82 SEC. 3. MEASUREMENT. 



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 Whitworth 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. 



4O7a. Chronograph for projectile experiments with the 

 recording apparatus of Deprez. Dumoulin Froment, Paris. 



411. Complete Apparatus for measuring the Velocity 

 of Projectiles in the bore of a gun, and for measuring the speed 

 of electricity. Siemens and Halshe. 



412. Vibration Chronograph, for measuring the time of 

 descent on an inclined plane, executed according to Beetz, by 

 M. Th. Edelmann, at Munich. (A description accompanies the 

 object.) I* ro f> Dr. Beetz, Munich. 



413. Edelmann's Apparatus for the descent of a fall- 

 ing Body accessory to Beetz's chronograph. 



M. Th. Edelmann, Munich. 



D. STROPIIOMETERS. 



395. Hearson's Patent Strophometer or Revolution 

 Indicator, an instrument for showing at a glance, by the position 

 of a pointer on a graduated dial, the number of revolutions per 

 minute an engine is at the time making. Elliott Brothers. 



415. Mercurial Gyrometer, or " orbit meter." 



Royal Polytechnic Academy (Prof. Reuleaux, Director}, 

 Berlin. 



The instrument indicates directly the angular velocity of an axle, shaft, 

 &c., in figures showing the rotations per minute. The reading takes place 

 on an alcohol column, w r hich shows on one side a millimeter scale, and on 

 the other the rotation numbers. The instrument is so arranged that the 

 scale of the rotations has uniform graduation. 



