or composition which is spread
over its surface. This movable piece rests on a wedge attached to a
flat rod running through a slot in the head; there is a slot in this
rod about four inches long, a pin passing through it into the staff.
To use the instrument, withdraw the rod as far as the slot will
permit, which will allow the movable piece on which the composition
has been spread to drop below the surface of the head, and protect it.
Push the head to the bottom of the chamber, and arrange the position
of the staff so that the movable piece will cover the vent, then press
the end of the rod home. This motion will throw out the composition,
and a distinct impression of the vent and of fire-cracks (should there
be any) will be left upon its surface; draw the rod back as far as
the slot will allow, and withdraw the instrument: the impression,
being protected thereby, will come out uninjured.

Impressions of injuries or cavities in the bore may easily be taken by
a similar contrivance.

27th. Hydraulic pump and apparatus for the water-proof.

Any of the various patterns of this machine may be applied to the
proof of guns. An iron cross-head is secured to a stout wooden block
which fits into the muzzle, and which has a flange or shoulder to
cover the muzzle-face; rings of caoutchouc or gutta-percha are placed
between them; an iron rod with a ring in one end, to fit over the
trunnion, and with a thread cut on the other end, is used on each side
of the gun, to connect the trunnion with the cross-head. The whole is
set up with nuts, and the pressure upon the rings makes a tight joint;
a coupling upon the cross-head receives the hose, and the water is
forced into the gun through a hole in the wooden block. Care should be
taken that the valve is loaded with the proper weight for proof.

28th. Dies for marking guns.--A full set of figures, with such capital
letters as may be required for the inspection-marks; these should be
one inch in length. Also, small letters of suitable size to mark
"lbs.," and a full set of half-inch figures.

USE OF THE INSPECTING INSTRUMENTS.

29. The guns having been freed from rust, and their foundry numbers
noted, in the order of their relative positions, on the field-book,
the inspecting officer will proceed to verify the instruments to be
used in their measurement, if this has not been previously done in a
manner entirely satisfactory to him.

He will then examine carefully the guns, insid