ed, 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 deg.. The
eye-piece is also capable of lateral adjustment to allow for the drift
as far as 10 deg., 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.