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Rh 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 dispatching 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 dispatching, 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

current is not interrupted. An air- piston, furnishes the power to swing the frame, the operation requiring an instant only. When the controlling 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 dispatched too frequently and overtaking each other a time lock is attached to the sending apparatus; this locks the controlling valve when a carrier is dispatched, 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 cushion and is brought to rest without injury or shock. The carriers are thin steel cylinders closed at the front end by a convex disk of the same material carrying a buffer of felt and 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 4 500 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.

REFERENCES.—The system, as used in the United Kingdom is fully described in a paper by Messrs Culley and Sabine (Min. Proc. Inst. Civ. Eng. vol. xliii.). The same volume contains a description of the pneumatic telegraphs of Paris and of experiments on them by M. Bontemps, and also a discussion of the theory of pneumatic transmission by Professor W. C. Unwin. Reference should also be made to a paper, by C. Siemens (Min. Proc. Inst. Civ. Eng. vol. xxxiii.), describing the Siemens circuit system; and to Les Télégraphes, by M. A. L. Ternant (Paris, 1881); General Post Office Technical Instructions, vol x., “Pneumatic Tubes”; Kempe's Engineers' Year-Book (1908 edition).

PNEUMATIC GUN. Air as a propellant has in recent years been applied to guns of large calibre, in which its comparatively gentle action has proved advantageous when high explosives contained in their shells are employed as projectiles. In 1883 Mr Mefford of Ohio utilized an air pressure of 500 lb per sq. in. in a 2-in. gun, and succeeded in propelling a projectile 2100 yds. The arrangement was of the simplest form-a hose with an ordinary cock by which the air was admitted into the gun behind the projectile. The question was then taken up by Capt. E. L. Zalinski (1849-1909) of the United States Artillery, who in 1888 reduced the so-called “dynamite gun” to a practical shape and obtained excellent firing results.

The principal features of his system are: (1) An extremely ingenious balanced valve admitting the air pressure into the gun. This valve is opened and closed by a simple movement of the firing lever, and is capable of adjustment so that the propelling force,

Dynamite gun, mounted at Sandy Hook, New York Harbour.

and consequently the range, can be regulated. (2) A light steel projectile carrying the bursting charge, and provided with a tail to which vanes are attached in order to give rotation. (3) Electric fuses of entirely original design. Each shell carries a wet battery, the current from which fires the charge on impact with any solid object, and a dry battery which becomes active after the shell has dived below the surface of the water, and ignites the charge after delay capable of regulation. For safety all the electric circuits are made to pass through a disconnector, which prevents them from being completed until the shell has been fired. The gun is a built-up smooth-bore tube, 15 in. or less in diameter. The full-calibre shell weighs 1000 lb, and carries a bursting charge of 600 lb of blasting gelatine, cut into the form of cheeses, fitting the steel envelope, and provided with a core of dry gun-cotton as a primer. Sub-calibre projectiles, 10 in. and 8 in., can also be used. In their case, rotation is given by vanes or fins attached to the body of the shell. Air at 1000 lb pressure is stored in tubes close to the gun, and is supplied from primary reservoirs, to which it is directly