ELECTRICITY APPLIED TO THE STUDY OF SEISMIC MOVEMENTS.
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.
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.
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.
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).
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 Rr, 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.
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.
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.
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.