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Show CHANGES OF TilE SURFACE 142 we dwell on a thin crust which covers a central ocean of liquid incandescent lava we ouO'ht still to be very reluctant to concede ' 0 b . . on slight evidence that the internal heat is varia le tn q~anht~. In our ignorance of the sources and nature of volcamc fire, 1t seems more consistent with philosophical caution, to assume that there is no instability in this part of the terrestrial system. We know that different regions have been subject in succession to a series of violent subterranean convulsions, and that fissures have opened from which hot vapours, thermal springs, and at some points red hot liquid lavas have issued to the surface. This evolution of heat often continues for ages after the extinction of volcanos and after the cessation of earthquakes, as in Central France, for example, and it seems perfectly natural, that each part of the earth's crust should, as M. Fourier states to be the fact, present the appearance of a heated body slowly cooling down. This may be owing chiefly to the shifting of the volcanic foci; but some effect may perhaps be due to that unequal absorption of the solar rays to which we have aUuded, when speaking of the different temperature of the earth, accord" ing to the varying distribution of its superficial inequalities. M. Cordier announces as the result of his experiments a1;1d observations on the temperature of the interior of the earth, that the heat increases rapidly with the depth, but the increase does not follow the same law over the whole earth, being twice or three times as much in one country as in another, and these differences not being in constant relation either with the latitudes or longitudes of places. All this is precisely what we should have expected to arise from variations in the intensity of volcanic heat, and from that change of position, which the principal theatres of volcanic action have undergone at different periods, as the geologist can distinctly prove. But M. Cordier conjectures that there is a connexion between such phenomena and the secular refrigeration and contraction of the internal fluid mass, and that the changes of climate, of which there are geological proofs, favour this hypothesis*. We cannot help suspecting that if it had appeared that the same species of animals and plants had continued to inhabit the "' See M. Cordier's Memoir on the Temperature of the Interior of the Earth, read to the Academy of Sciences, 4th June, 1827. Edin. New Phil. Journ., No; viii., p. 273. AND CLIMATE ' CONTEMPORANEOUS. 143 seas, lakes, and continents b f, mutations which the north~rne 1~:e ~nd after the great physical the .secon~ary strata were formed misp~ere has undergone since ancient climate of the 0'1 b I ' the difficulty of explaining th bl o o e wou d have a d e mounta e than at prese t I ppeare far more insur-t~ Ie elementary truths ofnn.teteotr~~uld ha~e been so contrary to twn to have fo1lowed from tl .. gy to suppose no refrigera- ] . . Ie nsmg of s c Jams m northern latitudes tl t . o many new mountain-been had in that case als~ tla Iecou;-se.would probably have might have been argued 't~ cosmo oglCal speculations. It accession of high rid WI 1 much plausibility, that as the I . ges covered w't] g aCiers had not occasioned an I _1 p:rpetual snow and as to affect the ~tate of . yl~erceptible mcrease of cold so " orgamc Ite th ' new source of heat which 'b ere must have been some ca use. Th' . counter alanced th t f . Is It might h b . a re ngerating 1 ' ave een satd 1 · . opment of cent·ral fire issuin fro . ' was t le mcreased deve~ m the crust of the earth 1 ~Innumerable fissures opened which rai~ed the Alps and :t~l:n It I waslshak.en by convulsions But, Without enteri . r co ossa chams. of th~ hypothesis of ;;a~~~ f~7:1~er di~cussion on the merits experiments will continue t ~e rlgeratwn, let us hope that there be internal heat in th o 1 ~ made, to ascertain whether its distribution. When 't e g.o e, and what laws may govern established, it will be tt' I s texistenc.e has been incontrovertibly se c~ I . me o enqmre h th . ar variations. Should th w e er It be subject to begm to indulge speculations ;:: als~ be confirmed, we may ~ot hastily assume that it h {I pectmg the cause, but let us ti?n o~ the planet, with whi:~l ~: er::ce to the original formaWith Its final dissolution I th mtoht be as unconnected as great changes in the exte;nal ~onfie me~n time we know that have at various times taken I guratwn of the earth's crust ~u.~t ~ave produced some :ff:~;' oand 1-v:·e may affirm that they e eir. Influence ought, therefore t: c Imate. .The extent of b nqUiry, more especially as th . ' form a primary ob1ect of etwe h e1e seems an b · . J . . en t e eras at which t] . . o vwus comcidence high latitudes were m d dle prmctpal accessions of land I·n dI' mm. uti.O n of temperaat e' an the successi.v e pen.o ds when the ure was most decided. |
