lieved to be at 11,700 feet, about 250 feet above the Peak of Teneriffe; in lat. 37°30', on Etna, at 9000 feet; in lat. 42°30′, in Asia, among the Caucasus, at 9000 feet; in lat. 46°, among the Alps, at 8220 feet; according to Hugi, the névé of the Alps is at a height of about 7800 feet. Among the Pyrenees, according to Bouvard, at from 1350–1400 toises, or a mean of 8892 English feet; but the Pic du Midi de Bigorre, which rises to a height of 1493 toises, or 9432 feet, is free of snow part of the year. At Edinburgh, were there a mountain sufficiently lofty, at 6000 feet; and in Iceland, in lat. 65°, at 2892 feet Farther north, in Lapland, in lat. 70°, where the winters are more rigorous, but the summers milder than in Iceland, according to Von Buch and Wahlenberg, the snow-line is 3517 English feet above the sea.' Humboldt' has given a valuable table of the limits of this line in both hemispheres, computed from measurements.

56. Upon volcanoes the snow-line is subject to occasional changes, depending upon the presence of internal fires. Thus, that of Mount Aconcagua, in S. lat. 32°30', to the north-east of Valparaiso, though higher than Chimborazo, has been seen without its snowy mantle; and in January 1803, the same was witnessed on Cotopaxi, the night before a dreadful eruption.

57. In lat. 30° 40', at the temple of Kedar-nath, on the southern side of the Himmalehs, elevated 11,897 feet above Calcutta, or 12,000 above the sea, snow falls, but does not lie throughout the year. Captain Webb could not find a vestige of snow on the Nitee Ghaut, in Kemaoon at a height of 16,814 feet3. On this the northern side, the snow-line rises 17,000 feet above the sea. What a singular anomaly! A place in Asia having a snowline 1200 feet higher than a mountain under the equator in

1 Gilbert's Annalen der Physik, 1812, xli; Annals of Philos. iii. 347.

2 Asie Centrale; Recherches sur les Chaines de Montagnes et la Climatologie comparée, par A. de Humboldt. 8vo. Paris, 1843, tom. iii. 359.

"On the 21st August 1818, on the crest of the Nitee Ghaut, in N. lat. 31°, at 3 P.M., Captain Webb found the average height of four barometers 16.27 inches, with a temperature of 47° F.; on the same day and hour, Colonel Hardwicke, at Dumdum, near Calcutta, about 50 feet above the sea, observed the barometer at 29.48 inches, and thermometer 84° F.

America! The village and temple of Milem among the Himmalehs, in lat. 30° 25', are far below the range of perpetual congelation at an altitude of 11,790 feet, where even luxuriant crops are gathered. In the same European latitude, the snow-line descends several hundred feet below this altitude. At a similar height, on the 18th June 1818, Captain Webb was encamped amidst the most flourishing oak and arborescent rhododendrons. On the following day, on the crest of the Pilgointi Churhai pass, more than 12,700 feet above the sea, no snow was visible. Instead of white-capt mountains, a profusion of rich flowers met the eye; and even 1000 feet higher, the goat-herds lead their flocks. The pass of Oota Dhoora, which lies east of the Nitee Ghaut, and a day's march beyond Milem, was ascertained barometrically to be 17,780 feet high, and even there the snow lies only part of the year. The Bhotiya traders frequent this pass more than any other throughout the entire range. Gerard observes that he crossed beds of snow, when proceeding, in 1821, by the Chárang pass, 17,348 feet above the sea, to the valley of Nangalti. He also states, that the lofty mountains in the neighbourhood of Charang, whose peaks tower to the giddy height of nearly 18,000 feet, are not covered with snow. On the right bank of the Tagla, at an elevation of 18,000 feet, snow was seen, but scanty; and those mountains which enclose the Tagla, rising between 19,000 and 20,000 feet are just tipped with it. He farther observes, “Zamsiri, a halting-place for travellers on the banks of the Shelti, is 15,600 feet above the sea,-a height equal to that of the passes through the outer range of the snowy mountains, and yet there is nothing to remind the traveller of the Himmalehs. Gently sloping hills and tranquil rivulets with banks of turf and pebbly beds, flocks of pigeons, and herds of deer, present the idea of a much lower elevation. But nature has adapted the vegetation to the country; for did it extend no higher than on the southern face of the Himmalehs, Tartary would be uninhabitable either by man or beast. On ascending the southern acclivity of the snowy range, the extreme height of cultivation is found to be 10,000 feet, and even there the crops are frequently cut green.

