h from the bore. This is to be pierced
lengthwise to receive the wire.

TO TAKE THE IMPRESSION.

36. Shove the wire through the vent; let it pass along the bore and
out at the muzzle; put it through the leaden button and tie a knot at
the end. Draw the wire back through the vent until the leaden button
is introduced firmly into the inner orifice.

[Illustration: TYPES OF VENTS. Lith. by J.F. Gedney, Washn.]

Apply the lever, making its shoe bear on the button, and force it well
in by repeated blows, the muzzle being the fulcrum. This done,
disengage the button by pushing in the priming-wire.

In taking impressions of the vent and cracks, each button in turn is
used as a pattern for moulding its successor, allowing for the
progressive enlargement of the vent, or the cracks emanating from it.
When the crack shows itself, the head of the button should be so
enlarged as to include it.

These examinations should take place after every twenty fires, at
least, and more frequently when any unusual enlargement of the vent or
extension of cracks shall be developed, and indicate its speedy
destruction.

Before each examination the bore of the gun is to be carefully washed
and dried.

In recording the measurements of the bore in extreme proof and after
service, distinguish between "indentation," which is the depression at
the "seat of the shot," which is always below, and the "wear of the
bore," which is generally above, and increase of bore, or
"enlargement" from any other cause.

When from the appearance of the bore at the interior orifice of the
vent, and especially when a crack or cracks appear to be extending
rapidly, the vent so enlarged may be filled with melted tin, zinc, or
Babbitt metal,--a tight-fitting sponge-head being pushed to the bottom
of the chamber to close the interior orifice,--and the other vent be
drilled through for the purpose of continuing the firing.

The precise time at which this is to be done will vary, according to
circumstances; such as quality of metal, charge, and elevation.

The endurance of a smooth-bored gun with service charges may be surely
predicted by observation of the progressive wear of the interior
orifice of the vent.

There are certain general forms in which this enlargement takes place.
They may be classed as triangular, lozenge, quadrilateral, star,
circular, and elliptic. (See Plate.)

With the ordinary central vent, when subjected to a rapid, continuous
fire, th