Until within a recent period, stone blocks were universally regarded as the best permanent support for the rails wherever they could be laid upon a solid and durable foundation, and wooden sleepers were only resorted to as tempo rary supports, to be ultimately super seded by stone blocks whenever the foundation of the road should be properly consolidated. Opinion has, however, undergone some change on this subject, and wooden sleepers are now always used in preference tc stone blocks for permanent purposes: whether they will prove economical, when submitted to the trial of a long period of time, experience alone can decide.
The chairs, which are the immediate supports or props on which the rails rest, are attached to the centre of the up per surfaces of the blocks in the follow mg manner : Two holes are drilled in the blocks to a sufficient depth, about 2 inches in diameter, into which plugs of oak or other hard wood are driven ; holes are then bored in these, 3-8ths of an inch in diameter, corresponding in position with two holes in the chair of cast iron, which is to support the rail. Iron pins of half an inch in diameter are then driven through the holes in the chairs into the holes in the block, a piece of patent felt being placed between the chair and the block ; and the chair is thus firmly fastened to the block..
The chairs are of cast or wrought-iron, formed with a cavity corresponding to the magnitude and form of the rail : they vary very much in their size and form, according to the opinion or judg ment of the engineer.
A great variety of expedients have been resorted to to maintain the rail fixed in its position in the chair. Pins and wedges of iron were first used of various forms, and applied in various ways. These, however, have now been very generally superseded by the simple con trivance of a wooden block or wedge, driven in between the side of the chair. These wedges are prepared by previous ly passing them through a hydrostatic press, so as to harden them by exposing them to a severe pressure. Besides af fording a very effectual fastening to the rail in the chair, these wedges, from the nature of the material, soften the jar which attends the transition of the wheels over the chairs.
The distance between the bearings or chairs has also been subject to change. The necessary strength or weight of the rail will evidently depend on this dis tance; the greater the distance between the props, the greater must be the strength of the rail • and, so far as re gards the expense, the engineer has to balance the cost of heavier rails against the saving effected by a diminished number of blocks and chairs. But, in
dependently of the consideration of ex pense, the effect upon the carriages and engines is to be considered. Between chair and chair, a slight flexure of the rail takes place, and the wheels have consequently to pass over a series of emi nences, so as to give to the carriages a pitching motion, the intervals and degree of which must depend conjointly on the strength of the rails and the distance between the chairs.
The least distance between the chairs now used is 3 feet, and the greatest 5 feet ; 50 lb. rails are very generally used on 3 feet bearings ; 65 lb. rails on 4 feet bearings and 15 lb. rails on 5 feet bearings.
Rails made entirely of malleable iron were first employed at the collieries near Edinburgh ; and were formed of rectan gular bars, which presented too small a surface for ale wheels, or otherwise re quire more materials than it would be consistent with economy to employ. To obviate this difficulty, a patent was obtain ed by Birkenshaw, of Bedlington, Eng., for an improved form, which consists in giving the bar the form of a triangular prism. The chief advantage of wrought iron rails is the reduction of the number of joints and the difficulty of making mst-iron rails perfectly even at the joints. Edge rails are best adapted for permanent works, and they are of such a nature that ordinary carriages cannot be em ployed upon them ; but on any railway where such carriages can be used they must do more injury to the surfaces of the rails than will be equivalent to the advan tage of suffering them to travel on them.
Tram-ways differ from the preceding in having the guiding flanch upon the rails, instead of being fixed upon the wheels of the carriages. It affords the advantage of employing such carriages as can be used where there are no rails. The tram-rail is exceedingly convenient for temporary uses, and, in its ordinary form, it is much used in quarries, in mines, in forming new roads, in digging canals, and in conveying large stones for buildings, and other purposes. Tram rails are very weak, considering the quantity of iron in them, and in some works it has been found necessary to strengthen them, by adding a rib on the under side.