PNEUMATIC DISPATCH (Lat. pnewniati eus. from (ilk. vevicartK6s, perimatikos, relating to wind or air, from rveiiiza, pneutna. air, wind, spirit). The name given to a method of sending mail matter. telegraphic dispatches, parcels. etc.. through a tube by means of air pressure. The matter to be transported is placed in a carrier so designed as to fit closely the inside of the tube while being free to move, this carrier being propelled forward by introducing air under pres sure behind it or by exhausting the air in front of it. Pneumatic dispatch was conceived as early as 1667 by Denis Papin, who, in a paper read be fore the Royal Society of London in that year, described a plan to exhaust the air from a tube in such a way that an interior piston would he propelled in the direction of the suction and haul a carrier atbiebed to it by means of a chord. The first practical results with pneumatic dis patch. however, seem to have been obtained by Medhurst, an Englishman. who first described his system in a pamphlet published in 1310. Medliurst was followed by a score or more of inventors, some of whom achieved mechanical success, but it was not until 1353-54, when a tube 220 yards long was built in London by the Electric and International Telegraph 1'o., to con vey telegraph dispatches, that a practical work ing pneumatic system was put in actual opera tion for commercial purposes. This system was designed by Josiah L. Clark, and employed a tube inches in diameter, in which the carriers were dispatched in one direction only. This system was improved by C. E. Varley, who succeeded \1r.Clark as the engineer of the company named, and who in creased the diameter of the tubes to inches, and operated the carriers in both directions. using vacuum for sending in one direction and compression for sending in the other direction.
The next improvement in pneumatic dispatch was made by Siemens Bros.. of Berlin, Oermany, who proposed a circuit system in which two tubes were used, the 'up' tube being connected to the 'down' tube at the distant end; the air was compressed into one end of the circuit and ex hausted at the other end, and furthermore it was kept in constant circulation, so that carriers were dispatched by inserting them into the tube without stopping the air current. To stop the carriers at intermediate stations, a wire screen could be inserted across the tube which would permit the air to pass, but would stop the carrier.
The apparatus for sending and receiving carriers consisted of two short sections of tube attached to a rocking frame so that either could be swung by hand into line with the main tube. One of the tube sections was open at both ends and was used for dispatching carriers; a carrier was placed in it, then it was swung into line with the main tube, when the air current passing through swept the carrier along. The other tube section contained a perforated screen at one end and was used to receive carriers; when it was swung into line the air passed along through the screen, but the carrier was caught.
The first extensive application of the Siemens system was made in Berlin. Germany, in 1865, when a circuit was built betweer4 the telegraph station and the Exchange requiring 5670 feet of inch wrought-iron tube. This experimental line proved so successful that the system was rapidly extended until in 189; there were 38 stations and 28 miles of tubes in use. ln extend ing the system the Siemens system of operation was discarded; air was not kept constantly cir culating, but power was stored up in large tanks. some being exhausted and others filled with com pressed air. The exhausted tanks were perma nently connected with the closed tubes, %%Idyll were opened when required for use, and the tanks con taining compressed air were connected to the tubes when messages were sent. The tubes were laid in circuits, including several stations in a circuit, and the carriers traveled only in one direction around the circuit. In 1860 the first stretch of pneumatic tube was laid in Paris and the system has been gradually extended since. As in Berlin, the tubes were laid in eircuils, hut instead of operating the circuit from a single power station each station of the circuit has a power plant and the sections of tubes between any two stations can he operated independently of the rest of the circuit. The carriers are run in trains. each train Teeing propelled by a piston which pushes the rear carrier of the train. and these trains are dispatched every 15 minutes. Every train stops at all stations, where the car riers for that station are taken out and others for sutc•ceeding stations arc inserted. The speeds of the trains are from 13 to 23 miles per hour. Vienim, Austria. has a pueumatie dispatch sys tem ri.sentbling that of Paris.