behind the piston: the aperture thus formed serves also for the free admission of air to press on the back of the piston. When the wheels have passed by, the valve falls again by its own weight.

But by the operation of raising the valve out of the trough, the composition between it and the main pipe has been broken, and the air-tight contact must be reproduced. To effect this, another wheel R (Fig. 4) is attached to the carriage, which serves to ensure the perfect closing of the valve by running over the top plates immediately after the piston rod has passed; and a copper tube or heater N, about five feet long, filled with burning charcoal, is also fixed to the under side of the carriage, and passes over the surface of the composition, softening it and pressing it down, so that when on cooling it becomes solid, it seals the joint air-tight as before. Thus each train, in passing, leaves the pipe in a fit state to receive the next train.

A protecting flap or cover I, formed of thin plates of iron about five feet long, hinged with leather, is made to lie over the valve, to preserve it from snow or rain; the end of each plate underlaps the next in the direction of the piston's motion, being lifted up by wheels D (Fig. 3), fixed under the advancing carriage, and allowed to close again as it retires.

The parts above described constitute the essence of the plan. Much ingenuity and care were bestowed on the arrangement of other details, such as the entrance, exit, and separating valves, the mode of effecting junctions and crossings, the construction of the tube, the manner of connecting together the pipes of which it was formed, the

arrangement of the exhausting pumps, &c. &c. But it is not necessary here to go into these particulars.

The exhausting pumps on the experimental line at Wormwood Scrubs were worked by a steam engine of fifteen horse-power, and produced in one minute a vacuum in the pipe equal to about 18 or 20 inches of mercury; and by maintaining this exhaustion, it was found that, even with the small pipe used, a load of 13 tons could be propelled up the incline at a rate of 20 miles an hour; or with a vacuum of 23 inches, a load of 5 tons would go 45 miles an hour.

These trials were considered so successful, and seemed to promise so much for the new system of propulsion, that they naturally attracted the attention of persons interested in railways, and among these was Mr. James Pim, treasurer of the railway between Dublin and Kingstown, who, after having carefully observed the experiments, became a most energetic advocate of the plan. It appears that the railway with which he was connected, had a short piece of line from Kingstown to Dalkey, which had been used for the transport of stone for the new works at Kingstown Harbour, and which, having steep gradients and sharp curves, offered what were then considered rather formidable difficulties to the working of the line.

About May 1841, Mr. Pim wrote a letter to Lord Morpeth, asking the permission of the Board of Public Works of Ireland (under whose care the Kingstown and Dalkey road was placed) for the parties interested in the experiment to lay down an atmospheric apparatus along this line.

It appears, however, that the sanction of the Board of Trade was needed to carry out this proposal, and Mr.

Pim, nothing daunted, wrote towards the end of the same year a letter to the Earl of Ripon, President of the Board, describing clearly the principle and mode of working of the atmospheric system, and giving a lucid and forcible statement of the arguments in its favour; the object of the letter being to ask the Board to refer it to such persons as their Lordships might select, to enquire into the several statements made, and in their report to state particularly whether the invention was entitled to a further and more extended trial.

The request was granted, and two scientific referees— Lieut. Col. Sir Frederic Smith, R.E., F.R.S., and Professor Barlow, F.R.S.-were accordingly appointed to investigate the merits of the plan.

Their report was dated February 15, 1842. It appears that they conducted experiments, in January, on the model line at Wormwood Scrubs, which generally corroborated those of the projectors; and after making the necessary calculations and deductions, they reported that they considered the principle of atmospheric propulsion to be established; that its economy of working would increase with the scale on which it was applied; and that it appeared well suited for such a line as that from Kingstown to Dalkey. On the points of first outlay, cost of working, safety, and convenience, as compared with the locomotive system, the referees did not venture any very decided opinion.

The report was, however, sufficiently favourable to warrant the Government in sanctioning the trial of the principle on the Kingstown and Dalkey line, and in granting a loan of £25,000 to the Company for the purpose. This determination was due to the influence of the late

Sir Robert Peel, then First Lord of the Treasury, who was on this occasion, as well as during its whole history, a strong supporter of the plan.

With this encouragement, the railway was accordingly prepared, the tubes laid down, the engines erected, and the apparatus was set to work in August 1843.

The line was single, and about one mile and threequarters in length. It had a short descent from Kingstown, after which it rose to Dalkey with an average gradient of about 1 in 116; the steepest part, 1 in 57. There was one considerable curve, of 518 feet radius; a shorter one of 570 feet radius; and a third, of 700 feet radius. The atmospheric tube was 15 inches internal diameter, placed in the middle of the road, between the two rails, and firmly attached to the cross transoms under the sleepers. It was in lengths of 9 feet each connected by socket joints carefully filled with cement. The width of the longitudinal valve opening was two and a half inches. The arrangements of the valve were made with all possible care, and with the benefit of all the experience gained by the previous experiment.

The pipe did not extend the whole length of the road, but stopped short of the summit of the hill by 560 yards, the carriages running up this distance by their momentum alone.

The steam engine was placed, for the sake of convenience, at about 500 yards from the upper end of the tube, being joined to it by a connecting pipe of equal diameter. The engine was of 100 horse-power, working an air pump of 67 inches diameter, with a stroke of 5 feet 6 inches.

Experiments made on the line, soon after the opening,

gave good results as to the action of the apparatus. It was found that a rarefaction of 13 to 14 inches could be obtained in two minutes, and 22 inches in five minutes; the pump making 22 strokes per minute. And in running trains up the incline, it was observed that 30 tons could be drawn up at a speed of about 30 miles an hour, and 70 tons at about 20 miles; which, considering the difficulties of the road, was certainly satisfactory.

This confirmation of the results previously obtained on a smaller scale served to increase the popularity of the new invention, and to stimulate its promoters to urge its claims upon the railway world, with a view to securing its more extended adoption.

It may be well here to give a summary of the chief arguments which, at various periods, were urged in favour of the atmospheric system, as compared with other modes of railway locomotion.

The more cogent of these took the shape of objections to the locomotive engine. It was said—

(1) In the first place, that to make a steam engine locomotive was eminently unfavourable for its economy of fuel; that the quantity consumed was excessive, and the kind expensive.

(2) That this was also a very unfavourable condition for keeping the engine in repair, and that the necessity of having a large stock of engines constantly under examination in hospital' led to a very large extra outlay of capital.

(3) That the locomotive engine had to overcome the friction and other resistances due to itself, and to the tender carrying its supplies of fuel and water; to which had