by the weight upon the rail. The thickness of the rib varies between half an inch to three-fourths of an inch; and the total depth of the rail from three to five inches. The thickness and breadth of the bottom have been varied according to the strength and stability demanded by the traffic.

736. Steel Rails. Rails made entirely of steel, or of wrought iron, with a thin bar of steel forming the top surface, or steel-top, or steel-headed rails as they are termed, from their superior strength and durability, are coming into general use in replacing the worn-out wrought-iron rails of old roads. Steel obtained from any of the usual processes, either cast, puddled, or Bessemer steel, may be used for the steel heads of rails.

From the experience of Swedish engineers it appears that solid Bessemer steel rails of the best charcoal pig-iron may be made 10 per cent. lighter than the best English wrought-iron rails, a result which has been carried into practice on the Austrian railways.

The durability of iron rails appears to depend principally upon the perfection of the welding, the chief cause of their want of durability arising from the lamination caused by imperfect welding.

Formerly wrought-iron rails were made partly by hammering and partly by rolling. At present rolling alone is used, and the results are said to be more satisfactory, whilst the process of manufacture is more simple.

The resistance to wear of rails, from English experience, it is said, may be measured by the product of the speed and of the weight passing over them. The rule proposed for the work that rails may be subjected to is 220,000,000 tons transported at the rate of one mile per hour. The length of time that iron rails will last in any given case will be found by multiplying the number of tons transported by the rate of speed per hour and dividing by 220.

737. Supports. The rails are laid upon supports of timber or stone. The supports should present a firm, unyielding bed to the rails, so as to prevent all displacement, either in a lateral or a vertical direction, from the pressure thrown upon them.

Considerable diversity is to be met with in the practice of engineers on this point. On the earlier roads, heavy stone blocks were mostly used for supports, but these were found to require great precautions to render them firm, and they were,. moreover, liable to split from the means taken to confine the rails to them. Timber is generally preferred to stone. It

affords a more agreeable road for travel, and gives a better lateral support to the rails than stone blocks, and the wear upon the locomotive and other machinery is less severe.

The usual method of placing timber supports is transversely to the track, each support, termed a sleeper, or cross-tie, being formed of a piece of timber six or eight inches square. The ordinary distance between the centre lines of the supports is three feet for rails of the usual dimensions. With a greater bearing, rails of the ordinary dimensions do not present sufficient stiffness. The sleepers, when formed of round timber, should be squared on the upper and lower surface. On some of the recent railways in England, sleepers presenting in cross section a right-angled triangle have been used, the right angle being at the bottom. They are represented to be more convenient in setting, and to offer a more stable support than those of the usual form. The sleepers are placed either upon the ballasting of the roadway, or upon longitudinal beams laid beneath them along the line of the rails. The latter is indispensable upon new embankments to prevent the ends of the sleepers from settling unequally. Thick plank, about eight inches broad and three or four inches thick, is usually employed for the longitudinal supports of the sleepers.

On some of the more recent railways in England, the rails have been laid upon longitudinal beams, presenting a continuous support to the rail, the beams resting upon cross-ties.

738. Ballast. A covering of broken stone, of clean coarse gravel, or of any other material that will allow the water to drain off freely, is laid upon the natural surface of the excavations and embankments, to form a firm foundation for the supports. This has received the appellation of the ballast. Its thickness is from nine to eighteen inches. Open or brokenstone drains should be placed beneath the ballasting to convey off the surface water. The parts of the ballasting upon which the supports rest should be well rammed, or rolled; and it should be well packed beneath and around the supports. After the rails are laid, another layer of broken stone or gravel should be added, the surface of which should be slightly convex and about three inches below the top of the rails.

739. Temporary Railways of Wood and Iron. On the first introduction of railways into the United States, the tracks were formed of flat iron bars laid upon longitudinal beams. The iron bars were about two and a half inches in breadth, and from one-half to three-fourths of an inch in thickness, the top surface being slightly convex. They were placed on the

longitudinal beams, a little back from the inner edge, the side of the beam near the top being bevelled off, and were fastened to the beam by screws or spikes, which passed through elliptical holes with a countersink to receive the heads of the spikes; the holes receiving this shape to allow of the contraction and expansion of the bar, without displac ing the fastenings. The longitudinal beams were supported by cross sleepers, with which they were connected by wedges that confined the beams in notches cut into the sleepers to receive them. The longitudinal beams were usually about six inches in breadth, and nine inches in depth, and in as long lengths as they could be procured. The joints between the bars were either square or oblique, and a piece of iron or zinc was inserted into the beams at the joint, to prevent the end of the rail from being crushed into the wood by the wheels.

