pirit-level on its
upper surface.

If then the bottom of the sight-notch coincides with the bottom of the
bar, the line of sight is parallel to the axis; otherwise, the
reinforce-sight or the sight-bar must be lowered until coincidence is
obtained.

A centre line on the bar verifies the coincidence of the line of
sight, and also the motion of the sight-bar in the vertical plane. The
bevel verifies the angle of the bar; and the distance between the
outer faces of the sight-notch and of the reinforce-sight, being also
marked on the levelling-bar, verifies this adjustment.

Another method is sometimes and more advantageously used in adjusting
the sights of guns which have not been turned.

Two iron or wooden disks are turned to the exact diameter of the bore,
and placed on a rod two or three feet longer than the bore. One of the
disks is placed near the bottom of the bore, the other just within the
muzzle. On the part projecting beyond the muzzle there is a double
square, each arm of which is divided into equal parts and traversed by
a fine slit.

The square being set vertical by a spirit-level or plumb-line, and a
waxed thread or wire stretched taut from the outer arm through the
slit, cutting equal divisions on each, and passing through the centre
of the slit, it is evident that a line of sight is obtained through
the centre of metal and parallel to the axis of the bore.

This method has the advantage of adjusting the sights in any plane
parallel to the axis, as in the case of the side-sights of rifled and
other cannon.

ADJUSTMENT OF THE SIDE-SIGHTS.

40. The gun having been levelled, and the trunnions placed horizontal,
a centre line is to be drawn on the top of the left (or right)
rimbase.

The support for the sight is then to be fitted to the breech, at the
distance from this line marked on the pattern-sight for its calibre,
with the bottom of the sight-notch in the bar, exactly the height of
the front sight (one inch) above the upper surface of the rimbase; the
sight-bar perpendicular.

It is advisable to place a very thin sheet of rubber under the
support, to prevent the heads of the screws from being jumped off by
the vibration. The screws have the same thread as those for the
present reinforce-sight.

After the breech-sight is adjusted, a parallel to the axis of the bore
is to be drawn in the usual manner, and the front sight screwed in on
the rimbase.

Some trifling adjustment may then be req