By John Frederic Daniell, Esq. 9 



Any portion of the surface of the globe which is fully 

 turned towards the sun receives more radiant heat than it 

 projects, and becomes heated ; but when, by the revolution 

 of the axis, this portion is turned from the source of heat, 

 the radiation into space still continues, and being uncom- 

 pensated, the temperature declines. In consequence of the 

 different degrees in which different bodies possess this power 

 of radiation, two contiguous portions of the system of the 

 earth will become of different temperatures, and if on a clear 

 night we place a thermometer upon a grass-plat, and another 

 upon a gravel walk or the bare soil, we shall find the tem- 

 perature of the former many degrees below that of the latter. 

 The fibrous texture of the grass is favourable to the emission 

 of the heat, but the dense surfaces of the gravel seem to 

 retain and fix it. But this unequal effect will only be per- 

 ceived when the atmosphere is unclouded, and a free passage 

 is open into space; for even a light mist will arrest the 

 radiant matter in its course, and return as much to the radiat- 

 ing body as it emits. The intervention of more substantial 

 obstacles will of course equally prevent the result, and the 

 balance of temperature w ill not be disturbed in any substance 

 which is not placed in the clear aspect of the sky. A portion 

 of a grass-plat under the protection of a tree or hedge, will 

 generally be found, on a clear night, to be eight or ten degrees 

 warmer than surrounding unsheltered parts, and it is well 

 known to gardeners that less dew and frost are to be found 

 in such situations than in those which are wholly exposed. 



There are many independent circumstances which modify 

 the effects of this action, such as the state of the radiating 

 body, its power of conducting heat, &c. If, for instance, the 



vol. vr. C 



