170 ESSAY 



In the observations hitherto given by me on 

 the cold connected with dew, the temperature 

 of grass has been chiefly considered, partly be- 

 cause my first experiments had been made upon 

 it, and partly from a wish, which arose after- 

 wards, to compare my own experiments with 

 those of Mr. Six, which had been confined to 

 that substance. I found it, however, very unfit 

 to furnish the means of comparing the degrees 

 of cold produced at night on the surface of the 

 earth, at different times and places ; as its state 

 on different nights, on the same parts of the 

 plat I commonly made use of, and in different 

 parts of the plat on the same nights, was often 

 very unequal, in point of height, thickness and 

 fineness, all of which circumstances influenced 

 the degree of cold produced by it. I observed, 

 in consequence, a much greater uniformity in 

 the results of experiments made with various 

 other bodies, whose condition, when first ex- 

 posed to the air, was always the same. Of these, 

 the most productive of cold were the filamentous 

 and downy, as wool of moderate fineness, very 

 fine raw silk, very fine unspun cotton, fine flax, 

 and swandown, all of which were not only more 

 steadily cold, upon clear and calm nights, than 

 grass, but also gave rise to a greater degree of 

 cold, than was almost at any time observed upon 

 it, even in its best state. Among the bodies of 



