It is obvious that the determinations will not be absolutely accurate, for when the gun is worn, should the stationary points be perpendicular, the movable points, being then horizontal, would fall below the true horizontal diameter, and the measurements would be more in error than it would be with the points in any other direction. Still, if care is taken to preserve the points at the greatest length possible, a very tolerable degree of accuracy may be attained. In the inspection of guns arranged on skids, the gun itself should be turned, which will insure accurate measurements. Care must also be taken not to allow the joints of the staff to become so loose that the coincidence of the centre line is destroyed when they are screwed together. If this should occur, however, a few turns of thread, placed between them at the time of putting the instrument together, would remedy the difficulty.
7th. An instrument for verifying the interior position of vents.
When the vent is drilled in the vertical plane of the axis, as in the guns of old patterns, a simple head, shaped to fit the bottom of the bore, or the chamber, with a staff fitted to it, is sufficient. But for the Dahlgren guns, with two vents, some other plan is better. The following has been found satisfactory:
A head of well-seasoned wood, which is fitted to the chamber, is attached to a wooden disk of the diameter of the main bore. The surface of the head corresponds with a longitudinal central section of the chamber; at the point where the projection of the vent would meet it a piece of hard wood is inserted. A central line drawn through its length, crossed at a right angle by another line at any known point from the smaller end, will afford convenient points to measure from. A stout square wooden staff is attached to the axis of the head; at a distance equal to the length of the bore, the end is jogged into the centre of a half disk of wood, which is fitted to the bore. The whole is so constructed that the straight edge of the half disk (or the chord) is in the same plane as a horizontal section of the head. A few holes are bored through the disk attached to the half head, to allow the instrument to pass freely into the gun and out of it.
A wire of untempered steel, of the size of the vent, with a sharp, well-centred point, and a small spirit-level, are required to use with this instrument.
The gun being levelled, and the instrument being pushed to the bottom of the bore, the upper edge of the half disk near the outer end of the staff is then brought to a level. The surface of the half head then corresponds with the horizontal central section of the chamber. The point of the wire being pushed gently to meet it, will show very accurately the interior position of the vent.
8th. Profile-boards for distances in front and rear of the base line.
Their lower edges are adapted to the shape of the gun, and the upper ones are parallel to the axis of the bore.
The distances from the base-line of the several parts, and of the points at which diameters are to be measured, are laid off accurately on the upper edge, and then marked in lines perpendicular to it on the sides and lower edges of the profile. An iron strip is attached to the upper edge to prevent warping, and the whole is well coated with shellac varnish, to keep it from absorbing moisture.
The following instruments are used in connection with the profile-boards: