TERRESTRIAL TEMPERATURE, DISTRIBUTION OF. The subject of the earth's temperature has already been treated of at some length under different headings. Generally, it was shown under arareiz and METEOROLOGY, that the temperature of the air in any region depends ou the inclination of the sun's rays to the surface of tho earth in that region ; on geographical position and physical eouforma Lion, the distribution of land and water, the state of the countries from which come the prevailing winds, the vicinity of the sea, the elevation of the land, the electrical state of the atmosphere, and numerous other circumstances and phenomena. Under Hereto:mt. ilex or 'rut Geese, the question of the influence of the proper, or iotcrnal, heat of the globe on its superficial temperature is examined.
The condition and properties of the ATMOSPIIERE are investigated under that title. Under SNOW, Penzeetae, the decrement of tem perature on ascending into the atmosphere is noticed. Under SEA, the temperature of the ocean Is considered. Whilst the effects of oceanic currents, glaciers, deserts, &e., are treated under those heads either in the present division, or In the NATURAL Htsronv and GEOGRAPUICAL DIVISIONS of this Cycloptedia. The important subject of Terrestrial Magnetism is reserved for a distinct article. Here it only remains to speak of some of those general deductions of recent investigators in climatology which have not been specifically mentioned ; and especially have we to do so in reference to isothermal lines, or lines of equal temperature, for an account of which, reference has been made to this article from Isoenznuee LINES; and to the tempo rature of the atmosphere over the sea, a branch of the subject which has been referred to the present article from Sze.
In the primitive condition of the earth, when the globe was a fluid mans, or in remote geological periods, when its central fluid mass was covered with a comparatively homogeneous crust, the effect of the radiation of its heat on the superficial temperature must, as Mr. Hopkins has shown, have been almost unlimited. But as by this radiation it would continue to part with heat till the superficial tem perature approximated to that of the circumambient space, unless the radiation were compensated by the generation of heat on the surface, which is known not to have been the case, it must necessarily happen at some indefinite time (it has been shown to require many millions of years), that the internal heat of the globe would cease to exert any appreciable Influence on its superficial temperature. And this is what has now very nearly come to pass OF TILE CLORE]; this influence, according to the of Mr. Hopkins, being now reduced to lose than the twentieth of a degree Fahrenheit. In fact, whilst volcanoes, thermal springs, borings for artesian wells, the con tinuous increase of temperature in descending deep mines, and other phenomena, afford irrefragable evidence of the higher temperature of the interior of the globe, it is as clearly seen that for a short distance from the surface the temperature of the earth is dependent on external hest and moisture, and varies with the seasons of the year and the hours of the day, whilst at a greater, but still small depth—which varies according to the latitude of the place, and the conducting power of the rock, but nowhere probably exceeds 100 feet—a point is reached at which there is no sensible change of temperature, and which has accordingly been designated the Invariable Stratum. The present
Influence of the internal heat of the globe, although almost inappre ciable, may, however, acording to Dove, be regarded as constant ; "lessening tho extremes, but not affecting the periods of the variations of temperature at the surface." When the influence of the internal heat ceased to be paramount on the surface, changes of temperature must have been in a large measure due to the altered conditions of land and water—the elevation of mountain regions, the subsidence of extensive areas, and the cense• quent changes of oceanic currents—to glacial action, &c.: circumstances of which the results are clearly traceable in the animal and vegetable remains preserved in the various strata which compose the crust of the earth, and some of which (as notably in the case of the Gulf Stream and the Arctic Current) are shown by the remarkable in flections of isothermal lines to be distinctly operating now. (Hopkins, Trans. of OM. Soc.; and Cambridge Phil. Trans.') In considering the present temperature of the earth, the sun must be regarded as the only source of heat and the ultimate cause of all climatic' change, and hence we arrive at the possibility of ascertaining, amidst all casual fluctuations, a regularly recurring periodicity, annual as well as dinrnal, for every variety of geographical position. The bearing of this periodicity on a theory of the general distribution of heat appears to have been first distinctly observed by Kirwan, who (in vol. viii. of the Irish Transactions') constructed a table of monthly temperatures for all parallels of latitude between 10° and 80*. Before the difference of temperature on the same parallel of latitude in the old and new continents was known or regarded, a simple formula was thought sufficient to express the temperature at any parallel of terrestrial latitude. The celebrated Tobias Mayer, from such mean temperatures as had in his time been observed, found that the tem perature t con Fahrenheit'a scale) at any place might be represented by sin, L, where T is the mean temperature at the equator, and L the geographical latitude of the place ; and in 1819 M. Daubuisson ("TraitZ do Geognosie ') proposed the more accurate formula t cos.' 1. (centigrade scale); which being adapted to Fahrenheit's scale, considering the mean temperature at the equator to be 81*, becomes 32' + 49' coral L. This formula luta been found to serve for tempera tures In Europe sa far north as the latitude of 60'; but beyond that parallel it is useless, as it suppomes the temperature at the geographical pole to be 32', which is much too high.