44 Emission and Transmission of Heat 



tained at a constant temperature, and through which passes a 

 current of air ; and let us suppose that all the elementary veins 

 have sensibly the same velocity or that a thin slice of air taken 

 perpendicularly to the axis of the pipe at the entrance preserves 

 its form while it traverses the pipe. 



During its passage the circumference of the section will be 

 at the temperature of the pipe, and the heat will propagate itself 

 from the circumference towards the centre. After a certain time 

 the whole section will have attained sensibly the temperature of 

 the pipe ; if at this moment it has not reached the end of the 

 pipe, the rest of its passage will evidently be without influence, 

 if at its exit from the pipe the heat has not had time to reach the 

 centre of the section the mean temperature of the section will be 

 the higher the longer it has been in the pipe. The temperature 

 of the escaping air will depend then on its velocity, and on the 

 length of the pipe. 



We have supposed the pipe to be circular and the slice of air 

 to be always limited by two plans, but all that we have said is 

 equally true for a pipe of any form whatever, and in spite of the 

 difference in velocity of the elementary veins which always takes 

 place ; except that the time necessary for the centre of the vein 

 to take the temperature of the circumference augments with the 

 difference of velocity. 



We may add that when the pipe is horizontal or more or less 

 inclined, the propagation of the heat depends not only on the 

 transmission through the air, but also on the movement of the 

 air due to its being heated. 



It is easy to see from these considerations how complicated 

 are the phenomena which take place during the heating of air 

 while it traverses a pipe maintained at a constant temperature, 

 we may however deduce certain general principles from the pre- 

 ceding reasoning, which will be useful under certain conditions. 



/ =temperature of objects to which radiation takes place in F. 

 /' = " of surrounding air in F 



For value of K see 794. 

 " " A" see 798-802. 



" C see Figure 2, page 21. 

 " C see Figure 3, " 



C' see Figure i, " " 



