ON DEW, &c. 181 



observations at length converted the doubt of 

 the justness of my ancient opinion, into a con- 

 viction of its error, and at the same time occa- 

 sioned me to conclude, that dew is the produc- 

 tion of a preceding cold in the substances, upon 

 which it appears. Wishing, however, to obtain 

 proofs, more striking in degree, of the validity 

 of these inferences, than such as had been 

 afforded to me by casual observation, while 

 attending to other parts of my subject, I insti- 

 tuted the experiments which will be next re- 

 lated. 



I had frequently remarked, early in the even- 

 ing, a considerable degree of cold on substances 

 exposed in calm weather to a clear sky, and I 

 had also sometimes seen, early in the evening, 

 the raised board altogether dry, while the grass 

 was much moistened. I therefore determined 

 to make the experiments in view on the raised 

 board, and to commence them as soon as the 

 sun should cease to shine upon it. The first 

 day I went to the country for this purpose, the 

 19th of August 1813, almost every circumstance 

 was favourable to its completion. There had 

 been no rain for three weeks; the wind was 

 northerly ; and the barometer was rising ; all 

 which indicated, that the atmosphere contained 

 little moisture. The air too was extremely still. 

 The only appearance in the least unfavourable 



