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Pneumatic Networks

Long ago, when the world was a more innocent place than it is now, my mother would take me with her to a large department store- I think it was the Co-op. When you bought something, the sales assistant would take the money and the paperwork, stuff it into a small metal cylinder, and offer it to the mouth of a large bronze pipe. This pipe inhaled it with a great rush of air, and a minute or two later the change and receipt would appear in another cylinder disgorged from an adjacent bit of pipework, having been dispensed by a cashier safely tucked away from the rigours of armed robbery. I was fascinated. It was a pneumatic message system.

This technology is still alive in some applications- it can deliver physical objects, which is hard to do with email. Obviously there are limitations. A transatlantic system might present a few engineering challenges...

HOW IT WORKS

EARLY HISTORY

The first known work on pneumatic transport was done by the Englishman George Medhurst (1759-1827). During a period of a few years about 1810, he invented not only the pneumatic dispatch tube, but also the pneumatic railway, which is somewhat outside the scope of this page. Medhurst's invention pushed a capsule along a tube a few inches in diameter, using positive air pressure; thus was born the pneumatic dispatch tube.

In 1853 J. Latimer Clark installed a 220 yard pneumatic tube between the London Stock Exchange in Threadneedle Street and the Central Station of the Electric Telegraph Company in Lothbury. The Electric Telegraph Co had been up and running long before pneumatics came along; the company was incorporated in 1846. There were similar installations in Berlin (1865) between the Central Telegraph Office and the Stock Exchange, and in Paris (1866) to the Place de La Bourse. Of these, more later.

PNEUMATIC DESPATCH IN ENGLAND: 1911

This is an edited and revised version of the information contained in the article on "Pneumatic Despatch" in the 1911 edition of Encyclopaedia Brittanica.

"Pneumatic Despatch is the name given to the transport of written despatches through 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. These stations were connected by a tube 1.5 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 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 R S Culley and R Sabine the radial system of pneumatic despatch was in 1870 brought to great perfection in connection 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 greatly increased. In 1909 there was in London a total length of 40 miles, 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 handpumps (when the tubes are very short and the traffic light) or by power, are usually 1.75 in. in diameter, and are used for the purpose of conveying messages from one part of a telegraph instrument-room to another, or from the instrument-room to the public counter."

The tubes were used to send telegraph forms from local post-offices to the Central Telegraph Office, where the skilled operators were concentrated.

"The underground, or street tubes are mostly 2.75 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 2.75 in. in diameter; but in Dublin, Gloucester, Lowestoft and Milford 1.75-in. tubes are employed. There are fifty street tubes in London, varying in length from 100 to 2000 yards. (Central Office to the Houses of Parliament), and also seventy-five house tubes; the pumps for the whole system are worked by four 100 horse-power steam-engines. At Cardiff, Edinburgh, Gloucester, Leeds, Lowestoft, Newport, Southampton and Swansea the air-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 2.75-in, tubes weighing 8 lb/foot, 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 plumbers 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 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 guttapercha 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 2.75-in, carriers hold 25 forms, and the 1.5-in. carriers 20 forms. The carriers are propelled from the central office by pressure, and drawn in the opposite direction by vacuum, the standard pressure and vacuum being 10 psi and 6.75 psi respectively, which values give approximately the same speed."

"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 10 lb pressure is the same as for 6.75 lb vacuum, but the horse-power required in the two cases is as 1.83 to 1. A tube 1 mile long, 2.75 in. in diameter, and worked at 10 lb per square inch pressure, will have a transit time of 2.75 minutes, and will theoretically require 335 horse-power to be expended in working it, (Editor's Note: I think this must be a misprint, as it seems a quite excessive amount of power) although actually 25% more horse-power than this must be allowed for, owing to losses through various causes. The transit time for a 2.75-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 1000 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 closed 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 the 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 beer inserted in the tube. By this arrangement carriers can bi despatched one after the other at comparatively short interval~ 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 impact each another and may become jammed."

PNEUMATIC SYSTEMS IN FRANCE

The first system was opened in Paris in 1874.

The Marseilles service opened in 1910 and closed on the 29th of February 1964

A pneumatic service was opened in Algiers on April 4, 1910. It continued to function for a few years after Algeria became independent on July 5, 1962.

