A HISTORY OF SCIENCE 



always found the mercury lower in a thermometer laid 

 upon the ground in a meadow in his neighborhood than 

 it was in a similar thermometer suspended in the air six 

 feet above the former; and that upon one night the 

 difference amounted to five degrees of Fahrenheit's 

 scale. Mr. Six, however, did 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 evap- 

 oration 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. Happen- 

 ing, in that season, to be in that country in 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 eight degrees lower, by 

 Fahrenheit's division, than the one in the air. Similar 

 results having been obtained from several similar ex- 

 periments, made during the same autumn, I deter- 

 mined in the next spring to prosecute the subject with 

 some degree of steadiness, and with that view went 

 frequently to the house of one of my friends who lives 

 in Surrey. 



"At the end of two months I fancied that I had 



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