position; so as the pipe may continue air-tight and the vacuum be again prepared ready for the propulsion of another train. For it will at once be seen that leakage of the pipe must be fatal to the retention of the vacuum or economical exhaustion of the pipe. Were the valve to continue open, the atmospheric air would then be pumped into the pipe instead of the attenuation of the air within the pipe itself proceeding. When it is considered, too, that there must be an open slit, with a continuous valve, from end to end of the pipe, whether one mile or three miles, or from stationary engine to stationary engine, over any extent of a railway, the practical difficulty arising from leakage must at once be apparent. On the Dalkey line the length of pipe being only one and a quarter mile, it was apparent that in short distances like this, to erect air-pumping engines would be a strong objection to this system. It was therefore proposed when that railway was opened, that the fixed engines might be placed at every three miles, or, perhaps, a greater distance along a line, and an arrangement made by which, when the travelling piston approached the end of one range of pipe, the piston would open a transverse valve at the end of the next pipe, which would admit it into the

next length of pipe, or within the influence of another air-pump, and so on, from one range of exhausted air-pipe to another; and so over the length of railway a train might proceed without stopping. There are, perhaps, few inventions in these inventive times that display more mechanical skill in overcoming difficulties than the atmospheric mode of propulsion, so far even as it has yet been practically tried; and should it ultimately prove unsuccessful, it will be exceedingly mortifying that so much labour should have been bestowed in vain; but should the result of experience be, to prove the system defective, it will sink into desuetude like many other inventions, to which, perhaps, the mental powers of the contrivers have been self-devoted, and which now lie buried in the tomb of time.

LONDON AND CROYDON AND EPSOM, AND DIRECT PORTSMOUTH ATMOSPHERIC RAILWAY.

As this railway is modelled on the preceding one, and has attracted much public attention, and as I have had opportunities of observing its progress, I shall give a short description of it, avoiding technicalities. The atmospheric machinery

is designed under the patent of Messrs Clegg and Samuda, by whom, and Mr. B. Cubitt, the plan has been carried into effect. The portion of the railway completed is 4 miles, namely, from the Dartmouth Arms station, five miles from London, to Croydon. In the absence of practical experience it became necessary to fix the length of the tube in which the exhaustion could best be effected without loss of power. On the portion of the railway yet formed the stationary engine-houses, which are of the most tasteful design, have been erected about three miles apart; namely, at Dartmouth Arms, Norwood, and Croydon. A strong cast-iron pipe of 15 inches internal diameter, being the same size as that on the Dalkey line, has been laid down and secured to the ground midway between the lines of rails, which are 4 feet 8 inches gauge. The pipe is cast with iron bands to strengthen it, and is made in pieces of 10 feet, firmly joined together, and so forming a continuous air-tight tube. A partial vacuum is formed within this tube by pumping the air out with powerful force-pumps worked by the stationary engines, which are of 50 horse-power each, constructed specially for the purpose by Messrs. Maudsley and Field. The engine power is 300 horses for the 47 miles of rail, there being two

engines at each station. The exhausting cylinder or air-pump is about 6 feet 3 inches in diameter. The piston is worked by direct action of the engine. On the top of the cylinder there are 24 spring valves for the escape of the air drawn from the vacuum tube by the ascent of the piston, and a similar number permits the discharge of the air from the bottom of the cylinder on the descent of the stroke. The vacuum tube commences at one side of the cylinder, having a closing valve between the two.

The travelling piston is placed inside the main pipe, and fits it air-tight with leather packing, but free for motion. The piston is linked to the leading carriage of the train by means of a strong iron slanting plate or arm. The arm projects from the tube, through a continuous slit on its upper surface, and is fixed to the bottom of the carriage. The pressure of the atmosphere upon the piston forces it along, and carries with it the connecting arm, and with the latter the carriages are drawn onwards, their wheels gliding along the rails. The arm traverses the slit or narrow opening on the top of the pipe, from end to end, without any obstruction being offered to its progress. On the continuous slit in the pipe the valve is placed, extending the whole length of the pipe,

and upon the regulation of its action in opening and shutting, and being kept air-tight, depends the vacuum.

The external appearance of the valve is an immense range of iron flaps or plates, about eight inches long, connected together, placed in a square groove or continuous box in the top of the tube, and hinged at one side, as shown in fig. 61., being a perspective view of a portion of the vacuum pipe.

The valve is formed of a broad strip of leather rivetted between two iron plates. The upper plate seen being wider than the slit prevents the leather from being pressed into the pipe from the pressure of the atmosphere as the vacuum is formed. The lower plate fits the opening when