920 APPENI>iX. 



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, and the tenth part or ten millionth of a second can be estimated. 



The measurement of the velocity of projectiles is effected by the passage of 

 an electric spark at the moment when the projectile touches an insulated wire 

 which reaches to the inside of the gun, and thus is free from retardation 

 caused by the inertia of matter or magnetism. 



The same apparatus can be used for the measurement of the velocity of 

 electricity in suspended wires. 



The complete apparatus comprises a Leyden jar, induction coil, commutator, 

 gun barrel, chronograph, and two batteries. 



407c. Recording Cylinder, with original marks by which 

 the speed of electricity in iron wires has been measured. 



Dr. Werner Siemens. 



Those marks which are surrounded by a halo or circle indicate the com- 

 mencement of the discharge, the successive series of small marks has been 

 formed by the electricity which has traversed the conductor ; the angular 

 distance between the first-mentioned mark and the first point of the series 

 gives the measurement of the speed of electricity. By measurement of the 

 time which the electricity requires to pass through lines of various lengths, 

 the electrostatic retardation, which is proportional to the square of the length 

 of the line, has been eliminated. By these researches it has been shown 

 that electricity is transmitted in conductors with a constant velocity which is 

 independent of the static retardation, and which for iron amounts to about 

 230,000 kilometres per second. 



(See Monatsberichte der Kgl. Pr. Acad. der Wissenschaften, 6 Dec. 

 875.) 



409a. Current Meter. 



Benjamin Theophilus Moore, M.A. 



This instrument will measure the velocity of running water at any depth 

 below the surface, with accuracy and facility. 



The frame is formed of thin brass bars united in front to a solid ogival 

 head, and terminating, in the opposite direction, in a double vane or tail. It 

 is suspended in the water by a stirrup. 



The frame supports a hollow cylinder which is provided with six screw 

 blades, and is free to revolve upon fine pivots at its extremities. This cylinder 

 contains a water-tight glass tube within which is a simple train of mechanism 

 to record the number of revolutions made by the cylinder. This train of 

 mechanism is suspended in such a manner, that it remains at rest while the 

 cylinder revolves, and the dials which record the number of revolutions are 

 seen through the glass without opening the cylinder. When the instrument 

 is suspended in running water by a cord attached to the stirrup, the frame 

 immediately takes up a position in which the axis of the revolving cylinder is 

 parallel to the direction of the stream, the cylinder is set in motion, and 

 stopped, at known instants, while under water, by means of a spring, operated 

 upon by a light cord. 



