CH. VII.] THE FRUIT AND SEED. 219 



few 3'ards of the preceding, but with the radiation of 

 the rajs of heat from the grass checked by no other 

 covering than a cambric pocket-handkerchief, decHned 

 no lower than 42°. Xo difference of result occurs, 

 whether the radiating surface be parallel or per- 

 pendicular to the horizon ; for when the mercury in 

 a thermometer, hung against an openly exposed wall, 

 fell to 38", another thermometer against the same 

 wall, but beneath a web of gauze stretched tightly at 

 a few inches distance, indicated a temperatui'e of 43°. 



These results explain the beneficial operation of 

 apparently such slight shelter to our wall-fruit when 

 in blossom. A sheet of canvas, or of netting, pre- 

 vents the direct radiation of heat from the wall — the 

 cooling goes on more slowly, and is not reduced to 

 that of the exterior air at night before the return of 

 day begins to re-elevate the external temperature. 



The colder the body surrounding another body, the 

 more rapid the radiation from the latter ; for it is a 

 law of heat that it has a constant tendency to be 

 diffused equally, and the greater the diversity of 

 temperatui'e between two bodies in contact with 

 each other, the greater is the rapidity with which the 

 progi'ess towards equihbrium goes on. This is one 

 reason why a temperatm'e of 32° with a brisk wind 

 attending it -^dll 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 



