136 APPLICATION OF PRINCIPLES. 



placed near it, out of the mirror ; the power of radia- 

 tion 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 it- 

 self 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 

 equalising 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 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 



