PNEUMATIC TUBES. Pneumatic tubes are employed to despatch letters, parcels, tele grams, etc., to a distance by the energy of compressed air. Their application was origi nally suggested by Denis Papin in 1667, when he read a paper before the Royal Society of London, describing a plan by which the air in a tube was exhausted in such a way that the piston working in the tube would be drawn in the direction of the suction, pulling with it an attached carrier. His idea was taken up and developed by several inventors, with more or less success, up to 1854, when the Electric and International Telegraph Company, of London, built in that city the first pneumatic despatch system which was actually operated for com mercial purposes. It consisted of a lead tube 220 yards in length and one and one-half inches in diameter, in which the carriers were pro pelled in one direction only. Subsequently, the diameter of the tubes was increased to two and one-fourth inches, which were arranged to send the carriers in two directions — onward from, and inward to, the central station. At the latest report the English Postal Department operates about 601 miles of tubes, 40 miles of which are situated in the London district. In the English system the tubes radiate from the central station to the branch stations located at various points, the larger stations being equipped with two tubes — one for outward and one for inward traffic, while the smaller stations carryon the work through a single tube. The outgoing carriers are propelled by pressure and the incoming carriers are drawn in by the suction of a vacuum. The standard pressure used is about 10 pounds to the square inch, and a vacuum of about six pounds to the square inch, giving about the same speed in both directions. The tubes consist of lead pipes one and one-half inches to two and one-fourth inches in di ameter, for interior use, such as the delivery of telegrams and parcels of small size, such as cash-carriers in stores, from one room to an other in the same building; while the under ground or street tubes are larger, ranging from two and one-fourth inches to three inches in diameter. The two and one-fourth-inch pipes weigh about eight pounds per linear foot and are made in lengths of 28 feet. When installed, they are enclosed in a cast-iron pipe three inches in diameter, made in sections of nine feet. The air-pumps are driven by steam-engines or by electric motors; those in the London district are worked by four 50 horse-power steam engines. All of the carriers have gutta-percha
bodies covered with felt, and are provided with buffers or pistons of felt attached to the front ends, which fit tightly in the tubes. The ends of the carriers are closed by elastic bands to prevent the messages from falling out, but they can be stretched sufficiently to allow the insertion of the message forms. The de spatching and receiving apparatus originally consisted of a very complicated double-sluice valve, but it has been superseded by the D-box, a contrivance much simpler in form, which al lows communication at will with either the pres sure or vacuum mains, so that it is available for both sending and receiving. The carriers run at a speed of about 25 miles an hour.
In Germany, the Siemens system is em ployed. Its first extensive use was made in Berlin, in 1865, when 5,670 feet of wrought-iron tubing, two and one-half inches in diameter, were laid and connected the telegraph station with the Exchange. In this system, the tube forms a com plete circuit, into one end of which the air is introduced under pressure while it is exhausted at the other end, thus maintaining a constant circulation, and also enabling the inserting and despatching of the carriers without temporarily shutting off the air current. The carriers were stopped at intermediate stations by placing a wire screen across the tube. The sending and receiving apparatus was constructed of two short pieces of tubing attached to a rocking frame so that either of them could be swung into the main circuit by the hand, whenever desired. One of them was open at both ends, and was used in sending; the other was pro vided with a wire screen at one end, and when swung into the line of the main tube, allowed the air to pass through but stopped the car rier. At the present time, Berlin has in opera tion about 30 miles of tubes serving about 40 stations. As in the first experimental line, the tubes are laid in circuits, each of which serves a certain number of stations, but the air instead of being kept in constant circulation is stored up in large tanks, and turned into the tubes whenever it is required to despatch a carrier. Paris and Vienna have systems similar to that of Berlin. The carriers are run in trains, des patched at 15 minute intervals, with a maximum speed of 23 miles an hour. They stop at all of the stations on the circuit so that the carriers belonging to each station can be taken off, and those for the succeeding stations put in.