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Show 470 CAUSE OF TilE WAVE-LIKE MOTION AND RJ<:TRJ<:A1' intermission from "stufas," as the Italians term them. Hot springs in gt·eat numbers, especially in tracts where earthquakes are frequent, serve als~ as regular condu~tors of l~cat from the interior upwards. Silex, carbonate of ltme, munate of soda, and many earths, alkalies and metals are poured out in a state of solution by springs, and the solid matter which is tranquilly removed in this manner may, perhaps, exceed that which issues in the shape of lava. It is to the efficacy of this ceaseless discharge of heat, and of solid as well as gaseous matter, that we probably owe the O'eneral tranquillity of our globe; for were it not that some kind ~f equilibrium is established between fresh accession.s of .heat and its discharO'e, we miO'ht expect perpetual convulsiOns, 1f we b b • conceive the land and the ocean itself to be incumbent m many extensive districts on subterranean reservoirs of lava. If there be reason for wonder, it is, as Pliny observed, that a single day should pass without some dreadful explosion. " Excedit profecto omnia miracula, ullum diem fuisse quo non cuncta conflagrarent * ." But the circulation of heat from the interior to the surface, is probably regulated like that of water from the continents to the sea, in such a manner that it is only when some obstruction occurs to the regular discharge, that the usual repose of Nature is broken. Any interruptio~ to the regular drainage of a country causes a flood, and, 1f there be any obstruction in the passages by which volcanic matter continually ris~s, an earthquake or a paroxysmal eruption is the consequence. . . Michell has observed, that the wave-hke motton of the ground during earthquakes, appears less extraordinary if we call to mind the extreme elasticity of the earth, and that even the most solid materials are easily compressible. If we suppose large districts to rest upon th~ surf~ce of su?terranean lakes of melted matter, through whtch viOlent motiOns arc propagated, it is easy to conceive that superincumbent soli.d m.asses may be made to vibrate or undulate. The foll.owmg t~gcnious speculations are suggested by the above mentloned wnter. ''As a small quanuty of vapour almost instantly generated ~t some considerable depth below the surface of the earth will "' Hist. Mundi1 Lib. ii., c. 107. OF TilE SEA DURING EARTHQUAKES. 471 p. mducc a vibratory mo tI' On, so a very large quanti. ty (whether It. be ge.n erated a1m os t m· s t ant 1y , or m. any small porti.O n of tH~e) Will produce a wave-like motion. The manner in which tins wave-hke motion ·u b . WI e propagated may m some measure be represented by the Iw! 11 ow·m g experi·m ent. Suppose a large cloth, or carpet (spread upon a floor) to be raised at one edge, and then suudenly brought down aO'at'n t th fl h .. d . b . b o o . e oor, t e au un er It emg Y th~s means propelled, will pass along, till it escapes .at the opposite side, raising the cloth in a wave all the way as It goe~. In like. manner, a large quantity of vapour m1a y beb conceived to ra1se th. e e.a rth in a wave , as 't c 1 passes a o~g etw~cn t~e strata wh1eh 1t may easily sepamte in an horizontal dn·ectiOn, there being little or no cohesion between one Rtratum and another. The part of the earth that · L! t . d b . b IS urs raise , emg ent from its natural form will endeav . t . If b . ' OUl 0 rhestore1 I~se .Y 1 Its elasticity, and the parts next to it being to ave t 1e1r we1g 1t supported by the vapoUt·, which will insin ·u Ia te itself under them, will be raised in their turn t'll 't L! .1 , I I eit 1er unus some vent, or is again condensed by the cold into water, and by that means prevented from proceeding any farther*., In order to account for the retreat of the ocean from the shores before or during an earthquake, the same author imagines a subsidence at the bottom of the sea fl'Om the giving way of the roof of some cavity in conscq~cnce of a vacuum produced by the condensation of steam. For such condensation, he observes, might be the first effect of the introduction of a large body of water into fissures and cavities already filled with steam, before there has been sufficient time for the h~at of the incan~escen~ lava to turn so large a supply of water mto steam, wh1eh bemg soon accomplished causes a greater explosion. Sometimes the risinO' of the coast must • • 0 giVe nse to the retreat of the sea, and the subsequent wave may be occasioned by the subsiding of the shore to its fot·mer level; but this will not always account for the phenomena. Duri.ng the Lisbon earthquake, fot· example, the retreat pt·eceded the wave not only on the coast of Portugal, but also at • On the Cause and Phenomena of Earth(1uakes, Phil. T1amJ. vol.li. "58- 1760. ' , 'i |