AIE TO THE MOTION OF ELONGATED PROJECTILES. 
419 
Various attempts have been made to measure the velocities of cannon-balls by the aid 
of electricity. The machines with revolving cylinders were in general failures, because 
their inventors made their success depend upon the known uniform angular velocity of 
the cylinder. These failures opened the way for the introduction of Major Navez’ 
electro-ballistic pendulum, and others of the same class, which worked with two screens, 
and therefore furnished no means for testing the probable accuracy of the velocity 
determined. The apparent convenience and portability of these instruments led to 
their general use both in Europe and America. Major Navez’ instrument, in its 
original complicated form, is now out of fashion ; whilst Colonel Benton’s two-pendu- 
lum instrument and Colonel Leurs’ modification of it are in common use, as they are 
simpler than Major Navez’ instrument, and give results quite as much to be relied 
upon. Even if the electro-ballistic pendulum were perfect in itself as a measurer 
of time, considerable errors might be expected to arise from the imperfections of the 
indications of two screens placed at moderate distances apart. A reference to a paper 
by Major Navez, “ Considerations sur les experiences de balistique en ce qui concerne 
la mesure du temps”*, will show how little had been accomplished when that was 
written (1865). And Colonel Benet has well remarked: — “Electro-ballistic machines 
heretofore used have been powerless to solve one of the most important problems in 
ballistics — the law of the movement of a projectile through the air, — and this because 
of the limited number of points of the trajectory that could be determined ”f. From 
preliminary experiments already made, I feel certain that a simpler, cheaper, and better 
instrument might be substituted with advantage for electro-ballistic pendulums where 
such instruments can be used. The time occupied by a body in falling from rest 
through a given space, or the time occupied by the sound of the explosion in travelling 
over a given space, might be made the foundation of the measurement of a velocity ; 
or the velocity of the shot might be directly compared with the velocity of the sound 
of the explosion. 
In the spring of 1864, when I was appointed Professor of Applied Mathematics to 
the Advanced Class of Artillery Officers at Woolwich, and Keferee of the Ordnance 
Select Committee, I strongly recommended the construction of a chronograph capable 
of recording the time occupied by a projectile in passing over nine or more successive 
equal spaces. The principle of the chronograph used at the Greenwich Observatory was 
plainly the one to be adopted. The chief difficulties to be overcome were found (1) 
in the arrangement of a proper system of screens, so that the ball in passing might 
merely cause a momentary interruption (not a rupture) of the galvanic current, and that 
the resistance of the circuit might be kept perfectly constant during the experiment, (2) 
in the arrangement of a system of marking, which should give definite records on the 
surface of the cylinder when moving with a velocity of about 10 inches per second, and 
(3) in the compensation for the want of uniformity in the angular velocity of the cylinder. 
* Revue de Technologie Militaire, t. iv. 
t Electro-ballistic Machines, 1866, p. 39. 
3 M 2 
