ntervals; each
wire forms a part of the primary of an induction coil, and as each is
cut a dot is made on the rotating drum by the induced spark.

Deprez.

In the chronograph of Marcel Deprez, a cylinder for receiving records
is driven at a high velocity, 4 to 5 metres per second surface
velocity. The velocity is determined by means of an
electrically-driven tuning-fork, the traces being read by means of a
vernier gauge. A mercury speed indicator of the Ramsbottom type
enables the rotation to be continuously controlled (A. Favarger,
_L'Electricite et ses applications a la chronometrie_).

Dent.

_Astronomical Chronographs._--The astronomical chronograph is an
instrument whereby an observer is enabled to register the time of
transit of a star on a sheet of paper attached to a revolving
cylinder. A metal cylinder covered with a sheet of paper is rotated by
clockwork controlled by a conical pendulum, or by a centrifugal clock
governor such as is used for driving a telescope. By means of a screw
longer than the cylinder, mounted parallel with the axis of the
cylinder and rotated by the clockwork, a carriage is made to traverse
close to the paper. In some instruments this carriage is furnished
with a metal point, and in others with a stylographic ink pen. The
point or pen is made to touch the paper by an electromagnet, the
electric current of which is closed by the observer at the transit
instrument, and a mark is recorded on the revolving cylinder. The
movement of the same point or pen is also controlled by a standard
clock, so that at the end of each second a mark is made. The cylinder
makes one revolution per minute, and the minute is indicated by the
omission of the mark. In E.J. Dent's form (_Nature_, 23, p. 59)
continuous observations can be recorded for 6-2/3 hours. The conical
pendulum used to govern the rotation of the cylinder was the invention
of Sir G.B. Airy. The lower end is geared to a metal plate which
sweeps through an annular trough filled with glycerin and water. When
the path of the pendulum exceeds a certain diameter it causes the
plate to enter the liquid more deeply, its motion being thereby
checked; also, when the pendulum moves in a smaller circle the plate
is lifted out of the liquid and the resistance is diminished in the
same proportion as the force. The compensatory action is considerable;
doubling the drivin