Then about 1882-83 it was found by some inventors if moderately fine wires were stretched level, and as tight as possible, they would afford such little friction and resistance to light and nicely balanced wheels, that no inclination of the tracks was necessary, and that the carriages mounted on such wheels and tracks would run the entire length of a long building and turn corners not too sharp by a single initial push of the hand. In other arrangements a carrier is self-propelled by means of a coiled spring on the carrier, which begins its operation as soon as the carrier is given a start; and to meet the exhausted strength of such spring, coiled springs at different points on the line are arranged to engage and give the carrier an additional push. Before the carrier is stopped its action is such as to automatically rewind its spring. A system of pneumatic transmission was invented, by which a carrier is caused to travel through a tube by the agency of an air current, created therein by an air compressor, blower, or similar device. The device is so arranged that the air current is caused to take either direction through the tube; and in some instances gravity may be used to assist a vacuum formed behind the carrier. The tube is controlled at each end by one or more sliding gates or valves, and the carrier is made to actuate the gates, and close the one behind it, so that the carrier may be discharged without permitting the escape of the air and consequent reduction of pressure. An interesting invention has been made by James M. Dodge of Philadelphia in the line of conveyors, whereby pea coal and other quite heavy materials introduced by a hopper into a trough are subjected to a powerful air blast which pushes the material forward; and as the trough is provided with a series of frequently occurring slots or perforations open to the outer air and inclined opposite the direction of travel, the powerful current from the blower in escaping through such outlets tends to lift or buoy the material and carry it forward in the air current, thereby greatly reducing frictional contact and increasing the impelling operation. The inventor claims that with such an apparatus many tons of material per hour may be conveyed with a comparatively small working air pressure. In order that a conveyor carriage may be automatically switched off at a certain place or station on the line, one mode adopted was to arrange at a gate or station a sort of pin or projection or other deflector to engage some recess or corresponding feature on the carriage, so as to arrest and turn the carriage in its new direction at that point. Another mode was the adoption of electro-magnets, which would operate at a certain place to arrest or divert the carriage; and in either case the carriage was so constructed that its engaging features would operate automatically only in conjunction with certain features at a particular place on the line.