380 



Proceedings of the Royal Institution, 



compared with the temperature of tlie thermometer, and the effect 

 ascribed to their difference. Tliis is not the correct way of expressing 

 the fact, and it diverts the mind from the circumstance, that the effect 

 is due solely to tlie difference between the temperature of the body e^ 

 and that of the various bodies comprised within the cone emu. Th^, 

 inaccuracy of this mode of describing the matter will be rendered 

 evident by imagining a thermometer as t in Fig. 2, suspended be- 



50 



o 

 75 



100 





tween two walls whose temperatures were 50° and 100° ; the ther- 

 mometer would, in this case, exhibit a temperature of about 75**. ff* 

 now a screen, with a temperature of 80° or 90°, were placed at a b^ 

 it would depress the thermometer, though of a temperature higher 

 than that of the thermometer. In like manner, a screen heated to 60*^^ 

 or 70°, and placed at c d, would elevate the thermometer, though m- 

 ferior in temperature. 



The experiments of Mr. Leslie on the effect of surface in influ- 

 encing radiation, added new difficulties to the subject, and revived 

 the idea that cold was something entirely distinct from heat, and not 

 merely its absence. Mr. Leslie found, it will be recollected, that a 

 rough blackened surface radiated about ten times as much heat as a 

 bright metallic surface, the temperatures being the same. He found, 

 that if the body e in Fig. 1, were a pohshed metallic canister, filled 

 with boiling water, it affected the thermometer at /only about one- 

 tenth as much as when the canister was rough and blackened ex- 

 ternally. Similar effects were produced when the canisters were 

 filled with ice, instead of boiling water ; that is to say, the rough 

 black canister depressed the thermometer nearly ten times as much 

 as the bright canister, thus appearing to indicstte, that cold was some- 



