PLYMOUTH ROCK. 137 PNEUMATIC DISPATCH. England principles. A beautiful granite canopy has lieen frected upon the rock. PLYMOUTH ROCK. A breed of domestic fowls, popular and numerous throughout the United States, in several varieties. It is of large size, and has admirable qualities for market pur- poses. The favorite variety is the 'barred,' of a grayish-white color, every feather marked with many curving black bands. There are also clear white and clear buff varieties. The standard weight of the cock is 0..50: of hens, 7..50 pounds. See (iihired Plate of FowL.S, under POULTRY. PLYMOUTH SOUND. A beautiful inlet of the English Channel between Cornwall and Dev- onshire, forming the harbor of Plymoutli and Devonport. which is an important British naval station (Map: England, B 6). See Plymouth, and the article Breakwater. PLYMPTON, pllmp'ton, Geoere Washixg- lox (ls:i7 — ). An American engineer and edu- cator, born in Waltham, Mass. He graduated as a civil engineer from the Rensselaer Institute, Troy, N. Y. in 1847; was appointed professor of engineering and architecture in Cleveland Uni- versity (1852), and the following year took the chair of mathematics in the New^ York State Normal School. He occupied a similar position in the State Normal School at Trenton, N. -J., from 18.57 till 1863, when he was made professor of physical science in the Brooklyn Polytechnic- Institute, and six years afterwards took the same chair in the New York Cooper Union, of whose night schools he assumed direction in 1879. He was also professor of chemistry and toxicology at Long Island College Hospital, Brooklyn, from 1864 to 1886. His publications include The Blowpipe (1859), The Starfinder (1878), and The Aiiernid (1880). PLYNLIM'MON, or PLINLIM'MON. A mountain uf Wales, on the boundary between the counties of Montgomery and Cardigan, 2481 feet in height. Although only 12 miles from the coast, it is in the midst of a wild waste of moor and bog. and on its slopes are the sources of the rivers "vc and Severn. PNEUMATIC CLOCKS. See Clock. PNEUMATIC DISPATCH (Lat. pneumati- ciis. from Gk. irfev/j.aTi.Kds, prriunatikos, relating to wind or air, from wveO/m, pneiima, 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 Ijeing 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 be propelled in the direction of the suction and haul a carrier attached to it by means of a chord. The first practical results with pneiimatic dis- patch, however, seem to have been obtained by Medhurst, an Englishman, who first described his system in a pamphlet published in 1810. Medhurst was followed by a score or more of inventors, some of whom achieved mechanical success, but it was not until 185.3-54, when a tube 220 yards long was built in London by the Electric and International Telegraph Co., to con- vey telegraph dispatches, that a practical work- ing pneumatic system was i)ut in actual opera- tion for commercial purposes. This system was designed by .Josiah L. Clark, and employed a tube 11-2 inches in diameter, in which the carriers were dispatched in one direction only. This system was im[)roved by C. F. Varley, who succeeded Mr.Clark as the engineer of the compan' named, and who in- creased the diameter of the tubes to 214 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 B<?rlin, Germany, who proposed a circuit system in wliieh 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 u.sed 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 betweeij the telegraph station and the Exchange recjuiring 5670 feet of 2Va inch wrought-iron tube. This experimental line proved so successful that the system was rapidly extended until in 1897 there were 38 stations and 28 miles of tubes in use. In 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 filleil with com- pressed air. The exhausted tanks were perma- nently connected with the closed tubes, which 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 1S6C the first stretch of pneumatic tube was laid in Paris and the sy.stem has been gradually extended since. As in Berlin, the tubes were laid in circuits, but in.stead 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 staticms can be operated independently of the rest of the circuit. The carriers are run in trains, each train being 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
