OCR Text |
Show 462 STATE OF TilE ATMOSPHERE DURING EARTHQUAKES. -value of some terms. So if a student of Nature, who, when he first examines the monuments of former changes upon our globe, is acquainted only with one-tenth part of the processes now going on upon or far below the surface, or in the depths of the sea, should still find that he comprehends at once the import of the signs of all, or even half the changes that went on in the same regions some hundred or thousand centuries ago, he might declare without hesitation that the ancient laws of nature have been subverted. Even after toiling for centuries, and learning more both of the present and former state of things, he must never expect to gain a perfect insight into all that formerly happened, so long as his acquaintance is very limited in regard to much that is now going on. So completely has the force of this line of argument been overlooked, that when any one has ventured to presume that all former changes were simply the result of causes now in operation, they have invariably been called upon to explain every obscure phenomenon in geology, and if they failed, it was considered as conclusive against their assumption. Whereas, in truth, there is no part of the evidence in favour of the uniformity of the system, more cogent than the fact, that with much that is intelligible, there is still more which is yet novel, mysterious, and inexplicable in the monuments of ancient mu· tations in the earth's crust. Before the immense depth of the sources of volcanic fire was generally admitted, the causes of subterranean movements were sought in peculiar states of the atmosphere. These were imagined to afford not only prognostics of the convulsions, but to have considerable influence in their production. But the supposed signs of approaching earthquakes were of a most uncertain and contradictory character. Aristotle, Pliny, and Seneca, taught that earthquakes were preceded by a serene state of the air; whereas several modern writers have been of opinion that a cloudy sky and sudden storms are the forerunners of these commotions. That there is an intimate connexion between subterranean convulsions and particular states of the weather is unquestionable ; but as Michell truly remarked, " it is more probable that the air should be affected by the causes of earthquakes, than that the earth sh"ould be affected in so extraordinary a manner, and to so great a depth, by a cause residing in the air.,, EARTHQUAKEs AND V • OLCANOs, THEIR RELATION. 463 After VIOlent earth k is obstructed; lakes a~~a esl the regular drainage of a country landslips, and the evapor~~~: :;e caused b! local subsidences or water produces unseaso bl a.n extensive surface of shallow damp soil which is trave~a deb rams. Fogs proceed from the filled with water. In add'et' y numerous rents and crevices tri·C a1 effect prou uced b It hI on to th ese C·i rcumstances, the elec-f k masses o roc against eYa he mh ovement an d f n·c t1·0 n of m-eat f . d I . c ot er may ca 1' h . ~- o wm ' u.m mous exhal a tI' ons, and otl use tg tnm. g ' ~m· ists mena. Rams, moreover a . lcr atmospheric pheno- . ' re sometimes d . d f eruptions accompanying earth uakes . enve ~om volcanic before stated, are attended with qa . ' for. eruptions, as we vapour. copiOus dtscharge of aqueous Before we attempt to enquire farth . of earthquakes we shall b . fl ~r Into the true causes ' · rie Y recaptt 1 consid.e ring them as origi'natm' g f rom uth ate our reasons for volcamc phenomena. In th fi I e same sources as I d b . e rst pace th . vu se y viOlent earthquakes i 1 d . '. e regiOns con-all the active volcanos. Earth n~a~e: :Ithm. them the site of times extending over vast areaq ' dometimes local, some- B I s, prece e vol · . ot.l the subterranean movement and th came. eruptions. agam and again, at unequal t'nt erva ls fe eruptiOn return 0 ti d . unequal degrees of force to tile l me, an with b h . , same P aces Th d . ot may contmue for a few hou rs, or 1.c 0r s•e verale · urat10n · of years. Paroxysmal convulsions of b th k' d consecutive lowed by long periods of tran uillit o T m s are us~ally folthose containing abundance ofqmi' Y·l hermal sprmgs, and h . . nera matter i 1 t' c aracteristlc of countries where active vole n so u IOn, are are frequent. In districts consider bl d'anos or earthquakes vents, the temperature of hot sp . ah y b Istant from volcanic b rmgs as een som t' · ·y subterranean movements · I n a dd'1 t1. 0n to th e· Imes raised tiOn and analogy, we may observe th t . . ese stgns of rela· conceive how columns of melted ~att:r It Is b not ;ery easy to great heights, as we know them t tt . c~n e raised to such exerting an llydrostatic pressure ~a~a~~~ ~~ volc~nos, without masses of land . nor can we b . o movmg enormous capable of forci~g up so great e s~rphnsefd that elastic fluids f · . a we1g to rock · f · o proJectmg large stones to imme h . . m usiOn, and also cause tremors vibrations ~se. ~tghts m the air, should solid crust of the e~rth Th ' a~ vto ent movements in the . e vo cano of Cotopaxi has thrown |