350 THE SCIENTIFIC PAPERS OF 



the dynamical effect produced in allowing air to expand from one 

 pressure to another. They presumed that if the air flowed into 

 a long tube, it expanded in the same manner as if it were 

 allowed to push a working piston forward, which, however, was 

 not the case. If compressed air were allowed to re-expand 

 behind a working piston the temperature would fall in precisely 

 the same ratio in which it would rise in compression, the heat lost 

 being the equivalent of the force communicated to the piston. 

 But was it the same if air expanded into a long pneumatic pipe ? 

 Certainly not. There was in that case no working piston with 

 resistance behind it, the carrier piston consisting only of a piece 

 of hose containing some slips of paper, which offered practically 

 no obstacle. All the resistance that had practically to be dealt 

 with in the pneumatic pipe was that of the air itself. Suppose 

 air of 2 atmospheres pressure were admitted at one end of 

 the pipe (which might be one mile or three miles long), the pres- 

 sure would taper down to atmospheric pressure at the opposite 

 end. No work was accomplished here, except that exerted upon 

 the air itself in being pushed through the tube, which, therefore, 

 became the recipient, in the shape of heat, of all the force 

 which had been exerted, and the result was that the expansion 

 of the air from two atmospheres to atmospheric pressure would 

 not be accompanied by any decrease of temperature. Therefore 

 the dynamical formula} regarding the force and volume of 

 air expanding behind a working piston did not apply to the 

 case of a pneumatic pipe. Assuming that the pipe itself was 

 a non-conductor of heat, and that the temperature of the air 

 on entering the pipe was the same as the temperature of the 

 pipe itself, he maintained that the air would flow out of the 

 other end of the pipe at exactly the same temperature as that at 

 which it entered. Taking the case of a pipe of conducting material, 

 and assuming that the air entered the pipe at two atmospheres 

 pressure and at the temperature of the pipe itself, the temperature 

 at which the air left the pipe must be in excess of that of the 

 compressed air when it entered, inasmuch as the latter had work 

 to perform ; it had to push forward the air and overcome its 

 friction against the side of the tube ; and inasmuch as work was 

 performed in the early part of the operation, the temperature of the 

 air would diminish. Heat would be communicated from the tube 



