222 PRINCIPLES OF GARDENING. [CH. YII. 



bottles, equal in size and thicknes of glass, and of 

 the same shape, be filled with warm water, and one 

 of the bottles be covered with an envelope of fine 

 muslin, this bottle will give out heat to the sur- 

 rounding air with much greater rapidity than the 

 other bottle : so that in a given time the bottle with 

 the envelope will be found colder than the one which 

 has no covering. 



Shelters such as the preceding, or the slighter 

 agents, sprays of evergreens, placed before the 

 branches of wall-trees, or other plants, as already 

 noticed, operate beneficially in another way — check- 

 ing the rapid passage of the air over them — such 

 passage is detrimental in proportion to its rapidity, 

 for the more rapid it is, the greater is the amount 

 of evaporation, and consequently of cold produced. 

 Mr. Daniell says, " That a surface which exhales 

 100 parts of moisture when the air is calm, exhales 

 125 parts when exposed to a moderate breeze, 

 and 150 parts when the wind is high. During 

 all high winds, but especially when blowing from 

 points varying between the east and the south — for 

 they are the driest in this country — the gardener will 

 always find shelters beneficial to his plants whether 

 in blossom or with fruit in its first stages of growth, 

 for these winds cause an evaporation much exceeding 

 in amount the supply of moisture afforded by the 

 roots." In March, such shelters are much required. 



