ON DEW, &c. 163 



It may be added here, that I have always 

 found, on dewy nights, the temperature of the 

 earth, \ an inch or an inch beneath its surface, 

 much warmer than the grass upon it. On five 

 such nights the differences were from 12 to 16 

 degrees. The earth, at the above-mentioned 

 depth, was also almost constantly warmer on 

 dewy nights than the air; sometimes it was con- 

 siderably so, for I once observed it to be 10 

 warmer, at another time 9, and at a third 7-J. 

 An exception will no doubt occur, if very mild 

 weather should follow a lorig frost ; but of this 

 I have had no experience. 



In the experiments upon my housetop in 

 London, I always found, during clear and calm 

 nights, wool lying on the wooden frame to be 

 colder than the air, at the same height; but the 

 difference was seldom more than 3. On the 

 evening, however, during which dew formed 

 there more copiously than usual, the difference 

 was 5. That the smallness of these differences 

 was not wholly occasioned by any thing special 

 in the air of cities was afterwards proved, by my 

 finding others much greater, in a garden nearly 

 in the middle of London, from which almost the 

 whole of the sky was visible. 



Metals, likewise, furnish proofs of the con- 

 nexion of dew with a cold in the substance, on 



M 2 



