1896.] on Chronographs and their Application to Gun Ballistics. 177 



The first I will further subdivide into two parts : — 



(a) Instruments depending upon the action of gravity. 

 (h) Instruments liaving revolving drums 



The latter into — 



(c) Appliances for ascertaining external ballistics. 



(d) Appliances for ascertaining internal ballistics. 



The lecturer here described, with the aid of lantern slides, several 

 instruments which had been used for ballistic work, such as Navez- 

 Leur, Boulenge, &c. 



About the same time as the Boulenge was introduced, I designed the 

 instrument shown in Fig. 1. In this a weight drops freely in air, 

 and the registration does not, as in the Boulenge chronograph, 

 commence from the moment of its liberation, but during its fall, thus 

 avoiding any inaccuracy of residual magnetism in the electro-magnets, 

 from the fact that registration takes place during the fall. When 

 small portions of time have to be measured, the experiments may be 

 so arranged that the weight under the accelerating force of gravity 

 shall have acquired a considerable velocity before registration com- 

 mences. Also the time of passing several screens can be recorded. 



The instrument consists essentially of two upright brass cylinders 

 revolving on pivots, those at the bottom being fixed, while the two at 

 the top consist of screws to allow of the cylinders being removed. 

 The cylinders are carefully insulated from one another, and connected 

 with two binding screws on the base board. On the bed of the instru- 

 ment are two levels at right angles to one another, by which, with the 

 aid of three levelling screws, the cylinders may be placed truly 

 vertical. Close to but not quite touching the cylinders are scales 

 divided into thousandths of a second, which by means of a peculiar 

 vernier subdivide these into hundred-thousandths of a second. On the 

 top is an electro-magnet which serves to hold up the weight equi- 

 distant between the two cylinders. 



The weight has two sharp points which nearly, but not quite, touch 

 the surface of the cylinders. 



The action of the instrument is simply this. The weight being 

 released a short time before the gun is fired, descends between the 

 cylinders ; the shot on passing through the first screen breaks the 

 continuity of the primary wire of an induction coil, thus causing an 

 induced spark to pass from one cylinder to the other through the brass 

 wire of the weight. As the cylinders are smoked, a minute spot 

 registers the exact position of the weight at that moment. The 

 weight continuing to fall, as the shot passes the second screen (the 

 primary current in the meantime having been re-established) the same 

 result follows ; and so on for any number of screens. The distances 

 between the spots, as read off from the velocity scale, give the time 

 of the shot passing the various screens. 



By means of a calculating scale the velocity may be determined 

 for any distance between the screens. For a second experiment the 



Vol. XV. (No. 90.) n 



