Illustration: FIG. 4. (‡ RAIL “CHAIR.”) With respect to the “level” at which the line should be laid, a section of the route, showing all the elevations and depressions, is first made, then such a course is chosen that the material produced by cutting through the higher parts shall be just sufficient to form the embankments for filling up the lower parts; fig. 1 will give some idea of this arrangement. Of course a line perfectly level would be the best, just as would be one perfectly straight; but as the difficulties of the one must be balanced, so must those of the other, and a line as nearly level should be obtained as is consistent with expense. For instance, suppose A and B, fig. 2, to be towns to be connected by a line of railway, and the chief of the intermediate ground to be above their level; of course it would be very expensive to cut through the whole distance, as shown at the dotted line _a_, this level would therefore be too low; but if a higher level were taken, as at the dotted line _b_, then only the centre of the distance would have to be cut through, and the material (earth, &c.) produced by the cuttings would suffice to fill up the hollows at the ends. These considerations and many others must therefore determine the level at which the railway shall be constructed; but the line is seldom (if ever) on one level from end to end, nor at one continuous “gradient” or slope, for the course of the line is so arranged as to make as little cutting and filling up as is consistent with a road whose gradients shall never exceed a specified amount, which must be determined by local circumstances. The excavation and filling up being finished, the “trams” have to be laid; these are bars of wrought iron about fifteen feet long, of the form shown at A and B, fig. 3. The most usual form is that marked A. They are made of wrought-iron, passed while hot between rollers cut at their edges into the form required. These trams are laid upon bars of wood called “sleepers,” at about four feet apart, and united to them by what are called “chairs,” which are pieces of cast-iron of the form shown at fig. 4, fastened to the sleepers by iron spikes, and into these the trams, or “metals,” as they are called by the workmen are wedged. These bars of iron are laid very evenly and perfectly parallel at a certain distance apart, which must exactly correspond to the distance between each wheel of a pair belonging to the carriages and engines; this distance is called the “gauge,” the wide gauge (as on the Great Western) is seven feet, and that called the “narrow gauge,” is four feet eight-and-a-half inches, and the space between the lines is of sufficient width to prevent any danger of collision in the trains on passing each other; they are generally six feet apart. Illustration: FIG. 5. (‡ RAILWAY SWITCH.) Illustration: FIG. 6. (‡ TURN-TABLES.) Illustration: FIG. 7. (‡ LOCOMOTIVE ON TURN-TABLE.) Illustration: FIG. 8. (‡ WHEEL WITH FLANGE.)