ON DEW, &c. 171 



this class, wool produced the least cold, and I 

 formerly mentioned that it attracted less dew, 

 than silk, cotton, and flax. The last mentioned 

 substances, and swandown, were found equal, 

 or nearly so, in their tendency to become cold. 

 Swandown, however, exhibited the greatest cold 

 rather more frequently than any of the rest ; on 

 which account, and from its being more easily 

 managed, as it was used while adhering to the 

 skin of the bird, I at length scarcely ever em- 

 ployed any other body of the same class. On 

 the night, during which grass was observed to 

 be 14 colder than the air, swandown, lying 

 upon a neighbouring piece of grass, was still 

 one degree lower. This difference of 15, be- 

 tween the temperature, at night, of a body on 

 the surface of the earth, and that of the air, a 

 few feet above the earth, is the greatest which 

 I have hitherto seen. 



Fresh, unbroken straw, and shreds of white 

 paper, though not properly to be ranked among 

 filamentous substances, were also found to be a 

 little more productive of cold, than the wool 

 which I used. 



The next class consisted of bodies in the state 

 of a powder, more or less fine. These were 

 clean river sand, glass, chalk, charcoal, lamp- 

 black, and a brown calx of iron. Chalk pro- 

 duced the least, and the three last substances, 



