124 ESSAY 



not suppose, agreeably to the opinion of Mr. 

 Wilson and myself, that the cold was occasioned 

 by the formation of dew ; but imagined, that it 

 proceeded, partly from the low temperature of 

 the air, through which the dew, already formed 

 in the atmosphere, had descended, and partly 

 from the evaporation of moisture from the 

 ground, on which his thermometer had been 

 placed. The conjecture of Mr. Wilson, and 

 the observations of Mr. Six, together with many 

 facts, which I afterwards learned in the course 

 of reading, strengthened my opinion ; but I 

 made no attempt, before the autumn of 1811, 

 to ascertain by experiment if it were just, though 

 it had, in the mean time, almost daily occurred 

 to my thoughts. Happening, in that season, 

 to be in the country on a clear and calm night, 

 I laid a thermometer upon grass wet with dew, 

 and suspended a second, in the air, two feet 

 above the other. An hour afterwards, the 

 thermometer on the grass was found to be 8 

 lower, by Fahrenheit's division, than the one 

 in the air. Similar results having been obtained 

 from several similar experiments, made during 

 the same autumn, I determined, in the next 

 spring, to prosecute the subject with some de- 

 gree of steadiness, and with this view went 

 frequently to the house of one of my friends, 

 who lives in Surrey. At the end of two months, 



