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Show CHAPTER VII. On the causes of vicissitudes in climate-Remarks on the present diffusion of heat over the globe-On the dependence of the mean tem11erature on the relative position of land and sea-Isothermal lines..:_Currents from equatorial regions- Drifting of Icebergs-Different temperature of Norlhem and Southern hemiS}Jheres-Combination of causes which might produce tho extreme cold of which the earth's surface is susceptible-On the conditions necessary for the production of the extreme of heat, and its probable effects on organic life. As the proofs enumerated in the last chapter indicate that the earth's surface has experienced great changes of climate since the deposition of the older sedimentary strata, we have next to inquire, how such vicissitudes can be reconciled with the existing order of nature. The cosmogonist has availed himself of this, as of every obscure problem in geology, to confirm his views concerning a period when the laws of the animate and inanimate world were wholly distinct from those now eiitablished ; and he has in this, as in all other cases, succeeded so far, as to divert attention from that class of facts, which, if fully understood, might probably lead to an explanation of the phenomenon. At :first, it was imagined that the earth's axis had been for ages perpendicular to the plane of the eclip ... tic, so that there was a perpetual equinox, and unity of seasons throughout the year :-that the planet enjoyed this ' paradisiacal' state until the era of the great flood ; but in that catastrophe, whether by the shock of a cornet, or some other convulsion, it lost its equal poizc, and hence the obliquity of its axis, and with that the varie<;l seasons of the temperate zone, and the long nights and days of the polar circles. 'Vhen the advancement of astronomical science had exploded this theory, it was assumed, that the earth at its creation was in a state of fluidity, and red hot, and that ever since that era it had been cooling down, contracting its dimensions, and acquiring a solid crust,-an hypothesis equally arbitrary, but more calculated for lasting LAWS GOVERNING THE DIFFUSION OF HEAT. 105 popularity, because, by referring the mind directly to. the beginning of things, it requires no support from observatiOns, nor from any ulterior hypothesis. They who are satisfied with this solution are relieved from all necessity of inquiry into the present laws which regulate the diffusion of heat over the surface, for however well these may be ascertained, they {:annot possibly afford a full and exact elucidation of the internal changes of an embryo world. As well might an ornithologist study the plumage and external form of a full-fledged bird, in the hope of divining the colour of its egg, or the mysterious metamorphoses of the yolk during incubation. But if, instead of vague conjectures as to what might have been the state of the planet at the era of its creation, we :fix our thoughts steadily on the connection at present between climate and the distribution of land and sea; and if we then consider what influence former fluctuations in the physical geography of the earth must have had on superficial temperature, we may perhaps approximate to a true theory. If doubt still remain, it should be ascribed to our ignorance of the laws of Nature, not to revolutions in her economy ;-it should stimulate us to farther research, not tempt us to indulge our fancies in framing imaginary systems for the government of infant worlds. In considering the laws which regulate the diffusion of heat over the globe, says Humboldt, we must beware not to regard the climate of Europe as a type of the temperature which all countries placed under the same latitudes enjoy. '".fhe physical sciences, observes this philosopher, always bear the impress of the places where they began to be cultivated; and, as in geology, an attempt was at first made to refer all the volcanic phenomena to those of the volcanos in Italy, so in meteorology, ~small part of the old world, the centre of the primitive civilization of Europe, was for a long time considered a type to which the climate of ·all corresponding latitudes might be referred. But this region, constituting only one-seventh of the whole globe, proved eventually to be the exception to the general rule; and for the same reason we may warn the geologist to be on his guard, and not hastily to assume that the temperature of the earth in the present era is a type of that which most usually obtains, since he contemplates far |
