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

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 /> 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.

Catalogues of earthquakes, showing their distribution in time and space, have been constructed by Wallet, Ferrey, Von Hoff, Cotte, and other seismologists. The most complete of these statis tical works is the catalogue raisonne compiled by Mr R. Mallet and his son, Dr J. W. Mallet, and published by the British Association between the years 1854 and 1858. This includes notices of all recorded earthquakes from 1606 B.C. to 1842 A.D., and is thence carried on to 1850 from Perrey's annual catalogues. Between 6000 and 7000 separate earthquakes are recorded as having occurred in almost every part of the world, both on land and at sea. But though seismic energy may thus become sensible at any point of the earth s surface, there are, as everyone knows, certain regions peculiarly subject to earthquakes ; and it is, in fact, possible to trace seismic bands of variable width following the great lines of elevation which divide the oceanic basins.

It is now several years since Professor Alexis Perrey, of Dijon, sought to trace a relation between the occurrence of earthquakes and the age of the moon. By careful analysis of his catalogue he believed that he had established the facts that earthquakes occur more frequently at the syzygies than at the quadratures, that their frequency increases at the perigee and diminishes at the apogee of the moon, and that shocks are more frequent when the moon is on the meridian than when 90 from it. Such a connection between seismic phenomena and the phases of the moon would accord with Zantedeschi s views on the existence of a terrestrial or terrene tide, views which were based, however, on the old hypothesis of a liquid nucleus in the earth, covered by a thin crust.

From Mallet s discussion of his catalogue for three centuries, lie was led to detect definite periods of maximum energy. Thus it is found that the greatest number of earthquakes are recorded about the middle of each century ; whilst a second epoch, less powerful than the first, occurs towards the close of the century. According to Perrey there is a preponderance of earthquake-shocks at particular seasons, the equinoxes and solstices, which he terms "critical epochs." Mallet s analysis of a large catalogue showed a decided maximum about the winter solstice, but Perrey s other epochs were less marked. In the present state of our knowledge it would be rash to regard seismic force, whatever it may be, as a distinctly periodic force, or to insist upon any of those relations between earthquakes and meteorological phenomena which have sometimes been discussed.

Annual reports on earthquakes have been published for many years by Professor Fuchs. During the year 1876 he recorded 104 earthquakes, which were distributed among the months as follows : In January 10, February 10, March 14, April 8, May 7, June 7, July 8, August 5, September 7, October 14, November 5, December 9. In the preceding year 97 earthquakes were noticed, occurring as follows : In January 15, February 7, March 12, April 7, May 9, June 10, July 6, August 5, September 3, October 2, November 9, December 12.

Bibliography.—The First Principles of Observational Seismology, by R, Mallet, 2 vols. 8vo, London, 1862. (This contains the Report on the Neapolitan Earthquake of 1857). Mullet s four "Reports on the Facts and Theory of Earth quake Phenomena," in Reports of the British Association, 1850 to 1858. (The Earthquake Catalogue is the 3d report of this series). Mallet s chapter " On Observation of Earthquake Phenomena" in the Admiralty Manual of Scientific Inquiry, 1849 ; his Introduction to the translation of Palmieri s Vesuvius, 1873 ; and several papers in the Philosophical Transactions and Transactions of the Royal Irish Academy. Report on the " Geological Theories of Klevation and Earthquakes," by W. Hopkins, in Rep. lirit. Assoc. for 1847: several Reports on Earthquakes in Scotland by Dr Bryce, in Rep. Brit. Assoc.; Oidham and Mallet on the "Caehar Earthquake," in Quart. Journ. Geol. Soc., xxviii. p. 2-J5, 1872; Sciope s Volcanos ; jye s. Principles of Geology, and Phiilips s Vesuvius; Perrey s " Memoirs," in Mem. dc fAcad. de Dijon, Comptes RenJus, <fec.; lioccardo s Sismopiro!o(jia, Genoa, 186!) ; Fuciis s BerU-lite iiber die vulkanischen Kreignissu des Jahres 1870," <kc., in Mineral. Mittheilungen, Vienna.

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under various modifications in different languages (e.g., Auricularia, Perce-oreille, Ohr-wurm, Oorblazer, Ormask, Oerentvist, Gusauo del oido, &c.) to the somewhat osculant

insects comprised in the old Linneau genus Forficula, au