The development of the pneumatic gun has been attended with some other important discoveries, which may be of interest.  It is well known that mortar fire is very inaccurate, except at fixed long distances, in consequence of the high angle, the slowness of flight of the projectile, the variability of the powder pressure, and the inability to change the elevation and the charge of powder rapidly.  In the pneumatic gun, the complete control of the pressure remedies the most important of the mortar’s defects and makes the fire accurate from long ranges down to within a few yards of the gun.  It is obvious that the pressure can be usefully controlled in two ways:  (1) by keeping the elevation of the gun fixed and using a valve that can be set to cut off any quantity of air, according to the range desired; (2) by keeping the pressure in the reservoir constant, and using a valve which will cut off the same quantity of air every time, changing the elevation of the gun according to the distance.  Another important discovery consists in the application of subcalibered projectiles for obtaining increased range.

The gun is smooth-bored and a full-sized projectile is a cylinder with hemispherical ends, to the rear of which is attached a shaft having metal vanes placed at an angle, which causes the projectile to revolve round its longer axis during flight.  A subcalibered projectile, however, being of less diameter than the bore of the gun, has the vanes on its exterior, and is held in the axis of the gun by means of gas checks which drop off as the projectile leaves the muzzle.  The shock to the explosive is, of course, greater than in the full-sized projectile, but the increase can be calculated, and so far a dangerous limit has not been reached.  From the fifteen-inch gun with a pressure of 1,000 pounds per square inch and a velocity of about 800 f.s., a range of 4,000 yards has been obtained at an elevation of 30 deg. 20, with a ten-inch subcalibered projectile, about eight calibers long and weighing 500 pounds.  This projectile will contain 220 pounds of blasting gelatine.  With improved full-sized projectiles weighing 1,000 pounds, a range of 2,500 yards will doubtless be obtained.

At elevations below 15 deg. these long projectiles are liable to ricochet, and what is now wanted is a projectile which will stay under water at all angles of fall and will run parallel to the surface like a locomotive torpedo.  Such a projectile has yet to be invented; but I have seen a linked shell, which has been experimented with from a nine-inch powder gun, that partially meets this condition.  It is made of several sections united by means of rope or electric wire in lengths of 100 to 150 feet.  When fired all sections remain together for some distance; the rear section then first begins to separate; then the next, and so on.  It is primarily intended to envelop an enemy’s vessel, and to remedy the present uncertainty of elevation in a gun mounted in a pitching boat; but it is found that when it strikes the water in its lengthened out condition, it will neither dive nor ricochet, but will continue for some distance just under the surface until all momentum is lost, when it will sink.  This projectile is at present crude, and has never been tried loaded, but it will probably be developed into something useful in time.