of the vent, particular attention will be
paid, in the reinspection of the guns, to this point. The standard
gauge will be used to ascertain the general enlargement, and the
searcher to detect defects which may have been developed in firing.
Impressions are to be taken of the lower orifice of the vent with
softened wax, and if they show that the vent is corroded in furrows
and enlarged considerably in diameter at its junction with the bore, a
permanent impression is to be taken in lead to show the conical
enlargement. (See mode of taking impressions, Arts. 35 and 36.)

63. When the number of rounds fired is not known, an estimate may be
made from an examination of the vent by cylindrical gauges differing
from each other by .01 of an inch passed through it. If the number is
estimated, it is to be entered in red ink.

64. In all the guns of the Dahlgren pattern the vents are (.2)
two-tenths of an inch in diameter. In all other guns .22 of an inch
exterior, .2 of an inch on the interior.

Observation of the wear of the vent in proof firing of smooth-bored
guns gives the following as the average diameter of the vent, after
the undermentioned number of fires:

No. of rounds 100 200 300 400 500
Diameter of vent .24 .26 .30 .35 .40

These, combined with examination of the interior orifice, will enable
a very correct judgment to be formed of the probable number of fires
sustained and duration of the gun.

The larger the calibre and the heavier the charge the more promptly
the wear is manifested on the interior and exterior.

This enlargement does not extend very far from the lower orifice until
the enlargement on the exterior has reached a diameter of .3 of an
inch.

65. So long as the wear is regular, and the cracks, although numerous,
do not exceed .5 of an inch in length, the indications are good. If
the cracks are but few or diminish in number, running into each other
and extending rapidly, it is a very unfavorable sign. In the rifle
cannon (Parrott's) cracks athwart the bore either running into the
bouching or in the rear of it are very unfavorable to the gun's
endurance.

66. Whenever any premature explosions of shells take place within the
gun or near the muzzle, a careful examination of the gun shall be
made; and all the circumstances of the case, together with the
opinions of the commander and officers in immediate charge as to the
cause thereof, reported to the Bureau; tak