the gun are measured by the profile-board,
marked with the true dimensions, the differences being measured by the
foot-rule, or, if minute, estimated by the eye.

The exterior diameters are measured with the calipers and square, or
by the set-gauges used in turning, and a graduated wedge.

To verify the position and alignment of the trunnions of a gun, it is
first necessary to ascertain, by means of the trunnion-gauge and of
the calipers, their cylindrical form and their diameters, which should
be the same, or allowance must be made for half the difference in
measuring their axial distances from the base-line, by the
trunnion-rule, which should next be done. These distances should be
equal, or their axes do not coincide--an error not tolerated.

The trunnion-square is then placed upon the trunnions in the plane of
their axis. The feet of its branches should coincide with the surfaces
of both trunnions, throughout their length, above and in rear, and
their inner edges with the faces of the rimbases. Then, with the
beam-compass, scribe on the upper surface of the gun the distance of
the axis of the trunnions from the base-line, and push the
sliding-point of the square down, till, at that distance, it touches
the surface of the gun, and screw it fast. Then turn the gun over, and
again scribe on it the same distance from the base-line. The square,
being again applied, will determine whether the trunnions are above or
below the axis of the bore, which will coincide with that of the gun,
if accurately bored, and turned on the same centres and bearings. If
the branches rest upon the trunnions before the point of the slider
touches the gun at the scribe, their axis is below; but if the point
touch first, above the axis of the bore, by half the space between.
The graduated wedge, being placed under the vertical sliding-point,
will determine the amount. If both touch at once, both axes are in the
same plane.

No gun can be received, the axis of the trunnions of which is above
that of the bore.

The lengths of the trunnions are measured with the foot-rule, and the
diameters of the rimbases by that of the exterior rim of the
trunnion-gauge.

If the alignment of the trunnions be correct, it will serve as a means
of determining the correctness of the line of sight, which, before the
gun is removed from the lathe, should be distinctly traced on the
sight-masses and the swell of the muzzle, and should be at right
angles to the