Pneumatic Despatch, the name given to a method of sending written documents, chiefly telegraphic despatches, through a comparatively narrow tube by means of compressed air and by a partial vacuum. Early in the 19th century Mr Medhurst proposed to construct a railway on this principle with carriages moving through an air-tight tunnel (see MURDOCK). But Mr Latimer Clark, C.E., was the first to carry out a plan for the transmission of telegrams by pneumatic power. In 1853 he got a tube laid between the central station of the International Telegraph Company and the Stock Exchange, London, through which a carrier containing despatches was propelled by a current of air produced by connecting the tube with a vacuum holder. An improvement on this was made in 1858 by Mr Varley, C.E., who introduced compressed air for the outward and retained the vacuum method for the inward traffic. This method is still in use.
The essentials of a pneumatic despatch are the exhausting and compressing pumps worked by a steam-engine or other motor; a metal tube, which in England is usually a lead pipe, since it is easily made air-tight by soldering the joints; a small carrier of gutta-percha or other material to contain the despatches; and a suitable arrangement of valves at the stations for connecting the tube or tubes with the compressed air or vacuum mains. It has been found by experience that with heavy traffic, such as that at the telegraphic department of the London Post-office, a lead pipe inches in diameter is a convenient size, and this is enclosed in an iron pipe for protection. The method of working is this: At the central station end of the tube there is a double sluice valve, and when the carrier is inserted into the message chamber (the widened end of the pipe) the lower slide of the valve is drawn so as to close the mouth of the pipe, but on the rod of the slide there is a stop which actuates a lever and rack, and this opens the upper slide in front of the carrier. At the same time, by a separate arrangement, a valve is opened to admit compressed air, which forces the carrier forward. When its arrival at the other end of the tube is signalled electrically, the slide is moved so as to cut off the air from the pressure main, and then the chamber at the mouth of the pipe is ready for another carrier. From the distant end of the pipe the carrier is drawn or sucked to the central station by making a connection at that station with the vacuum main instead of the pressure main. For a distance of 1000 yards with a -inch tube the time of transit is one minute when the air pressure is 10 lb. per square inch or with an equivalent effective vacuum of 6 lb. per square inch. With the air pressure and vacuum usually employed, a speed of from 25 to 35 miles per hour is attained in tubes not exceeding a mile in length. The speed varies inversely as the square root of the length of the tube. The pneumatic service of the British post-office has been gradually increasing until the system, which in 1854 was represented in London by one 6 horse-power engine working a single tube of a few hundred yards in length, comprised in 1900, in London alone, four 50 horse-power engines (each indicating 130 horse-power) and 81 tubes of an aggregate length of 34 miles, in constant operation between 42 stations. The newspaper offices in several of our large towns have pneumatic despatches in connection with the telegraphic instrument rooms of their respective post-offices. In Paris (which has 20 stations for post-office work) and some other towns on the Continent, instead of several tubes radiating from a central to outlying stations as in London, one tube or tubes alongside each other from the central telegraph office form a continuous circular line with intermediate stations, and several carriers linked together are sent at one time. In some of these continental pneumatic despatches the pipes are of iron instead of lead, and there are other differences of detail. Berlin has 28 miles of tubing between 38 stations. In the United States great progress has been made in the use of pneumatic power for post-office work. Mr B. C. Batcheller, an American engineer, has invented an improved system which, from its completeness and simplicity, gives promise of very extended use in future. It consists of double tubes of wrought iron running parallel to each other. At the central station a steam-engine of 25 horse-power for every mile of tube compresses the air and forces it into one of the tubes, along which it rushes, returning by the other, a constant current being kept up. Cylinders of steel weighing about 13 lb., and having a capacity of about 800 cubic inches, are loaded with the letters, or whatever is to be sent, and, by an ingenious arrangement, introduced into the tube without interrupting the current. They are propelled at the rate of the current, and on arrival at the terminus are delivered into an air-chamber which stops them, also without interrupting the current, which returns by the other tube. At intermediate stations another ingenious device delivers cylinders, specially fitted for each station, at the proper places. The system was first tried in Philadelphia in 1893, the tubes used being 6 inches in diameter. One of the lines, 1 mile in length, was traversed in 85 seconds. In Boston 8-inch tubes were used, and carried over 360,000 letters per day between the post-office and the railway station. New York post-office has three sets of tubes: one to the Grand Central Station (3 miles), with 3 intermediate stations, carries 6000 lb. of letters daily at the rate of 7 minutes each way. In 1900, by sanction of the authorities, Mr Batcheller formulated a complete scheme for London. Its use for small parcels as well as letters and telegrams might do much to relieve the congested traffic in the streets.
It has been several times proposed to utilise the pneumatic tube for passenger traffic, and a piece of railway of this kind was laid down in the Crystal Palace grounds in 1865 by Mr Rammel. It consisted of a single line of rails in a tunnel 600 yards in length, along which a carriage carried passengers. Motion was given by alternate blowing and suction; but it was little more than an experiment, and was not permanent. Electricity has proved a much more useful power than air for this purpose.
Pneumatic power is capable of application in many other directions. It is extensively used in working tools of all kinds for riveting, hoisting, drilling, metal-cutting, hammering, &c.; and it is in many cases superseding steam and rivalling electricity (see COMPRESSED-AIR MOTOR). The London and North-western Railway Company were in 1901 preparing to introduce into their system a method of working their signals by pneumatic power, which has been successfully used in many American lines. The motive-power will be compressed air contained in an underground pipe, the pressure of which will, by means of pistons, be made to work signals at any reasonable distance. The action will be quicker; fewer signalmen will be required; the complicated system of rods and levers will be dispensed with; the new apparatus will not be so apt to get out of order as the old, and will be much cheaper to maintain. See also AIR-ENGINE.