In some instances the bars were fastened to long stone blocks, but this method was soon abandoned, as the stone was rapidly destroyed by the action of the wheels; besides which, the rigid nature of the stone rendered the travelling upon it excessively disagreeable.

This system of railway, whose chief recommendation is economy in the first cost, has gradually given place to the solid rail. Besides the want of durability of the structure, it does not possess sufficient strength for a heavy traffic.

740. Gauge. The distance between the two lines of rails of a track, termed the gauge, which has been adopted for the great majority of the railways in England, and also with us, is 4 feet 8 inches. This gauge appears to have been the result of chance, and it has been followed in the great majority of cases up to the present time, owing to the inconvenience that would arise from the adoption of a different gauge upon new lines. The greatest deviation yet made from the established gauge is in that of the Great Western Railway, in which the gauge is seven feet. Engineers are generally agreed that with a wider gauge the wheels of railway cars could be made of greater diameter than they now receive, and be placed outside of the cars instead of under them as at present; the centre of gravity of the load might be placed lower, and more steadiness of motion and greater security at high velocities be attained. All roads having a gauge above 4 feet 8 inches are inclined rather to reduce them to that gauge or use a third rail so as to run the cars of that gauge

over their own.

Within the last four or five years the subject of roads of very narrow gauge has been much discussed. The advan

tages principally claimed for roads of this kind are: 1st, great reduction in first cost; 2d, allowing steeper grades and curves of smaller radius; 3d, less wear and tear on the road on account of the rolling stock being much lighter; 4th, the ratio of live to dead weight is much less. Some lines have been made with a 23-foot gauge, but the advocates of narrow gauge generally recommend a 3-foot gauge. The latter is gauge of the Denver and Texas narrow-gauge road.

the

In a double track the distance between the two tracks is generally the same as the gauge; and the distance between the outside rail of a track, and the sides of the excavation, or embankment, is seldom made greater than six feet, as this is deemed sufficient to prevent the cars from going over an embankment were they to run off the rails.

741. On all straight portions of a track, the supports should be on a level transversely, and parallel to the plane of the track longitudinally. The top surface of the rail should incline inward, to conform to the conical form of the wheels; this is now usually effected by giving the chair the requisite pitch, or by forming the top surface with the requisite bevel for this purpose.

742. Curves. In the curved portions of a track the centrifugal force tends to force the carriage towards the outside rail of the curve, and by elevating the outer rail the force of gravity tends to draw it towards the inside rail. From the above conditions of equilibrium the elevation which the exterior rail should receive above the interior can be readily calculated. The method adopted is to give the exterior rail an elevation sufficient to prevent the flanch of the wheel from being driven against the side of the rail when the car is moving at the highest supposed velocity; or, in other words, to give the inclined plane across the track, on which the wheels rest, an inclination such that the tendency of the wheels to slide towards the interior rail shall alone counteract the centrifugal force.

743. Sidings, etc. On single lines of railways short portions of a track, termed sidings, are placed at convenient intervals along the main track, to enable cars going in opposite directions to cross each other, one train passing into the siding and stopping while the other proceeds on the main track. On double lines arrangements, termed crossings, are made to enable trains to pass from one track into the other, as circumstances may require. The position of sidings and their length will depend entirely on local circumstances, as the length of the trains, the number daily, etc.

The manner generally adopted, of connecting the main track with a siding, or a crossing, is very simple. It consists (Fig. 231) in having two short lengths of the opposite rails

of the main track, where the siding or crossing joins it, movable around one of their ends, so that the other can be displaced from the line of the main track, and be joined with that of the siding, or crossing, on the passage of a car out of the main track. These movable portions of rails are connected and kept parallel by a long cross-bolt, to the end

Fig. 232-Represents a plain M, and section N, of a fixed crossing plate. The plate A is of cast-iron, with vertical ribs c, c, on the bottom, to give it the requisite strength. Wroughtiron bars a, a, placed in the lines of the two intersecting rails d, d, are firmly screwed to the plate; a sufficient space being left between them and the rails for the flanch of the wheel to pass.

of which a vertical lever is attached to draw them forward, or shove them back.

At the point where the rails of the two tracks intersect, a cast-iron plate, termed a crossing-plate (Fig. 232), is placed to connect the rails. The surface of the plate is arranged either with grooves in the lines of the rails to admit the flanch of the wheel in passing, the tire running upon the surface of the plate; or wrought-iron bars are affixed to the surface of the plate for the same purpose.

The angle between the rails of the main tracks and those of a siding or crossing, termed the angle of deflection, should not be greater than 2° or 3°. The connecting rails between