n induction coil, so that when
the primary circuit is broken an induced spark removes a speck of
black from the paper and leaves a mark. The time period is deduced by
counting the number of vibrations and fractions of vibration of the
tuning-fork as recorded by a sinuous line on the cylinder. In later
forms of this instrument the cylinder advances as it rotates, and a
spiral line is traced. To obtain good results the spark must be very
small, for when large it often leaps laterally from the end of the
style, and does not give the true position of the style when the
circuit is broken. The same arrangement of tuning-fork and revolving
cylinder, with the addition of a standard clock, has been used by A.M.
Mayer (_Trans. Nat. Acad. Sci. U.S.A._ vol. iii.) and others for
calibrating tuning-forks, and comparing their vibrations directly with
the beats of the pendulum of a standard clock the rate of which is
known. The pendulum marks and breaks the primary circuit by carrying a
small platinum wire through a small mercury meniscus. Better and
apparently certain contacts can be obtained from platinum
contact-pieces, brought together above the pendulum by means of a
toothed wheel on the scape-wheel arbor. Sparking at the contact points
is greatly reduced by placing a couple of lead plates in dilute
sulphuric acid as a shunt across the battery circuit.

Fick.

_For Physiological Purposes._--A. Fick's pendulum myograph or
muscle-trace recorder is described in _Vierteljahrsschr. der
naturforsch. Ges. in Zuerich_, 1862, S. 307, and in _Text-book of
Physiology_, M. Foster, pp. 42, 45. It was used to obtain a record of
the contraction of a muscle when stimulated. In many respects the
instrument is similar to the electro-ballistic chronograph of Navez. A
long pendulum, consisting of a braced metal frame, carries at its
lower end a sheet of smoked glass. The pendulum swings about an axis
supported by a wall bracket. Previous to an experiment, the pendulum
is held on one side of its lowest position by a spring catch; when
this is depressed it is free to swing. At the end of its swing it
engages with another spring catch. In front of the moving glass plate
a tuning-fork is fixed, also a lever actuated by the muscle to be
electrically stimulated. When the pendulum swings through its arc, it
knocks over the contact key in the primary circuit of an induction
coil,