

 same temperatui'e 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 warmth 

 from them, but if there be a brisk wind, a constant 

 current of air at the lowest temperature then occur- 

 ing is brought in constant contact with them, and 

 the cooling is rapid in accordance ^^ith 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. 



It is not altogether immaterial of what substance 

 netting is formed. Worsted is to be preferred, not 

 only because it is the most durable, but because it is 

 the best preventive of a wall's coolhig. I have found 

 the thermometer under a hemp net sink during the 

 night from two to four degrees lower than that under 

 a net of worsted, the meshes being small and of equal 

 size in both nets. This can only be because worsted 



