Although, as we have shown, the results obtained on the Dalkey line are not universally regarded as in favour of the atmospheric principle, many eminent engineers contending that they decidedly establish its inferiority to the locomotive system, it is clear, from the many inventions which have since been brought forward in connexion with the subject, that the principle of atmospheric, or at least pneumatic propulsion has numerous partisans, who consider that all that is required to establish its superiority is either new modes of applying the principle, or improvements in the details of execution. From the interest excited by the subject we shall proceed to notice a few of these inventions. We shall commence with Mr. Pilbrow's atmospheric railway, the distinguishing characteristic of which is, that the atmospheric main or tube is closed throughout its length; the connexion between the leading carriage on the rails and the travelling piston inside the tube being effected by a singular and extremely ingenious arrangement. The apparatus consists of a cylindrical cast iron tube, having a covered rectangular channel extending along its upper side.
Along this channel, at intervals of about 30 feet, are fixed small square boxes, with standards to each to support a pair of vertical spindles, placed one on each side of the channel. On the lower end of each spindle, within the boxes, and on a level with the channel, is fixed a pinion with oblique teeth, and to the upper side of the travelling piston is attached a bar or rack with similar teeth on each side; the progress of the piston brings the rack between the wheels, and turns the spindles round with great velocity. To the upper ends of the spindles are fixed pinions exactly like those below, and to the leading carriage is attached a rack similar to the piston rack. This upper rack lies immediately over the lower or piston rack, and gears into the upper pinions; the advance of the piston, therefore, by imparting motion to the upper pinions, causes a corresponding advance of the carriage. Each rack is of sufficient length to reach from one pair of pinions to another, and consequently the racks are always in gear with the pinions on one pair of spindles.
The annexed cuts will help to render the foregoing description more intelligible.
Fig. 1. is a cross section of the atmospheric main, Fig. 2, a section at the boxes, and Fig. 3. a longitudinal section of the main, with the piston and a portion of the carriage racks; a is the tube, and b the rectangular covered channel extending along the upper side of the tube; c c are the spindles, and d d the lower, and e e the upper, pinions fixed thereon; f is a valve to admit the air to the back of the piston, and g is the key by which it is raised; h is the piston, and k the piston rack which moves along the covered channel, and gearing into the lower pinions gives motion to the spindles; m is the external rack which is attached to the leading carriage, and gearing into the upper pinions, the carriage is impelled with the same velocity as the piston; this rack in its passage depresses the key g, which raises the valve f, by which the air is admitted to the back of the piston.
Fig. 4 is a representation of the leading carriage with the whole of the apparatus.
It cannot be denied that Mr. Pilbrow's arrangements exhibit great ingenuity and skill, but experience is wanting to show how farthe system is adapted to meet the various exigences of railway traffic. The point on which failure is most to be apprehended is precisely that on which the whole system depends, viz., the mode of connecting the piston with the train. For the perfect working of the plan each pair of pinions should stand in such position that the teeth of the piston rack shall, on reaching the pinions, fall into the space between two of the pinion teeth, instead of encountering the teeth themselves; should the latter event happen, the rack and pinions would be liable to become jammed, and a fracture of some part of the apparatus must ensue; but even supposing this objection to be of no practical moment, still, when we consider the momentum of a heavy train, which, moving with a velocity of 30 miles, comes in contact with the pinions, which are at rest, it is to be feared that the teeth of the racks and pinions would soon be destroyed, or at least that a series of shocks would take place, as the racks became engaged with the pinions in succession, which would prove highly unpleasant to passengers, and tend to check the velocity and to increase greatly the wear and tear of the carriages.
Should these difficulties, however, prove to be imaginary, or greatly overrated, we think it will be found to possess many of the advantages over the Samuda system which Mr. Pilbrow has claimed for it in a pamphlet which he has published. Among the leat disputable of these advantages are the following:-
1st. As regards facility and economy of construction; public roads may cross the line on a level, whereby bridges are rendered unnecessary; no cranes or elevated rails are required for taking carriages on or off the line, and different lines of railway may cross each other at any required angle, (one tube passing below the other tube,) which on Mr. Samuda's plan cannot be done. Mr. Pilbrow further claims a saving as regards the cost of his main, and also in the difference of expense between the continuous valve and his spindles and pinions, but we think this latter item more doubtful.
2d. As regards the loss of power from leakage; the piston valves are less liable to leak than the continuous valves, as the former are ground into their seats, and they expose a less surface at which leakage can take place; for the pinion valve or seat is but about nine inches in circumference at the aperture where the air is admitted, and there are but two of them to every 30 feet of main=l 1/2 feet, whereas in the continuous valve the whole 30 feet is liable to leakage; hence, were the piston valves even to leak as much as the long valve, surface for surface, the leakage would amount to only one-twentieth of the leakage of the continuous valve. From this circumstance Mr. Pilbrow assumes that one engine to every 10 miles will be found sufficient, whereby the number of engine establishments upon a line would be greatly reduced: but leakage is not the only source of loss of power in the atmospheric tube; the resistance from the friction of the air in the tube is very considerable, increasing rapidly with the length, and Mr. Herapath, from M. Mallet's experiments on the Dalkey line, calculates that with a main 5 3/4 miles long, the engine could not exhaust the main to one half of that which it would if only 1 1/2 mile long; and he contends, therefore, that nothing is gained in applying this system by employing longer mains than 1 1/2 mile, and is of opinion that mains of only a mile would be more powerful, command greater loads and velocities, and prove on the whole more economical.