counted out on a sloping glass desk, and the time
of flight of a projectile between two or more screens is found. When
very close readings are required, they are made by means of a
traversing geometric micrometer microscope. When the distance between
the screens is known, and also the time of flight, the midpoint
velocity is found by applying Bashforth's formula. When the velocity
of shot from a shot-gun has to be found, a thin wire stretched across
the muzzle takes the place of the first screen, and a thin sheet of
metal or cardboard carrying an electric contact, or a Branly coherer,
the conductivity of which is restored by means of an induced current,
takes the place of the second screen. The electric firing circuit is
provided with a safety key attached by a cord to the man who loads the
gun and prepares the electric fuse. The firing circuit is closed by
inserting the key in a switch at the rear of the gun, thus preventing
him from getting into the line of fire when the gun is fired by the
chronograph. The tram, when the instrument is adjusted, has a
practically constant velocity of traverse.

Crehore-Squier.

The polarizing photo-chronograph, designed and used by A.C. Crehore
and G.O. Squier at the United States Artillery School (_Trans. Amer.
Inst. Elect. Eng._ vol. 14, and _Journal United States Artillery_,
1895, 6, p. 271), depends for its indications upon the rotation of a
beam of light by a magnetic field, produced by a solenoidal current
which is opened and closed by the passage of the projectile. The
general arrangement is as follows:--A beam of light from an electric
lamp traverses a lens, then a Nicol prism, next a glass cylinder
furnished with plane glass ends and coiled with insulated wire, then
an analyser and two lenses, finally impinging on a photographic plate
to which rotation is given by an electric motor, the plane of rotation
being perpendicular to the direction of the beam of light. The same
plate also records the shadow of a pierced projection attached to a
tuning-fork, light from the electric lamp being diverted by a mirror
for this purpose. The solenoid used to produce a magnetic field across
the glass cylinder, which is filled with carbon bisulphide, is in
circuit with a dynamo, resistances, and the screens on the gun range.
It is a well-known phenomenon in physics that when, with the
above-mentioned combination of p