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iron frame-work, as shewn in the drawing, must be of larger dimensions, to prevent its being broken by the wheels of the carriages.

§ 17.-Mode of passing yielding Ground, or Mosses.

We have thus described the different modes of executing, and laying down railways upon ordinary ground. It would be going beyond the proper sphere of a work of this kind, to attempt to offer plans to meet difficulties in extraordinary cases, the intention being, rather to describe what has been done, than to devise plans for executing unforeseen difficulties. The modes of passing yielding ground, mosses, or bogs, adopted by some engineers, may, however, be described in this place; and the Chat Moss, on the Liverpool and Manchester railway, being the most extensive moss yet passed over, we shall give the plan adopted by Mr. Stephenson, for carrying the railway over this moss.

This moss is of considerable extent, comprehending an area of about twelve square miles, being of so soft and spongy a nature, that cattle cannot walk upon it, and an iron rod sinks with its own weight. The depth varies from ten to thirty-five feet, resting on clay and sand. The distance which the railway was to be carried over it, was upwards of four miles and a half, an undertaking which required some degree of nerve to contemplate. It is necessary to premise, that, in carrying the railway across the moss, the level required, that it should, in some places, be twelve feet above, in others, nine feet below, and to vary from these, to level with the original surface of the moss. We have, therefore, three distinct operations, viz., embanking the railway above; forming a cut below; and forming the road

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Embanking the railway above the moss. another moss, of considerably less extent than this, over which the railway passed, and which, at one end, was terminated by an extensive cutting of clay and gravel. As an embankment, of four feet in height, had to be formed over this moss, the materials from the excavation were used for this purpose. The moss was about twenty feet deep, and it was soon found, that as the materials were successively laid upon the moss, the whole mass gradually sunk; and when the embankment was finished, although the actual level of the railway was only four or five feet above the original surface of the moss, the quantity of the metal deposited would have formed, on ordinary ground, an embankment twenty-four or twenty-five feet high; with such materials, therefore, (clay and gravel,) it would have been impossible to form an embankment over Chat Moss. The quantity required, and the consequent expense, would have been enormous. Mr. Stephenson had recourse, therefore, to the moss itself, for materials to form the embankment, which, by its inferior specific gravity, would not sink to such an extent as gravel and clay. In its natural state, the moss was unfit for this purpose, but drains were cut, five yards apart, which laid the moss between the drains dry, and rendered it excellent material for the purpose. With this material, embankments were formed upon part of the moss, and it was found to require only about four times the quantity of material, that would have sufficed for sound ground, and the road appears in quite as good order, as in any other part of the line.

Forming a cut below the level of the surface of the moss, was accomplished entirely by draining; the drain was cut along the line of the railway, eighteen inches to two feet deep, which laid dry that portion of the

moss between them. About twelve inches in thickness, thus dried, was excavated at a time; and it was, in that manner, successively drained and excavated, until the proper depth was obtained. The permanent road was then formed in the manner hereafter described.

Laying down the road upon the surface of the moss. Drains were first of all cut on each side of the line, and lateral ones, where necessary, to carry off the water. By this means, a certain depth of moss on the top was partially consolidated, and formed a layer or surface of dry moss, of considerable tenacity; upon this, hurdles, nine feet long, and four feet broad, wickered with heath, were laid down transversely. In many places, only one layer of hurdles was required; but when the moss was very soft, two layers were used. Upon this, was laid about two feet of ballast, or gravel, to form the permanent road; and wooden sleepers, stretching across each line of road, were used to lay the rails upon. The stability of the road, therefore, depends solely upon the tenacity of the materials, supported by the buoyancy of the moss. When we consider, however, the area of base thus firmly united and bound together, and the support which even so spongy a substance as the moss must give to so extensive a platform; it is natural to suppose, that the impression made upon so great an area, by the pressure of so inconsiderable a proportion of the whole weight as that of a train of carriages, must be slight indeed; and we find, that, since the opening of the railway, the passage of the traffic over the moss proves that the road is exceedingly stable. It may be necessary to remark, that the surface of the moss is higher than that of the country bordering its edge.

Figs. 13 and 14, Plate V., shew this mode of forming a railway over a moss; a a are the hurdles, or wickered foundation, which may either be single, or two or three

layers, according to the tenacity of the moss; bb is the coating of sand or gravel, resting upon the wicker work, small branches of trees, brushwood, or furze, being laid down upon the hurdles; upon this gravel, either longitudinal sleepers, cc, are laid down, with transverse sleepers, ddd, laid upon them, or transverse sleepers, without the longitudinal ones, according as the moss may require.

The longitudinal sills, ee, are then laid along the transverse sleepers, upon which the chairs and rails are laid down in the usual way. A moss has been passed, in the manner shewn in the above figures upon the Garnkirk railway, near Glasgow, by Messrs. Grainger and Miller.

The drainage is effected in the same manner as shewn in Fig. 4, side drains, cd, cd, being cut, to consolidate the surface of the moss, and carry off the water from the surrounding country; and centre and cross drains, a b, ef, to effect the drainage of the road. x, Fig. 14, shews one half of the centre drain.

§ 18.-Construction of Passing Places, and Turn

Tables.

Having thus given a description of the various modes of cutting, embanking, and forming the superstructure of railways, and of laying down the rails; we shall now proceed to give an outline, and shew the various modes of laying down single and double lines of rails, with the different kinds of crossings, and rails required for carriages passing each other, or from one line to another.

In almost all the private lines of railway in the neighbourhood of Newcastle-upon-Tyne, and other districts of Great Britain, one main line of road is laid the whole distance; with short lengths of double road at certain

intervals, and proper passings between them, for the carriages going in one direction, to pass the others returning in the opposite direction.

In public lines, and for general traffic, especially where slow and quick travelling trains are to work, it is almost necessary to have double lines of road laid the whole distance. Quadruple lines, being so extremely expensive, will only be resorted to in extraordinary cases, or near the termination of extensive lines of railway.

The same description of passing places, will suffice for a quadruple, as for a double line of railway; we shall, therefore, divide the subject into two kinds :-double lines of railway, with the kinds of crossings required for them; and single lines, with their crossings.

Fig. 25, Plate III., represents a double railway, with crossings, from one line to the other, for the carriages to pass each other, or for a quick train to pass one moving at a slower rate. A A' is one line, along which the carriages travel in the direction shewn by the darts, or from A to A'; BB' being the other line, whereon the carriages travel in the opposite direction, or from B' to B, those two lines are supposed to extend the whole length of the railway. When the goods to be conveyed, are to travel at the same, or nearly at the same, rates of speed, perhaps few, if any, passings will be required from one road to the other; but, when it is intended for the conveyance of passengers also, or for the transit of light goods, at a swifter pace than that which it is deemed advisable to convey heavy goods; then it will be necessary to have certain passings, so that the carriages moving slow, can cross to the other road, and allow those moving faster to pass them, when the former can again pass upon the same road again,

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