This system is used when the manufactory is situated below the level of the pit, and is separated from it by an excavation. This occurs in the neighbourhood of Paris where, above the plaster-stone, brick-clay is often found. Under these circumstances, as the working of the plaster-stone leaves great hollows, and the brickworks are situated at the bottom of the hill, we want to pass the clay over the excavation by a simple and cheap method. The object is attained by the installation of a cable system. The full waggon suspended below a cable causes an empty waggon, which is suspended below another parallel cable, to ascend. The sketch in Fig. 17 explains the working of this aerial railway.
Fig. 17. Fig. 16. Suspension Pulleys. Fig. 17.Sketch showing the Working of Two-way Aerial Railway.
Two iron cables, C and C', firmly attached at their extremities to fixed points, are stretched over the space to be crossed. On these cables run two grooved pulleys, which support beneath them the body of a waggon. The system of pulleys (Fig. 16) is joined by means of a cross-bar, B, to two other cables, C and C. One of these, C', passes over a return pulley, R, and joins the bar B' of the descending waggon W; the other, C, is turned several times round the windlass T, and joins the bar B' of the waggon W.
When the loaded waggon W is hung in its place, the system is set in movement by giving a few turns to the windlass T, so as to start the empty waggon W ; and as soon as the apparatus has gained a certain speed, it is left to itself, the difference of weight causing the empty waggon to ascend. The next time, the windlass T is turned in the opposite direction so as to pull the empty waggon W, and so on.
On the arrival of the waggons, they are placed on their framework, which has remained on the rails, and are taken, the full ones to the factory, the empty ones to the clay-pit.
When there is no excavation to be crossed, and the manufactory is separated from the clay-pit by a gradual slope, or one that can be made gradual, it is advantageous to use inclined planes if the inclination is more than 8 or 10 in 100.
There are an infinite number of ways in which inclined planes can be arranged. The best and the most economical is that by cables where the apparatus is self- working, i.e. where the descending load pulls up the empty cars.
The installation comprises a pulley or a windlass, roller-frames, rollers, and a cable. In the one we describe, the pulley over which the cable passes is furnished with a lever friction-brake (Fig. 18); it is placed between the uprights of a wooden frame which is fixed firmly into the ground, at the top of the incline, with stout piles, the axis of the pulley being quite vertical. After the pulley is placed the roller - frame with its four return rollers; two of these, with vertical axes, help to give the two branches of the cable their necessary distance and direction; and the two others, which have horizontal axes, are placed at the beginning of the slope, at the point where the plane changes from horizontal to inclined.
Fig. 18. Return-pulley of an Inclined Plane, with Brake.
The rollers, which support the cable and prevent it from rubbing on the ground, are placed in a straight line at distances of from 5 to 10 metres according to the girth and tension of the cable.
The latter may be metallic (steel or iron), or of vegetable fibres (hemp, manille, agave); its length is that of the incline with 10 or 15 metres extra to enable it to go round the pulley and pass the roller-frame.