In Paris, you went to a pneumatic station, and wrote your message. This was sent pneumatically to the office nearest its destination; its journey was completed by a courier. The process was called sending a "petit blue", from the colour of the forms used. La Bourse is the Paris stock exchange.

THE OPERATION OF THE PARIS SYSTEM IN 1874

The first circuit was centred on the central electric telegraph station at Rue de Grenelle. Trains departed every quarter of an hour from Rue de Grenelle, covering the 1500m to the next bureau (Rue Boissy-d'Anglas) in 90 seconds. The canister carrying messages for local distribution was removed, another carrying messages handed in at that bureau was added, and the train sent off to the next destination, Grand-Hotel. From there it proceeded to La Bourse, Place Theatre-Francais, and Rue des Saint-Peres. It finally returned to Rue de Grenelle, having taken 12 minutes on its round trip.

Each train was composed of ten canisters, together weighing about four kilograms. Under either vacuum or pressure of about 10 psi the train had an average speed of 1 kilometre in 60 seconds.

The tubes were of iron, with an internal diameter of 65mm, the sections being joined with stepped flanges to minimise air leakage. The curves in the line were between 5 and 20 metres in radius.

There were two other circuits, centred on La Bourse:

• Bourse
• Rue J J Rousseau
• Rue des Vielles-Haudriettes
• Place du Chateau-d'Eau
• Porte St-Denis
• Bourse

• Bourse
• Rue Ste-Cecile
• Gare du Nord
• Boulev. Rocheouart (Montmartre)
• Rue Lafayette
• Bourse

For a contemporary map of these three circuits, as published in La Nature, see http://www.dself.dsl.pipex.com/MUSEUM/COMMS/pneumess/naturepneu1b.gif.

The outward journey was under air pressure applied from the rear; the return was by a vacuum applied in front of the canisters. This method was presumably adopted to minimise the pressure difference between tube and atmosphere, to reduce air leakage. There were also branch lines to Champs-Elysee, Place du Havre, and Rue des Halles.

The trains of canisters arrive via the tube at the bottom, and appear in the vertical column P. The tank B is initially full of with air, which is compressed by the introduction of water from the Paris mains via the upper pipe b. This method was used in districts where noisy machinery was not permitted. In less sensitive zones conventional steam-powered air pumps were said to be more economical; presumably the Paris water had to be paid for.

PNEUMATIC DESPATCH IN GERMANY

Berlin's pneumatic dispatch system began in 1865 and closed in 1976. It was 400 kilometres in total length at its maximum extent. The system is mostly dismantled but a few stations still exist.

PNEUMATIC DESPATCH IN THE USA

The first American pneumatic tubes were introduced in Philadelphia, in 1893. Boston, Brooklyn, New York, Chicago and St Louis also installed pneumatic systems. By 1915, these six cities had in total more than 56 miles of tubes in use.

The date of opening of the pneumatic postal service in New York so far remains obscure, but was between 1893 and 1898, as by the latter year the NY system connected 21 neighborhood post offices in Manhattan to the main post office branch there. Well into the 20th century, 30% of the first class letters passing through the New York main post office were sent out to the branch offices by the pneumatic tubes.

On the right Postmaster Frederick E. Coyne places the first bundle of mail into a pneumatic carrier; note its large diameter compared with European versions. On the left is R W Morrell, a pneumatic tube expert. The Chicago postal pneumatic tube ran between the post office and the Winslow rail station. The tubes were rented from the Chicago Pneumatic Tube Company.

The angled appliance in left foreground is for sending canisters. Behind it is a table for the arriving ones.

The angled appliance for sending canisters is in the upper middle of the picture.

PNEUMATIC DESPATCH IN CZECHOSLAVAKIA: STILL OPERATING

Prague, now capital of the Czech Republic, has an underground pneumatic tube network (potrubní postí) that is still operating at the time of writing. The network is approx 60 kilometers in extent, and has been in operation since the 1920s. The Prague system currently handles about 9,000 transmissions per month, down from more than a million messages per month in its heyday in the 1960s and '70s.