The weights of the heavy chairs on the London and Birmingham railway are, joint chairs 31 lbs., and intermediate ones 26 lbs.

Pieces of felt, when stone blocks are made use of, are now very commonly interposed between the iron chair and the block. When the chair is firmly pressed down by the pins, the felt forms a bed for it, giving it a firmer seat, and likewise diminishing the jolting and concussions of the carriage wheels, and the rigidity of motion upon the hard stone block. It is found, however, that in a few years the good effect is nearly lost, from the constant action of heavy loads on the rails, and that the chair bears hard upon the block.

The chairs upon most of the principal lines where stone blocks are used, such as the London and Birmingham, Edinburgh and Glasgow, &c., have felt placed under them.

WEDGING OR KEYING THE RAILS TO THE CHAIRS.

When the iron rails have been laid into the open sockets of the chairs, the next operation to be performed is called keying the rails, which is done by means of wedges called keys, one of which is driven hard into a small vacant space left to receive it, between each chair and the outer side of the

rail, which is intended to firmly bind and secure the rail in its seat. Various sorts of keys have been prepared: they are most commonly made of oak or elm, both compressed and uncompressed, and also prepared by the patent process. Both solid and latterly hollow iron keys have been tried.

Fig. 30. is a sectional end-view of Mr. Barlow's patent hollow iron key, applied to wedge a double parallel rail. This form

Fig. 30.

of

key has been very favourably thought of.

As on the proper keying of the rail much of the stability depends, too much care and attention cannot be bestowed upon the subject: hence the contrivances are numerous. The chief object of the key is to keep the rail firmly down in the chair, and at the same time not to prevent the longitudinal expansion of the rail. The kind of key understood to answer best, depends much on practical experience. The general use of wooden keys for wedging rails, arises from the greater facility they afford for their being tightened and replaced; but to attain this object, the wedges should be of ample size. Nothing demonstrates more the imperfections attending the construction of rail

ways, where chairs and keys are used, than the constant watching the line of way requires, for tightening the keys. Figs. 25, 26, 27, 28. illustrate the manner of

fixing the rails with wood

keys.

RAILWAYS ON CONTINUOUS BEARINGS.

Having now traced the progress of railways constructed on the plan first introduced by the late Mr. Jessop, in 1789, to the present time, it may be observed, that the laying the rails on alternate props or supports over the entire length of the road, whether they were made of wood or stone, has been the plan in general use, and seemingly has been considered by engineers to be the best. It is at first view extremely difficult to account for this partiality; for after the introduction of steam power on railways, difficulties presented themselves in the construction of this form of railway, which had not been foreseen. Instead, however, of attempting the introduction of a new principle of laying the rails, large sums have been expended by the older companies in altering the form of rails first laid down. It is indeed amusing to observe how much has been written on vertical and lateral deflection, on the strength

and rigidity of iron, and on railway chairs and keys; when it seemingly required but little ingenuity to devise a scheme, where the obvious deflections of a bar suspended on two fulcrums, as a bridge, could be got the better of, by placing the bar on a solid foundation; while greater safety and stability would be attained, and complicated contrivances to fix the bar become unnecessary.

That any diminution of vertical deflection, by placing the bars on a solid base, is important, must be apparent to all, when it is considered that a heavy load depressed, must be like ascending an inclined plane on the rail, the height of which is equal to the central deflection. It was assumed, however, when the Great Western Railway was under consideration by the parliamentary committee, that as much power was gained in the descent as was lost in the ascent, the odds being made even; and thus the deflection would be no impediment; but Mr. Barlow has shown that this assumption is erroneous, both in theory and practice, and that in fact the gain from the descent is so exceedingly small, in such short planes, that it may be wholly rejected; so that in a plane supposed perfectly horizontal, the retardation or additional resistance to the carriages, caused by the deflection of the bar, will be equivalent to

the carriage being carried up a plane of half the whole length on a slope.

Solidity of the base of rails, upon which heavy carriages are to run, being, as a preliminary point, so obviously and essentially requisite, the only wonder is how it can be discussed at all. Yet we find engineers stating, that between rails firmly fixed to solid longitudinal bearers, and rails suspended in the cominon way between two points, not much difference of deflection exists. The fallacy of the statement is, however, so apparent, that it is hardly deserving of notice, were it not that such assertions have the dangerous effect of misleading the judgment of impartial inquirers. It may be therefore laid down as the rule of common sense, that the more firmly a rail can be laid continuously on a solid bed, the less vertical deflection or bending it will have; and unless iron bars, when suspended, can be made equal in strength, and to bear the same weight, ceteris paribus, the first plan must have most stability, not only in bearing the vertical strain, but also the lateral one; for surely it requires no argument to show that an iron bar laid upon a series of points, or fulcra, must be much easier bent vertically and laterally by the heavy blows or jolts of a carriage, than when the same bar is made to form a part of the solid