used with vents in guns of Dahlgren's
pattern.

This instrument is made of bronze or composition. When placed upon the
gun, one of its branches coincides with the curve of the cylinder, and
the other, starting from its centre, lies along the cylinder in
contact with it longitudinally. The lower edges of the branches are a
right line and a curved line, making two right angles with each
other. The length of that of the transverse branch is equal to the
distance between the centre of the two vents. The rear surface of the
transverse branch is curved and quadrilateral. Its sides are inclined,
so that their rear edges show the exact direction of the vents. Every
point in the upper edge lies in the same horizontal plane. The height
is sufficient to permit the edges to give an accurate direction to the
drill.

The upper edge of the other branch runs off in a sloping curve to its
extremity.

A centre line is drawn through the lower edge of the longitudinal
branch, and is continued upwards on the rear surface of the transverse
branch to the top.

The guide being placed with its centre upon the centre mark of the
gun, and the centre line of the longitudinal branch being made to
coincide with the centre line scribed upon the cylinder, the rear
lower edge of the transverse branch will then coincide with the
base-line, its extremities will indicate the centres of the vents, and
the rear edges of the sides will show their true direction.

15th. Vent-gauges of untempered steel wire, with shoulders to prevent
them from slipping into the vent. One should be of the proper diameter
of the vent, one of the greatest, and one of the least diameter
allowed.

16th. A vent-searcher, a steel wire of the length of the vent, bent to
a right angle at the lower end and pointed. It is used for detecting
imperfections in the sides of the vents.

17th. A semicircular protractor of metal for measuring the inclination
of vents, or for ascertaining their deviation from the guide.

18th. A set of templates for verifying the shape of lock-lugs, the
angle of the rear sight mass, the curve between the base-line and the
front of rear sight mass, that at the end of the cascabel, the bevel
of the breeching-hole, the opening of the cascabel, and the shape of
the muzzle swell.

If the inspection should take place at the foundry, the templates used
in chipping might be verified and used for inspection.

For guns of Dahlgren's pattern, a bronze model,