USE OF THE INSPECTING INSTRUMENTS.
29. The guns having been freed from rust, and their foundry numbers noted, in the order of their relative positions, on the field-book, the inspecting officer will proceed to verify the instruments to be used in their measurement, if this has not been previously done in a manner entirely satisfactory to him.
He will then examine carefully the guns, inside and out, for defects of metal or of manufacture, and note the results.
The interior of the bore is to be examined by reflecting the rays of the sun into it from a mirror or mirrors; or, if the sun is obscured, and there can be no delay, by means of a spirit-lamp, or of a wax taper, on the end of a rod, taking care not to smoke the surface of the bore.
The cylinder-gauge is then to be introduced, which must pass freely to the bottom of the cylindrical part of the bore. If obstructed, the depth to which it reaches should be noted.
The star-gauge is used to ascertain the exact diameter of the bore, and of the cylindrical part of the chamber. The bore must be measured at intervals of 1/4 inch from the bottom of the cylindrical part to the seat of the shot; of 1 inch from that point to the trunnions; and of 5 inches from the trunnions to the muzzle. If any marks of the reamer or other defects are seen in the bore, they are to be searched for, and their depths and positions noted. These results are to be tabulated according to the blank forms furnished. The whole length of the bore is ascertained by means of the measuring-staff, with the point screwed on, supported in the axis of the bore by the disks and half-tompion.
In the absence of this instrument, a pine rod, having the proper length of the bore marked on it, and the end rounded to the curve of the bottom of the bore, will answer as well, using a thread or a straight-edge across the face of the muzzle.
The shape and dimensions of the chamber, and the position of the interior orifice of the vent, are verified by means of the chamber-gauge, the description of which will explain its use. An inspection of the chamber-reamer will be generally satisfactory in determining the size and shape of the chamber.
The vent is measured by the appropriate gauges, the smaller of which must enter freely, and the larger not at all. It is searched for roughness, or for cavities in the metal around it, by means of the searcher, the point of which should feel every part of it carefully.
Its inclination to the surface, and its position externally, are verified by means of the vent-guide furnished for the Dahlgren guns, and by the semicircular protractor and the vent-gauge.