tself should be turned,
which will insure accurate measurements. Care must also be taken not
to allow the joints of the staff to become so loose that the
coincidence of the centre line is destroyed when they are screwed
together. If this should occur, however, a few turns of thread, placed
between them at the time of putting the instrument together, would
remedy the difficulty.

7th. An instrument for verifying the interior position of vents.

When the vent is drilled in the vertical plane of the axis, as in the
guns of old patterns, a simple head, shaped to fit the bottom of the
bore, or the chamber, with a staff fitted to it, is sufficient. But
for the Dahlgren guns, with two vents, some other plan is better. The
following has been found satisfactory:

A head of well-seasoned wood, which is fitted to the chamber, is
attached to a wooden disk of the diameter of the main bore. The
surface of the head corresponds with a longitudinal central section of
the chamber; at the point where the projection of the vent would meet
it a piece of hard wood is inserted. A central line drawn through its
length, crossed at a right angle by another line at any known point
from the smaller end, will afford convenient points to measure from. A
stout square wooden staff is attached to the axis of the head; at a
distance equal to the length of the bore, the end is jogged into the
centre of a half disk of wood, which is fitted to the bore. The whole
is so constructed that the straight edge of the half disk (or the
chord) is in the same plane as a horizontal section of the head. A few
holes are bored through the disk attached to the half head, to allow
the instrument to pass freely into the gun and out of it.

A wire of untempered steel, of the size of the vent, with a sharp,
well-centred point, and a small spirit-level, are required to use with
this instrument.

The gun being levelled, and the instrument being pushed to the bottom
of the bore, the upper edge of the half disk near the outer end of the
staff is then brought to a level. The surface of the half head then
corresponds with the horizontal central section of the chamber. The
point of the wire being pushed gently to meet it, will show very
accurately the interior position of the vent.

8th. Profile-boards for distances in front and rear of the base line.

Their lower edges are adapted to the shape of the gun, and the upper
ones are parallel to the axis of the bore.

The dist