348 THE SCIENTIFIC PAPERS OF 



similar open circuit between the General Post Office, Cannon 

 Street, and Thames Street, which he had no doubt worked equally 

 well, and went far to prove the advantages of the system. Another 

 complaint was, that the iron pipes employed by him (Dr. Siemens) 

 in laying down the Charing Cross Circuit were apt to rust in con- 

 sequence of the use of injection water in the air-pump which had 

 since been discontinued ; it was also stated that injecting water 

 into the air-pump was accompanied by a waste of power. He 

 entirely dissented from the latter proposition. He had prepared a 

 diagram (Plate 20) showing the curve of compression : if a piston 

 travelled in the cylinder, the pressure would rise, in the manner 

 indicated by the dynamical curve, which compression was accom- 

 panied by a rise of temperature from 60 to 170 Fahr., in bring- 

 ing up the pressure to double that of the atmosphere. By inject- 

 ing cold water, not only was the cylinder lubricated as stated in 

 the paper, but the heat was absorbed by the water, the result being 

 that the increase of pressure would not take place in the ratio 

 indicated by the dynamical line, but in that indicated by the 

 other line, which represented the ratio of isothermal compression. 

 Injecting water therefore was not a source of loss of power, but of 

 gain of power. Probably the quantity of water injected had been 

 too small, and in that case no doubt vapour would be carried over 

 into the reservoir. The postal authorities had done away with the 

 reservoir altogether, which, he thought, was a mistake, because it 

 was necessary to allow the water to settle and the air to become 

 dry and cooled down to the point at which it was fit to enter the 

 pipes. As regarded rusting, the authors themselves stated that in 

 Paris, where the air was compressed by water, no inconvenience 

 had been observed on that score, nor had any such effects been 

 experienced in Berlin. If rust had given trouble in the circuit in 

 question, he considered it was entirely due to the mode of working. 

 No doubt a lead pipe was better in some respects for small 

 diameters, but when he designed the circuit, cost was a very im- 

 portant element. He could not afford to have lead pipes inside- 

 cast-iron, the cost of which might suit the Post Office, now that 

 the authorities were accustomed to spend their millions somewhat 

 freely ; but at the time to which he ah 1 uded they were in the habit of 

 going closely into estimates. Xo doubt it would have been better 

 to line the inside of the pipes with softer metal, such as tin or lead, 



