EMISSION AND TRANSMISSION OF HEAT* 

 CHAPTER I. 



EMISSION OF HEAT FROM A SURFACE MAINTAINED AT A CON- 

 STANT TEMPERATURE. 



775. The case under consideration is that of a pipe heated 

 within by steam, and with its outer surface exposed to the air ; 

 that of a vessel full of warm water, and so on. The quantity of 

 heat emitted by a surface maintained at constant temperature, 

 and exposed to the air depends on the area of its surface, on its 

 form, on its temperature, and on that of the air to which it is ex- 

 posed. It is important to know the quantity of this heat in heat 

 units, per unit of surface, during unity of time, as a function of 

 the elements which cause it to vary, at least for the cases which 

 ordinarily occur in practice. 



In order to understand how this quantity of heat emitted 

 may be determined, consider the case of a metallic vessel full of 

 warm water ; the metals being very good conductors of heat, the 

 exterior surface of the vessel will be at the temperature of the 

 water which it contains, let the weight of water, augmented by 

 that of the vessel, multiplied by its specific heat, be P kilograms, 

 .S the area of the surface of the vessel in square meters, O C the 

 temperature of the surrounding air, the time in seconds which 

 is required for the water to cool from T to T i degrees. 



The quantity of heat units lost during the time is evidently 

 equal to P, and ought to be sensibly the same as that quantity 

 which would have escaped from the vase in the same time if the 

 temperature had been constant and equal to the mean of T and 

 T i , that is to say to T % . According to this the quantity of 

 heat M which the surface of the vessel would emit per hour per 

 square meter, if the temperature were maintained at T y 2 would 

 be 



M - *L v 3 6o _ J_ p x 

 S * * o 



* From " Trait6 de La Chaleur " by E. PSclet. 



29 



