VI. VELOCITY. 101 



tion coil, the circuit is complete with the exception of the small spaces on 

 either side of the weight. When taking velocities of shot, the primary 

 circuit is led through screens, constructed so that the current is broken and 

 immediately made again during the passage of the shot. The gun being 

 fired, the weight begins to descend ; the shot in passing the first screen causes 

 a spark to flash from one cylinder to the other through the weight which 

 having been previously smoked registers by a white spot the position of the 

 weight at that instant. As the weight continues to descend the same result 

 is obtained at the next screen, and so on. Adjacent to the cylinder is a time 

 scale divided into thousandths of a second, subdivided by a vernier of novel 

 construction into hundred thousandths of a second, by which the absolute 

 time taken by the shot between the screens is easily read off. For other uses 

 to which the instrument may be applied, see Royal Artillery Institution Papers. 



407. Clock-Chronograph, contrived for the purpose of 

 measuring the time occupied by projectiles in passing over a 

 succession of equal spaces, with a view to determine accurately 

 the resistance of the air to their motion. Rev. F. Bashforth. 



If the fly-wheel be spun by hand, and the markers be brought down, 

 they will trace two uniform spirals on the cylinder ; each marker is, however, 

 under the control of an electro-magnet. When the galvanic current is inter- 

 rupted, a record is made by the corresponding marker being suddenly drawn 

 aside. The circuit of the lower electro-magnet is interrupted once a second 

 by a clock beating half-seconds, which gives a scale of time. The circuit of 

 the upper electro-magnet passes along the tops of all the screens, as is 

 shown in the case of one screen. When one or more threads are broken 

 in any screen, a record is made on the cylinder. Thus, when an experi- 

 ment is to be made, the fly-wheel is spun briskly by hand, the markers 

 are brought down, and the gun is fired. The times of passing the screens are 

 recorded on one spiral, opposite a scale of time on the other. This instru- 

 ment was used in making all the experiments referred to in " Reports on 

 " Experiments made with the Bashforth Chronograph to determine the 

 " Resistance of the Air to the Motion of Projectiles, 1865-1870," published 

 by authority. Generally 10 screens were placed at intervals of 150 feet, but 

 in the experiments with the WhitAvorth gun (p. 162), 16 screens were placed 

 at intervals of 75 feet ; some of these records are shown. For a full descrip- 

 tion of the chronograph, see Proceedings of the Royal Artillery Institution, 

 Woolwich, for 1866, which description is also published separately. 



407a. Chronograph for projectile experiments with the 

 recording apparatus of Deprez. Diimoulin fromcnt, Paris. 



407b. Electric Chronograph. Dr. Werner Siemens. 



This instrument, which was described in the year 1845 in Pogg. Ann. 

 (Bd. 66, p. 435), serves for the measurement of high velocities, especially 

 those of projectiles both along the barrel and in their further flight, and also 

 that of electricity. 



It is based on the circumstance that an electric spark leaves a sharp mark 

 on polished steel, and that this mark can easily be discovered when the cylinder 

 has been previously blackened. The cylinder is turned rapidly by clock- 

 work, and each hundredth revolution is marked by the stroke of a small bell. 

 By means of a regulator the rapidity of rotation is so arranged that the 

 stroke of the bell coincides exactly with the beat of a second pendulum ; the 

 reading is made with a microscope with cross wires, the clockwork being 

 stopped. The graduation of the micrometer head gives 0-0001 of a revolu- 

 tion of the cylinder or millionths of a second if the cylinder rotates 100 times 

 a second. 



