—In 1870 Mr. J. W. Wilmot designed a double sluice-valve by means of which carriers could be despatched continuously without stopping the flow of air in the tubes. Mr. Wilmot further increased the working capacity of pneumatic tubes when, in 1880, he invented an intermediate automatic signaller, by means of which a carrier signals the passage of a given point on its journey, showing that the section of the tube traversed is clear, thus allowing a second carrier to be despatched before the first has reached its destination.
Fig. 2.
DIAGRAM ILLUSTRATING THE PNEUMATIC TUBE SYSTEM
LONDON RADIAL SYSTEM.
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From this beginning the English system developed into what has been termed a “radial system;” that is to say, one principal and several minor central pumping stations have been established, and from these radiate tubes to numerous sub-stations (see Fig. 2). Some of the stations are connected with double lines for sending in opposite directions. The out-going tube from the pumping station is worked by compressed air, and the incoming tube by exhaustion. Other stations are connected by single tubes, and they are operated alternately by compression and exhaustion. Intermediate stations are located on some of the lines. For the central station the Varley valves were found too expensive and troublesome to keep in order, so they were replaced by the Wilmot double sluice-valves, which are operated manually. In recent years the sluice-valves have been in turn replaced by what are termed D-boxes, a simpler form of apparatus. At the sub-stations the tube terminates in a box into which the carriers drop. As the system has been gradually extended, tubes two and three-sixteenths inches inside diameter have been used for short lines, and three-inch tubes for long lines. The tubes are of lead laid inside a cast-iron pipe which serves as a shield, protecting them from injury. They are laid in twenty-nine foot sections, the joints being made by soldering over a steel mandrel, which is afterwards drawn out by a chain. The joints in the cast-iron protecting pipe are made by caulking with yarn and lead. “Electric signals are used between the central and sub-stations, consisting of a single stroke bell and indicator, giving notice of the arrival and departure of carriers, and to answer the necessary questions required in working. Where there are intermediate stations the tubes are worked on the block system, as if it were a railway. Experience shows that, where great exactness of manipulation cannot be obtained, it is necessary to allow only one train in each section of a tube, whether worked by vacuum or pressure. But where there is no intermediate station, and where the tube can be carefully worked, carriers may be allowed to follow one another at short intervals in a tube worked by vacuum, although it is not perfectly safe to do so in one worked by pressure. In working by pressure it has been found that, notwithstanding a fair interval may be allowed, carriers are apt to overtake one another, for no two carriers travel in the same times, because of differences in fit, unless they are placed end to end. If signalling be neglected and a carrier happens to stick fast, being followed by several others, a block will ensue which it will be difficult to clear, while the single carrier could readily have been dislodged.” (Proceedings Institute of Civil Engineers, London, Vol. XLIII. p. 61.)
No changes have been made in the carriers from those used in the early experiments which have already been described.
The London system has grown until it now includes no less than forty-two stations and thirty-four miles of tubes. Similar systems have been established in connection with the telegraph in Liverpool, Manchester, Birmingham, Glasgow, Dublin, and New Castle. The tubes give a cheaper and more rapid means of despatching telegrams between sub-stations and central stations than transmission by telegraph, and local telegrams can be delivered in the sender’s handwriting.
An Underground Pneumatic Railway for Transportation of Mail.
—Before describing the systems used in the cities on the Continent of Europe, we must notice a very large pneumatic tube, or more properly called a pneumatic tunnel railway, constructed in London for the transportation of mail from one of the railway stations. The first railway of this type was constructed in 1863 by the Pneumatic Despatch Company of London, and extended from Euston to the district post-office in Eversholt Street, a distance of about eighteen hundred feet. The tunnel was flat on the bottom, having a D-shaped cross-section two feet eight inches by two feet eight inches. The carriers or carriages were cradle-like boxes fitting the tunnel, and they moved at a speed of seventeen miles per hour, carrying fifteen mails daily. In 1872 two similar but larger tunnels were built from Euston Station to the general post-office, a distance of fourteen thousand two hundred and four feet, or two and three-quarters miles. One was for the up traffic, and the other for the down. The tunnels were four and a half feet wide by four feet high, the straight part being built of cast iron and the bends of brick. The line was operated by a fan twenty-two feet in diameter, which forced the air into one tunnel and exhausted it from the other, producing a vacuum of ten inches of water, or six ounces per square inch. The carriages occupied twelve minutes in traversing the tunnels, and there was one gradient of one to fourteen. The carriages were ten feet four inches long and weighed twenty-two hundredweight. “The system was able to transport over the whole line, allowing for delays, an average of a ton per minute.” The system was used to transport the mails in bulk, but it was found to be slow and unsatisfactory, and was very soon abandoned.