PNEUMATIC DESPATCH. The transport of written despatches through long narrow tubes by the agency of air-pressure was introduced in 1853, by Mr Latimer Clark, between the Central and Stock Exchange stations of the Electric and International Telegraph Company in London. The stations were connected by a tube 1 1/2 inches in diameter and 220 yards long. Carriers containing batches of telegrams, and fitting piston-wise in the tube, were sucked through it (in one direction only) by the production of a partial vacuum at one end. In 1858 Mr C. F. Varley improved the system by using compressed air to force the carriers in one direction, a partial vacuum being still used to draw them in the other direction. This improvement enables single radiating lines of pipe to be used both for sending and for receiving telegrams between a central station supplied with pumping machinery and outlying stations not so supplied. In the hands of Messrs Culley and Sabine this radial system of pneumatic despatch has been brought to great perfection in connexion with the telegraphic department of the British post office. Another method of working, extensively used in Paris and other Continental cities, is the circuit system, in which stations are grouped on circular or loop lines, round which carriers travel in one direction only. In one form of circuit systemthat of Messrs Siemensa continuous current of air is kept up in the tube, and rocking switches are provided by which carriers can be quickly introduced or removed at any one of the stations on the line without interfering with the movement of other carriers in other parts of the circuit. More usually, however, the circuit system is worked by despatching carriers, or trains of carriers, at relatively long intervals, the pressure or vacuum which gives motive power being applied only while such trains are on the line. On long circuits means are provided at several stations for putting on pressure or vacuum, so that the action may be limited to that section of the line on which the carriers are travelling at any time.
The following particulars refer to the radial system of pneumatic despatch as used in the British post office. In London most of the lines connect the central office with district offices for the purpose of collecting and distributing telegrams. Iron tubes were used in some of the earliest lines, but now the tubes are always made of lead, with soldered joints, and are enclosed in outer pipes of iron for the sake of mechanical protection. The bore, which is very smooth and uniform, is normally 2 1/4 inches, though in a few cases it is as much as 3 inches, and in some only 1 1/2 inches. The greatest single length of any of the existing London lines is 3873 yards, but a more usual length is from 1000 to 2000 yards. In most cases a single tube serves both to send and to receive, but where the traffic is heavy a pair of tubes are used, one to send by pressure and the other to receive by vacuum. The pumps, which supply pressure and vacuum to two mains, are situated in the central office. At the outlying stations the tubes terminate in a glass box, open to the atmosphere. At the central station the end of each tube is a short vertical length, facing downwards, and provided with a double valve, consisting of two sluices, one at the end and the other a little way above the end, the distance between the sluices being somewhat longer than the length of a carrier. The sluices are geared together in such a manner that a single movement of a handle closes one and opens the other, or vice versa. To send a carrier from the central station, the carrier is introduced into the tube, the lower sluice being open; a single movement of the handle then successively (1) closes the lower sluice, (2) opens the upper sluice, and (3) opens a valve which admits air behind the carrier from the compressed-air main. As soon as the arrival of the carrier is signalled (electrically) from the out-station, the handle is pushed back, thus (1) cutting off the compressed air, (2) closing the upper sluice, and (3) opening the lower sluice. The tube is then ready for the sending of another carrier. When an electric signal arrives from an out-station that a carrier is inserted there for transmission to the central station the handle is pulled forward far enough to close the lower and open the upper sluice, but not far enough to put on the air-pressure. The vacuum main is then put in connexion with the tube by a separate stop-cock. When the carrier arrives the vacuum is shut off and the lower sluice opened to allow it to drop out. This arrangement of double sluices admits of the insertion or removal of a carrier while other carriers are travelling in the same tube, and without sensible dis-turbance of their motion. But great caution requires to be exercised in allowing two or more carriers to follow one another on a single section of line, especially on lines worked by pressure, since no two carriers travel at precisely the same speed. When the same tube is used alternately for sending and receiving the upper sluice is dispensed with. On some lines there are intermediate stations, and the sections are then worked by a block system like that used on railways. The carriers are cylindrical cases of gutta-percha covered with felt, which is allowed to project loosely at the back, so that the pressure makes it expand and fit the pipe closely. In front the carrier is closed by a buffer or piston composed of disks of felt of the diameter of the pipe. The despatches are held in by an elastic band at the back. An ordinary carrier weighs 2 3/4 oz., and holds about a dozen despatches. During business hours carriers are passing through the London tubes almost incessantly. With a pressure of 10 lb per square inch, or a vacuum of 7 lb, the time of transit, if through a 2 1/4 inch tube, is 1 minute for a length of nearly 1000 yards, and 5 1/2 minutes for a length of 3000 yards.
The following statistics show the growth of the pneumatic despatch in the post office during ten years (the figures for 1875 are taken from a paper by Messrs Culley and Sabine, cited below, and those for 1885 have been furnished by Mr W. H. Preece):
== TABLE ==
In Paris large areas of the city have been covered by pneumatic circuits made up of iron pipes round which omnibus trains of carriers are sent at intervals of fifteen minutes, The trains consist of several carriers much heavier than the English type, linked to one another and to a leading piston. The trains are stopped at the suc-cessive stations to take up and deposit despatches. The pneumatic despatch took root in Paris in 1866, and has been developed there in a way which differs greatly in mechanical details from the English system. An arrangement like that used in Paris has been followed in Vienna and in Berlin, where the Siemens system has also been used. In New York the English system is adopted, but with brass instead of lead tubes.
Interruptions occurring in the pipes can be localized by firing a pistol at one end and registering by a chronograph the interval of time between the explosion and the arrival of the air-wave reflected from the obstacle.
In addition to its use for postal and telegraphic purposes the pneumatic despatch is occasionally employed for internal com-munication in offices, hotels, &c., and also in shops for the transport of money and bills between the cashiers desk and the counters.
References.The system as now used in the United Kingdom is fully described in a paper by Messrs Culley and Sabine (Min. Proc. Inst. Civ. Eng., vol. xliii.). The same volume contains a description of the pneumatic telegraphs of Paris and of experiments on them by M. Bontemps, and also a discussion of the theory of pneumatic transmission by Prof. W. C. Unwin. Reference should also be made to a paper by C. Siemens (Min. Proc. Inst. Civ. Eng., vol. xxxiii.) describing the Siemens circuit system; and to Les Télégraphes, by M. A. L. Ternant (Paris, 1881). (J. A. E.)
The above article was written by: James Alfred Ewing, M.A., B.Sc., F.R.S.; M.Instit.C.E.; Professor of Mechanism and Applied Mechanics, Cambridge; Fellow of King's College, Cambridge; Professor of Mechanical Engineering at the Imperial University, Tokyo, Japan, 1878-83; author of Treatise on Earthquake Measurement, Magnetic Induction in Iron and other Metals, The Steam Engine and other Heat Engines, etc.