cardinal points. When a horizontal shock occurs, the weight, P, falls in a
direction opposite thereto, and into one of the corrugations, where it
rests, so that the direction of the shock is indicated. But, in falling,
it causes the lever, F, to tilt, and this brings about an electric contact
between the screw, _h_, and the column, _n_, which sends a current into
the electro, E, so that the armature of the latter is attracted. In its
position of rest this armature holds a series of parts, S, A, L, which
have the effect of stopping the pendulum of a clock placed upon the same
apparatus. At the moment, then, that the armature is attracted the
pendulum is set free and the clockwork is started. As the current, at the
same time, sets a bell ringing, the observer comes and arranges the
apparatus again to await a new shock. Knowing the hour at which the hand
of the clock was stopped, he sees how long it has been in motion again and
deduces therefrom the precise moment of the shock.

The small rod, _f_, which is seen at the extremity of F, is for the
purpose of allowing electricity to be dispensed with, if need be. In this
case the screw, _h_, is so regulated that F descends farther, and that _f_
may depress the armature of the magnet just as the current would have
done.

[Illustration: FIG. 4.--APPARATUS FOR THE STUDY OF VERTICAL MOVEMENTS.]

_Apparatus for the Study of Vertical Movements._--In this apparatus (Fig.
4), the contact is formed between a mercury cup, T, and a weight, D. The
cup is capable of being raised and lowered by means of a screw, so that
the two parts approach each other very closely without touching. At the
moment of a vertical shock a contact occurs between the mercury and
weight, and there results a current which, acting upon the electro, E,
frees the pendulum of the clock as in the preceding apparatus. In this