olarizing Nicol prism and analyser,
the light is shut off by rotating the analyser, it is instantly
restored when the carbon bisulphide is placed in a magnetic field.
This phenomenon is utilized in this instrument. The projectile, by
cutting the wire screens, causes the magnetic field to cease and light
to pass. By means of an automatic switch the projectile, after cutting
a screen, restores the electric circuit, so that successive records
are registered. After a record has been made it is read by means of a
micrometer microscope, the angle moved through by the photographic
disk is found, and hence the time period between two events. In the
photo-chronograph described in _Untersuchungen ueber die Vibration des
Gewehrlaufs_, by C. Cranz and K.R. Koch (Munich, 1899), also note on
the same, _Nature_, 61, p. 58, a sensitive plate moving in a straight
line receives the record of the movement of the barrels of firearms
when discharged. It was mainly used to determine the "angle or error
of departure" in ballistics.

Watkin.

In a second chronograph by Watkin ("Chronographs and their Application
to Gun Ballistics," _Proc. Roy. Inst._, 1896), a metal drum, divided
on its edge so that when a vernier is used a minute of angle may be
read, is rotated rapidly by a motor at a practically uniform speed.
The points of a row of steel-pointed pins, screwed into a frame of
ebonite, can be brought within 1/200 in. of the surface of the drum.
Each pin is a part of the secondary circuit of an induction coil, the
space between the pins and the drum forming spark-gaps. The drum is
rubbed over with a weak solution of paraffin wax in benzol, which
causes the markings produced by the sparks to be well defined. The
records are read by means of a fine hair stretched along the drum and
just clear of it, the dots being located under the hair by means of a
lens. The velocity of rotation is found by obtaining spark marks, due
to the primary circuits of two induction coils being successively
broken by a weight falling and breaking the two electric circuits of
the coils in succession at a known distance apart. This chronograph
has been used for finding the velocity of projectiles after leaving
the gun, and also for finding the rate at which a shot traverses the
bore. For the latter purpose the shot successively cuts insulated
wires fixed in plugs screwed into the gun at known i