OF ATMOSPHEEICAL TEMPERATURE. 137 



will not be disturbed in any substance wliicli is not 

 placed in tbe clear aspect of the sky. A portion of 

 a grass-plat under the protection of a tree or hedge, 

 ■vfill generally be found, on a clear night, to be eight 

 or ten degrees warmer than surrounding unsheltered 

 parts ; and it is well known to gardeners, that less 

 dew and frost are to be found in such situations, than 

 in those which are wholly exposed." {Hort. Trans., 

 vi. 8.) 



These laws plainly direct us to the means we are 

 to employ to moderate atmospherical temperature. 

 A screen, of whatever kind, interposed between the 

 sun and a plant, intercepts the radiant heat of the 

 sun, and returns it into space ; and thus, in addition 

 to the diminution of perspiration by the removal of a 

 part of the stimulus that causes it, actually tends to 

 lower the temperature that surrounds the plant. In 

 like manner, the interposition of a screen, however 

 slight, between a plant and the sky, intercepts the 

 radiant heat of the earth ; and, instead of allowing it 

 to pass off into space, returns it to the ground, the 

 temperature of which is maintained at a higher point 

 than it otherwise would be. Hence it is that plants 

 growing below the deep projecting eaves of houses, or 

 guarded by a mere coping of thatched hurdles, suffer 

 less in winter than if they were fully exposed to the 

 sky. 



It is also obvious from what has been stated, that 

 plants growing upon grass will be exposed to a greater 

 degree of cold in winter than such as grow upon 

 gravel : but it does not therefore follow that hard 



