Page:The American Cyclopædia (1879) Volume VI.djvu/376

 368 EARTHQUAKE thickness very small. The amplitude of the vibrations of the particles at the surface of the earth near the seismic vertical was deduced as 2-5 in. : at a greater distance it was found to be somewhat greater, and Mallet remarks that it thus becomes evident that the earthquake is it- self not an agent capable of directly producing any considerable permanent elevation of the land, but that it occurs along with and as part of a train of circumstances that do occasionally produce such elevations. The velocity with which the shock is propagated along the sur- face, or the velocity of transit, was found from seven determinations to vary between 989 and 658 ft. per second, depending on the nature of the strata concerned. This agrees well with other determinations made by Mallet in Eng- land and Sch weitz in Hungary ; but the velocities are 300 ft. less than those found by Noggerath in southern Germany, where however differ- ent strata come into consideration, and Mallet has shown that owing to discontinuity and oth- er causes the actual velocity of translation may be so low as one tenth of that which would take place were the .rock continuous. These veloci- ties refer to the surface of the ground, and re- quire a slight increase in order to obtain the true velocity normal to the front of the wave. The maximum velocity of the wave particle in its orbit, or the velocity of the shock itself, was determined in 13 cases ; the results vary from 9*8 to 21'2 ft. per second, their mean being 12*4 ft., and only three of the determinations mate- rially differ from this. There were strong indi- cations of a diminution of wave velocity with increase of distance from the focus. It is there- fore evident that the velocity of the shock that causes the destruction during an earthquake is far less than the rapidity with which it is prop- agated. It is not necessary to presume that a similar velocity prevails in all other earth- quakes ; indeed, in the case of the tremendous shock at Riobamba in 1797, Mallet shows that the velocity of the shock must have been about 80 ft. per second, and this is very probably one of the most violent on record. Owing to the imperfect elasticity of the earth, the return movement in the orbit of each particle is not so great or rapid as the forward movement; in two cases Mallet was able to determine the existence of a slight difference of this kind. A definite idea of the severity of the maximum shock experienced during this earthquake may be formed by considering that it was such as would have been given to an obstacle run into by a locomotive moving at the rate of 10 m. per hour. The nature of the fleeting earth- quake forces that instantaneously desolate the earth having been thus most successfully in- vestigated by Mallet, the same path has been followed by Oldharn in an equally successful and instructive manner in reference to the earthquake of Cachar, India, on Jan. 10, 1869 ; but the details of this work have as yet been only partially published. The Origin ofEarth- qiialces. If our ideas as to the true nature of earthquakes have but recently come into some degree of accordance with the truth, still more is this the case with our views as to their ulti- mate cause and origin. Oh this difficult sub- ject it might still be hazardous to offer any conjectures, were it not that the many unsuc- cessful attempts of the past have resulted to a certain degree in limiting our views to one or two causes that are not improbably in continual and effective operation. The theories relative to the origin of earthquakes may be divided into two classes : first, those that relate to the immediate active cause of all classes of earth- quake motions; second, those that relate to the origin of the internal heat of the earth. The views of the first class have already been incidentally partially indicated in the preceding section. Considering them chronologically, we come first to the ancient indefinite notion of a blow or concussion somewhere within the earth, which was more definitely explained by Fromondi (1527) and by Travagini (1679). In 1693 Flamsteed suggested that explosions of gas communicated tremblings to the air and thus to the earth ; in which view he was fol- lowed by Amontons (1703), who invoked heat- ed air as the exploding gas. Lister regarded lightning and earth quakes .as the results of the firing of "the inflammable breath of pyrites," i. e., sulphur; and Lemery (1700) considered earthquakes to be due to the explosions of any chemical mixtures containing sulphur and iron. Bouguer also invokes the inflammation and explosion of gases. Stukely (1750) finds their origin in electrical discharges within the earth, in which he is followed by Beccaria, Donati, Percival, and Priestley ; and even Humboldt and Poey do not deny the possibility of some connection with electricity. Buffon held that caverns exist in the interior of the earth, and that the falling of their ceilings down upon their floors produced the initial concussions and noises; Humboldt, Darwin, and others, have shown that such caverns may have some connection with earthquakes, though not ne- cessarily in the manner indicated by Buffon. The theory of the Rev. J. Michell (1760) de- manded the sudden formation or condensation of aqueous vapor between the crust and the molten interior of the earth, and the passage of waves of this vapor in between the sedi- mentary strata of the crust, lifting the upper strata in waves like those of a carpet when it is gently shaken on the floor. In 1785 Dolo- mieu explained how water penetrating into the interior of JEtna might flash into steam, pro- ducing eruptions and earthquakes; this idea, extended by Bakewell (1813), has since become a favorite with many, and in some shape or other probably has been the prevailing one of the past 30 years. To meet certain very se- rious objections to Dolomieu's theory, Scrope (in 1825 and 1872) considers that the water is already in place, being confined in cavities within the earth, long before a given earth- quake, but that it is exploded into steam by