Art. LI.—Description of a Compound Seismograph.
[Read before the Philosophical Institute of Canterbury, 1st November, 1893.]
The object of this seismograph is to register an exact record of earthquakes, at whatever time and at whatever angle they may come, both horizontally and vertically, as well as the number of waves, their magnitude, direction, and the exact time of the commencement and termination of the disturbance, in hours, minutes, seconds, and fractions of seconds. To gain these results I have united a number of the standard instruments of the present day into one compact machine, all working together, and recording on smoked-glass plates, all driven by the same agency—clockwork.
I will begin by describing the pendulum seismograph which is to be used for marking the hour, &c., when the disturbance takes place, and which gives a condensed record on an almost stationary plate.
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The frame of this machine is about 3ft. 8in. high by about 16in. wide, with a foot of about 2ft. 10in. long. From the top of this frame descends the pendulum for 2ft. 6in., terminating in a lead ring 6 ¾in. in diameter, and 1 3/16in. in thickness and depth. This will weigh about 101b. On the top of this ring is a glass plate, upon which works a screw, carried by an arm from the framework, to give the friction required to stop the pendulum from swinging owing to inertia. Through the centre of the ring is a metal rod with a conical hole in its centre, in which works the top end of the indicator. The indicator is a piece of steel wire working on a bearing at 2in. from its top, and 6in. down it terminates in a slide to carry a needle which rests on a smoked-glass plate, and gives record magnified three times.
The bearing referred to above is composed of two knife-edges placed in a piece of wood, which crosses the frame about 2in. below the ring of the pendulum, on which rests a ring with two Λ-shaped niches cut to receive the knife-edges. These are to prevent the ring being carried off the knife-edges in an earthquake. Another pair of knife-edges are fastened on to the indicator, and rest in V-shaped niches in the ring at right-angles to the knife-edges under the ring.
It will be seen by this that the record on the glass will be drawn in the opposite direction to the movement of the pendulum.
Beside the pendulum is a spiral spring, which forms the vital part of a vertical-action seismograph, and which is 2ft. 6in. long when stretched. This descends from the top of the frame beside the pendulum, and terminates on a lever at about 3in. from its fulcrum. This lever is made forked, and provided with two parallel fulcrums so that it cannot swing from side to side—as it would if not so provided—but only rise and fall; after passing the point at which it receives the spring it is carried out 3ft., when it is met by the short end of a bent lever, the fulcrum of which is carried by two arms coming out from the uprights of the stand. Situated on top of the forked lever, near its junction with the bent lever, is a lead weight with a glass plate suspended from it, on which work friction-screws. The long end of the bent lever carries a needle, and marks on another glass plate.
Besides the pendulum for recording horizontal motion, there are a pair of bracket seismographs working at right-angles to each other. They are made as follows: A piece of wood, which must be substantial and about 8in. high, with a piece projecting 2 ½in. from top and bottom, carries a steel rod, both ends of which are held in place by steel bearings. Standing out from this about 3in. is another framework to carry a column of lead measuring 6in. high and 2in. in diameter. To the bottom of the lead is fixed an indicator 2ft. long, with a place at its end to carry a needle. This will record on the same plate that the vertical action is recorded upon.
To receive all these records, and to give the exact time of the shock, I have designed that two glass plates should be used—the one, 1ft. in diameter, under the pendulum, to receive a condensed record with small amount of magnification, turning round once in twenty-four hours; the other, 3ft. in diameter, turning once in the hour, to receive the record from the vertical action and the duplex-bracket seismograph.
To economize space I intend that the small plate shall be put as far under the pendulum as possible, and the large one allowed to go under it, or over, as far as will be allowed by the bearing of the small plate, or the indicator of the pendulum. The large plate will be supported from the front of the stand which is carried out at each side to give the ground-work for the brackets, the two indicators of which will reach out away from the pendulum, and record on the furthest part of the plate from it.
Each of the plates is carried by a star-shaped support made of thin wood, to give them stability.