Principles of Geology; Being an Attempt to Explain the..., v. 1

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106 .. DEPENDENCE OF CLIMATE mightier alterations in the position of land an.d sea, than those which now cause the climate of Europe to differ from that of other countries in the same parallels. . It is now well ascertained that zones of equal warmth, ~oth • 1 t osphcre and in the waters of the ocean, are neither m t 1e a m -Yv I · 1 d' para1 1 e1 t o the equator nor to each other· . t IS a so. l.S • covered that the same mean annual temperature may exist m two places which enjoy very different climates, forth~ seasons b nearly equalized or violently contrasted. 'Ihus the may e . 'd 'I h lines of equal winter temperature. do not c01~c1 e wit 1 t c lines of equal annual heat, or the 1sothermal hnes. T~e de­viations of all these lines from the same parallel of latitude, determined by a multitude of circumstances, among the are . . d l . f principal of which are the position~ ?IrectiOn, an e evat10n o the continents and islands, the position and depth of the sea, and the direction of currents and of winds. It is necessary to go northwards in Europe in order to find the same mean quantity of annual heat as in North Arne· rica. On comparing these two continents, it is found that places situated in the same latitudes, have sometimes a mean difference of temperature amounting to 11° or eve.n someti~es 17o of Fahrenheit; and places on the two contments whiCh have the same mean temperature, have sometimes a difference in latitude of from 7° to 13° t. The principal cause of greater intensity of cold in corresponding latitudes of North A.merica and Europe, is the connexion of the former count~·y ~Ith the polar circle, by a large tract of land, some of which 1s from three to five thousand feet in height, and, on the other hand, the separation of Europe from the arctic circle by an ocean. The ocean has a tendency to preserve every where a mean temper~· ture, which it communicates to the contiguous land, so that 1t tempers the climate, moderating alike an excess of heat or cold. The elevated land, on the other hand, rising to the * We are indebted to Baron Alex. Humboldt for collecting together in a benut~ul essay, the scattered data on which some o.p11roximation to a true theory of the .di~ tribution of heat over the globe may be founded. Many of these data are deriV t from the author's own observations, and many from the works of M. Pr~vos on the radiation of heat, and other writers. See Humboldt on lsot~:~rmal Ll~;6. Memoiresd'Arcueil, tom. iii. translated in the Edin. Phil. Journ. vol.m.July, 1 t Humboldt's tables, Essay on I~othermal Lines, &c. ON TIIE POSITION OF LAND AND SEA. 107 colder regions of the atmosphere, becomes a great reservoir of ice and snow, attracts, condenses, and congeals vapour, and communicates its cold to the adjoining country. For this reason, Greenland, forming part of a continent which stretches northward to the 8~nd degree of latitude, experiences under the 60th parallel a more rigorous climate than Lapland under the 7~nd parallel. But if land be situated between the 40th parallel and the equator, it produces exactly the opposite effect, unless it be of extreme height, for it then warms the tracts of land or sea that intervene between it and the polar circle. ll'or the surface being in this case exposed to the vertical, or nearly vertical rays of the sun, absorbs a large quantity of heat, which it dif­fuses by radiation into the .atmosphere. For this reason, the western parts of the old continent derive warmth from Africa, "which, like an immense furnace," says Malte-Brun * " dis- . ' tnbutes its heat to Arabia, to 'J'urkey in Asia, and to Europe." ~n the. contrary, Asia in its north-eastern extremity, expc­l'lences m the same latitude extreme cold, for it has land on the north between the 60th and 70th parallel, while to the south it is separated from the equator by the North Pacific. In consequence of the more equal temperature of the waters of the ocean, the climate of islands and coasts differs essentially from that of the interior of continents, the former beinO' cha­racterized by mild winters and less temperate summers; fur the sea breezes moderate the cold of winter, as well as the summer ~eat. . When, therefore, we trace round the globe those belts m which the mean annual temperature is the same, we often find great differences in climate; for there arc insulm· climates where the seasons are nearly equalized, and excessive climates as they have been termed, where the temperature of winter and summer is strongly contrasted. 'rhe whole of Em·ope, ~omparcd with the eastern parts of America and Asia, has an ms~lar climate. The northern part of China, and the Atlantic reg10n of the United States, exhibit " excessive climates.'' 1Ve find at New York, says Humboldt, the summer of Rome and. the winter of Copenhagen; at Quebec, the summer of Par1s and the winter of Petersburgh. At Pekin, in China, • Phys. Gcog. Book xvii.