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 the heat conducted from some source perhaps at a great horizontal distance ; he thinks that no concussion in the proper sense of the word takes place, but that when the expansive power of the water and vapor overcomes the adhe- sion of the rock and splits it, then the sudden tearing is accompanied by a reaction, giving rise to waves of shock, sound, &c. The idea of the formation of fissures as the result of protracted increasing tension seems also to have been adopted by Hopkins, Darwin, and others. In 1834 Keferstein, and in 1836 Sir John Herschel, independently of each other, advocated certain views that have been more elaborately treated by many geologists, such as Hunt (1858-72), Sorby (1863), Deville, Nau- mann, Forbes, and Daubree. Starting with a suggestion of Scrope and Babbage that the heat slowly conducted from the solid interior of the earth would be concentrated near the surface under the increasing accumulation of sedimentary deposits that are poor conductors of heat, these chemical geologists show that such heat would probably slowly bring about chemical changes, metamorphism, evolution of gases, the transformation of the strata into a pasty mass of lava, and the explosion of the confined water, if such existed or were formed by the chemical changes. In 1854 Zantedeschi maintained that the fluid nucleus of the earth and the exterior thin crust are subject to solar and lunar tides, the crust being flexible like a thin sheet of iron ; and that the upward pres- sure against the crust of a high tide in the in- terior ocean of lava elevates and cracks the solid strata, causing earthquakes and volca- noes. These hypothetical tides have been ren- dered quite improbable by the researches of Hopkins and Thomson. Lament (1862), Var- ley (1870), and Sumichrast (1871) have con- nected together the phenomena of terrestrial magnetism and earthquakes, and have ex- pressed the belief that the latter are due to magnetic action. The earlier papers of Mal- let (1846 and 1862) made highly probable the general truth of the theory that the sudden expansion of steam under high tension is the immediate cause of the earthquake ; he showed that the phenomena of the earthquake of 1857 may be thus explained in a very reasonable manner; but in 1873 he concludes that more causes than one may and probably do produce these concussions, as will be seen in our next section. The second class of theories of the origin of earthquakes includes those that seek "or the source of the internal heat of the globe. Passing by the ancient idea of internal fire, we come to Lemery (1700), Lister, Boyer, Breis- Sir William Hamilton, Sir Humphry avy, Daubeny, and many others, who at- tributed the heat to some form or other of in- ternal chemical changes. Stukely (1750) and his followers sought its origin in electric cur- rents. The nebular hypothesis of Laplace (1780) afforded Poisson, Scrope, Hopkins, Lyell, Phillips, and others, a base for the idea, EARTHQUAKE 369 now widely accepted, that the present inter- nal heat is but the remnant of that which pre- vailed when the earth was in its original con- dition of entire fluidity, and which it has slowly lost by radiation. Finally, when the mathematical investigations of Sir William Thomson had confirmed the idea of Hop- kins that the earth must be in general solid, perhaps even to the centre, and when the ideas of permanent internal lakes of lava and of areas of special heat were .beginning to be looked upon as too purely hypothetical, Mallet (1873) has with great success under- taken to solve the problem, and his views, covering as they do the whole field of vulcan- icity, seem to meet with universal approbation. He shows that the secular cooling of the earth, supposing it to have been once a hot fluid globe, gave rise at first to a thin crust, whose contraction while thin was more rapid than that of the interior, thus causing it to be torn into segments by forces of tension ; but that as the crust thickened it became subject to strains of the nature of tangential pressures as soon as the interior began to contract more rapidly than the exterior (the existence of such pressures was first pointed out by Prevost in 1835) ; the bending and crushing consequent upon these crushing pressures formed the chief mountain chains and ocean beds, as urged by PreVost, Dana, and others ; the nucleus shrink- ing more than the shell, and the latter not be- ing able to resist the strain upon it, it must crush and sink down upon the shrinking nucleus, the crushing being more or less paroxysmal ; this crushing is accompanied by an immense evolution of heat, the quantity of which may be calculated by means of the theorems of the mechanical theory of heat, in connection with experiments instituted by Mallet (the heat re- sulting from crushing is a reality ; that which Vose in 1866 suggested as resulting from pres- sure only cannot exist unless the pressure pro- duce crushing or sudden compression) ; this heat, acting on the already heated interior, abundantly suffices to melt the adjacent por- tions thereof, producing lava, or to convert water into the steam that produces both earthquakes and volcanic eruptions. Mallet has made such experiments on the crushing of rocks and such calculations for all the volca- noes of the earth as prove that of the telluric heat known by the observations of the tem- perature of the earth to be annually dissipated into space, one fourth part is sufficient to ac- count for the annual volcanic energy. He dis- cerns therefore in the secular cooling of the earth and crushing of its shell a true physical cause for the earthquake; the crushing goes on per saltum, and is attended with either partial permanent elevations or with heat suf- ficient to melt rock and produce volcanoes and earthquakes. The function of the earthquake and volcano is thus seen to be preservative and not destructive ; these are the instruments by means of which the solid crust of the globe