Italy, with her volcanic nature, has very naturally made a specialty of
movements of the ground, or seismic perturbations. So the larger part of
the apparatus designed for such study are due to Italians. Several of
these instruments have already been, described in this journal, and on the
present occasion we shall make known a few others that will serve to give
an idea of the methods employed.
For the observation of the vertical and horizontal motions of the ground,
different apparatus are required. The following is a description of those
constructed for each of such purposes by the Brassart Brothers.
[Illustration: FIG. 1.--APPARATUS FOR THE STUDY OF HORIZONTAL SEISMIC
MOVEMENTS.]
_Apparatus for Studying Horizontal Movements._--A lever, (Fig. 1), movable
about a horizontal axis, carries a corrugated funnel, _i_, at one of its
extremities. At the other extremity it is provided with a counterpoise
which permits of its being exactly balanced, while not interfering with
its sensitiveness.
[Illustration: FIGS. 2 AND 3.--DETAILS OF THE APPARATUS.]
The opening of the funnel passes freely around a column, _v_ (Fig. 2),
upon which is placed in equilibrium a rod that terminates in a weight, P.
The corrugations of the funnel carry letters indicating the four cardinal
points, and the funnel itself is capable of revolving in such a way that
the marked indications shall always correspond to the real position of the
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
case, in order that the contact may be continuous and that the bell may be
rung, the piece, A, upon falling, sets up a permanent contact with the
part, _a_ (Fig. 3).
[Illustration: FIG. 5.--BRASSART'S SEISMIC CLOCK.]
_Brassart's Seismic Clock._--This apparatus is designed for being put in
connection at a distance with an indicator like the ones just described.
It is a simple clock to which a few special devices have been added.
Seismic clocks may be classed in two categories, according as they are
stopped by the effect of a shock or are set running at the very instant
one occurs. The Messrs. Brassart have always given preference to those of
the second category, because there is no need of watching them during a
seismic calm, and because they are much more easily constructed. It is to
this class, then, that their seismic clock belongs. It is capable of being
used for domestic purposes in place of any other clock, and of becoming a
seismoscopic clock as soon as it is put in electric communication with the
seismic telltales.
To the cross-piece that holds the axle of the drums the inventors have
added (Fig. 5) a support formed of a strip of brass, S, with whose
extremity is jointed (at the lower part) a double lever, A. This latter is
held in a horizontal position by a small counterpoise, _i_, so that the
finger at the opposite extremity shall prevent the pendulum, P, from
swinging. To keep the latter in a position of rest a bent lever, _n n'_,
is jointed to the upper part of the support, S. The longer arm, _n'_, of
this lever is bent forward at right angles, so that it may come into
contact with and repel the small rod of the pendulum as soon as the lever
has been lifted by means of a small cord which is connected with the
larger arm, _n_, and runs up to a small hook, from whence it descends and
makes its exit under the clock-case.
In order to stop the clock, then, it is only necessary to pull on this
cord slightly, when, by moving the pendulum to the left, it will thrust
it against the inclined plane of the finger of the lever arm, A. It is
clear that the extremity of the pendulum, upon striking against the
finger, will depress it slightly and go beyond the projection against
which it remains fixed owing to the counterpoise, _i_. The fever, _n n'_,
is brought back to its position of rest by means of a small counterpoise
at the extremity of the arm, _n_. When the lever, A, is depressed, the
pendulum escapes and sets the clock running. This depression is effected
by means of an electro-magnet, E, whose armature, which is connected with
the rod, _t, t_, lifts the arm, _i_, of the lever, and depresses A. The
wires of the two bobbins of the electro-magnet end in two clamps, 1 and 2.
The second of these latter is insulated from the clock-case. Both
communicate with the extremities of the circuit in which is interposed the
seismic telltale that brings about a closing of the current. Having noted
the position of the hands on the dial when the clock was running, one can
deduce therefrom the moment at which the shock occurred that set the clock
in motion.
In addition to the parts that we have described, there are other accessory
ones, R R_r_, and a third clamp, 3, which constitute a sort of rheotome
that is designed to keep the circuit closed after the momentary closing
that is produced by the telltale has occurred. This little mechanism is
indispensable when the disturbed telltale has also to act upon an electric
bell. This rheotome, which is very simple, is constructed as follows: A
small brass rod, R, which is screwed to the support, S, carries at its
left extremity a brass axis, X, which is insulated from the rod, R, by
means of an ivory piece. Toward the center of this small rod, the bent
lever, _r_, carries a small arm that is bent forward, and against which
abuts the axis of the pendulum, thus causing it to be thrust toward the
left when the pendulum is arrested by the projection of the finger, A. As
soon as the pendulum is set free, the lever, _r_, redescends and places
itself against the axis, X. This latter communicates with clamp 3, which
is insulated, while the rod, R, communicates with clamp 1. The external
communications are so arranged that the circuit in which the bell is
interposed remains definitely closed when the lever, _r_, is in contact
with the rod, X.
[Illustration: FIG. 6.--ROSSI'S TREMITOSCOPE.]
_Rossi's Tremitoscope._--This instrument (Fig. 6) unites, upon the same
stone base, three different arrangements for showing evidences of
trepidations of the earth. On one side we find (protected by a glass tube)
a weight suspended over a mercury cup by a spring, and designed to show
vertical motions. The two other parts of the apparatus are designed for
registering horizontal motions. The first is a pendulum which causes a
contact with four distinct springs, and whose movements are watched with a
spy-glass. The second is a steel spring which carries at its upper part a
heavy ball that vibrates at the least shock. This ball is provided with a
point which is movable within a second ball, so that its motion produces
a contact. All these different contacts are signaled or registered
electrically.
[Illustration: FIG. 7.--SCATENI'S SEISMOGRAPH.]
_Scateni's Registering Seismograph._--This apparatus, which is shown in
Figs. 7 and 8, consists of two parts--of a transmitter and of a
registering device.
[Illustration: FIG. 8.--REGISTERING APPARATUS.]
The transmitter consists of a glass vessel supported upon a steel point
and provided beneath with a platinum circle connected with a pile. All
around this circle are four strips of platinum, against one of which abuts
the circle at every movement of the glass. Each strip of platinum
communicates, through a special wire, with one of the electro-magnets of
the registering device (Fig. 8). This latter consists of an ordinary clock
that carries three concentric dials--one for minutes, one for hours, and
one for seconds. In a direction with the radii of these dials there are
four superposed levers, each of which is actuated by one of the electros.
On another hand, each dial is divided into four zones that correspond to
the four cardinal points. When a shock coming from the north, for example,
produces a contact, the corresponding electro is affected, and its lever
falls and marks upon each of the dials a point in its north zone. We thus
obtain the exact hour of the shock, as well as its direction. As may be
seen, the apparatus, as regards principle, is one of the simplest of its
kind.--_La Lumiere Electrique._
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