3d. A standard cylinder gauge. This is a hollow cylinder of iron, turned to the least allowed diameter of the bore, and one calibre in length. It has a cross-head at each end, one of which has a smooth hole through its axis to fit the staff, and the other is tapped to receive the screw in the end of it.
D. Van Nostrand Publisher. Julius Bien, pr.
4th. A measuring-staff of steel or iron, in joints of suitable length, connected together by screws. Each joint is provided with a light brass disk, the diameter of which is 0.05 inches less than that of the bore. Through the centre of the disk there is a hole which fits upon a shoulder at the joint; the whole is so arranged that when the joints are screwed together the disks between them are held firmly in place, while the length of the staff is not affected by them. A steel point is screwed on to the end. When pushed to the bottom of the bore, the staff coincides very nearly with its axis. The outer joint is graduated to inches and tenths. A slide is made to play upon it with a vernier scale, graduated to hundredths of an inch. On the inner end of the slide a branch projects at a right angle, sufficiently long to reach across the muzzle face, and, when in contact with it, to indicate the precise length obtained from that point to the end of the measuring-point on the other end of the staff. A half disk of wood, made to fit the bore, with a groove for the staff to rest in, placed just inside of the muzzle, is useful in preventing any springing of the staff.
The point being taken off, the staff can be used with the cylinder gauge, to measure the distance to which the latter descends. But as the graduation is intended for the points, care must be taken in this case to allow for the difference.
5th. A chamber-gauge for verifying the shape and size of conical chambers.
The head should be made of close-grained, well-seasoned wood, and of the exact dimensions of the chamber. Two planes, crossing each other at a right angle, coinciding with the vertical and horizontal central sections, have been found better than a solid block. The edges should be bevelled. A metal socket in its centre connects it with the measuring-staff. Being pushed to the bottom of the bore, if the length coincides with that obtained by the point, it is obvious that the chamber is large enough, provided the cylindrical part has not been bored too deep, in which case a shoulder would be found at the junction. The edges of the gauge should be chalked before it is inserted. When withdrawn, if the chalk-marks are visible all around the chamber, it is evident the chamber is not too large. With slight modifications, this arrangement may be applied to the slope of cylindrical chambers, and to the curve at the bottom of the bore of any guns. Should the inspection of guns with conical chambers or slopes take place at the foundry, an examination of the chamber reamer will be very satisfactory. If found correct in size and shape, the impossibility of making the chamber too large will be apparent.
6th. A star-gauge, for measuring the diameters of the bores and of cylindrical chambers. This instrument is composed of the staff, the handle, and a set of points for each calibre.
The staff is a brass tube, made in three pieces, for convenience of stowage, and connected together, when required, by screws. Its inner end expands into a head, in which are placed four steel sockets, at equal distances from each other, which receive the points. Two of the sockets opposite to each other are secured permanently. The two others are movable. A tapering plate or wedge, the sides of which are cylindrical, runs through a slit in the head; an aperture in the inner ends of the movable socket embraces the cylinder, so that when the plate is moved forward or backward, the sockets are projected or withdrawn. The tapering of the plate has a certain known proportion to its length, so that if it is moved in either direction a given distance, a proportional movement is imparted to the sockets, and to the points which they contain. It is easy to see how, in this manner, a movement of .10 in. may increase or diminish the distance between the points .01 in. Therefore there would be no difficulty in estimating, to a considerable degree of accuracy, a difference of .001 in. between the points. In general, however, the distance on the plate required to move the points .01 in. is about .06 in. only.
A square sliding rod is connected with the tapering plate, and runs through the whole length of the tube, projecting some inches beyond the outer end. This rod has as many parts as there are joints in the staff, and, like them, connects by screws. Each section of the rod works in its proper joint, through a square socket at each end, and is prevented from falling out by pins. When screwing the joints together, if the ends of the rod are pressed up to each other they become connected by the same motion.