ON DEW, &c. 161 



any diversity of exposure to the sky. Thus, the 

 mercury in a thermometer placed upon wool, 

 lying on the raised board, was found to be at 

 the 44th degree, while that in another, pendent 

 in the air, at the same height from the ground, 

 and wrapped in wool, was at the 48th. Wool 

 also, on the raised board *, was commonly a little 

 colder than the same substance on grass, when 

 the night was very still ; and the leeward end 

 of that board was generally colder than the 

 windward extremity. 



But, the most remarkable examples of this 

 kind were exhibited by the gravel walk, and the 

 bare garden mould. In still and serene nights, 

 the surfaces of these bodies were always warmer 

 than the neighbouring grass, and frequently 

 warmer than the air. On one night of this 

 description, I observed, 2|- hours after sunset, 

 the surface of the gravel walk to be 16J, and 

 that of the garden mould to be 12j, warmer 



* The greater cold of the raised board, in my experiments, 

 most probably depended on the grass being very short; since 

 Mr. Wilson Found, that snow on the ground was colder than 

 the same body on a raised board. If 1 , 2, or 3 degrees were 

 added to the cold of the grass at my place of observation, 

 agreeably to the difference found by Mr. Six, between the 

 temperatures of long and short grass in dewy nights, the cold 

 on my raised board would, upon such nights, have been 

 always less than that of the grassplat. 



M 



