airs of set screws on the clamp-head
embracing the cascabel.

The centre is then removed, and a drill inserted in the lower
extremity of the boring-shaft, which, being held firmly by a shoulder
and turned by a four-armed wrench, while pressed up to the metal by
slowly turning the cogged driving-wheel, cuts the hole. This is
successively enlarged, by two or more counterbits, to the size of the
body of the screw.

The cutter is then inserted in the leading bar, and the thread cut.

ADJUSTMENT OF THE SIGHTS.

39. The bore having been thoroughly cleaned, the axis is levelled by a
spirit-level; this may be very conveniently done by the aid of the
levelling-bar. The axis of the trunnions is to be laid horizontal,
either by placing a small level on the trunnions, or, as more exact,
by using the trunnion-square. If the trunnion-square is used it will
be proper to verify the position of the line of sight, which is
frequently incorrectly placed at the foundries.

The breech-sight is then to be adjusted.

A brass head or tompion, fitted with a vertical arm, on which there is
a ledge for a spirit-level, is then introduced into the bore, and the
arm placed vertical by the spirit-level and a tangent screw.

The arm is pierced on its centre line with two holes--one at the
height of the prescribed diameter of the muzzle, the other at a height
equal to the proper distance of the bottom of the sight-notch from the
axis of the bore. A waxed thread or fine wire, being stretched from
the upper hole to the centre of the sight-notch, will coincide with
the line of sight traced on the swell of the muzzle, the top of the
reinforce sight-mass, and the base-line, if they are correctly placed.
It will also be parallel to the axis of the bore if the adjustment of
the breech-sight is exact, and the top of the reinforce-sight is made
to coincide with it.

This is, however, seldom the case, and after the adjustment of the
reinforce-sight it is necessary to verify it.

This is done by the levelling-bar--a square steel bar with parallel
faces, somewhat longer than the distance between the sights, the rear
end of which is bevelled at 60 deg. (the angle at which the sight is
placed).

The outer end of this bar is placed on the reinforce-sight, which has
been previously adjusted to the proper height, and the bevelled end in
contact with the outer face of the sight-bar. The bar is then levelled
by two screws placed near the inner end, and a s