The highest habitation is 9500 feet; 11,800 feet may be reckoned the upper limit of forests, and 12,000 feet that of bushes, although in a few sheltered situations dwarf-birches and small bushes are found almost at 13,000 feet. But if we go to the Baspa river, the highest village will be found at an elevation of 11,400 feet, cultivation reaching to the same altitude, and forests extending to 13,000 feet at least. Advancing farther, we find villages at the last-mentioned elevation, cultivation 600 feet higher, fine birches at 14,000 feet, and tama bushes, which furnish excellent fire-wood, at 17,000 feet. Eastward, according to the accounts of the Tartars, crops and bushes thrive at a still greater height." In 1823, from the crest of the Hangarang pass, whose height Gerard measured barometrically at 14,837 feet, this traveller beheld in front " a granitic range of most desolate aspect; not a blade of vegetation visible, the snow itself only finding a resting-place at 19,000 feet." One longs to stand where this officer stood, and behold the sublime vista which he describes,-" Beyond it, through a break, were seen snowy mountains, pale with distance, appearing to rise out of the table-land on the banks of the Indus; and from the angles of altitude which I observed, their pale outline, and the broad margin of the snow, they cannot be less elevated than 29,000 feet."

58. The snow-line among the Himmalehs, is thus shewn. to be several thousand feet higher on the northern than upon the southern side, a fact of high value, as it proves the fallacy of theoretical calculations in this peculiar instance, which would have assigned for the snow-lines a position very different from that which it occupies. Doubtless the dry, serene, and transparent atmosphere of these lofty mountains, together with the radiation of heat from Thibet, are the cause of the anomaly described. Conclusive as the evidence appears, supported by the testimony of other travellers and Baron Humboldt,' that the snow-line is higher on the northern than upon the southern side of the Himmalehs, it is not a

Ann. de Ch. et de Phys. tom. iii. 303; ib. xiv. 5-55; Asie Centrale, tom. iii. 281-327; Cosmos-Sabine, vol. i. p. 465; Calcutta Jour. of Nat. Hist. 1844, vol. iv., compared with Jour. of Asiat. Soc. of Bengal, 1840, vol. ix. pp. 575, 578, 500.

little singular, that the fact should be doubted by some and denied by others. It seems, however, beyond question, that the general opinion is correct, and though the peculiarity is singular, yet when we consider the cause, it is not remarkable.

59. We have at the beginning of this section stated that the solar beams contain, besides luminous and thermal, chemical or actinic rays. The two first are of variable intensity, as our senses prove, and this mutability is chiefly dependent on the earth's position in her orbit, and her diurnal revolution; but the actinic rays suffer changes independent of these causes, and as yet not well understood. This has been proved by the actinograph, an instrument constructed for registering the amount of actinism in the solar beams. Mr Hunt found in his observations, that though the photographic papers employed were of uniform quality, he could not obtain a constant impression. Thus during the intensity of the heat and light of June and July 1846, the actinograph failed to reach the maximum. If that extreme be indicated by 100, then during these months the instrument ranged between 67 and 83. Practically the same truth was demonstrated, for it was with much difficulty that during these months photographic pictures could be obtained, notwithstanding an intense light and clear sky. Mr Bingham supposes this to have arisen from the condensation of moisture upon the plates, from the bromine solution employed. Probably it arose from the same unexplained cause which towards the equator produces a similar inconvenience. The cause we are yet unable to understand, but the fact is deeply interesting. May it not contain the germ of what, under the vague term of "atmospheric influence," exercised a force so powerfully destructive in this kingdom? It may be observed, that, during the same months, vegetation participated in the effects of unusual intensity of light and heat, by the development of leaves within the flowers. Results similar to those which follow the impact of solar rays, attend when photographic papers are placed in connexion with the negative wire of the electric apparatus,

D

CHAPTER IV.

60. Isothermal lines. 61. Illustrated. 62. Divide the globe into botanical regions. 63. Mean annual temperature. 64. Its remarkable steadiness. 65. Not materially changed since early ages. 66. Range of temperature, extreme heats. 67. Extreme colds. 68. Excessive cold may be borne with impunity. Extraordinary heats borne without injury. 70. Effect of intense cold on sound. 71. Provision made by the Esquimaux for spending their long winters; effect of cold and want of light upon human race. 72. Dangers escaped by a scientific party. 73. Physiological effects of great cold. 74. Antiseptic property of intense cold. 75. Poles of maximum cold. 76. Their connection with the magnetic poles. 77. Hypotheses. 78. Isogeothermal, isotheral, and isochimenal lines. 79. Formulæ for computing mean temperatures objectionable. 80. Influence of ocean upon temperature. 81. Influence of continents upon temperature. 82. Historical records of intense frosts. 83. Temperature of Southern Hemisphere. 84. Difference explained. 85. Temperature of space. 86. Whence

its source.

60. As we recede from the equator, the extremes of annual temperature increase. The mean temperature of various latitudes attracted the attention of Baron Humboldt,' and with the view of tabulating the results, so that they might be presented in a tangible form, he constructed his Isothermal lines ("os equal, and guos heat), or lines passing through places possessing the same mean annual temperature. This renowned philosopher detected a considerable difference in temperature between the old and new continents under the same parallels. Thus, at lat. 30°, the temperature in the old continent is 70°.52, while in the new continent it falls to 66°.9. Again, in lat. 40°, it is 63°.14 in the former, and 54°.5 in the latter; in lat. 50°, it equals 50°.9 in the one, and 37°.9 in the other;

Mém. d' Arcueil, tom. iii. p. 462; Fragments Asiatiques, tom. ii. p. 398; Ed. Phil. Jour. vols. iii. iv. and v., 1820, 1821.