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 used with vents in guns of Dahlgren's pattern.
This instrument is made of bronze or composition. When placed upon the gun, one of its branches coincides with the curve of the cylinder, and the other, starting from its centre, lies along the cylinder in contact with it longitudinally. The lower edges of the branches are a right line and a curved line, making two right angles with each other. The length of that of the transverse branch is equal to the distance between the centre of the two vents. The rear surface of the transverse branch is curved and quadrilateral. Its sides are inclined, so that their rear edges show the exact direction of the vents. Every point in the upper edge lies in the same horizontal plane. The height is sufficient to permit the edges to give an accurate direction to the drill.
The upper edge of the other branch runs off in a sloping curve to its extremity.
A centre line is drawn through the lower edge of the longitudinal branch, and is continued upwards on the rear surface of the transverse branch to the top.
The guide being placed with its centre upon the centre mark of the gun, and the centre line of the longitudinal branch being made to coincide with the centre line scribed upon the cylinder, the rear lower edge of the transverse branch will then coincide with the base-line, its extremities will indicate the centres of the vents, and the rear edges of the sides will show their true direction.
15th. Vent-gauges of untempered steel wire, with shoulders to prevent them from slipping into the vent. One should be of the proper diameter of the vent, one of the greatest, and one of the least diameter allowed.