1 826.] Mr . Daniell on Climate. 1 1 9 



If a body be so situated that it may receive just as much radiant 

 heat as itself projects, its temperature remains the same ; if the 

 surrounding bodies emit heat of greater intensity than the same 

 body, its temperature rises, till the quantity which it receives 

 exactly balances its expenditure, at which point it again becomes 

 stationary ; and if the power of radiation be exerted under cir- 

 cumstances which prevent a return, the temperature of the body 

 declines. Thus, if a thermometer be placed in the focus of a 

 concave metallic mirror, and turned towards any clear portion 

 of the sky, at any period of the day, it will fall many degrees 

 below the temperature of another thermometer placed near it, 

 out of the mirror ; the power of radiation is exerted in both 

 thermometers, but to the first all return of radiant heat is cut off, 

 while the other receives as much from the surrounding bodies 

 as itself projects. This interchange amongst bodies takes place 

 in transparent media as well as in vacuo ; but in the former case 

 the effect is modified by the equalizing power of the medium. 



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 uncompensated, the temperature 

 declines. In consequence of the difierent 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 temperature 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 perceived 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 

 radiating body as it emits. The intervention of more substantial 

 obstacles will of course equally prevent the result, and the 

 balance of temperature will 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 gene- 

 rally be found, on a clear niglit, 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, 8cc. If, for instance, the body be in 

 a liquid or aeriform state, although the process may go on freely, 

 aa in water, the cold produced by it will not accumulate upon 



