It may bees well here to observe, that in Laying rails, the allowance for tho elongation produced by summer heat must be very carefully made. Iu a 15 feet rail, the difference of length for an increase of 76' Fehr. will be about Lth of an inch ; but at the same time that pro vision must be made for the free expansion of the rails, they must not be laid with so open a joint as to give rise to concussions. It is usual, also, to give a slight inclination to the upper surface of the rails, inclining inward about 1 in 30, for the purpose of giving a better bearing to the conical surfaces of the wheels ; and in curves, the outer rail is elevated above the inner one, by a height regulated by the maxi mum velocity attained upon the curve, and by its radius. Upon level crossings, very sharp curves, viaducts, or bridges, it is usual to fasten counter-rails; but they have so often proved sources of danger, through their independent movements, that these counter-rails are never placed unless under very exceptional circumstances. On level crossings they are necessary in order to protect the rails from the shocks of passing waggons. A very common elevation of the outer rails, on curves of 1000 yards radius (and this seems to be the minimum radius which should bo allowed in railways upon which ordinary express speeds are attained) is about 2 inches ; and in curves near stations, of 500 yards radius, the elevation becomes about 4 inches. Fractures of the rails occur on changes of temperature, and chairs are frequently broken when wedged up too tightly. Great care is therefore required in the inspection of the rails at these seasons ; and it is worthy of remark, that there is a marked tendency of the rails to displace themselves in the direction of the movement of the trains, especially on inclines and near the stations. In some cases, every 10th or 20th sail is notched upon its chairs, in order to resist the tendency to this kind of displacement.
The battle of the gauges, which raged so furiously a few yeara since, has now been practically solved by the adoption of the mixed gauge on the Great Western line itself ; and indeed the advantages of a uniformity of gauge are so much greater than those which can be secured even by a more logical arrangement than the one adopted originally by Mr. Stephenson, that it is almost idle to discuss the question at present. For all useful purposes, it is ascertained that the 4 feet 8i inches gauge is as efficient as the 6 or 7 feet gauge; and it Is immeasurably cheaper, both as regards the formation of the roadway and the con struction of the rolling plant. It seems that the preciso gauge of 4 feet S4 inches was the result of a mere accident ; but as it has been adopted on the majority of railways, it is now desirable to carry it out on all similar works. The space between the rails is usually made 6 feet from edge to edge.
Accessories of the Boatitray.—It is often necessary to pass engines and carriages from one line of rails to another, and this is effected by MOULDS of switches or points, and crossings by turntables, or by traversing platforms. if we in fig. 13, a series of cases in alowly, and must be repeated for every separate carriage. Turn-tables consist of circular platforms of wood and iron, fixed on a level with the rails, and mounted on friction-rollers, so as to turn freely upon their central pivots ; four rails are fastened on the upper face, and made to correspond with the positions of the longitudinal and trans verse rails at each quarter revolution, in such cases as those repre sented in fig. 16, wherein, if it were desired to transfer a carriage from
which such changes of direction may be necessary, we may perceive that some contrivance is required to guide the flanges of the wheele in such wise as to cause them to follow the new line of rail they are intended to travel upon. This contrivance consists of two moveable rails (sly. 14) working simultaneously and parallelly upon fixed pivots, so that the flange of the wheel on ono side should bear against one of the rails, and should thus be forced to follow it, whilst the other flange would find an open passage which would allow it to follow the new line of direction given. Many varieties of points and crossings have been introduced from time to time, of which the most concise account is to be found in Perdonnet's Trait6 Elementaire '; but for our present purposes it may suffice to say that the system now gene rally adopted is the one known by the name of Wyld's patent, in which the surfaces of the rails in the principal line are not touched, but the tongues of the points are made to pass gradually under the heads of the main rails. On the Great Western line many of the points were originally made as in fig. 15. The radius of the curves of the best points and crossings is usually made equal to l650 feet, and care is taken that counterpoises should be fixed, to keep the main line open ; as far as possible, also, no points should bo fixed so as to meet the advancing trains on the main line. At the points of turns out of single lines of railways, and at the junction points of embranehments, signal posts e:re Invariably fixed for the purpose of indicating which line is open, and the radius of curvature is made aA large as possible. On the Belgian lines, the embranchrnenta are made near the stations, and some times the radius of the points does not exceed 1000 feet ; but it is to be observed that these einbranchments have no back curves like those of ordinary crossings. It may be as well to add, that where the two rails cross one another, grooves are cut to allow the flanges to pass, and guard-rails are placed to check any tendency of the rails to leave the road, as shown in fig. 13.
Turntables present a marked advantage over points and crossings in this respect, that they allow the carriages, &c., to pass from one line to the other in a much smaller space; hut the operation itself is performed line a to the line b, the carriage would be rolled to d ; a quarter revolution of tha table is then made, and the carriage is pushed on to e ; a similar quarter revolution of the table e is made, and the carriage could then be advanced in either direction on tho lane d. The dimensions of turn-tables depend on the sizes of the carriages to be moved : thus, for goods waggons, an 8-feet table will suffice; for passenger carriages a 12-feet table is necessary ; whilst for locomotives and tenders coupled together, tables of as much as 36 or even 50 feet diameter have been made. Sometimes the turn-tables of stations are made to serve a series of converging rails ; and in the engine-houses of some railways a large table is placed in the centre, so as to serve the purposes of as many as 16 lines.