Pneumatic Despatch, a contrivance for sending packages through tubes by means of atmospheric pressure. The first idea of a plan for pneumatic transmission appears to be due to Denis Papin, who in 1667 presented a paper to the royal society of London on the " Double Pneumatic Pump." This consisted of two large cylinders to exhaust the air from a long metal tube containing a travelling piston, to which a carriage was attached by means of a cord. More than a century elapsed before any further effort in this direction was made. Panckouke's Dictionnaire encyclopedique des amusements des sciences (1792) gives a description of a machine by M. Van Estin, by means of which a hollow ball holding a small package was propelled by a blast of air through a tube several hundred feet in length and having many curves. This plan seems, however, to have been more an amusement than an attempt to introduce an industrial scheme. With more regard to practical results, Med-hurst, an engineer of London, published a pamphlet on the subject in 1810. He proposed to move small carriages on rails in air-tight tubes or tunnels, by compressed air from behind, or by creating a partial vacuum in front.

In 1812 he published another pamphlet, but the plan was not put into successful operation, principally from insufficient means of exhaustion. About 1832 he proposed to connect the carriage inside of such a tube with a passenger carriage running on the top of the tube; and although the latter project has never been commercially successful, it was the first to be practically attempted. (See Pneumatic Railway.) More than a score of patents were taken out on the continent and in England and America, none of which met with any practical success. Returning to the original idea of Denis Papin, inventors attempted to accomplish pneumatic transmission by moving the load inside the tube, and in course of time achieved success. In France MM. Jarroux and Taisseau presented a project for atmospheric telegraphy before the academy of sciences, and they were succeeded in the same direction by MM. Brochet and Ador. In 1857 Mr. Latimer Clark patented in England his system of pneumatic transport, and in 1858 he laid down a tube in London between Moorgate street and the general post office. Several stations were connected by a line of tubes in which cylindrical carriers holding despatches were placed.

The cylinders were surrounded by India-rubber bands to make them fit accurately, and a partial vacuum was created in front, and compressed air was also employed to act behind. In 1860 M. Sebillot of Paris published a scheme by M. Kieffer, his pamphlet being entitled "A Reform in the Postal Service of Paris." - In England, in 1861, an iron tube, somewhat semi-cylindrical, about 30 in. in diameter and a quarter of a mile long, was laid down near Battersea, with gradients and curves like an average road. Iron carriages were made to fit the tube by means of flexible flanges, and a centrifugal fan moved by a steam engine constituted the exhausting apparatus. It was found that two iron carriages of about 7 cwt. each could in this way be driven at a speed of about 30 m. an hour. A pneumatic despatch company began operations in 1863. The experimental tube was removed from Battersea and laid down with some additions from the Euston station of the London and Northwestern railway to the N. W. district post office, a distance of about a third of a mile. The packages were blown through the tube to the N. W. station by compressed air, but moved in the other direction by the excess of normal atmospheric pressure against a partial vacuum.

This system has been extended to embrace several miles of tunnel about 4 1/2 ft. in diameter, besides about 13 m. of small pneumatic tubing for sending telegraph messages between various stations. The pneumatic tunnel extends from the station at Euston square to the general post office in St. Martin's-le-Grand. The central station is in Holborn, where the machinery for driving the package trains is placed. The distance between Holborn and Euston square is a mile and three quarters, and that between Holborn and the post office 306 ft. less than a mile. The tubes have a horse-shoe cross section, 4 ft. 6 in. high with an area of 17 sq. ft. The straight portions are of cast iron, and the curves of brickwork faced with cement. The chief gradients in the Euston square section are 1 in 45 and 1 in 60; those in the post office section are 1 in 15. The carriages weigh 22 cwt. and are 10 ft. 4 in. long, having a cross section conformable to the tube, leaving a space of about an inch all around, occupied by a flange of India rubber which causes a carriage to fit the tube in the manner of a piston. The trains are drawn from Euston square and from the post office by exhaustion, and driven to those stations by pressure.

The engine is supplied by three boilers, each 30 ft. long and 6 ft. 6 in. in diameter. The experiments by Mr. Barlow, consulting engineer to the Midland railway company, led him to conclude that the greatest working economy was in moving a great amount of weight at a low speed, on the high as well as the low gradients. The system can transport over the whole line, allowing for delays, an average of a ton per minute. By means of an improvement invented by Messrs. Siemens and Halske, while a current of air circulates through any number of stations, a carriage may be introduced or stopped at any intermediate station. - In Paris a system of pneumatic transmission was decided upon in 1865, and an experimental line was laid between the place de la Bourse and the Grand H6tel (boulevard des Capucines) in 1866. It was found by experiments that the water of the town could be used to produce, by being forced into reservoirs, or allowed to run from them, alternate compression and expansion of air in connecting reservoirs, the action being changed at will by a system of cocks; and this was the mode adopted for the propulsion of packages through the tubes.

Since then a complete and comprehensive system has been carried out from time to time, the motive power in many of the recent extensions, however, being steam. In 1874 the station in the place du Havre was connected with the one at the Grand Hotel, and the water at this station was employed to produce pressure and vacuum for working in both directions. There are now (1875) 45 stations in the city, and the number of despatches of all kinds transmitted is about 25,000 a month, or an average of 830 a day, of which the Bourse station sends over 2,500. - A pneumatic despatch system is now (July, 1875) in operation in the Western Union telegraph building at the corner of Broadway and Dey street, New York. The moving of the packages is done by exhausting, accomplished by a Root's rotary blower. Packages are sent from all parts of the building to the operating room in the seventh story, but most of them from the receiving room on the ground floor. In the centre of the operating room stands a chest about 5 1/2 ft. high, 18 in. wide, and about 12 ft. long. The upper part of it, about 6 in. deep, forms one chamber, connecting by openings, which may be closed or shut at pleasure, with a dozen or more chambers beneath.

A large exhaust pipe about 8 in. in diameter descends from the middle of the upper chamber to the exhausting engine in the basement. From each receiving desk in the room below a tube about 1 1/2 in. in diameter descends to the floor, and then bending in a gradual curve is carried to the centre of the building, where it ascends vertically with its two dozen fellows to the chest in the operating room. Each compartment in the chest receives two tubes. A cylindrical box about 6 in. long and 1 1/4 in. in diameter, made of stout leather and open at one end, with a flange at one or both ends, as may be preferred, so as nearly to fit the tube, is used as the carrier for the light paper parcel, which is rolled up and held to its place inside the box by its elasticity. The weight of the whole load is but a few ounces, and consequently it needs a propelling force of less than half a pound to the square inch to force it up the tube with considerable velocity. At the orifice in the chamber of the exhausting chest is a bent spring, which arrests the box at its exit, so that it falls with little force in the chamber, at the same time that a lever is moved which closes a galvanic circuit, by which means an alarm is rung to call a messenger.