Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/316

Rh 300 the combination of the two instruments, when used with a 38-ton gun, firing an 800-H) projectile, with 130 8&amp;gt; of 1 5-inch cubical powder. The last velocity is calculated from that taken by a Le Boulenge&quot; instrument outside the gun. This chronograph, the invention of an officer of the Belgian artillery, is shown in figs. 9, 10, and its mode of application in fig. 11. FIG. 9. Le Boulenge Chronograph, FIG. 10. Le Boulenge Clirono- side view. graph, front view. The shot, after leaving the gun, cuts the wire of the first screen, and subsequently the wire of the second screen. The wire oi the first screen conveys a current through an electromagnet A (figs. 9, 10), which then supports a cylindrical rod C. This rod is sheathed with a zinc tube D, which is renewed from time to time as required ; several short zinc tubes are slipped on, instead of one long one, if no great variation in the velocities is expected, so as to save material. Directly the first screen is cut, the rod C, Z&quot;* Screen. / Battery FIG. 11. Mode of using Le Boulenge s Chronograph, which is called the chronometer, drops. The wire of the second screen conveys a current through another electromagnet B, which supports a much shorter rod F, called the registrar. The shot, in cutting the second screen, releases this short rod, which falls on a disk 0, setting free a spring M. This spring carries a horizontal knife N, which flies forward and nicks the zinc tube covering the long rod. The nick made by the knife, if released while the long rod is still suspended, is the zero point. The currents from both screens pass through a disjunctor by which they can be cut simul taneously. When this is done the long and the short rods fall at the same moment. A certain time is consumed by the short rod in reaching the disk, setting free the spring, and cutting a nick on the zinc. During this time the long rod is falling into a recess in the stand, deep enough to receive its full length. The instrument is so ad justed that the nick thus made is 4 435 inches above the zero point, corresponding to 15 sec. This is the disjunctor reading, and requires to be frequently corrected during experiments unless the weather is exceptionally favourable to the electrical conditions. Instead now of using the disjunctor to cut both currents simultaneously, suppose the shot to cut them in succession ; then the long rod will be fall ing for a certain time, while the shot is travelling from one screen to the other, before the short rod is rehased. The longer the shot takes to travel this distance, the farther the long rod falls, and the higher up on it will be the nick made by the knife. A simple cal culation connects the distance fallen through by the rod with tie time occupied by the shot in travelling over the distance between the screens, and hence its velocity at the middle point is known. A graduated rule is generally supplied with each instrument, arranged for a certain distance between the screens ; from this any velocity obtained can be read off at once. The electric chronograph, invented by Captain Watkin, Watkii T5.A. is shown in figs. 12, 13, 14. clu-ono c graph. FIG. 12. Watkin Chronograph. Two brass cylinders A, A revolve on pivots B, B, C, C. Between them, at the top, hangs a brass weight (fig. 13), suspended freely by means of a piece of stet-1 B, which pivots in another piece A let into the base of the weight. Two arms E enter, on falling, two V springs on the bed of the instrument, which catch the weight, A finely-pointed brass wire, CD, enclosed through part of its length in an ebonite tube, passes through the upper end of the weight from side to side ; the piece B of tin- weight is placed between the ends A, C of the lever arms of the holder (fig. 14). The arm CD pivots on the pin F; AB is fixed. K, K is an electromagnet; on its keeper H is fixed a strong spring G, G. When this spring is placed between the ends B, D of the arms, it forces the opposite end C to