EARTHQUAKE 609 His illustration of the movement of the ground is that of a loose carpet thrown into undulations by being shaken at one corner. Although MichelFs hypothesis is still accepted, in a modified form, by some geologists, it should be remembered that many arguments of considerable weight have been urged by modern physicists against the doctrine of a liquid nucleus and a thin crust. Whatever the merits of Michell s theory, he failed to understand the true nature of wave-motion, and the way in which it is transmitted during an earthquake shock. Modern seismologists believe that an earthquake is a vibratory motion propagated through the solid materials of the earth, much in the same way that sound is propagated by vibrations in the atmo sphere. It appears that this view was first suggested by Dr Thomas Young in his Lectures on Natural Philosophy, published in 1807. The development of this view, especially in its quantitative results, lies at the very base of seismology. In 1846 Mr Mallet communicated to the Royal Irish Academy his first paper " On the Dynamics of Earthquakes ; " and in the following year the late Mr Hopkins, of Cambridge, presented to the British Association a valuable report in which earthquake-phenomena are dis cussed in some detail. Since that date the great advances in this country have been made by Mr Mallet, assisted occasionally by the Rev. Professor Haughton and other mathematicians. Even at the present day, after all that has been written on the subject, but little is really known as to the origin of earthquakes. Probably several distinct causes should be recognized, for it is hardly to be supposed that all sub terranean disturbances, differing as they do so widely in intensity and in duration, should be referable to one common mechanism. Any great concussion, even upon the surface, is competent to produce tremors which may be regarded as diminutive earthquakes ; thus the great landslip at the Rossberg in Switzerland in 1806 was accompanied by a local quaking of the ground. Volger and Mohr have suggested that some of the small earthquakes which have been felt in Germany may be referred to the falling-in of the roof of enormous subterranean cavities formed by the long-continued solvent action of water on deposits of rock- salt, limestone, and gypsum. Such causes, however, can have given rise to only very petty shocks, and must be quite subordinate to subterranean disturbances of a more general character. The late Mr Poulett Scrope was led to refer most earthquakes to " the snap and jar occasioned by the sudden and violent rupture of solid rock-masses, and psrhaps the instantaneous injection into them of intumescent molten matter from beneath." He believed that the rupture of the rocks was due to expansion of deeply seated masses of mineral matter, consequent upon either increased temperature or diminished temperature. It is argued, however, by Mr Mallet, on mechanical principles, that such fractures could produce only very weak impulses ; but he believes that some earthquakes, especially those marked by long continued tremors, may be due to the movement and crushing of rock masses by tangential pressures produced by secular cooling of the earth. Steam has always been a favourite agent with seismologists, since it is clearly com petent to produce great effects by its sudden generation or by its sudden condensation. It has been suggested that water, finding its way through fissures in the earth s crust, might reach highly-heated rocks and remain quietly, in the spheroidal condition, until a local reduction of temperature suddenly caused it to flash into steam. After all, the origin of earthquakes is probably to be regarded as part only of a much wider question. Whatever causes are competent to produce volcanic action are, in all likelihood, equally competent to produce the ordinary manifestations of seismic energy. A relation is clearly traceable between the geographical distribution of volcanoes and the chief earthquake-areas ; and although it is not for a moment to be supposed that the volcano and the earthquake stand to each other in the relation of cause and effect, it is never theless highly probable that they represent merely different expressions of the same subterranean forces. Whatever may be the real origin of the earthquake shock, it is cjnvenient to regard its effects as proceeding from a concussion or sudden blow delivered underground at some definite centre. This centre of impulse ia called the seismic focus. It must be borne in mind, however, that such a centre, so far from being anything like a mathe matical point, is in nature a subterranean region, which iu many cases is no doubt of very large dimensions, measuring perhaps some miles in diameter. From the seismic centre waves are propagated in all directions through the solid materials of the earth s crust ; and if the focus be situated beneath the sea, the vibrations of the ground will be accompanied by undulations of the water. Those waves which pass through the elastic materials of the earth consist, for the most part, of longi tudinal vibrations, like those of atmospheric sound-waves, and consequently not like ordinary water-waves. In the sound-wave the air is alternately condensed and rarefied, tbe molecules advancing and retreating in the line of direc tion in which the wave is travelling. In a water-wave, on the contrary, the molecules of liquid rise and fall, or rather describe closed curves in planes which are transverse to the dirsction in which the undulation or wave-form advances. According to Mr Hopkins, both orders of vibra tion longitudinal and transversal coexist in the earth quake-wave, and call for investigation. When, for example, the molecules of an iron bar are disturbed by a blow delivered at one end, both kinds of vibration are generally excited, and hence two waves are sent through the bar, the longitudinal, however, having a much greater velocity than the transversal wave. But it may be doubted whether the seismologist need concern himself with any but longi tudinal vibrations. For, admitting that small transversal vibrations are generated at the seismic focus, it is probable that they would be cut off to a great extent during trans mission from stratum to stratum. Indeed, the planes of junction between the several beds in stratified deposits would hinder the transmission of transversal vibrations travelling in a direction normal to the strata. Hence Mr Mallet maintains, that in studying the effects of an earth quake, attention may be restricted, without danger of error, to the longitudinal or normal vibrations, the transversal or tangential vibrations being neglected. Around the seismic centre, or mean focal point, the molecules of the rock will first be squeezed together by the concussion, and then separated by virtue of the elasticity of the solid medium ; the onward motion is then rapidly taken up by the next set of molecules, which in like manner are pushed against each other, and then spring apart. In this way the pulse, or form of the wave, may be propagated to an enormous distance, whilst the excursions of the individual particles are confined within very narrow limits. It is therefore of great importance to distinguish between the transit of the wave and the movement of the material particles. Each molecule may move only through a few inches, but the undulation may travel for hundreds of miles. The distance through which the individual particles oscillate is called the amplitude of the wave. After the Neapolitan earthquake of 1857 Mr Mallet found from actual observa tion at a place called Polla, situated nearly above the seismic centre, that the amplitude of a wave which caused certain fractures in masonry could not have been more than 2| inches. He is thus led to believe, contrary to the opinion of most geologists, that earthquakes are net great agents of
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