OF ATMOSPHERICAL TEMPEEATUEE. 135 



As to temperature in tlie open air, unconnected 

 witli atmosplierical humidity, there seems to be no 

 means of regulating or modifying it to any consider- 

 able extent. In some respects, however, we have 

 even this powerful agent under our control ; but, in 

 order to exercise such control, it is necessary to un 

 derstand correctly the theory of what is called radia- 

 tion, which cannot be better explained than in the 

 words of Mr. Daniell. " The power of emitting heat 

 in straight lines in every direction, independently of 

 contact, may be regarded as a property common to 

 all matter ; but differing in degree in different kinds 

 of matter. Coexisting 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 pro- 

 perties in different substances may be measured. 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 tem- 

 perature rises, till the quantity which it receives ex- 

 actly 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 temperature of the body declines. Thus, if a 

 thermometer be placed in the focus of a concave me- 

 tallic mirror, and turned towards any portion of the 

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

 grees below the temperature of another thermometer 



