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Show TIERRA DE L FUEGO. June, 1834. There are no direct observations, by which to judge of the m,ean temperature of the year in these southern islands. But after reading the above accounts, it will readily be granted that it must be very low. Even in Georgia, in lat. 54°-55°, it is not improbable that the soil is perpetually frozen at a few feet beneath the surface. At Deception Island in lat. 62°-63° from the preservation of the dead body alluded to, and the interstratification of ice with the volcanic ashes, we may feel almost sure that such must be the case. In the northern hemisphere, it is only on the great continents that so low a mean temperature is found in corresponding latitudes. In North America, according to Richardson,* north oflat. 56°, the thaw does not penetrate to a greater depth than three feet. In the Steppes of Siberia, Humboldtt states that to the northward of 62°, the ground between twelve and fifteen feet below the surface is always frozen. In the space, however, between these two great northern continents, the line of perpetual congelation rises considerably towards the north. It is a remarkable meteorological fact, that in the northern and southern hemispheres, a low mean temperature, in latitudes without the frigid zone, is the result of a directly opposite condition of things. In the northern hemispl~ er~ the atmosphere is rendered extremely cold, from the ~a~1ation of a large extent of country during a long winter ; nor 1s 1t moderated by the warmer currents of any neighbouring sea : hence the extreme cold of the winter more than counterbalances the heat of summer. In the southern hemisphere, on the other hand, although the winter is moderate, the summer is cold ; for a sky constantly clouded rarely permits the rays of the sun to warm the surface (itself a bad absorbent) of the great ocean : hence, the mean temperature of the year falls below the freezing point. It will .at once be evident, that a kind of vegetation which reqmres an equable temperature, will approach much nearer * Appendix to Back's Expedition. t Fragmens Asiatiq ues, vol. ii., p. 386. June, 1834. IIEIGIIT OF SNOW-LINE. the line of perpetual congelation in a climate such as this of the southern hemisphere, than in the opposite one subject to extremes. The height of the plane of perpetual snow in any country, seems chiefly to be determined by the extreme heat of summer, rather than by the mean of the year. As the summer in Tierra del Fuego is so very wretched, we ought not to feel surprised at the fact stated by Capt. King, -that in the Strait of Magellan, the line descends to about 3500 or 4000 feet. In the northern hemisphere, we must travel about fourteen degrees nearer the pole to meet with so low a limit, namely, between lat. 67° and 70° on the mountains of Norway. In the Cordillera of South America, between latitudes 41° and 43° 30', the culminant peaks have altitudes pretty nearly equal. Several were measured by the o~cers of th.e BeaO'le with considerable care, by angles of elevatiOn, the posl-o 0 . tions of the mountains being accurately known. sorno IS 7550 feet; mountain south of Osorno 5609; Minchinmadiva 7046 ; northern end of same range 6862 ; Corcovado 7 510 ; Yntales 6725. Not only these points, but a great ~art. of the range* was thickly clothed with snow, in .the begmmng of February (answering to our August), whiCh desc~nded some way down the mountains, and presented to a d1stant beholder a perfectly horizontal line. We were assured that the snow, which it appeared must inevitably be the case, remained throughout the year. On January 26th, afte~ a week of uncommonly fine weather, Mr. King measured w1t? a pocket sextant, the angle of this line with the summit of the Corcovado ; and subtracting the result from the total height, the snow-line was found to descend to 4480 feet. It is possible that there may have existed some unk~own cause of error; but as the average height of the few lzzghest • Mr. Sulivan, who surveyed this part of Chilt1e, informs me, t~1at between Osorno and Yntales, there are probably many mountains whtch rise to a height of nearly 6000 feet. He says he does .not recollect any one summit, which (during January) was not covered With snow. T 2 |