8 



On Climate, fyc. 



" radiating their heat to the heavens, I perceived immediately 

 " a just reason for the practice which I had before deemed 

 " useless." 



The power of emitting heat in straight lines in every di- 

 rection, independently of contact, may be regarded as a pro- 

 perty common to all matter ; but differing in degree in different 

 kinds of matter. Co-existing with it, in the same degrees, 

 may be regarded the power of absorbing heat so emitted from 

 other bodies. Polished metals, and the fibres of vegetables 

 may be considered as placed at the two extremities of the scale 

 upon which these properties in different substances may be 

 measured. Tf a body be so situated that it may receive just 

 as much radiant heat as itself projects, its temperature re- 

 mains 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 ex- 

 penditure, at which point it again becomes stationary : and if 

 the power of radiation be exerted under circumstances 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 ther- 

 mometers, 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. 



