This bouching has heretofore been made of cast zinc. Others with a flange and washer and the thread cut are now supplied, and the use of the old rings is prohibited.
402. If it be desired to explode the shell in front of or in the midst of a body of troops, or after having penetrated some resisting obstacle, the time-fuze should be used. This is the only fuze to be used with shrapnel.
403. The Vent is made in a bouching of pure copper screwed into the gun. In the largest calibres the interior orifice is lined with platinum.
The upper portion of the copper in naval guns is replaced by steel, to obtain a harder surface for receiving the blow of the hammer. The steel is three-fourths (3/4) of an inch thick.
A new vent can be readily put in, after getting out the old one, without injury to the screw-thread. This can be done by boring out the bouching with a drill, which leaves a thin shell containing the thread. Into the hole thus made insert a square mandrel about four inches, driving it lightly; by wrenching it, a portion of the shell of the bouching can be detached and removed by unscrewing. This may be repeated, and the whole of the old copper removed. The screw-thread is then to be cleaned out, and the new vent-plug screwed in.
404. Sights.—These consist of a fixed sight upon the right rimbase, and a brass movable sight placed in a socket which is screwed into the rear of the reinforce at the breech of the gun. The movable sight is furnished with a sliding eye-piece, and is graduated up to 10°. The eye-piece is also capable of lateral adjustment to allow for the drift as far as 10°, and for the effect of the wind. It is desirable that the sights should be placed on both sides of the breech; otherwise, in firing from a port at extreme train, there is a considerable loss of lateral aim. Furthermore, with the sight on the right rimbase, it is not convenient for the 2d Captain to attend the screw without interfering with the aim.
405. These guns are all rifled to the right, by which it is understood that the upper surface of the projectile is made to turn from left to right, the observer looking from the breech towards the muzzle of the gun.
406. Drift.—This is a deviation caused by the direction of the rifling, is always to the right when uninfluenced by the wind, and is to be allowed for.
407. The drift is in practice confounded with the deviation produced by the direction and force of the wind, which may either annul or increase it, according to whether it blows from right or left across the line of fire. At long range it is also necessary to consider the motion of the vessel across the line of fire. Suppose this to be at the rate of six knots, and the gun is elevated 15°, the time of flight would be by the Tables, 18 sec., while the deviation arising from this cause would be upwards of 60 yards. It is therefore of great importance that the Captain of the gun shall be carefully instructed in making this adjustment of the eye-piece.
408. Elevating Screw.—To obtain readily the changes of elevation necessary in the use of rifled cannon, the heavier calibres are made with very small preponderance, and are supplied with an elevating screw which is attached to the carriage at the lower end, while the nut is connected with the cascabel of the gun. Both screw and nut admit of movements by which the screw can take any position required in the various degrees of elevation. The parts should be allowed a certain amount of play; if binding is prevented, it is believed that the evident advantages of the screw may always be obtained.