PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. O/O 



tlio tiiimol. AVlicMi it is required to make the down journey tlie breaks are 

 taken oft' the wheels, and the carriag'e passes by its momentum a few yards 

 into the tunnel, where it travels over a vault or air-well covered with an 

 open iron •grating', from which, as soon as it has passed, a gale of wind 

 rusiies, and a pair of iron doors, hung like lock gates, close firmly over the 

 mouth of the tunnel, and the train is lost to the sight of tiie outside spec- 

 tator. With every revolution of the eject(jr — it makes from 120 to 150 a 

 minute — the strength of the gale increases, and the carriage flies more 

 (jiiiekly along its dark iron roadway. 



Down the steep gradient no pressure behind the carriage is needed, as it 

 travels by its own momenluni, and it is brought up at the other end by the 

 application of the ordinary break power. 



The return voyage is made by the exhausting process, just as one would 

 draw water through a straw. At a signal given by electric telegraph a 

 valve is openc^d, the doors of the iron case are thrown back, and the air 

 sucking discs are again in full swing. Near the upper end of the tunnel 

 and in its side there is a vaulted passage, eight feet in height, built of 

 brick, and which communicates with the well near the engine house. 

 Through this side vault the throat of the tube, the air from the tube is 

 inhaled, the iron doors at the upper end still remaining closed. Instantly 

 the carriage, which is at the mouth of the tunnel at the lower end, feels 

 the effect of the air rushing into this suction vault, the external atmosphere 

 acts upon the carriage from behind with its ordinarj^ pressure of some 

 fourteen pounds to the inch, and being relieved of a portion of the pressure 

 in front, the carriage is forced forward on its return journey. The iron 

 doors at the upper station open as the pressure increases and the advancing 

 carriage emerges into daylight. 



There is this important point of difference between the pea-shoveller of 

 our school days, and the pneumatic tube, that in the former the pea or 

 pellet is projected violently from the mouth o^ the tube, while in the latter 

 the carriage glides gently along, gradually slackening till the destroyed 

 ecpnlibrium of the atmosphere is restored. Compared with the old atmo- 

 spheric principle, when a pressure of from seven to ten pounds was required, 

 one cannot fail to be convinced of the mnny advantages which the present 

 system possesses over its predecessor, AVith a view of making the experi- 

 ment as complete as possible, and of testing the sj'stem under the disad- 

 vantages to which it would be subject in its more extended application, 

 one portion of the tunnel has a gradient of one in fifteen, or one-sixth 

 greater than that of Holborn-hill, while another is upon a curve of not 

 more than eight chains radius. These difficulties are, however, overcome 

 in a manner which proves that the system could be adopted upon any line, 

 however crooked or steep in its ascent. 



The tunnel is dark, it is true, as all tunnels must be, but the carriage is 

 well lighted, and tJjere is no reason why gas should not be employed, as is 

 the case in the carriages of the Metropolitan Railway. There are luxu- 

 riously cushioned seats placed along each side of the carriage, as in an 

 omnibus. There is a glass door at each end, and lamps are carried in front 

 and behind the carriage, so that the tunnel is really illuminated as the 

 train pa.sses through. The motiop of the carriage is so smooth aod easy, 



