382 Proceedings of the Royal Institution. 



ft 6, Fig. 3, represent parts of two opposite walls of a chamber ; sup- 

 pose c to be one of Mr. Leslie's canisters filled with ice, and Ho be 

 a thermometer. The t^nperature of the canister is 82°; that of the 

 walls we will suppose to be 64° : they will therefore radiate heat, as 

 regards its temperature, with double the intensity of the canister. 

 Imagine the thermometer to have acquired the temperature of the 

 walls before the canister is introduced into the apartment. Ori 

 placing the latter in the position shewn in the diagram, it will in- 

 tercept, from the thermometer, a pyramid of rays emanating from the 

 wall bbj whose section is tde, and it will substitute a smaller pyra- 

 ttiid, whose section is tfg, which would be equal in effect to the 

 larger pyramid, if the temperatures were equal, because the smaller 

 makes up by its nearness to the thermometer what it wants in extent 

 of base. The temperatures are, however, different ; the rays of the 

 larger pyramid are from two sources, one being the. direct radiation 

 of the wall b 6, and the other reflection from that wall of rays arriving 

 from the opposite wall a a ; but both these sources are at 64° ; those 

 of the smaller pyramid are also from two sources ; one, which is the 

 radiation of the canister at 32°, and the other, the reflection from 

 the canister of the opposite pyramid of rays from the wall «a, 

 having the base h k, and arising from a source at 64°. It is evident, 

 Iherefore, that the heat flowing to the thermometer is diminished 

 by the interposition of the cold canister, because the sum of the 

 radiating and reflecting powers of any surface or surfaces is nearly 

 a constant quantity ; therefore, if one surface, as the wall b 6, both, 

 radiates and reflects heat at 64°, it must give more than the canister, 

 which reflects at 64°, but radiates only at 32°. This difference will 

 be greater or smaller, according to the surface given to the canister 

 being greatest with a black surface, which radiates nine parts, and 

 reflects but one out of ten ; and least with a bright surface, which 

 radiates but one part and reflects nine. The whole may be stated 

 and compared in figures ; in doing which, it is not necessary ^o take 

 into account the heat intercepted by the canister, because that i^tt*^ 

 same whatever be its surface. 



In the first place, when the surface /g" is bright, the canister ra- 

 diates one-tenth of its own heat, and reflects nine-tenths of that which 

 strikes it from the wall a a. , i^ ij^^ Uuj>ino ' 



