Boulenge chronograph ("Chronograph le Boulenge," par M.
Breger, Commission de Gavre, Sept. 1880) two screens are used. The
wire of the first forms part of the circuit of an electromagnet which,
so long as it is energized, supports a vertical rod called the
"chronometer." Hence when the circuit is broken by the passage of a
shot through the screen this rod drops. The wire of the second screen
conveys a current through another electromagnet which supports a much
shorter rod. This "registrar," as it is called, when released by the
shot severing the wire of the second screen, falls on a disk which
sets free a spring, and causes a horizontal knife to fly forward and
nick a zinc tube with which the chronometer rod is sheathed. Hence the
long rod will be falling for a certain time, while the shot is
travelling between the two screens, before the short rod is released;
and the longer the shot takes to travel this distance, the farther the
long rod falls, and the higher up on it will be the nick made by the
knife. A simple calculation connects the distance through which the
rod falls with the time occupied by the shot in travelling over the
distance between the screens, and thus its velocity ascertained. The
nick made by the knife, if released while the chronometer rod is still
suspended, is the zero point. If both rods are released
simultaneously, as is done by breaking both circuits at once by means
of a "disjunctor," a certain time is consumed by the short rod in
reaching the disk, setting free the spring and cutting a nick in the
zinc; and during this time the long rod is falling into a recess in
the stand deep enough to receive its full length. The instrument is so
adjusted that the nick thus made is 4.435 in. above the zero point,
corresponding to 0.15 sec. This is the disjunctor reading, and
requires to be frequently corrected during experiments. The instrument
was modified and improved by Colonel H.C. Holden, F.R.S. For further
information respecting formulae relating to it see _Text Book of
Gunnery_ (1857).

Watkin.

The electric chronograph of the late H.S.S. Watkin consists of two
long cylinders rotating on vertical axes, and between them a
cylindrical weight, having a pointed head, is free to fall. The weight
is furnished with an insulated wire which passes through it at right
angles to its longest axis. When the weight falls the ends of th