UNCLE IN ENGLAND. 209 



cold, such as wool, cotton, and flax, and still 

 more fine raw silk and swan-down ; all these 

 were more steadily cold upon clear nights than 

 even the grass ; but swan-down showed the 

 greatest cold. 



" I have already explained to you," con- 

 tinued my uncle, " that the surface of the earth, 

 and all substances upon it, radiate back into the 

 sky, at night, the heat which they receive in the 

 day ; and that, when this radiation is unob- 

 structed by clouds, the cold it produces is pro- 

 portionably greater. But the degree of cold is 

 very much augmented when the form or situa- 

 tion of these substances prevents their deriving 

 fresh supplies of heat from warmer bodies in 

 contact with them> or in their neighbourhood. 

 Most of the substances which I have named are 

 not only naturally bad conductors of heat, but 

 their form scarcely permits them to transmit from 

 fibre to fibre any heat they might acquire. This 

 is the reason why dew appears in greater quan- 

 tity on shavings of wood, than on a thick piece 

 of wood ; and why filamentous substances be- 

 come colder than all others. 



" On a dewy evening the Doctor depressed 

 a small tumbler into the soft garden mould, so 

 that the brim was level with the ground ; and he 

 placed another standing on the surface of the 

 mould : in the morning the former was dry in 

 the inside, while that which stood on the surface 



T3 



