On exceptionally sharp curves, wrought-iron or steel tie-bars, as in Fig. 335, are sometimes used to maintain the line to gauge. They may be made out of bars 3 inches wide by ½ an inch thick, turned over at 225 the ends to grip the outside flanges. Being made to exact template, they have to be threaded on to the rails before spiking down, and are placed between the sleepers at distances from 7 to 10 feet apart. Laying Permanent Way.--To preserve a good line and level to the permanent way, it is absolutely necessary that the road-bed should be kept thoroughly drained. If provision be not made for quickly carrying away the rain-water, and if it be allowed to collect under and around the sleepers, the action of the passing trains will work the finer particles of the packing into the consistency of soft mud, which will be gradually squeezed away, leaving the sleepers imperfectly supported and insecure. A loose sleeper involves a depression in the rails, and a corresponding lurch in the vehicles of the train, and a series of these depressions may produce such an oscillation in the train as to cause it to leave the rails. The height or space from formation-level to rail-level is generally about 1 foot 9 inches for a flange railroad, and about 2 feet for a chair railroad. Figs. 336 and 337 show cross-sections of both descriptions of road as laid down for a double line in cutting. The same arrangement applies to similar roads laid down in embankment, merely omitting the side-drains or water-tables. The bottom layer of ballast or road-bed should consist of good hard, quarried, or broken stones, each 6 inches deep, set on edge, firmly and closely hand-packed, forming a foundation through which the rain-water can be quickly carried away. On the top of this bottom pitching should be placed a 6-inch layer of broken stone ballast or strong clean gravel, of which none of the stones should be larger than will pass through a 2-inch ring. When the sleepers and rails have been laid on this second layer, and properly packed to line and level, the top ballasting, or boxing, of either broken stones or strong clean gravel, should be filled in to the form and extent specified. Where broken stones are used for the top ballasting none of them should be larger than will pass through a 1½-inch ring. Broken stone ballast should only be made from the hardest and soundest description of rock or boulders, so that, however small the particles, they will remain sharp and clean. [Illustration: Fig. 336, 337, 339, 338, 340, 341, 342, 343, 344]