The cableway generally consists of a single span between two towers, which are either fixed or movable on rails according to the requirements of the work to be done. In addition to the main cable which bears the weight, and the rope which moves the skips along it, the cableway has the "fall" rope, which lowers the skip to the ground and raises it; the dumping rope, which discharges it; and the "button" rope, which pulls blocks off the horn of the skip truck at intervals as the latter moves, to support the "fall" rope from the main cable. If the fall rope sagged its weight would, after a certain amount had been paid out, overcome the weight of the skip, and render it impossible to lower the skip to the filling point. So a series of fall-rope carriers are, at the commencement of a journey from one end of the cableway, riding on an arm in front of the skip carriage. The button-rope, passing under a pulley on the top of the skip carriage, is furnished at intervals with buttons of a size increasing towards the point at which the skip must be lowered. The holes in the carriers are similarly graduated so as to pass over any button but the one intended to arrest them. If we watched a skip travelling to the lowering point, we should notice that the carriers were successively pulled off the skip carriage by the buttons, and strung along over the main cable and under the fall rope.
When the skip has been lowered and filled the fall and hauling ropes are wound in; the skip rises to the main cable, and begins to travel towards the dumping point. As long as the dumping rope is also hauled in at the same rate as the hauling rope it has no effect on the skip, but when its rate of travel is increased by moving it on to a larger winding drum, the skip is tipped or opened, as the case may be, without being arrested.
The skip may be filled by hand or made self-filling where circumstances permit.
The cableway is so economical in its working that it has greatly advanced the process of "open-pit" mining. Where ore lies near the surface it is desirable to remove the useless overlying matter (called "over-burden") bodily, and to convey it right away, in preference to sinking shallow shafts with their attendant drawbacks of timbering and pumping. An inclined railway is handicapped by the fact that it must occupy some of the surface to be uncovered, while liable to blockage by the débris of blasting operations. The suspended cableway neither obstructs anything nor can be obstructed, and is profitably employed when a ton of ore is laid bare for every four tons of over-burden removed. In the case of the Tilly Foster Mine, New York, where the removal of 300,000 tons of rock exposed 600,000 tons of ore from an excavation 450 ft. long by 300 ft. wide, the saving effected by the cableway was enormous. Again, referring to the Chicago Drainage Canal, "the records show that while labourers, sledging and filling into cars, averaged only 7 to 8 1 / 2 cubic yards per man per day, in filling into skips for the cable ways the labourers averaged from 12 to 17 cubic yards per day."[19] The first cableway erected by the Lidgerwood Manufacturing Company for the prosecution of this engineering work handled 10,821 cubic yards a month, and proved so successful that nineteen similar plants were added. The cableways are suspended in this instance from two towers moving on parallel tracks on each bank of the canal, the towers being heavily ballasted on the outer sides of their bases to counteract the pull of the cable. From time to time, when a length had been cleared, the towers were moved forward by engines hauling on fixed anchors.
The cableway is much used in the erection of masonry piers for bridges across rivers or valleys. Materials are conveyed by it rapidly and easily to points over the piers and lowered into position. Spans of over 1,500 feet have been exceeded for such purposes; and if need be, spans of 2,000 feet could be made to carry loads of twenty-five tons at a rate of twenty miles an hour.
TELPHERAGE
On most ropeways the skips or other conveyances are moved along the fixed ropes by trailing ropes working round drums driven by steam and controlled by brakes. But the employment of electricity has provided a system called telpherage, in which the vehicle carries its own motor, fed by current from the rope on which it runs and from auxiliary cables suspended a short distance above the main rope. "Telpher" is a term derived from two Greek words signifying "a far carrier," since the motor so named will move any distance so long as a track and current is supplied to it. The carrier—for ore, coal, earth, barrels, sacks, timber, etc.—is suspended from the telpher by the usual hook-shaped support common to ropeways, to enable the load to pass the arms of the posts or trestles bearing the rope. The telpher usually has two motors, one placed on each side of a two-wheeled carriage so as to balance; but sometimes only a single motor is employed. Just above the running cable is the "trolley" cable, from which the telpher picks up current through a hinged arm, after the manner of an electric tram. The carriers are controlled on steep grades by an electric braking device, which acts automatically, its effect varying with the speed at which the telpher runs. The carrier wheels, driven by the motors, adhere to the cable without slipping on grades as severe as three in ten, even when the surface has been moistened by rain. "In order to stop the telpher at any desired point, the trolley wire is divided into a number of sections, each controlled by a switch conveniently located. By opening a switch the current is cut off from the corresponding section, and the telpher will stop when it reaches this point. It is again started by closing the switch. At curves a section of the trolley wire (i.e. overhead cable for current) is connected to the source of current through a 'resistance' which lowers the voltage (pressure of the current) across the motors at this point. Thus, upon approaching a curve, the telpher automatically slows down, runs slowly around the curve until it passes the resistance section, and is then automatically accelerated."[20]
The telpher line is very useful (for transporting material considerable distances) in districts where it would not pay to construct a surface railway. On plantations it serves admirably to shift grain, fruits, tobacco, and other agricultural products. Then, again, a wide field is open to it for transmitting light articles, such as castings and parts of machinery, from one part of a foundry or manufactory to another, or from factory to vessel or truck for shipment. When coal has to be handled, the buckets are dumped automatically into bins.
The telpher has much the same advantages over the steam-worked ropeway that an electric tram has over one moved by an endless cable. Its control is easier; there is less friction; and the speed is higher. And in common with ropeways it can claim independence of obstructions on the ground, and the ability to cross ravines with ease, which in the case of a railway would have to be bridged at great expense.