Chap. XXX.

RAILROADS.

1567

quantity of yards as when wet; other drains were then cut on each side the embankments,2 feet deep, and when the water had drained out of the moss under the intended roadway,double layers of hurdles, covered with heath, were laid upon it, and on these a stratum ofballast. The drains were entirely effectual; 670,000 cubic yards of moss in its natural£tate when dry were required to make 277,000 cubic yards of embankments, so greatis its capacity for water.

When the fall is 10 feet in a mile, the gradient is 1 in 528; when 20 feet, 1 in 264 feet; andwhen 30 feet per mile, a third, or 1 in 176; when the gradients are established, allowance canbe made in the calculations of quantities, and the average depths and heights determined on.

The plans and sections of the intended line being prepared, the quantities of earthworkmay be accurately ascertained, and contracts made for the cuttings and embankments, andthe centre of the line being marked out, the operations of cutting may immediatelycommence. The first contractor usually provides engines, rails, waggons, and whateveris required for the transport of the earth, and sublets the work to gangs of navigators.Horse-power has been abandoned, the locomotive or stationary engine, drawing the waggonsat the rate of 8 or 10 miles an hour when loaded, being much ipore convenient. Muchjudgment is required in establishing the situations for the waggons to tip or shoot theirload: where the depth of cutting is considerable, inclined lines may be formed, alongwhich very little labour is required to move the carriages. One man will throw into awaggon in the course of a day 15 cube yards of stiff clay, or 25 of sand, and, according tothe nature of the ground, from 60 to 90 men can throw up and load 1000 cubic yards inthat time.

Ashley Cutting , on the Great Western Railway, about 5 miles on the London side ofBath, at the base of Kingsdown Hill, is 23 feet above the level of Box Brook, which runs inone of the two valleys separating the parallel ridge of the upper oolite that caps the hills inthis district, and upwards of 800 feet above the level of the sea. The top of KingsdownHill, behind the cutting, is above Box Hill, where the tunnel passes, and the land slopesdown to the railway at the rate of 1 in 11, exposing an inclined plane of at least 4000 feet inlength to the action of the atmosphere. The cutting was made through a drift from thehigh lands, and not through the original stratification: and after its completion, the upperportion of this deposit slipped, producing considerable inconvenience : this arose from itstendency to absorb water, from which it had no means of clearing itself the lower portionof the bed, on which it rested and abutted, being removed. To remedy this the whole wascut to a slope of 4i to 1, and shafts were sunk until they pierced the water-bearing strata;these were connected with several headings, driven on the surface of the marl, parallel to therailway, which most effectually decreased the quantity of water; but 1500 yards of shaftsand headings were driven at a cost of 30s, per yard run before this was entirely accom-plished, or the drift could be made to stand at a slope of 2 to 1.

j Equalising the Excavations and Embankments , so that the parts excavated will furnish earthsufficient to fill up the valleys or low ground to be crossed: to effect this we must resortto a system of successive approximations, by assuming different slopes, and shifting the lineto one side or the other till we arrive at the proper result. The solid contents of bothexcavations and embankments must be calculated, bearing in mind that the earth in itsnatural state occupies less than when broken up, and that an artificial embankment settlesconsiderably : this allowance must depend upon the nature of the soils. The axis of theroad is then set out by piquets, and its width with stakes, on which is marked either theheight to be filled in, or the cutting to be made.

The removal of the earth by the most economical method is the next consideration, andthen the inclination to be given to the side slopes : when the earth is a mixture of clay andsand, 2 of base to 1 of perpendicular, and where the latter material predominates, a greaterslope is necessary: in solid rock the sides may be left nearly or quite perpendicular ; butit must also be remembered that slaty rocks disintegrate very rapidly when exposed to theaction of the frost, and it is better to cut them into steps and cover their surfaces withvegetable mould, on which grass seeds should be sown.

The stratified rocks, which dip towards the horizon, being liable to slip when cutthrough, require great attention; where clay and sand alternate, they must be effectuallydrained all the surface water collected, and the springs working in them conducted away,for which purpose deep trenches must be cut, and filled in with gravel or loose stones with

an easy fall. .

Drainage of the subsoil is also necessary, and this is often performed by making alongitudinal drain under the roadway, or by side-cuttings in the manner of catch-waterdrains on the slopes, running them obliquely along the surface, and then allowing themto empty into the cross drains or natural watercourses.

In marshy soils it is often requisite to make an entirely new artificial bed to receive theroadway, and this is best done by taking out the spongy elastic earth, and substituting aharder material.

Gradients , or the proportionate ascent or descent of the several planes constructed on the