e
insulated wire move very close to the surfaces of the cylinders which
form part of a secondary circuit of an induction coil, the primary
circuit of which is opened when a screen is ruptured by a shot. A
minute mark is made by the induced spark on the smoked paper with
which the cylinders are covered. The time period between events is
deduced from the space fallen through by the weight, and by means of a
scale, graduated for a given distance between the screens, the
velocity of a shot is at once found. It may be noted that the method
of release is such that the falling weight is not subjected, after it
has begun to fall, to a diminishing magnetic field, which would be the
case if it were directly supported by an electromagnet. An iron rod
when falling from an electromagnet, during a minute portion of its
fall, is subject to a diminishing force acting in the opposite sense
to that of gravity, whereby its time of fall is slightly changed.

Colonel Sebert (_Extraits du memorial de l'artillerie de la marine_)
devised a chronograph to indicate graphically the motion of recoil of
a cannon when fired. A pillar fixed to the ground at the side of the
gun-carriage supported a tuning-fork, the vibration of which was
maintained electrically. The fork was provided with a tracing point
attached to one of the prongs, and so adjusted that it drew its path
on a polished sheet of smoke-blackened metal attached to the
gun-carriage, which traversed past the tracing point when the gun ran
back. The fork used made 500 complete vibrations per second. A central
line was drawn through the curved path of the tracing point, and every
entire vibration cut the straight line twice, the interval between
each intersection equalling 1/1000 second. The diagram so produced
gave ihe total time of the accelerated motion of recoil of the gun,
the maximum velocity of recoil, and the rate of acceleration of recoil
from the beginning to the end of the motion. By means of an instrument
furnished with a microscope and micrometers, the length and amplitude,
and the angle at which the curved line cut the central line, were
measured. At each intersection (according to the inventor) the
velocity could be deduced. The motion at any intersection being
compounded of the greatest velocity of the fork, while passing through
the midpoint of the vibration and the velocity of recoil, the tangent
made b