ances from the base-line of the several parts, and of the
points at which diameters are to be measured, are laid off accurately
on the upper edge, and then marked in lines perpendicular to it on the
sides and lower edges of the profile. An iron strip is attached to the
upper edge to prevent warping, and the whole is well coated with
shellac varnish, to keep it from absorbing moisture.

The following instruments are used in connection with the
profile-boards:

A rule for verifying the marks, of such a length that not more than
one fleeting may be necessary, to be graduated decimally according to
the standard.

A small square of steel, to be used in referring the marks on the
board to those on the rule.

A steel straight-edge, long enough to extend across the muzzle-face,
and several inches on the board, to ascertain the extreme length from
base to muzzle. It is also used for the same purpose at the extreme
end of the cascabel.

A steel scratcher, to mark the gun at points not otherwise indicated,
where diameters are to be measured.

9th. A trunnion-square of steel or iron for ascertaining the position
of the trunnions, with reference to the axis of the bore. This
instrument is a square with two branches, one of which is fixed and
the other movable. The foot of each branch is in the same plane, and
is parallel to the upper edge of the main piece which connects them.
The latter is graduated to inches and tenths. The movable branch
slides on the main piece, and may be secured to it by two
thumb-screws. It is provided with a vernier scale graduated to
hundredths of an inch. Between the branches there is a slide, also
provided with a vernier graduated as before, with a thumb-screw to
secure it firmly; in its centre there is a sliding-point, moving
vertically, with a thumb-screw to fasten it. Above the foot of each
branch there is a slit to receive the shank of a plate, on the end of
which a thread is cut; the lower edge of the plate forms a right angle
with the branch, and the plate is fastened to the branch by a nut, at
a point from the end equal to the semi-diameter of the trunnion, which
is marked on each branch.

When the feet of the branches, or the lower edge of the plates, rest
upon the trunnions, the upper edge of the main piece is parallel to
their axis, if their alignment is correct. When in the latter
position, the edges of the feet will lie close against the sides of
the trunnions.

A graduated steel w