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Originally appearing in Volume V21, Page 867 of the 1911 Encyclopedia Britannica.
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PNEUMATIC DESPATCH, the name given to a system of transport of written despatches through long narrow tubes by the agency of air pressure. It was introduced in 1853 by J. 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 11 in. in diameter and 220 yds. 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 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. Radial System.—In the hands of R. S. Culley and R. Sabine the radial system of pneumatic despatch was in 1870 brought to great perfection in connexion with the telegraphic department of the British post office, since that date the total length of tubes (which are employed for telegrams only) has been very largely increased (in 1909 there was in London a total length of 40 m.), whilst in all large and also in very many smaller provincial towns there are installations; these are constantly being added to, as it is found more economical to transmit local message-work by tube rather than by wire, as skilled telegraphists are not required, but only tube attendants. In some cases only a single tube is necessary, but three or four, or even more, are in use in some towns, according to local circumstances. Short tubes, known as " house tubes " are in use in a great number of offices; such tubes, which are worked either by hand-pumps (when the tubes are very short and the traffic inconsiderable) or by power, are usually 12 in. in diameter, and are used for the purpose of conveying messages from one ?art of a telegraph instrument-room to another, or from the instrument-room to the public counter. The underground, or " street " tubes are chiefly 24 in. in diameter, but there are also a number of 3-in. tubes in use; those in the large provincial towns (Birmingham, Bradford, Cardiff, Edinburgh, Glasgow, Grimsby, Liverpool, Manchester, Newport, Leeds, Newcastle, Southampton and Swansea) are 21 in. in diameter; but in Dublin, Gloucester, Lowestoft and Milford 11-in. tubes are employed. There are fifty street tubes in London, varying in length from too to 2000 yds. (central office to the Houses of Parliament), and also seventy-five house tubes; the pumps for the whole system are worked by four loo horse-power steam-engines. At Cardiff, Edinburgh, Gloucester, Leeds, Lowestoft, Newport, Southampton and Swansea the pumps are driven by electric motors; at Bradford and Grimsby gas-engines are used, and at Milford an oil-engine. The tubes are in all cases of lead, the 24-in. tubes weighing 8 lb per foot run and being made in lengths of 28 ft.; they are enclosed in 3-in. cast-iron pipes made in lengths of 9 ft. Great care is exercised in making the joints in the lead pipes. Before the tube is placed in its trench a strong chain is passed through it, and a polished steel mandrel, 6 in. long and slightly less in diameter than the diameter of the tube, is heated and attached to the chain, and pushed half its length into the end of the tube already laid; the new length of tube is then forced over the projecting end of the mandrel until the tube ends (which have been previously cut flat) butt perfectly together; an ordinary plumber's joint is then made. By this means the tube is made perfectly air-tight, and the mandrel keeps the surface of the tube under the joint as smooth as at any other part of its length. After the joint is completed the mandrel is drawn out by the chain attached to it, the next length is drawn on, and the above process repeated. The tubes are laid about 2 ft. below the surface of the ground. The tubes radiate from the central to the branch offices, the principal offices having two tubes, one for " inward " and Working, the other for " outward " traffic. At the smaller offices both the inward and the outward traffic is carried on through one tube. The " carriers " are made with gutta-percha bodies, covered with felt, the front of the carrier being provided with a buffer or piston formed of several disks of felt which closely fit the tube; the messages are prevented from getting out of the carrier by the end being closed by an elastic band, which can be stretched sufficiently to allow the message forms to be inserted. The 3-in. carriers will hold 75 ordinary message forms, the 24-in. carriers 25 forms, and the 12-in. carriers 20 forms. The carriers are propelled in one direction (from the central office) by " pressure," and drawn in the opposite direction by " vacuum," the standard pressure and vacuum being ro lb and 6; lb per sq. in. respectively, which values give approximately the same speed. The time of transit of a carrier through a tube when the air pressure does not exceed 20 lb per square inch is given very approximately by the empirical formula: 1= .00872 1; where l=length of tube in yards, d=diameter of tube in inches, P=effective air-pressure in pounds per square inch, I= transit time in seconds. For vacuum the formula : .00825 _ 1, 1=1-•2341115.5-P1V d' where Pi effective vacuum in pounds per square inch. The horse-power required to propel a carrier is approximately, for pressure: H.P. _ (.574+.0011P) Pad'; for vacuum: db H.P. For a given transit time the actual horse-power required is much less in the case of vacuum than in the case of pressure working, owing to the density of the air column moved being much less: thus, for example, the transit time for io lb pressure is the same as for 61 lb vacuum, but the horse-power required in the two cases is as 1.83 to 1. A tribe 1 m. long, 24 in. in diameter, and worked at to lb per square inch pressure, will have a transit time of 24 minutes, and will theoretically require 3.35 horse-power to be expended in working it, although actually 25 % more horse-power than this must be allowed for, owing to losses through various causes. The transit time for a 2-,-,-in. tube is 16% more than for a 3-in. tube of the same length, when both are worked at the same pressure, but the horse-power required is 50 % less; it is not advisable, therefore, to use a tube larger than is absolutely necessary to carry the volume of traffic required. The somewhat complicated pattern of " double sluice valve " originally used at the central stations has been superseded by a simpler form, known as the "D" box—so named Despatching from the shape of its cross section. This box is of and cast iron, and is provided with a close-fitting, Receiving brass-framed, sliding lid with a glass panel. This