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 Spanish-American 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.
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 pumped at a pressure of about 2000 lb. 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 full calibre shell (1000 lb) these guns have a range of 2400 yds.; with a sub-calibre 8-in. shell (250 lb) the maximum range is 6000 yds. The official trials showed remarkable accuracy. At 5000 yds. 75%; of the (projectiles fell in an area of 360×90 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.