Since the plan of Medhurst mentioned in the preceding article, it has been sought to connect a passenger carriage with a pneumatic tube so as to afford a practical method of transit, and many patents have been taken out. The plan of Clegg and Samuda, patented in England in 1838, was one of the first invented, and is still as good as any since proposed. It was adopted on the atmospheric railways of Kingstown in Ireland, Croydon in England, and St. Germain in France. Along the upper side of the pneumatic tube there was a slit running throughout its entire length. Over this slit was placed a strip of flexible leather fastened hinge-like on one side. Beneath the continuous leather strip there were short pieces of iron plating placed end to end, which just fitted into the slit, and on the upper side were plates of iron, somewhat wider than the slit. A knee-shaped piece of iron connected with the passenger carriage passed beneath this continuous valve, and was attached to a cylinder several feet long, which acted as a piston, fitting the inside of the pneumatic tube by means of an India-rubber flange. The piston and pipe were lubricated with tallow.
It was difficult to keep the apparatus in order, and notwithstanding that a speed of over 30 m. an hour was attained, the enterprise in this form has been abandoned. But it has been proposed in another form, which is essentially the same as that of the postal pneumatic despatch. In the summer of 1864 Mr. Ram-mell, C. E., constructed a brickwork tunnel on the crystal palace grounds at Sydenham, England. The tunnel was about 10 ft. high by 9 ft. wide, and capable of admitting the largest carriage on the Great Western railway. Rails were laid upon the bottom of the tunnel, which was about 600 yards long. The route was laid with severe gradients and curves, to afford a practical test. A carriage capable of holding 30 passengers was provided with a flange of bristles at each end, by which it was made to fit the brickwork of the inside of the tunnel with such accuracy as to allow of the accumulation of sufficient pneumatic force to propel it. A small stationary steam engine worked a fan in the form of a hollow disk 22 ft. in diameter, so arranged as either to condense or rarefy the air in the tunnel. It is thought that much of the objection against the old system of Clegg and Samuda has been overcome by Rammell's adoption of the pneumatic despatch system.
In the old system the pressure required within the pneumatic tube was from 8 to 10 lbs. to the square inch, while when the tube receives the whole car a pressure of 2 or 3 oz. is all that is required to carry it over a steep gradient. The smoke and bad air of ordinary passenger tunnels traversed by locomotives are avoided, as the pneumatic carriage carries its own supply of pure air, and drives the foul air before it.