182

RAILWAY.

The values in the preceding Table depend almost entirely upon quantities whichare continually varying j hence the mode adopted has been to assume such as aremost probable for the present day; and since in comparing the values of differentgradients the differences alone will be looked to, the errors will be of less im-portance.

The following are the assumed dimensions of the engine, from which the Tablehas been calculated: diameter of cylinders 16 inches—length of stroke 21 inches—steam cut off at fths of the stroke—diameter of driving-wheel 6 feet 6 inches—weight of engine and tender 32 tons—adhesion 3700 tbs.—evaporating power 200cubic feet per hour—elasticity of steam in the boiler 95 fts. per square inch—pres-sure 80 lbs. per square inch—load 50 tons, exclusive of engine and tender.

All gradients above 1 in 330 are considered first-class gradients. From 1 in 330to 1 in 150 are fair working gradients.

The gradients on the London and Birmingham and Trent Valley lines are notsteeper than 1 in 330; on the Dalkeith Branch of the North British Railway thereis a gradient 8 miles long at 1 in 70; on the South Devon line (broad-gauge) thereis a gradient of 1 in 43, 2) miles long; and the Lickey incline on the Birminghamand Gloucester Railway is 2 miles long, at 1 in 37J.

Laying out a Railway. — It is usual in laying out a railway to mark a centre line bystumps, for the levels, placed at intervals of 1 chain, and a spitlock for the direction,on each side of which the width required for the roadway and slopes of the cuttingsand embankments is laid off: posts are also fixed in secure positions at every 10 chainsalong the line, as well as at the crossings of roads and the ends of viaducts andtunnels, as references for the levels. In laying out curves on railways, numerousmethods will readily suggest themselves, according to the circumstances in each case;as, for instance, by measuring offsets from the tangents and chords of the arcs, or bythe intersections of angles from two theodolites, one placed at each extremity of thecurve, &c., &c.

Quantity of Land necessary. —The quantity of land required for a double lineof railway will vary with the nature of the country: the actual width of the railway,including fences, is about a chain, or 8 acres per mile, and when the space for theslopes of embankments and cuttings has been added, it may be considered that theaverage quantity per mile is not less than 10 acres in very level country, 12 acres inordinary country, and from 15 to 20 acres in very unequal country; and 1 acre permile additional should be allowed for stations.

Earthworks.- —-The amount of earthwork, of course, depends upon the nature of thecountry; but an average taken from twelve railways gives 104,900 cubic yards ofexcavation per mile, which may not be considered a very exorbitant amount; theexcavation on the London and Brighton Railway being 156,000, and that on theBirmingham and Gloucester 79,000, cubic feet per mile.

To avoid a useless expenditure of labour, the amount of cutting should be propor-tioned to that of embankment; but when the quantity required in the embankmentcannot be equalized with that obtained from the cutting, a convenient spot is selectedfor depositing the surplus, or excavating for the supply of the deficiency. When theearth from the excavation is in excess of that required for the embankment, it wouldperhaps in many cases be found advantageous to use the surplus in wideningadjacent embankments. It is generally considered cheaper to throw to spoil than tolead three miles, though this would of course depend partly upon the height of theembankment for which side-cuttings would have to be resorted to, and partlyon the facility of obtaining a site for depositing the surplus. In estimating the relativequantities, regard should be had to the nature of the soil.