REFERENCE TO HORTICITLTURE. 



69 



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

 the surrounding hodies emit heat of greater intensity than the same body, its 

 temperature rise?, 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 circumstances which prevent a return, the tem- 

 perature 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 places in transparent 

 media as well as in vacua ; but in the former case the effect is modified by 

 the equalising powder of the medium/' This description is clear and satisfac- 

 tory ; but it must not be supposed, that though the balance of temperature 

 will not be disturbed from the effects of radiation when the body is completely 

 enclosed, yet that it may not be so by the other law of heat, conduction. 



222. "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 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 surface 

 of the gravel seems 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 generally be found, on a clear night, to 

 be eight or ten degrees warmer than surrounding unsheltered parts ; and it 

 is well known to gai"deners that less dew and frost are to be found in such 

 situations than in those wiiich are wholly exposed. There are many inde- 

 pendent circumstances which modify the effects of this action, such as the 

 state of the radiating body, its power of conducting heat, &c. If, for in- 

 stance, the body be in a liquid or aeriform state, although the process may 

 go on freely, as in water, the cold produced by it will not accumulate upon 

 the surface, but will be dispersed by known laws throughout the mass ; and 

 if a solid mass be a good radiator but a bad conductor of heat, the frigorific 

 effect will be condensed upon the face which is exposed. So upon the sur- 

 face of the earth absolute stillness of the atmosphere is necessary for the 

 accumulation of cold upon the radiating body ; for if the air be in motion, 

 it disperses and equalises the effect with a rapidity proportioned to its 

 velocity." (^Ho7't. Trans, vol. vi. p. 10.) 



22t3. AU the phenomena connected with dew or hoarfrost have been ex- 



