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 steel tanks charged from a special compressor with air at a pressure of 500 to per sq. in. The capacity of the

tank depends on the power required and the distance to be traversed by a single charge of air. The air passes through a reducing valve from the main to an auxiliary tank, in which the pressure is, say, , and thence to the driving cylinders. By using compressed air vitiation of the mine air is avoided, as well as all danger of fire or explosion of gas. Electric locomotives usually work on the trolley system, though a few storage battery locomotives have been successfully employed. Trolley haulage lacks the flexibility of steam or compressed air haulage, and is limited to main lines because the wires must be strung throughout the length of the line. By adopting modern non-sparking motors there is but little danger of igniting explosive gas. Electric and compressed air locomotives are durable, easily operated, and can be built to run under the low roofs of thin veins. Their power is proportioned to requirements of load and maximum gradient; the speed is rarely more than 6 or 8 m. per hour. Electric locomotives are in general more economical then either steam or compressed air.

For heavy gradients rope haulage has no rival, though for moderate grades it is often advantageously replaced by electric and compressed air haulage. Gravity or self-acting planes are for lowering loaded cars, one or more at a time, from a higher to a lower level. The minimum grade is that which will enable the loaded cars in

travelling down the plane to pull up the empty cars. At the head of the plane is mounted a drum or sheave, and around it passes a rope, one end of which is attached to the loaded cars at the top, the other to the empty cars at the foot. The speed due to the excess of weight on the loaded side is controlled by a brake on the drum. The rope is carried on rollers between the rails. There may be two complete lines of track or three lines of rails, one being common to both tracks, and the cars passing on a middle turnout or “parting”; or a single track with a parting. An engine plane is an inclined road, up which loaded cars are hauled by a stationary engine and rope, the empty cars running down by gravity, dragging the rope after them. This is similar to shaft hoisting, except that the grades are often quite flat. In the tail-rope system of haulage, best adapted for single track roads, there are two ropes—a main and a “tail” rope—winding on a pair of drums operated by an engine. The loaded train is coupled to the main rope, and to the rear end is attached the tail-rope, which reaches to the end of the line, passing there around a large grooved sheave and thence back to the engine. By winding in the main rope the loaded cars are hauled towards the engine, dragging behind them the tail-rope, which unwinds from its drum. The trip being completed, the empty train is hauled back by reversing the engine. The ropes are supported between the rails and guided on curves by rollers and sheaves. High speeds are often attained. Branches, operated from the main line, are readily installed. In the endless rope system the rope runs from a grip wheel on the driving engine to the end of the line, round a return sheave, and thence back to, the engine. Chains are occasionally used. The line is double track and the rope constantly in motion, the cars being attached at intervals through its length by clips or clutches; the loaded cars move in one direction, the empties in the other. There are two modes of installing the system: either the rope passes above the cars and is carried by them, resting in the clips, or it is carried under the cars on rollers, the cars being attached by clips or a grip-carriage. (For details see Hughes, Text-book of Coal Mining, pp. 236–272; Hildenbrand, Underground Haulage by Wire Rope.) Rope haulage is widely used in Collieries, and sometimes in other mines having large lateral extent and heavy traffic. With the tail-rope system, cars are run in long trains at high speed, curves and branches are easily worked, and gradients may be steep, though undulating gradients are somewhat disadvantageous. In the endless-rope systems cars run singly or in short trains, curves are disadvantageous, unless of long

radius, speed is relatively slow, and branch roads not so easily operated as with tail-rope. The tail-rope plant is the more expensive, but for similar conditions the cost of working the two systems is nearly the same. An advantage of the endless system is that the cars may be delivered at regular intervals.

Hoisting.—When the mine is worked through shafts, hoisting plant must be installed for raising the ore and handling men and supplies. On a smaller scale hoisting is also necessary for sinking shafts and winzes and for various underground services. As ordinarily constructed, a pair of horizontal cylinders is coupled to a shaft on which are mounted either one or

two drums (fig. 12). The diameter of the cylinders is such that each alone is capable of starting the load. As the cranks are set 90° apart, there is no dead centre, and the engine is able to start under full load from any point of the stroke. This is important in mine hoisting, which is intermittent in character and variable as to power and speed required. The cylinders are generally single-expansion, though compound engines are occasionally used for heavy work. The engine is direct-acting, the drums making one revolution for each double stroke. In geared hoists the drums are on a separate shaft, driven from the crank-shaft by tooth or friction gearing, and make one revolution for, say, 4 or 5 double strokes. The hoisting speed is therefore slower, and as less engine power is required for a given load the cylinders are smaller, though making more strokes per minute. Large and powerful geared hoists are not uncommon. The dimensions of the drum depend on the hoisting speed desired and the depth of shaft or length of rope to be wound. Drums are either cylindrical or conical. Conical drums (fig. 12) tend to equalize the varying load on the engine due to the winding and unwinding of the rope. On starting to hoist, the rope winds from the small towards the large end of the drum, the lever arm, or radius of the coils, increasing as the weight of