1574 THEORY AND PRACTICE OF ENGINEERING, Book II.

weighing 4692 pounds. These materials, including the labour of fixing them, and theballast over an average breadth of 26 feet to a depth of 18 inches, cost 52117. 4s. per mile.

An iron rail of this form, resting on a continued sleeper, is considerably stronger thanwhen placed on chairs; a 48-pound rail, so applied, is as strong as one of 75 pounds whenresting on points of support 3 feet distant. The sleepers were all of Memel, and thescrew-bolts, which secure the rails, were 18 inches apart.

Helton Rail has a half lap joint; the bottom of the chair is let into the stone and se-cured by two wooden pins; a convex boss is cast in the bottom of the chair, upon which the

Fig. 2951.

Fig. 2952.

ends of the rails are halved, and a pin, passing through a hole cast in them, secures them to thechairs.

The Southampton Railway bars are of a parallel form, and weigh 75 pounds per linealyard: they were placed in two varieties of chairs, the weights of which were 26 and 22pounds. The rails are in lengths of 15 feet, their depth is 5 inches, the width of the topsand bottoms 2j{ inches, and the thickness of the middle Jjj of an inch.

London and Greenwich Rails are laid to a gauge of 4 feet 8^ inches, and are of the edgeform, 2J inches wide at top, 4\ inches in depth, and 1 inch in thickness at ’bottom : they aresupported in cast-iron chairs placed at distances of 3 feet. The chairs are fixed in roughblocks of granite or Bramley Fall stone, each containing about 4 cube feet.

Railway Curves . — When circumstances oblige a departure from the straight line, thecurvature to be given to the rails is a point of the highest importance ; if it be struck froma radius of ^ mile only, the inclination of the rail rises but 16 feet in the mile, and the speedof the locomotive engine is reduced nearly on -half; but when the rails are perfectly level,the curvature does not materially impede the progress of the carriages. The flanges of thewheels on the outside rails of the curve, acting* against the rails, prevent the carriagesbeing thrown off; but to make them travel along the curve without friction, the wheelshave the outside of their rims made conical, or their diameter is increased on the side nextthe flange. When the carriages move around a curve the wheels eing connected togetherform conical rollers, moving upon the rails with different radii, the smallest being that ofthe inner curve, and the increase of that diameter of the wheel which runs on the outercurve makes up to a certain degree for its increased length. The diameter of the wheelsnext the flange is generally 1 inch more than on the outside of the tire: the breadth is 3£inches; when moving in a straight line, they are usually placed upon the axle, so that theywork at about 1 inch from the rail, but some latitude is allowed for their changing into acurved direction ; and it is found that for 3 feet wheels, this kind of tire is quite efficient inpreventing their rubbing against the inside of the rails at all ordinary curves.

When a carriage moves^ound a curve, its tendency will be to move in the direction of atangent to that curve, which is altered by the force or velocity, as well as by the radius ofcurvature. By raising the outside rail, an inclination may be given to the axles, whichwill produce upon the carriages a gravitating force towards the centre of the curve, equalto the outward centrifugal force, and there will consequently be no extra pressure againstthe rail, nor any chance of the carriages being thrown off.

Turn- Tables only admit of one or two carriages being turned at the same time, andconsequently are not generally useful; there is some trouble attending their manage-ment, and they require great attention. The length of the locomotives employed on theline determines their diameter: those on the London and Birmingham Railway are 12feet, those on the Leeds and Selby 8 feet; they are made of cast-iron, and turn on a pivotin the centre; on the outer rim are rollers, which facilitate the rotary motion : that onthe London and Birmingham line has 8 cast-iron rollers, 10 inches in diameter, workingupon iron arms, which radiate from a wrought-iron hoop that works round the centre pivot.A circular ring of cast-iron, 12 feet 7 inches in diameter, surrounds the turning-table,and is in depth 1 foot 9 inches; at the bottom is a circular iron plate, 12 inches wide,