and 18 inches square, and placed at 3 feet apart, centre to centre, each block supporting both rails, the gauge being 4 feet 8 inches. This plan was found, however, to produce too rigid a road. In some instances, to attain a greater connection between the blocks when placed separately, iron tie or connecting rods have been used at

curves.

SEATING THE CHAIRS UPON THE BLOCKS.

When the blocks and sleepers have been placed along the line of a road, the next thing is to set the chairs, or pedestals, which are to support and fix the rails. These are usually made of cast iron, for convenience, on account of the irregularity of shape. Malleable iron is, however, much stronger ; but though a patent has been obtained for making them of it, they have not been yet introduced. As great a variety of forms has been proposed for chairs, as for rails, because the former must be adapted to the latter. The chair generally stands vertically, having an open socket or groove to receive the rail, the base of which being, when set in the chair, about an inch clear of the block or sleeper. The distances the chairs are placed apart, are of course regulated by those of the blocks: they are fixed to the blocks in the following manner :

Holes are drilled in the stones, from 1 to 2 inches in diameter, into which plugs of oak, or tree-nails, are driven; and the chair being placed on its seat, and accurately levelled, iron pins or spikes, with heads, are driven through the holes in the base of the chair, into the wooden plugs, which fixes the chair fast to the block. When chairs are to be fastened to wooden sleepers, the ends of the latter are cut to the proper level, and the chairs are nailed down with strong spikes. There are usually two perforations in the intermediate chairs, as on the North British railway; but on some railways the chairs have three, as on the London and Brighton; two on one side, and one on the other. The holes are sometimes placed diagonally, and generally so in the joint-chairs.

The weight of the chair is regulated by the size and strength of the rail, and both are made much heavier than formerly. The 76 lbs. rail on the London and Brighton railway has a chair at the joinings 10 inches wide by 5 inches high, and the intermediate chairs are 10 by 4 inches. The chairs on the North British railway are 10 inches wide, 5 inches high, 5 inches broad, and the intermediate ones 10 inches wide, 5 inches high, and 4 inches broad.

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, ap

Fig. 30.

plied to wedge a double

parallel rail. This form 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