176 ESSAY ON DEW, &c. 



free from clouds, and nearly quite calm, but # 

 good deal hazy. 



Before another proper evening arrived, my 

 health became so infirm, that I was obliged to 

 relinquish this pursuit. I conclude therefore 

 my account of it, with two remarks. 1. If Mr. 

 Wilson had been accustomed to examine the 

 temperature of swandown, or any similar sub- 

 stance, placed upon snow, he would, probably, 

 have observed a cold, on the surface of the 

 earth, exceeding that of the atmosphere by 20* 

 or more, on the night of his actually observing 

 an excess of 16. 2. Since upon one evening, 

 when the atmosphere was neither very clear nor 

 very still, a difference of 14^ was found by me, 

 between the temperatures of air and of swan* 

 down, which is only \ a degree less than the 

 greatest difference I have ever observed, be- 

 tween the same substances on the stillest and 

 clearest nights in summer, a corroboration is 

 hence derived of a conclusion, made by Mr. 

 Six from his experiments, that the greatest dif- 

 ferences at night, in point of temperature, be- 

 tween bodies on the surface of the earth, and 

 the atmosphere near to it, are those which take 

 place in very cold weather. 



