edge is used to measure the deviation of the
trunnions from the feet of the square.

10th. A trunnion-gauge, which is an iron ring of the proper diameter
of the trunnions. Its outer edge coincides with the diameter of the
rimbases.

11th. A trunnion-rule, to measure the distance of the trunnions from
the base ring, or line. This is an iron rod with a head at one end,
through which passes one branch of a small square. The centre of the
rod is marked on the end, and the square is set so that the inner edge
of the branch which is parallel to the rod is at a distance equal to
the semi-diameter of the trunnion from the centre. It is secured in
this position by screws and clamps.

The upper side of the rod is graduated to inches and tenths. A slide
with a slot through it, to show the graduation beneath, traverses upon
it, and is kept from turning by a guide on the lower side. There is a
vernier upon the slide, graduated to hundredths of an inch; a
thumb-screw serves to secure the slide at any point on the rod. That
end of the slide from which the graduation of the rod commences has
both of its sides drawn out, to form knife-edges; the knife-edges and
the end of the slide are in the same plane. When the square at the end
is placed on the trunnion, the end of the rod will touch its side at
the point of its greatest diameter. The rod being held parallel to the
axis of the bore, with the side of the head pressing the rimbase, the
knife-edge will be in a proper position to fall into the base line
when moved to find it.

12th. A beam-calliper for measuring diameters is a square of steel or
iron, with two branches, one of which is fixed and the other sliding.
The inner edges of the two branches, when pushed together, lie, of
course, in contact with each other throughout their length. The beam
is graduated to inches and tenths. A vernier is attached to the
sliding branch, graduated to hundredths of an inch. The latter is
provided with a thumb-screw, to fasten it at any point.

The length of the beam must be rather greater than the diameter; and
that of the branches than the semi-diameter of the guns to be
inspected, at their largest points.

13th. A cascabel-block is a wooden cylinder of the proper diameter of
the breeching-hole, the size of which it is used to verify.

The opening between the jaws may be ascertained by measuring the iron
block which is fitted to go between them, or by a template.

14th. A vent-guide, to be