The whole of this lateral opening is covered by the valve before described, and that part of it through which the arm passes is lifted to allow it to pass, and also for the admission of air to the piston by means of an apparatus connected to the arm. The carriage to which this arm is attached we call the driving carriage; to the hinder part of this carriage a long heater is attached, which is drawn along by it upon the tallow contained in the trough, and reseals the valve ready for the next train, which repeats the operation above described. At certain distances, which are regulated by the nature of the road, steam engines and air pumps or other apparatus are fixed, for exhausting the pipes, and at a short distance beyond the connextion from the engine to the pipe valves are placed, closing the end of one length or section of pipe and the beginning of the next, between which a space is left for stopping the trains if required; these valves also divide the pipe into suitable lengths, to be exhausted by each apparatus, or close to the end where it is not required to be continued, as on acclivities where the carriages will run by their own gravity; thus every section of pipe is enclosed at the two ends by these valves, and is exhausted by its own steam-engine and apparatus.
These valves, which I call the separating valves, are opened by the driving carriage, to allow the piston to pass, and are closed after the train has passed."
Figs.1 and 2 represent the construction of the valve, Fig. 1 showing a section of the valve when closed, and Fig. 2 a section of the valve when open; a is a section of the cast iron vacuum tube or atmospheric main, b is the continuous valve of leather extending over the whole length of the opening on the upper side of the tube, and strengthened by short plates of iron c c, attached to the upper and under side of the leather, so as to impart firmness whilst admitting a sufficient degree of flexibility. d is the composition by which the edge of the valve is hermetically sealed, e. is a protecting cover formed of thin plates of iron about 5 feet long, hinged with leather for protecting the valve from rain, snow, etc.; this cover however was, we believe, subsequently dispensed with when the invention was brought into actual operation.
An experimental line of railway was subsequently laid down at Wormwood Scrubbs, by Mr. Clegg and Mr. Samuda, who had an interest in the patent. The length of the line was about half a mile long, with a rise of 1 in 120 for about half the way, and 1 in 115 for the remainder; the diameter of the atmospheric main was 9 inches, and the exhaustion was produced by means of an air pump, of 37 inches in diameter, and 22 inches stroke, worked by a condensing engine of 16 horse power. On this line several public experiments were made, which were attended by many engineers, and other scientific persons, some of whom formed a highly favourable opinion of the plan, and the Dublin and Kingstown railway company were induced by the report of their engineer, Mr. Vignoles, to adopt it in the extension of their line from the terminus at Kingstown to the village of Dalkey.
The length of this line is 3,050 yards, or nearly 1 mile and 3/4, with a rise of 71 1/2 feet from the commencement at Kingstown to the termination at Dalkey, the average rise from the lowest point being 1 in 140, but the last 365 yards have a rise of 1 in 57; there are also several sharp curves on the line, so that upon the whole it was well adapted to show the capabilities of the system for overcoming steep ascents, and sharp curves. The line is worked only one way by the atmospheric apparatus, the return being effected by the force of gravity.
As stated above, the length of the line is 3,050 yards, but the atmospheric main is only 2,490 yards long, the remainder of the way, 560 yards, being run by the momentum previously acquired. The diameter of the main is 15 inches, and near its extremity branches out a pipe of the same diameter, which leads to the exhausting apparatus, distant nearly 500 yards. The air pump, which is double acting, is 66 1/2 inches diameter, with a stroke of 66 inches. It is worked by a high pressure condensing engine, of 34 1/4 inches, and 66 inches stroke, working expansively, the cut off valve being regulated by a governor, so as to vary with the speed of the engine, from 1/3 at the slowest to 1/5 at the quickest. Mr. Samuda reckons this engine to be of 100 horse power, but it appears that he assumes the horse power to be equal to66,000lbs, raised one foot high, per minute; other engineers, calculating by Bolton and Watt's standard, estimate that on an average it works up to 180 horse power.
At the entrance end, and some 30 feet from it, is a kind of balance valve, (corresponding to Pinkus's vertical sliding valve,) very ingeniously contrived to open by the pressure of the compressed air in front of the piston; and at the other or exit end is another valve, opening outwards by means of the compression of the rarefied air, after the piston has passed the tube leading from the main to the air pump.
The continuous valve which covers the slit on the surface, is composed of a strip of oxhide of the best quality, strengthened above and below by iron plates, and the leather is double in the part which closes the opening. This valve is fastened down by one edge, (as described in the extract from Mr. Clegg's specification,) and when down fits closely over the slit, its edge being covered by a composition of wax and tallow. From near the centre of the leading carriage descends an arm, which passes through the aperture of the main to the middle of the piston rod within the main. Before the point where this arm is connected to the piston rod are two rollers, (the foremost of which is smaller than the other,) for the purpose of raising the valve, and behind the arm are two similar rollers, for the purpose of keeping open the valve at that part, and thus admitting the atmosphere to press on the back of the piston. After the arm and wheels have passed, the valve drops by its own gravity, and is forced down close by a wheel at the end of the first carriage passing over it: a heater filled with red-hot charcoal was attached to a second carriage, for the purpose of melting the composition, but we believe has since been laid aside, as unnecessary or ineffective.
The advance of the train as it approaches the station is retarded and finally arrested by means of a very powerful brake; it may likewise be arrested on any part of the line, without stopping the engine, by means of a valve in the piston, connected to a lever attached to the leading carriage, by opening which valve the vacuum in front of the piston is destroyed, and the air before and behind the piston is in equilibrio.
Shortly after the Dalkey line came into operation, the French government deputed a Mons. Mallet to examine and report upon the operation of the system as developed on that line. A translation of his report was subsequently published, from which we select the following extracts, showing the results obtained by M. Mallet in his experiments.
"In the following experiments the velocity has been quoted with great care. At each division of the road, forming lengths of 44 yards (40m. 22.) as I have before stated, the time has been marked by means of an instrument in the form of a watch, from which by touching a spring a drop of ink falls upon paper prepared to receive it. This paper takes of course a regular rotatory motion. From the time thus obtained the velocity is calculated. In my notes are the velocities of several trains calculated per hour, for each division, and in order to present their results in the most comprehensible form, I have arranged them in quarters of miles, and in this manner formed the following table.