SUPPORTS FOR THE RAILS.
The rails are supported upon transverse sleepers, or cross-ties, of tim ber. On some roads the sleepers are of iron or steel, inore rarely of stone. Track systems have been tried recently, in which such methods of support are entirely dispensed with.
Cross-tics are usually 9 feet long, TO inches wide, and 6 inches thick, though their dimensions may vary slightly. They may have a variety of forms, as seen in the illustration (fi/. 24, fig. i7). In the United States the invariable rule is to use hewn timber, sawed timber being employed only when it is impossible to get a sufficient supply of the hewn material. In the eaily history of railroads granite sills were utilized for supporting the rails, but were found to be very objectionable, on account of their want of elasticity, and were soon abandoned in favor of wooden sleepers.
The of the is subject to some variation. On German roads the rule appears to be to space them 3 feet from centre to centre. Where the practice is followed of supporting the rail-joints (/5/. 24, fig. I 7), the supporting cross-tie is wider than the others. Where, as is usually the case, the joints fall between the ties (suspended joints), the adjacent ties are spaced closer—say about 2 feet. The average English prac tice is to provide each rail-length (3o feet) with eleven cross-ties, which would correspond to a uniform spacing- of 23% feet between centres. It is customarv, however, to space them more widely in the middle portion of the rails (as much as 3 feet apart), and more closely toward the rail-ends, to afford greater support at the joints. On American roads the spacing- of ties is closer, the average being sixteen ties to a 3o-foot rail, making the mean distance between the ties about 2 feet from centre to centre. At suspended joints the common rule is to allow ro inches in the clear between the two joint-ties.
Timber Used for England and North Germany the wood most commonly used for ties is pine or fir, the former lasting from SeVell to eight years, and the latter from four to five. In South Germany and France oak is cominonly used, having a life of from fourteen to sixteen years. In the United States a variety of timber is used for this purpose— namely, chestnut, the life of which is from five to twelve years; cedar, from six to fifteen; hemlock, from three to eight; while oak lasts from five to twelve years, and spruce pine from four to seven years.
Anlisefitic Treatment of the rapid decay of the ties involves their frequent renewal, and correspondingly heavy expense, the impregnation of the ties with antiseptic materials, to preserve them against rotting-, is very g-eneraIly practised in European countries. In the
United States, where timber is still relatively cheap, the impregnation of ties has not vet come into general use, although the time is near at hand when the increase in cost of timber will force the adoption of this expe dient. As the use of suitable impreg-nating substances will double the life of ties of fir and pine and triple that of ties of beech wood, nearly all the principal European railroad companies, in view of these facts, have erected a suitable plant for treating their ties bv one or another of the numerous processes that have been proposed for the purpose. The following- methods are those which are in most general use, and which have exhibited the best results in actual practice.
Prescrz,ative Processes: the method devised by Ryan corrosive sublimate is the preservative agent. The thoroug-hly-seasoned timber, hewn to shape, is immersed for about ten days in a dilute solution of this salt. The liquid is then pumped out, and the sediment that has deposited itself upon the ties is washed off. The impregnated ties are then allowed to remain for two or three weeks, to dry thoroughly before being laid down.
The Process of Boircherie, chiefly used in France, employs a solution of the sulphate of copper, which is forced by hydrostatic pressure into the newly-felled tree, causing the sap to be displaced. The impregnating mate rial is contained in wooden tanks placed upon towers from 25 to 4o feet high, whence it is conducted through pipes to the felled trunks. By suit ably capping the end of the trunk to be impregnated and connecting- the pipe conveying the solution to the cap, the pressure of the liquid column is sufficient to carry the preservative fluid throng-1i the log, the sap being forced out at the other end. When the blue copper solution makes its appearance at the opposite end of the log, the operation is finished, the period varying, according to the character of the wood, from forty-eight to one hundred hours. The process of Boucherie is open to the objection that the impregnating material (sulphate of copper) is decomposed by contact with iron, so that the spikes driven into the ties to hold the rails are rapidly corroded.