Emission and Transmission of Heat 45 



i st. When air traverses a tube maintained at a constant 

 temperature greater than that of the air and supposing that the 

 velocity of the air, at first very small, increases as it progresses 

 through the tube, the air will emerge at the temperature of the 

 tube up to a certain limit of velocity, depending on the perimeter 

 of the tube, on the shape of its section and the inequality of the 

 velocities of the different elementary veins. This velocity will 

 increase in proportion as the section of the pipe diminishes. It 

 is impossible to foresee whether, under the same conditions, this 

 velocity would be greater in a vertical pipe than in a horizontal 

 one, because in the first case the increments of velocity resulting 

 from heating against the walls brings out the air from the centre, 

 while in the second case the layers of air in contact with the lower 

 surface are constantly displaced, circumstances which both tend 

 to distribute the heat. 



2nd. When the limit of velocity which I have just mentioned 

 has been reached, the air escapes at a decreasing temperature be- 

 cause in each section the temperature is decreasing from the cir- 

 cumference to the centre, and the greater the velocity of the air 

 the more is this the case. But the quantity of heat carried off 

 by the air increases with its velocity, this fact is thoroughly 

 proved by experiment and is easily explained by admitting that 

 the sum of the quantities of heat diffused through each section 

 increases very rapidly with time, for the number of sections pass- 

 ing in unity of time being proportional to the velocity, and the 

 time of passage of each section being inversely proportional to 

 the velocity, it follows that the quantity of heat carried off by the 

 air will increase with the velocity provided that the quantity of 

 heat which diffuses itself through a section increases very rapidly 

 with the time. 



822. In practice we may admit as a sufficiently close ap- 

 proximation that the quantity of heat emitted by the pipe is sen- 

 sibly equal to that which it would emit in the open air by air 

 contact to the surrounding air at a temperature the mean of the 

 observed temperatures of the air at entering the pipe and at leav- 

 ing it. 



I have verified this principle by means of a cylindrical vessel 

 1 6 inches high, and 8 inches in diameter, pierced at the centre by 



