GUNNERY 299 but, having the same character as other velocity instruments, is described here. The screens used for recording the flight of the shot through the air are now discarded, and the FIG. 5. Cutting plug, knife down. FIG. 6. Cutting plug, knife up. "cutting plugs," shown in figs. 5, 6, 7, are substituted for them. In figs. 5 and 6 B is the bore, and A a flat-headed shot or cylinder passing along it in the direction of the arrow. The plugs are screwed into the sides of the gun, as in fig. 7, which gives an end view of the bore. As Jutting the shot passes, its plugs, foremost edge pushes up the knife D, cut ting the wire e, which passes in at one side of the plug C, then through a hole in the knife D, and out again at the other side of the plug. The ends of this wire are con nected with the main wires leading to the instrument, and the FlG - 7. Cutting plug, ready for firing, knife D, in cutting it, interrupts the current. Fig. 4 shows a 10-inch gun pierced to receive cutting plugs in eighteen places. The main wires run from a plug to the primary of a Knhmkorff coil, fig. 2, near the instrument, with which the secondary communicates. Figs. 2 and 3 show the instrument in plan and elevation. A, A are disks, 36 inches in circumference, fixed on a horizontal shaft S. This shaft is caused to rotate, so that the edge of the disks travel at about 1100 inches per second, by a series of toothed wheels F, F, gearing as shown, and worked into rapid motion by the handle C. This motion is kept up with regularity by the descent of the weight B acting on the hindmost shaft, the speed of which is multi plied 200 times at the shaft S. At V is a vernier, reading a gradu ated circle, which is fixed on the shaft S. The edges of the disks are covered with a coating of lampblack. One of the secondary wires G is brought from the coil, and placed in communication with the revolving disk ; the other secondary wire H is fixed in a discharger Y, just clear of the edge of the disk. A stop-clock is placed on a stand D, and records the rate of revolution of one of the shafts. When the gun is fired the shot begins to move, and, as it travels along the bore, cuts the wires of the plugs in succession, breaking the primary currents, and causing sparks to pass in the secondary current from the dischargers Y, Y, to the disks A, A, which are in rapid revolution. The sparks burn off the lampblack and record their positions on the disks by a minute spot. Now if the shot could cut all the plug-wires simultaneously, all the spots on the disks would be in a straight line, but it does not do so, and during the intervals between the cutting of successive plug-wires the disks continue to rotate, so that the line of spots forms a spiral round the cylinder represented by the disks, as is apparent on bring ing the machine to rest. The shaft S is then turned so as to place each spot in succession opposite an index, and the spaces travelled by the circumferences of the disks between the spots are read off on the graduated circle. As the circumference of the disks is made to travel at the rate of 1100 inches per second, and the vernier reads to thousandths of an inch, the machine indicates portions of time rather less than a millionth of a second ; it is, in fact, practically correct to hundred-thousandths of a second. A millionth is about the same fraction of a second as a second is of a fortnight. Thus the distance between two successi /e spots gives the actual time oc cupied by the shot in moving over the portion of the bore between two corresponding successive plugs, and therefore the velocity acquired at some point of this tra pel. As the increment of velocity is constantly decreasing after the charge is fairly lighted, the point at which the measured velocity is acquired is very slightly behind the middle of the space between the two plugs. The observations are worked out, accidental errors eliminated, and the results put in the form shown in column 3 of Table I. The " crusher gauge " (fig. 8) is so closely associated in Crush* its use with the Noble chronoscope that it is convenient gauge, to give a description of it here, though its indications are FIG. 8. Crusher gauge. valuable to the gunmaker rather than to the gunner. In fig. 4 are shown three plugs, A, B, and C, driven through the walls of the gun, and terminating flush with the bore. These plugs contain gauges for measuring directly the pressure of the gas on the surface of the bore. They are of the same diameter as the cutting plugs, and can be used in any of the twenty-one holes pierced in the gun. Fig. 8 shows the construction. The bottom of the plug is chambered out, and a steel anvil E is placed at the top of the chamber. Against this anvil is pressed a cylinder of pure copper A, carefully tested for quality, which is held in its place by a spring B. A piston C fits accurately the lower part of the TABLE I. chamber, any ingress of gas being prevented by the gas-check D, which is flush with the bore. When the gun is fired the pow der-gas presses the piston hard against the copper cylinder A, which yields and becomes shorter, ex panding laterally. The plug is then removed from the gun, and the nozzle unscrewed; the cylinder is extracted and measured. Its compression is com pared with those given in a table constructed from experiment, and the pressure of the gas per square inch re corded. With the chro noscope the velocities are obtained directly from the time shown by the instrument to have been taken by the shot in passing over the interval be tween two cutter plugs. From these velocities calculation deduces the pressures required to produce the increments of velocity. These calculated pressures are compared with and confirmed by those ob tained directly from the indications of the crusher gauge. Table I. shows the nature of the information obtained from Distance of Plug from bottom of Bore. Mean of 4 Rounds. 1 00
Velocity. Difference in Velocity. tons per sq. in. f.s. f.s. Axis 23-8 1 ft. 23-8 2 22-5 137 3 19-9 533 C70 4 ,, 14-2 244 914 5 11-6 127 1041 6 ,, 7-0 69 Press 1110 and 7 3-8 55 cities 1165 side 8 ,, 2-9 47 1212 9 ,, 40 1252 10 34 1286 11 29 ; 1335 12 1-9 25 1340 13 ,, 1-6 21 1361 14 1-3 18 1379 15 ,, 1-1 16 1395 16 ,, 0-6 14
! 1409
