ON DEW, &c. 



159 



tions for 1788, " fogs did not, as far as I could 

 perceive, at all impede, but rather increase, the 

 refrigeration." But this was a mistake ; which 

 in all probability arose from his ascribing the 

 effect of a clear night to an ensuing foggy 

 morning, as he examined his thermometers only 

 in the daytime. He afterwards discovered his 

 error $ for, in his posthumous work, thick fogs 

 are ranked among the circumstances, which 

 always impede, and sometimes prevent alto- 

 gether, the appearance of a cold upon the sur- 

 face of the earth, greater than that of the atmo- 

 sphere. During a very dense fog, Mr. Wilson 

 found no difference, at night, between a ther* 

 mometer laid upon snow, and another suspended 

 in the air*. 



When, during a clear and still night, different 

 thermometers were examined, at the same time, 

 which had been placed in different situations, 

 those which were situated, where most dew was 

 formed, were always found to be the lowest. 

 Thus, upon one such night, I found a thermo- 

 meter placed upon a little wool, lying upon the 

 middle of the upper side of the raised board, to 

 be 9 lower than another thermometer, in con- 

 tact with an equal quantity of wool, attached to 

 the middle of the underside of the board. On 



* Edin. Phil. Trans. I. 170. 



