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pie, a thermometer placed upon a grass-plot, exposed 

 to a clear sky, fell to 35 degs. ; but another thermo- 

 meter, within a few yards of the preceding, but with 

 the radiation of the rays of heat from the grass 

 checked by no other covering than a cambric pocket- 

 handkerchief, declined no lower than 42 degs. No 

 diiFerence of result occurs whether the radiating sur- 

 face be parallel or perpendicular to the horizon ; for 

 when the mercury in a thermometer, hung against an 

 openly-exposed wall, fell to 38 degs., another ther- 

 mometer, against the same wall, but beneath a web of 

 gauze stretched tightly, at a few inches distance, in- 

 dicated a temperature of 43 degs. 



These results explain the beneficial operation of 

 apparently such slight screens to our wall-fruit when 

 in blossom. A sheet of canvass or of netting prevents 

 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 

 temperature between two bodies in contact with each 

 other, the greater is the rapidity with which the pro- 

 gress towards equilibrium goes on. This is one rea- 

 son why a temperature of 32 degs., with a brisk wind 



