When it has been decided to construct a railway between two places, the laying-out of the line is a subject requiring great consideration and the highest engineering skill—for the matter is, on account of the great cost, much more important than the setting-out of a common road. The idea of a perfect railroad is that of a straight and level line from one terminus to another; but there are many circumstances which prevent such an idea from being ever carried into practice. First, it is desirable that the line should pass through important towns situated near the route; and then the cost of making the roadway straight and level, in spite of natural obstacles, would be often so great, that to avoid it detours and inclines must be submitted to—the inconvenience and the increased length of road being balanced by the saving in the cost of construction. It is the business of the engineer who lays out the line to take all these circumstances into consideration, after he has made a careful survey of the country through which the line is to pass. The cost of making railways varies, of course, very much according to the number and extent of the tunnels, cuttings, embankments, or other works required. The average cost of each mile of railway in Great Britain may be stated as about £35,000. The road itself when the rails are laid down is called _the permanent way_, perhaps originally in distinction to the temporary tramways laid down by the contractors during the progress of the works. The permanent way is formed first of _ballast_, which is a layer of gravel, stone, or other carefully chosen material, about 2 ft. deep, spread over the roadway. Above the ballast and partly embedded in it are placed the _sleepers_, which is the name given to the pieces of timber on which the rails rest. These timbers are usually placed transversely—that is, across the direction of the rails, in the manner shown in Fig. 42. This figure also represents the form of rails most commonly adopted, and exhibits the mode in which they are fastened down to the sleepers by means of the iron _chairs_, _b_ _c_, the rail being firmly held in its place by an oak wedge, _d_. These wedges are driven in while the rails are maintained at precisely the required distance apart by the implement, _e f_, called a _cramp gauge_, the chairs having previously been securely attached to the sleepers by bolts or nails. The double ⟙ form of rail has several important advantages, such as its capability of being reversed when the upper surface is worn out, and the readiness with which the ends of the rails can be joined by means of _fish-plates_. These are shown in Fig. 43, where in a we are supposed to be looking down on the rails, and in B to be looking at them sideways. In Fig. 44 we have the rail and fish-plates in section. The holes in the rails through which the bolts pass are not round but oval, so that a certain amount of play is permitted to the ends of the rails. [Illustration: