e enlargement usually takes the form of an isosceles triangle,
the apex of one of the angles towards the muzzle, and the other two
perpendicular to it.

With the lateral vent of the Dahlgren system it usually takes the
lozenge form, the cracks extending from the opposite angles lengthwise
of the bore.

With those rifled cannon in which the vent is bouched, the cracks
appear around the bouching; and although the bouching preserves the
vent, yet the formation of fissures around the enlarged orifice, when
once commenced, causes a greater tendency to rupture. With the vent
not bouched, the wear in rifled cannon is about double that of the
smooth-bore.

So long as the wear of the vent is regular and without cracks, a mere
enlargement is not indicative of danger; but when it reaches a
diameter of four-tenths (.4) of an inch the vent should be closed and
a new one opened.

A gun of large calibre should not in service be expected to endure
more than 400 or 500 rounds before it will be necessary to open the
new vent, which, however, will be of no advantage, unless the old one
be closed at its interior orifice, on which the gases would otherwise
continue to act as a wedge.

The first distinct appearance of the cracks, as shown by the button,
is the proper limit.

After the gun bursts, make a sketch or draft showing lines of
fracture, and reserve specimens to be sent to the Ordnance Yard at
Washington for trial of density and tensile strength; and, if
practicable, a photograph should be taken.

[Illustration: XI IN. GUN No. 897. Lith. by J.F. Gedney, Washn.]

PREPARATION OF GUNS FOR SERVICE.

37. After the guns have been received at the Navy Yards it is
necessary to adjust the sights, and, in the guns of the Dahlgren
pattern, cut the screw hole in the cascabel.

CUTTING THE SCREW-HOLE.

38. The boring and screw-cutting machine is a convenient portable hand
drill-press, the use of which is readily understood by any machinist.

The gun being carefully levelled, and the trunnions placed horizontal,
the position of the centre of the screw-hole, which in the guns of the
Dahlgren pattern is tangent to the radius of the breech, is marked on
the neck of the cascabel with a centre punch.

The machine is placed on the cascabel, the boring shaft inserted in
the hollow leading bar, and its movable centre placed on the mark. The
instrument is then set vertical, by a spirit-level on the cogged
driving-wheel, and the four p