NOCTURNAL RADIATION. OO 



will be cooled, and will deposit on the glass exactly that 

 quantity of moisture which is due to the difference be- 

 tween the temperature of the two bodies. Different 

 substances, independent of colour, have the property of 

 parting with heat from their surfaces at different rates. 

 Rough and porous surfaces radiate heat more rapidly 

 than smooth ones, and are consequently reduced in tem- 

 perature ; and, if exposed, are covered with dew sooner 

 than such as are smooth and dense. The grass parterre 

 glistens with dew, whilst the hard and stony walk is un- 

 moistened.* 



Colourless glass isvery readily suffused with dampness, 

 but polished metals are not so, even when dews are 

 heavily condensed on other bodies. To comprehend 

 fully the phenomena of the formation of dew, we must 

 remember that the entire surface of the earth is constantly 

 radiating heat into space ; and that, as by night no ab- 

 sorption is taking place, it naturally' cools. f As the 

 substances spread over the earth become colder than the 

 air, they acquire the power of condensing the vapour 

 with which the atmosphere is always charged. The 

 bodies which cover this globe are very differently consti- 

 tuted; they possess dissimilar radiating powers, and con- 



* See the Researches on Heat, by Professor James Forbes, in the 

 Transactions of the Royal Society of Edinburgh ; also Melloni's 

 papers on the same subject in the Annales de Chimie, several of 

 which have been translated into the Scientific Memoirs, edited by 

 Mr. Richard Taylor. 



f The phenomena of dew have constantly engaged the attention 

 of man. Aristotle, in his book De Mundo, puts forth some just 

 notions on its nature. An opinion has almost always prevailed 

 that dew falls. Gersten appears to have been the first who opposed 

 this motion. He was followed by Muschenbroek, and then by 

 Du Fay. The researches of Leslie were of a far more exact cha- 

 racter. Dr. Wilson, in the Transactions of the Royal Society of 

 Edinburgh, 1st vol., published a Memoir on Hoar Frost of much 

 interest; but the questions involved remained unsettled until the 

 researches of Dr. Wells, which were published in his Essay on Dew, 



