the straight portions of the tracks should be of the shape of an S curve, in order that the passage may be gradually effected. At the present time switch rails and frogs of peculiar construction are in use, which are so made and arranged as to leave the main track unbroken, so that if the switch is wrongly placed the train on the main track will not run off. There are many devices for securing this result.

744. Turn-plates. Where one track intersects another under a considerable angle, it will be necessary to substitute for the ordinary method of connecting them, what is termed a turn-plate, or turn-table. This consists of a strong circular platform of wood or cast iron, movable around its centre by means of conical rollers beneath it running upon iron rollerways. Two rails are laid upon the platform to receive the car, which is transferred from one track to the other by turning the platform sufficiently to place the rails upon it in the same line as those of the track to be passed into.

745. Street crossings. When a track intersects a road, or street, upon the same level with it, the rail must be guarded by cast-iron plates laid on each side of it, sufficient space being left between them and the rail for the play of the flanch. The top of the plates should be on a level with the top of the rail. Wherever it is practicable a drain should be placed beneath, to receive the mud and dust which, accumulating between the plates and rail, might interfere with the passing of the cars along the rails.

746. Gradients. From various experiments upon the friction of cars upon railways, it appears that the angle of repose is about, but that in descending gradients much steeper, the velocity due to the accelerating force of gravity soon attains its greatest limit and remains constant, from the resistance caused by the air.

The limit of the velocity thus attained upon gradients of any degree, whether the train descends by the action of gravity alone, or by the combined action of the motive-power of the engine and gravity, can be readily determined for any given load. From calculation and experiment it appears that heavy trains may descend gradients of, without attaining a greater velocity than about 40 or 50 miles an hour, by allowing them to run freely without applying the brake to check the speed. By the application of the brake, the velocity may be kept within any limit of safety upon much steeper gradients. The only question, then, in comparing the advantages of different gradients, is one of the comparative cost between the loss of power and speed, on the one hand, for

ascending trains on steep gradients, and that of the heavy excavations, tunnels, and embankments on the other, which may be required by lighter gradients.

In distributing the gradients along a line, engineers are generally agreed that it is more advantageous to have steep gradients upon short portions of the line, than to overcome the same difference of level by gradients less steep upon longer developments.

747. In steep gradients, where locomotive power cannot be employed, stationary power is used, the trains being dragged up, or lowered, by ropes connected with a suitable mechanism, worked by stationary power placed at the top of the plane. The inclined planes, with stationary powers, generally receive a uniform slope throughout. The portion of the track at the top and bottom of the plane should be level for a sufficient distance back, to receive the ascending or descending trains. The axes of the level portion should, when practicable, be in the same vertical plane as that of the axis of the inclined plane.

Small rollers, or sheeves, are placed at suitable distances. along the axis of the inclined plane, upon which the rope

rests.

Within a few years back flexible bands of rolled hoop-iron have been substituted for ropes on some of the inclined planes of the United States, and have been found to work well, presenting more durability and being less expensive than ropes.

On very steep gradients the expedient of a third rail in the centre of the track, and raised rather above the plane of the other two rails, has been used. Two horizontal wheels underneath the locomotive run on this rail, and may be tightened to any desirable degree of compression on it. In this way a gradient of 440 feet per mile is used over Mont Cenis. Without the intermediate rail grades as steep as 280, and in one case 304 feet per mile, have been ascended by means of the adhesive power of the locomotive only. But such grades will never be sought; on the other hand, they will be avoided when possible. Grades of 50 and 60 feet to the mile are very common. The maximum grade allowable by law on the Central Pacific Railroad is the same as that of the Baltimore and Ohio Railroad, viz., 116 feet per mile.

748. Tunnels. The choice between deep cutting and tunnelling, will depend upon the relative cost of the two, and the nature of the ground. When the cost of the two methods

would be about equal, and the slopes of the deep cut are not liable to slips, it is usually more advantageous to resort to deep cutting than to tunnelling. So much, however, will depend upon local circumstances, that the comparative advantages of the two methods can only be decided upon understandingly when these are known.

749. The operations in Tunnelling will depend upon the nature of the soil. The work is commenced by setting out, in the first place, with great accuracy upon the surface of the ground, the profile line contained in the vertical plane of the axis of the tunnel. At suitable intervals along this line vertical pits, termed working shafts, are sunk to a level with the top, or crown of the tunnel. The shafts and excavations, which form the entrances to the tunnel, are connected, when the soil will admit of it, by a small excavation termed a heading, or drift, usually five or six feet in width, and seven or eight feet in height, which is made along the crown of the tunnel. After the drift is completed, the excavation for the tunnel is gradually enlarged; the excavated earth is raised through the working shafts, and at the same time carried out at the ends. The dimensions and form of the cross section of the excavation will depend upon the nature of the soil and the object of the tunnel as a communication. In solid rock the sides of the excavation are usually vertical; the top receives an arched form; and the bottom is horizontal. In soils which require to be sustained by an arch, the excavation should conform as nearly as practicable to the form of cross section of the arch.

