84) RADIATION FROM CLOUDS. 



sequently present, when examined by delicate thermome- 

 ters, varying degrees of temperature. By the researches 

 of Dr. Wells,* which may be adduced as an example of 

 the best class of inductive experiments, we learn that the 

 following differences in sensible heat were observed at 

 seven o'clock in the evening : 



The air four feet abo^e tbe grass . . GOf 



Wool on a raised board .... 54^ 



Swandown on ditto . . . . . 53 



The surface of the raised board . . . 57 



Grass plat 51 



Dew is most abundantly deposited on clear, calm 

 nights, during which the radiation from the surface of 

 the earth is uninterrupted. The increased cold of such 

 nights over those obscured by clouds is well known. The 

 clouds, it has been proved, act in the same way as the 

 screens used by gardeners to protect their young plants 

 from the frosts of the early spring, which obstruct the 

 radiation, and, in all probability, reflect a small quantity 

 of heat back to the earth. 



It is not improbable that the observed increase in 

 grass crops, when they have been strewn with branches 

 of trees or any slight shades, may be due to a similar 

 cause.f 



* By far the most complete set of experiments on the radiation 

 of heat from the surface at night, Y/hich have been published since 

 Dr. Wells' s memoir On Dtw, are those of Mr.Glaisher, of theKoyal 

 Observatory at Greenwich. Instruments of the most perfect kind 

 were employed, and the observations made with sedulous care. 

 The results will be found in a memoir On the Amount of the Radia- 

 tion of Heat , at night, from the Earth, and from various bodies 

 placed on or near the Surface of the Earth, by James Glaisher, Esq., 

 Philosophical Trans, for 1847, part 2. 



j- Dr. Wells noticed the practical fact that very light shades pro- 

 tected delicate plants from frost, by preventing radiation. Mr. 

 Goldsworthy Gurney lias maie a series of interesting experiments, 



