The above simple diagrams will explain some of the principal requirements to be kept in view when working out signalling arrangements. Where the lines and sidings are very numerous, as at important junctions and large terminal stations, the signalling becomes very intricate, and may require three or four cabins, slotted together in such manner that the necessary co-acting may be insured for the proper controlling of the mainline signals. Many of these signal-cabins contain a large number of levers, some of them having as many as a hundred, and a few of them two hundred and forty levers, or more, all of them so carefully arranged that no conflicting signal can be given. Not only has much skill to be exercised in the accurate adjustment of the interlocking machinery, but much study must be devoted to determine the exact duty of every lever, for the locking or releasing of other levers. Signal-cabins may be built of stone, brick, or wood. They should be roomy, well ventilated, and have abundance of light. Every signal-cabin should be placed in the position from which the signalman can obtain the best view of the signals and points under his charge. The height of the cabin floor will depend upon any obstacles that may intervene between the cabin and the signals, such as over-line bridges, station roofs, buildings, or other obstructions. Sometimes the floor has to be kept as low as five feet above rail-level, to secure a line of sight under the over-line bridges; and in others the floor has to be raised twenty, or even thirty feet above rail-level. [Illustration: Fig. 500, 508, 501, 502, 506, 507] Figs. 500, 501, and 502 show plan, transverse section, and elevation of a signal-cabin suitable for a small roadside station. The lower story and chimney-stack are of brick, and the upper story of wood, with slated roof. There is room for an interlocking frame of twenty or twenty-five levers, and space at the end of the cabin for the block-telegraph instruments, or electric train-staff instruments. The roof-work is open up to the slateboards, to obtain as much air capacity as possible. In the transverse section a winch for working mechanical gates is shown at the end of the interlocking frame. There 330 is a liberal amount of glass, and two or three sliding windows, which the signalman can open to enable him to speak to the engine-drivers or others during shunting operations. The lower story of the cabin can be utilized for trimming lamps and keeping a small supply of coals and other stores. When working after dark the lamps in the cabins should be well protected by shades, to prevent the lights being seen by engine-drivers, and mistaken for signals.