1871.] Pneumatic Transmission. 311 



in the two operations are different, the mean speed of a very- 

 light piston being the same in its transits through the same 

 tube, or through two tubes of equal dimensions, the utilised 

 engine power is directly proportionate to the mean specific 

 gravity of the air on the two sides of the piston. It follows from 

 this, therefore, that in working by exhaustion less engine 

 power is required, other things being equal, than in working 

 through the same tube by means of compression. And it 

 would also follow that in hot weather, and when the baro- 

 meter is low, the working of a pneumatic tube should be less 

 costly in engine power than in cold weather and when the 

 barometer is high." 



" With given utilised horse-power operating upon a given 

 line, the velocity of a very light carrier would be reciprocally 

 proportional to the cube root of the mean specific gravity of 

 the air moving in it. Mr. Siemens has proposed to take 

 advantage of this fact by the employment of hydrogen gas 

 for propulsion in letter tubes instead of atmospheric air. 

 The specific gravity of hydrogen is 0*07 ; that of air being 1. 

 The speed attainable, therefore, by the substitution of this 

 gas would be as — 



1 



or as 1 to 2j, nearly. 



3 / i 



This plan would be easily practicable with Messrs. Siemens's 

 system of complete circuit tubes, in which the same air is 

 pumped round without being changed. With any of the 

 ordinary systems by which the tube is open at one end, of 

 course only the atmospheric air could be used in practice." 

 A by no means unimportant matter in connection with the 

 working of pneumatic tubes is the mechanical means em- 

 ployed for producing the vacuum or plenum, as the case may 

 be. Several methods have been introduced with this ob- 

 ject. The first system adopted by Vallance in his model at 

 Brighton, in 1828, was to produce the vacuum by means of 

 air pumps worked by two steam engines, and this was the 

 plan afterwards most generally employed on the several ex- 

 perimental lines of pneumatic railway laid down in different 

 parts of the United Kingdom and elsewhere. In the Prus- 

 sian pneumatic dispatch tube both compression and expan- 

 sion are employed. The tube itself consists of two tubes of 

 welded iron, z\ inches in internal diameter, laid parallel to 

 one another beneath the pavement. A transverse coupling 

 connects them together at one end, whilst at the other end 

 they terminate in two reservoirs, between which an air-pump 



