on a new Theory of Dew, 487 



indicate a higher temperature than the air above it. He had 

 supposed the cold of evening to descend from above, and could 

 scarcely believe his eyes when he found the thermometer at 

 the height of 75 feet to read more than 2° R. above one at 

 5 feet. " It is then from the ground that this coldness proceeds, 

 for the thermometer at 4 lines from the ground generally read 

 lower than the one at 5 feet." All this is correctly observed ; 

 but the buried thermometer led him astray; for this naturally 

 indicating a higher temperature than any of the other instru- 

 ments, he supposed that the earth retained a considerable 

 portion of the heat it had acquired during the day, that a layer 

 of air cooled by evaporation from the surface produced the 

 cold to the height of four lines, while at higher elevations the 

 warmer air escaped this chilling influence. 



In January 1781 Professor Patrick Wilson noticed at the 

 Glasgow Observatory, between 1 and 3 a.m., the temperature 

 of the air 24 feet from the ground to be 7° F., while the snow 

 in the Park at the depth of 6 inches was 24°. He also noticed 

 the effect of clouds and screens in raising the temperature, 

 and Dr. Black suggested a screen of gauze, which was tried, 

 with a like effect. 



That dew rose out of the ground is a very old notion. Mr. 

 Aitken refers it to Gersten ; but his Treatise appeared so 

 lately as 1748, under the title Eocercitationes recentiores circa 

 Roris meteora. In a volume in my library, entitled De Rore 

 disquisitio physica D. Joannis Nardil (Florentise, 1642), it is 

 stated : — " Rorem obseruant hi fieri eoshalitu e terra eleuato, 

 silente vento, celo sereno, anni tempore, loco, et regione tempe* 

 ratis" 



The Florentine Academicians were the first to show that the 

 moisture which supplied dew already existed in the air. The 

 Hon. Robert Boyle/ in his ' Experimental History of Cold ' 

 (1665), showed that the beautiful exhibition of frost on the 

 window-pane is condensed from the vapour of the air in the 

 room, and he attempted to estimate, by weighing, the amount 

 of vapour condensed within a given time upon a phial con- 

 taining a mixture of salt and snow. Boyle distinctly recog- 

 nized the fact that dew and hoar-frost are formed by the 

 precipitation of the vapour of the air upon a colder body. 



In 1752 M. le Roi, of Montpellier, doubting the received 

 notion that dew rose from the ground, sealed up a bottle of 

 white glass containing air at 20° R. (77° F.), and he noticed 

 that as the temperature cooled down there was a considerable 

 deposit of dew within the bottle, which again disappeared as 

 the temperature became higher. Le Roi made a number of 

 admirable observations, among others the method of deter- 



2M2 



