From the opening of that railway, the 3UCCOSS of the great system was at once de rided. Nothing could check the spread of so cast an improvement ; and we have since seen such a twenty years of commercial energy as he world never saw before.
In the laying down and construction of a the engineer so adjusts his inclinations .
or gradients as to make the nearest practicable approach to a level, avoiding if possible any loss of power from undulations of surface, by mating all the inclinations on one side of the summit, or highest point to be passed over, rise towards it, and all on the opposite side descend from it. Curves on a main line of railway being objectionable, the engineer so adjusts his line as to avoid them when possible, and to make those which are inevitable of as large a radius as circumstances will admit. The works of a railway consist chiefly of tunneling, excavation, embankrnent,arching,viaducts, bridges ; concern ing each of which a few words may be said.
Tunnels are, in general, the most formidable works, and the time and expense of forming them can be least accurately calculated, because unforeseen circumstances often arise to retard their progress. Being objectionable also on other accounts, tunnels are avoided as much as possible in the more recently-designed railways. .Bicavations of great depth and extent are of frequent occurrence where the railway passes through high ground, but not at such a depth from the surface as to require a tunnel. The depth of cuttings is frequently from fifty to a hundred feet; and the slope of the sides depends on the nature of the rock or soil. Embankments are the artificial ridges' of earth formed to support the railway on a higher level than the natural surface of the ground. Their dimensions are often fully commensurate with those of cuttings, from which their materials are mostly procured. In the ordinary mode of proceeding, an embank ment is formed simultaneously with a cutting, the earth waggons proceeding filled from the excavation along a temporary railway to the embankment, where they are tipped up to dis charge their contents. Arching of almost every kind is more or less required in viaducts, bridges, culverts, and drains ; and simpler work in the retaining walls, station buildings, and other necessary erections. • Viaducts of
great magnitude are often executed for the purpose of crossing valleys at an elevation greater than could be conveniently obtained by embankment, and also for entering or pas sing through towns. They are usually of stone or brick, but sometimes of wood or iron. Bridges are required occasionally for crossing rivers, and very frequently at the intersection of roads, and as communications between severed property. They are formed of stone, brick, wood, iron, or combinations of two or more of these.
In order to obtain a firm dry foundation for the blocks or sleepers on which the rails rest, a layer or stratum of broken stone, technically called ballast, is spread over the road to the thickness of a foot or more, varying according to the construction adopted and ether circum stances. The rails used to he spiked down to stone blocks; but it is found better either to rest them on sleepers placed at intervals, or on continuous timber bearings, as on the Great Western line. Mr. Barlow has recently invented cast iron bearings for the rails, which are found to he a valuable improvement. Ile has also introduced a very broad wrought iron rail which requires no sleepers, the broad bottom of the rail being laid upon the ballast itself. The rails now used vary from 70 to 90 lbs. per yard. In the old colliery tramways, four feet was not an uncommon width for the gauge of the rails, but many lines were less. Some of the colliery railways of Northumber land are four feet eight inches and a half; and from these the .Stockton and Darlington, Liverpool and Manchester, and other lines, took their gauge. The advantage of uni formity has led most companies to follow this example. The ordinary width being consi dered by him inconveniently limited, Mr. Brunel fixed upon seven feet as the gauge of the Great Western and its tributary lines ; and hence has arisen the late ' battle of the gauges,' which may be truly said to have cost the respective companies millions of money, in law and legislation.