28 HEAT, CONSIDERED WITIf 



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 vacua ; but in the former case the effect is modified by the 

 equalizing power of the medium." This description is clear and satis- 

 factory ; but it must not be supposed, that though the balance of tem- 

 perature will not be disturbed irom the effects of radiation when the 

 body is completely enclosed, yet that it may not be so by the other 

 law of heat, conduction. 



" Any portion of the surface of the globe which is fully turned to- 

 wards the sun receives more radiant heat than it projects, and becomes 

 heated ; but when, by the revolution of the earth on its axis, this por- 

 tion is turned from the source of heat, the radiation into space still 

 continues, and, being uncompensated, the temperature declines. In 

 consequence of the different degrees in which different bodies possess 

 this power of radiation, two contiguous portions 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 still more 

 prevent radiation, and the balance of temperature will not be dis- 

 turbed 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 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 surface 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 equalizes 

 the effect with a rapidity proportioned to its velocity." (' Hort. Trans.,' 

 vol. vi. p. 10.) 



All the phenomena connected with dew or hoarfrost have been ex- 

 plained by Dr. Wells on these principles. The deposition of moisture 

 is owing to the cold produced in bodies by radiation, which condenses 

 the atmospheric vapour on their surfaces. The deposition of dew takes 



