ntacts
attached to diaphragms on which the sound wave acted. The contacts
consisted of minute hammers resting on metal points fixed to the
centre of diaphragms which closed the end of the experimental pipes.
The signal marked the instant at which a sound wave impinged on a
diaphragm. The markings on the paper band gave the period of time
between two events, and the number of vibrations of the tuning-fork
per second was estimated by means of markings due to the clock. The
sound wave was usually originated by firing a pistol into the pipe
furnished with diaphragms and contact pieces.

Ayrton and Perry.

In the chronographic use of the Morse telegraph instrument (Stewart
and Gee, _Elementary Practical Phys._ p. 234) a circuit is arranged
which includes a seconds' pendulum furnished with a fine platinum wire
below the bob, which sweeps through a small mass of mercury forming a
part of the circuit. There is a Morse key for closing the circuit. A
fast-running Morse instrument and a battery are placed across this
circuit as a shunt. A succession of dots is made on the paper ribbon
by the circuit being closed by the pendulum, and the space between
each adjacent dot indicates a period of one second's duration. Also,
when the key is depressed, a mark is made on the paper. To measure a
period of time, the key is depressed at the beginning and end of the
period, causing two dots to be made on the ribbon; the interval
between these, when measured by the intervals due to the pendulum,
gives the length of the period in seconds, and also in fractions of a
second, when the seconds' interval is subdivided into convenient equal
parts. This apparatus has been used in determination of the velocity
of sound. In the break circuit arrangement of pendulum key and Morse
instrument the markings appear as breaks in a line which would
otherwise be continuous. This combination was employed by Professors
W.E. Ayrton and J. Perry in their determination of the acceleration of
gravity at Tokio, 1877-1878 (_Proc. Phys. Soc. Lond._ 3, p. 268).

Hipp.

Mayer.

In the tuning-fork electro-chronograph attributed to Hipp a metal
cylinder covered with smoked glazed paper is rotated uniformly by
clockwork, a tuning-fork armed with a metallic style being so adjusted
that it makes a clear fine line on the smoked paper. The tuning-fork
is placed in the secondary circuit of a