46 Emission and Transmission of Heat 



a tube 4 inches in diameter with its surfaces entirely covered by 

 paper. By observing the cooling when the orifice of the tube 

 was open and then when closed, I have found that in the last case 

 the loss of heat was very nearly equal to half of that of the central 

 tube when surrounded by free air. 



823. Emission of heat to air traversing a conduit enclosing a 

 pipe maintained at a constant temperature. 



This case is very similar to the preceding one, except that 

 the diffusion of the heat takes place more rapidly, because the 

 concentric layers of air undergo a continual increase of surface 

 as they recede from the surface of the pipe, and at the same 

 time the interior surface of the conduit, heated by radiation, 

 heats the layers of air from the opposite direction. Here, as in 

 the preceding case, there is a limiting velocity, below which the 

 heating of the air is complete, and beyond which the temperature 

 of the air diminishes, although the quantity of heat carried away 

 increases with the velocity. 



824. For this case we may estimate the quantity of heat 

 emitted to be approximately equal to that which the pipe would 

 emit in the open air by radiation and air contact, the temperature 

 of the surroundings being taken as the mean of the observed 

 temperatures of the air at entering the conduit and at leaving it. 



