Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/634

612 spiral spring. During the passage of au earthquake wave the spring is compressed and the balls displaced, their displacement breaking contact in an electric circuit which had previously been completed through the balls. That ball which first moves gives the time at which the shock commences, whilst the other gives the elements of the shock. Two such instruments are necessary to form one seismometer, the two being placed at right angles to each other. An elaborate electro-magnetic seismograph has been con structed by Professor Palmieri, and has done good service in the observatory on Mount Vesuvius. The vertical move ments are recorded by a helix of copper wire, the lower end of which is caused by even the slightest shock to dip into a basin of mercury, and thus complete a galvanic circuit. An electro-magnet, brought into action when the connec tions are completed, strikes an alarm bell which calls an attendant, and also stops a clock, so that the instant at which the shock occurs is permanently marked. At the same time a second electro-magnet releases the pendulum of another clock,, which being thus set in motion unrolls a band of paper, while a pencil continues to mark upon the paper as long as the shock lasts. To record the vertical element a system of four U-shaped tubes is employed, the tubes being placed in different azimuths. Each limb is partly filled with mercury, and any oscillation in the level of the liquid is indicated by movement of a little float con nected with an index. The oscillation of the quicksilver also completes a galvanic circuit, and brings into action the electro-magnets already described.

Fig. 3.

Although the limits of this article forbid reference to some other seismometers, such as that of Kreil of Vienna, mention should certainly be made of one instrument which is marked by its extreme simplicity. Its construction, which is due to Mr Mallet, will be understood by reference to fig. 3. Two sets of right cylinders are turned in some hard material, such as boxwood. The cylinders are all of the same height, but vary in diameter. Two planks of wood are fixed to a level floor, one having its length in a north-and-south, and the other in an east-and-west direction. The cylinders star.d upright on the planks in the order of their size, with a space between each pair greater than their height, so that when one pillar falls it does not strike its neighbour. The surrounding floor is covered up to the level of the planks with dry sand. When a shock passes, some of the cylinders are overturned, the number depend ing on the velocity of the wave. Suppose the shock knock? over the narrow-based cylinders 4, 5, G, leaving Nos. 1, 2, 3 standing ; then the velocity of the horizontal component must have been greater than that needed to overturn No. 4, but not great enough to overturn No. 3. Hence the velocity (V) can be approximately obtained by using a formula due to Professor Haughton, viz.:

-TTQ V 2 = 16a 2 g /&amp;gt; 0) ;

where a is the altitude of the column, b the diameter of its base, and 6 the angle formed by the side and a line drawn through the centre of gravity to the extremity of the base. The direction in azimuth is indicated by the position in which the overturned pillars are found, since the bed of sand prevents rolling. It is possible to obtain the exact time at which the shock commences by connecting the narrowest-based pillar with the pendulum of a clock so as to stop it at the instant of overthrow. Where the angle of wave-emergence is very steep, this instrument is not to be recommended, since it ignores the vertical element of the shock.

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