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 R> per sq. in. in a 2-in. gun, and succeeded in propelling a projectile 2 1 oo 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 (1840-1909) 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: (i) 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.
automatically into To prevent carriers from being despatched too frequently and overtaking each other a time lock is attached to the sending apparatus; this locks the controlling valve when a carrier is despatched, 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 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 ft, and carries a bursting charge of 600 ft 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 ft pressure is stored in tubes close to the gun, and is supplied from primary reservoirs, to which it is directly pumped at a pressure of about 2000 Ib. There is always, thereFore, a considerable reserve of power available without pumping. Pneumatic guns of this description (see figure) have been mounted for the protection of New York and San Francisco. With a fullcalibre shell (1000 Ib) these guns have a range of 2400 yds.; with a sub-calibre 8-in. shell (250 Ib) the maximum range is 6000 yds. The official trials showed remarkable accuracy. At 5000 yds. 75 % of the projectiles fell in an area of 360 X 90 ft. When the gun was tried at Shoeburyness the accuracy was far greater than could be obtained with howitzer shells propelled by explosives. On account of the power of exploding, the shell under water, and thus securing a torpedo action, a direct hit upon a ship is not required, and the target offered is largely in excess of the deck plan. The gun is, in fact, capable of replacing systems of submarine mines with economy, and without the great objection of interfering with a waterway.
The only employment of the dynamite gun afloat has been in the case of the U.S. gunboat " Vesuvius," carrying three in the bows. These guns are fixed at a constant angle of elevation, and the range is regulated by the air valve, training being given by the helm. Thus mounted on an unstable platform, the accuracy of fire obtainable must evidently be much less than on shore. The " Vesuvius " was employed during the SpanishAmerican War of 1898, when on several nights in succession she approached the defences of Santiago under cover of darkness and discharged three projectiles. Fire delivered under such conditions could not be sufficiently accurate to injure coast defences; but the shells burst well, and made large craters. A small dynamite gun on a field-carriage was used in the land operations above Santiago in the same war.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)