In guns of the ordinary construction, the position of the vent is marked on the profile-board, and its inclination to the surface is determined by the protractor and vent-gauge.
The exterior lengths of the gun are measured by the profile-board, marked with the true dimensions, the differences being measured by the foot-rule, or, if minute, estimated by the eye.
The exterior diameters are measured with the calipers and square, or by the set-gauges used in turning, and a graduated wedge.
To verify the position and alignment of the trunnions of a gun, it is first necessary to ascertain, by means of the trunnion-gauge and of the calipers, their cylindrical form and their diameters, which should be the same, or allowance must be made for half the difference in measuring their axial distances from the base-line, by the trunnion-rule, which should next be done. These distances should be equal, or their axes do not coincide—an error not tolerated.
The trunnion-square is then placed upon the trunnions in the plane of their axis. The feet of its branches should coincide with the surfaces of both trunnions, throughout their length, above and in rear, and their inner edges with the faces of the rimbases. Then, with the beam-compass, scribe on the upper surface of the gun the distance of the axis of the trunnions from the base-line, and push the sliding-point of the square down, till, at that distance, it touches the surface of the gun, and screw it fast. Then turn the gun over, and again scribe on it the same distance from the base-line. The square, being again applied, will determine whether the trunnions are above or below the axis of the bore, which will coincide with that of the gun, if accurately bored, and turned on the same centres and bearings. If the branches rest upon the trunnions before the point of the slider touches the gun at the scribe, their axis is below; but if the point touch first, above the axis of the bore, by half the space between. The graduated wedge, being placed under the vertical sliding-point, will determine the amount. If both touch at once, both axes are in the same plane.
No gun can be received, the axis of the trunnions of which is above that of the bore.
The lengths of the trunnions are measured with the foot-rule, and the diameters of the rimbases by that of the exterior rim of the trunnion-gauge.
If the alignment of the trunnions be correct, it will serve as a means of determining the correctness of the line of sight, which, before the gun is removed from the lathe, should be distinctly traced on the sight-masses and the swell of the muzzle, and should be at right angles to the base-line, to the axes of the trunnions, and to the connecting piece of the trunnion-square, when its branches rest against their rear, with the plates across their upper surfaces.
The Inspector will further satisfy himself of the correct tracing of the line of sight on the gun, by examining the lathe and the manner of tracing it in the plane of the axis of the bore, at right angles to the axis of the trunnions, as by it are placed the sights and vent, and in their absence it serves as a line of metal sight.
The positions of the sight-masses are verified by the profile-board, and by reference to the line of sight, traced on them; their form and dimensions by the templates.