apparatus, lid fits air-tight, and closes the box after a carrier has been inserted into the mouth of the tube; the latter enters at one end of the box and is there bell-mouthed. A supply pipe, to which is connected a " 3-way " cock, is joined on to the box and allows communication at will with either the " pressure " or " vacuum " mains, so that the apparatus becomes available for either sending (by pressure) or receiving (by vacuum) a carrier. Automatic working, by which the air supply is automatically turned on on the introduction of the carrier into a tube and on closing of the D box, and is cut off when the carrier arrives, was introduced in 1909. On the long tubes (over about r000 yds.) a modification of the " D " box in its simplest form is necessary; this modification consists in the addition of a " sluice " valve placed at a distance of about 9 in. (i.e. rather more than the length of a carrier) from the mouth of the tube. The sluice valve, by means of an interlocking arrangement, is so connected with the sliding lid of the box that the lid cannot be moved to the open position unless the sluice valve has dosed the tube, nor can the sluice valve be opened unless the sliding lid is closed. The object of this sluice valve is to prevent the back rush of air which would take place into the tube when the sliding lid is opened to take out a carrier immediately on the arrival of the latter; for although the vacuum may be turned off by the 3-way cock, yet, owing to the great length of the tube, equilibrium does not immediately take place in the latter, and the back rush of air into the vacuum when the lid is opened to extract the carrier will cause the latter to be driven back into the tube. The sluice also prevents a similar, but reverse, action from taking place when pressure working is being carried on. As a rule, only one carrier is despatched at a time, and no second carrier is inserted in the tube until the arrival of the first one at the farther end is automatically signalled (by an electric apparatus) to the despatching office. On some of the long tubes a carrier, when it passes the midway point in the tube, strikes a trigger and sends back an electrical signal indicating its passage; on the receipt of this signal a second carrier may be despatched. This arrangement has been almost entirely superseded by a signalling apparatus which by a clock movement actuates an indicating hand and moves the latter to " tube clear " a certain definite time (30 to 40 seconds) after a carrier has been inserted in the tube. By this arrangement carriers can be despatched one after the other at comparatively short intervals of time, so that several carriers (separated by distinct intervals) may be travelling through the tube simultaneously. It is necessary that the carriers be separated by a definite interval, otherwise they tend to overtake one another and become jammed in the tube. Although the stoppage of a carrier in a tube is of exceedingly rare occurrence, it does occasionally take place, through picks being driven into the tube by workmen executing repairs to gas or water pipes, but the locality of such a stoppage is easily determined by a simple inspection along the route of the tube. In no case is any special means of testing for the locality from the central office found necessary. Circuit System.—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 system—that of Messrs Siemens—a 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. In America, in New York, Boston and Philadelphia, tubes (Batcheller system) up to 8 in. in diameter are in use. The tubes are of cast iron made in 12-ft. lengths and are carefully bored; they resemble ordinary water pipe. Short bends are made in seamless brass tube carefully bent to a uniform radius of twelve times the diameter of the tube, the tube being slightly larger in diameter than the main tube. The sending apparatus, or transmitter, is similar to the Siemens switch before described, and consists of two sections of the tube supported in a swinging frame so arranged that either section can be brought into line with the main tube, in which a current of air is constantly flowing. One of these tube sections maintains the continuity of the main tube, while the other is swung to one side to receive a carrier. In despatching, a carrier is placed in an iron trough and then pushed into the open tube section. The frame carrying the two tube sections is then swung until the section containing the carrier is brought into line with the main tube, when the carrier is swept along with the current of air. When the frame is swinging from one position to another the air is prevented from escaping by plates that cover the ends of the tube, and a by-pass is provided so that the current is not interrupted. An air-motor, consisting of a cylinder and piston, furnishes the power to swing the frame, the operation requiring an instant only. When the con-trolling ',lever is pulled and latched the frame swings, and as the carrier passes out of the apparatus it trips the lever, and the frame swings back automatically into position to receive another carrier. To prevent carriers from being despatched too frequently and overtaking each other a time lock is attached to the sending apparatus; this locks the controlling valve when a carrier is despatched, and keeps it locked for a given period of time, varying from five to fifteen or twenty seconds, according to the adjustment of the lock. The carrier is received at the farther end of the tube into an air cushion formed by closing the end of the tube with a sluice-gate, and allowing the air to flow out into a branch pipe through slots in the tube located about 4 ft. in the rear of the sluice-gate. When a carrier arrives it passes over the slots, enters the air leather; the rear end is closed by a hinged lid secured by a lock. The shell of the carrier is 24 in. long and 7 in. in diameter for the 8-in. tube; it is secured by two bearing-rings of woven cotton fabric clasped between metal rings; the rings are renewed after about 2000 M. of travel. The tubes are worked at a pressure of 6 lb per sq. in.., and for a distance of 4500 ft. require about 30 horse-power, the transit speed being 30 M. per hour. In addition to its use for postal and telegraphic purposes the pneumatic despatch is employed for internal communication in offices, hotels, &c., and also in shops for the transport of money and bills between the cashier's desk and the counters.

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