In tunnels through unstratified rocks, the sides and roof may be safely left unsupported; but in stratified rocks there is danger of blocks becoming detached and falling; wherever this is to be apprehended, the top of the tunnel should be supported by an arch.

Tunnelling in loose soils is one of the most hazardous operations of the miner's art, requiring the greatest precautions in supporting the sides of the excavations by strong rough framework, covered by a sheathing of boards, to secure the workmen from danger. When in such cases the drift cannot be extended throughout the line of the tunnel, the excavation is advanced only a few feet in each direction from the bottom of the working shafts, and is gradually widened and depended to the proper form and dimensions to receive the masonry of the tunnel, which is immediately commenced below each working shaft, and is carried forward in both directions towards the two ends of the tunnel.

750. Masonry of Tunnels. The cross section of the arch of a tunnel (Fig. 233) is usually an oval segment, formed of

a

Fig. 233-Represents the general form of the cross section c of a brick arch for tunnels.

a, a, askew-back stone between the sides of the arch and the bottom inverted arch.

arcs of circles for the sides and top, resting on an inverted arch at bottom. The tunnels on some of the recent railways in England are from 24 to 30 feet wide, and of the same height from the level of the rails to the crown of the arch. The usual thickness of the arch is eighteen inches. Brick laid in hydraulic cement is generally used for the masonry, an askew-back course of stone being placed at the junction of the sides and the inverted arch. The masonry is constructed in short lengths of about twenty feet, depending, however, upon the precautions necessary to secure the sides of the excavation. As the sides of the arch are carried up, the framework supporting the earth behind is gradually removed, and the space between the back of the masonry and the sides of the excavation is filled in with earth well rammed. This operation should be carefully attended to throughout the whole of the backing of the arch, so that the masonry may not be exposed to the effects of any sudden yielding of the earth around it.

751. The earth at the ends of the tunnel is supported by a retaining wall, usually faced with stone. These walls, termed the fronts of the tunnel, are generally finished with the usual architectural designs for gateways. To secure the ends of the arch from the pressure of the earth above them, castiron plates of the same shape and depth as the top of the arch, are inserted within the masonry, a short distance from

the ends, and are secured by wrought-iron rods firmly anchored to the masonry at some distance from each end.

752. The working shafts, which are generally made cylin drical and faced with brick, rest upon strong curbs of cast iron, inserted into the masonry of the arch. The diameter of the shaft within is ordinarily nine feet.

753. The ordinary difficulties of tunnelling are greatly increased by the presence of water in the soil through which the work is driven. Pumps, or other suitable machinery for raising water, placed in the working shafts, will in some cases be requisite to keep them and the drift free from water until an outlet can be obtained for it at the ends, by a drain along the bottom of the drift. Sometimes, when the water is found to gain upon the pumps at some distance above the level of the crown of the tunnel, an outlet may be obtained for it by driving above the tunnel a drift-way between the shafts, giving it a suitable slope from the centre to the two extremities to convey the water off rapidly.

In tunnels for railways, a drain should be laid under the balasting along the axis, upon the inverted arch of the bottom. Tunnelling in rock is greatly facilitated at the present day by power-drilling-machines, which are driven by compressed air. By this means they are able to advance three times as fast as by hand labor. The compressed air greatly facilitates ventilation. The Mont Cenis tunnel (nearly 7 miles long) and the Hoosac tunnel (about 4 miles long) have been driven in this way, and the St. Godard tunnel (nearly 13 miles long) is now in process of construction on the same plan.

754. The following extracts are made from a series of papers, published in the London Engineering, from Oct. 7, 1870, to December 30, 1870, giving a translation of a work by Baron von Weber, Director of the State Railways of Saxony, with running comments by the translator, detailing the experiments made by the author, and giving his deductions from them, on the Stability of the Permanent Way.

Baron von Weber desired, in the first place, to ascertain what was the minimum thickness which would be given to the web of a rail, in order that the latter might still possess a greater power of resistance to lateral forces than the fastenings by which it was secured to the sleepers.

755. Resistance of Rail to Lateral Forces. From the experiments the result was deduced, that the least thickness ever given to the webs of rails in practice is more than sufficient, and that if it were possible to roll websin. thick, such webs would be amply strong, if it were not that there