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Show 118 CAUSES OP . d * nd that a correspon d1'n g• interchange rents is not dispute ' a. demonstrated, accordmg to Hum· takes place in the seas~ IS • found to exist at great depths boldt, by the cold wlnch IS other proofs, may be men- . · and among · between the tropics ' ' f t wlJich the Gulf stream IS . lume o wa er tioned the great vo h ·d while another current flows constant1 y b ear ing nort war st, f Greenland an d L ab rad o r, firom the north along the c?l~bs . om i· tl e equt 1 rm · and helps to restore . 1. and colder water pass from the poles Currents of heavier . h l the inferior parts of the ocean; towards the equator, whlC 'dcoo and the cold of the polar 1 t f the torn zone, so that the 1ea 0 Tl refriO'eration, therefore, of the circle, balance each o~her ·f. 1 ;he su~posed alteration in the Polar regions, resultmgd tom uld be immediately commu- . . f 1 d an sea, wo distnbut10n ° an. nd from them would extend to the ant-nicated to the tropics, a here and the ocean would be arctic circle, where the atmosp uld augment. Although the h t · e and snow wo . cooled, so t a IC f hiO'her latitudes in the southern hemJS· mean temperature 0 0 d .r the most part lower than that · e have state , J.Or · bl phere ts, as w 1 . th orthern yet for a cons1dera e of t h e same Par.a lle s f mh e n ' f h 1. the mean annual heat o t e space on eac h side o t he me, . 11 I 1• n correspondmg para e s. • .c d to be t e same waters IS 1oun 't' f the land the gene· £ b the new pos1 1on o ' When, there ore, Y f f equent occurrence in the tern- . f · b shad become o r h ratmg o Ice erg the were frequently drifted as far as t c Perate zone, and when Y f ld would immediately be com-b degree o co equator, t e same . f Capricorn and from thence . d f as the tropic o ' h mumcate as ar 1 th The freedom, then, oft e to the l~nds or ocean tod t Je l~u fr~m pole to pole being duly circulation of h~at an ·~o t tl at the mean quantity of heat considered, it will be evl en 1 . bl 'tb great force from the Island of St. * The trade wind continually ?ws Wld' b'lch during tho eruption of the b d . notw1thstan mg w ' . t Vincent to that of Bar a oes '. t in 1812, ashes fell in profusiOn from~ grea volcano in the Island of St. Vmcen ' b . does Thill apparent tro.nsportahou of height in the atmosphere upon Bar a . . . of the existence of a counter-cur· matter against the wi~d, con~rmecl the o~~:o; rested on theoretical conclusions. . t . the higher rerrlons, which bad pre y ren m "' & 103 · · Daniell's Meteorological ~ssays~ c., P·. d water in' this chapter, no allusiOn IS t In Slleaking of the cuculabou oflat~t~n . the direction of currents, caus~d . d t 'rregu an 1es m f m d to the trade wm s, or o 1 t the movements ro ma e th These causes preven afli t by the rotatory motion of the ~~r . f om south to north, but they do not ~c ·'- . direct from north to sou ' or r uemg . ul t' the theory of a constant crrc a lOll. CHANGES OF TEMPERATURE. 119 which at two different periods visits the same point, may differ far more widely than the mean quantity which any two points receive in the same parallels of latitude, at one and the same period. For the range of temperature in a given zone, or in other words, the curves of the isothermal Jines, must always be circumscribed within narrow limits, the climate of each place in that zone being contro1led by the combined influence of the geographical peculiarities of an other parts of the earth. But, when we compare the state of things as existing at two distinct epochs, a particular zone may at one time be under the influence of one class of disturbing causes, as for example those of a refrigerating nature, and at another time may be affected by a combination of opposite circumstances. The lands to the north of Greenland cause the present climate of North America to be colder than that of Europe in the same latitudes, but they also affect, to a certain extent, the temperature of the atmosphere in Europe; and the entire removal from the northern hemisphere of that great source of refrigeration would not assimilate the mean temperature of America to that now experienced in Europe, but would render the continents on both sides of the Atlantic much warmer. To return to the state of the earth, after the changes before supposed by us, we must not omit to dwell on the important effects to which a wide expanse of perpetual snow would give rise. It is probable that nearly the whole sea, from the poles to the parallels of 45°, would be frozen over, for it is well known that the immediate proximity of land, is not essential to the formation and increase of field ice, provided there be in some part of the same zone a sufficient quantity of glaciers generated on or near the land, to cool down the sea*. Field ice is almost always covered with snow, through which the sun's rays are unnble to penetrate t, and thus not only land as extensive as our existing continents, but immense tracts of sea in the frigid and temperate zones, would now present a * See Scoresby's Arctic Regions, vol. i. p 320. f Captain Scoresby, in his account of the arctic regions, observes that when the sun's rays" fall upon the snow-clad surface of the ice or land, they are in a grt!at measure refl~cted, without producing any material elevation of ttlmperature i. but when they impinge on the black exterior of a ship, the pitch ou one side occas1onally becomes fluid, while ice is rapidly generated at the othllr." vol. i. p. 378. |