172 Mn. J. Bkyck on the Recent Progress of the 



according to the principles of the dynamical theory of heat lately 

 developed by Joule, Professor \V. Thomson remarks, — " Some idea of 

 the actual amount of mechanical energy of the luminiferous motions and 

 forces within our own atmosphere may be given by stating that the 

 mechanical value of a cubic mile of sunlight is 12,050 foot pounds, 

 equivalent to the work of one horse power for the third of a minute." — 

 (On the Density of the Luminiferous Medium, Transactions of the Royal 

 Society of Edinburgh, vol. xxi., Part I). We shall again recur to the 

 solar influence when speaking of terrestrial magnetism. 



(13.) Some very curious facts regarding atmospheric temperature have 

 been made known to us within a few years by scientific travellers, 

 among whom two townsmen of our own are distinguished — Dr. Joseph 

 D. Hooker, and Dr. Thomas Thomson. These relate chiefly to the 

 situation of the snow line in Central Asia, and show us how dependent 

 the elevation of this line is upon other causes than latitude merely. 

 While the mean level of the snow line on the equator in S. America 

 is from lo,200 to 15,800 feet, it rises on the inner ranges of the 

 Tibetan mountains, about lat. 28° to 30°, and on the Karakorum 

 mountains, lat. 33° to 35°, to about 20,000 feet! In the outer 

 ranges of the Himalaya about Sikim, the level ranges from 13,000 

 to 18,000 feet, the mean being about 16,000 feet. In the same 

 latitude in Spain, the snow line is 9,500 feet. The mean at lat. 

 0° being thus 15,500 feet, and at lat. 28° 16,000 feet, or 500 feet 

 higher, the remarkable difierence has no relation to latitude. The 

 cause no doubt is the greater heating of the atmosphere from the 

 larger extent of land in the latitude of Sikim, while in South America 

 the land is narrower, covered with moist forests, and the Andes press 

 close on the vast body of waters in the Pacific Ocean. In Bhotan, on 

 the southern Himalaya, towards the head of the Bay of Bengal, where 

 the movmtains are open to the influence of the moist ciu-rents of the 

 monsoon, and are protected from direct solar radiation by the fogs and 

 mists thus generated, the snow line sinks to 13,000 feet. As we advance 

 northwards, the line rises uniformly, reaching, as already stated, 20,000 

 feet. This remai'kable anomaly arises from the small supply of mois- 

 ture, the lofty spurs of the chain towards the Indian plains intercepting 

 the greater part of it, by the cleai'er sky permitting a more fervid radia- 

 tion, by the great amount of heat reflected from the bare arid plains, 

 and by the dry winds sweeping over these elevated tracts, under whose 

 influence snow and ice evaporate without melting. Facts of exactly 

 the same significancy have been made known to us regarding Norway 

 by Professor James D. Forbes. Here, in lat. 60°, the snow line is at 

 1,450 feet near the coast : inland it rises 1,000 feet, or to 5,500; 



