INTKUIJUCIION. 3) 



ahi in a zone that is nearly 3000 feet lower than that to which 

 it attains in the equinoctial region of the Cordilleras. 



plateau, this limit is at 16,625 feet from 20^° to 32° of latitude, whilt 

 Bt the equator, iu the Andes of Quito, it is 15,790 feet. Such is the 

 resuh I have deduced from the combiuatiou of numerous data famished 

 by Webb, Gerard, Herbert, and Moorcroft. (See my two memoirs on 

 the mountains of India, in 1816 and 1820, iu the Ann. de CJdmie et di 

 Physique, t. iii., p- 303 ; t. xiv., p. 6, 22, 50.) The greater elevation to 

 which the Hmit of perpetual snow recedes on the Tartarian declivity 

 is owing to tlie radiation of heat from the neighboring elevated plains, 

 to tlie purity of the atmosphere, and to the infrequent formation of snow 

 in an air which is both very cold and veiy dry. (Humboldt, Asie Cen 

 trale, t. iii., p. 281-326.) My opinion on the ditference of height of 

 the snow-line on the two sides of the Himalaya has the high authority 

 of Colebrooke in its favor. He wrote to me in June, 1824, as follows: 

 " I also find, from the data in my possession, that the elevation of the 

 iiue of perpetual snow is 13,000 feet. On the southern declivity, and 

 at latitude 31*^, Webb's measurements give me 13,500 feet, consequently 

 500 i'eet more than the height deduced from Captain Hodgson's oh 

 servations. Gerard's measurements fully confirm your opinion tha' 

 the hue of snow is higher on the northern than on the southern side.' 

 <t was not until the present year (1840) that we obtained the complett 

 and collected journal of the brothers Gerard, published under the su 

 pervision of Mr. Lloyd. {Narrative of a Journey from Caionpoor h 

 the Boorendo Pass, in the Himalaya, by Captain Alexander Gerard ant 

 John Gerard, edited by George Lloyd, vol. i., p. 291, 311, 320, 327, an< 

 ^Ml.) Many interesting details regarding some localities may be fount ■ 

 in tlie narrative of A Visit to the Shatool,for the Ptirpose of determinin. 

 the Line of Perpetual Stiow on the southern face of the Himalaya, in At 

 gust, 1822. Unfortunately, however, these travelers always confoun 

 the elevation at which sporadic snow falls with the maximum of th 

 height that the snow-line attains on the Thibetian plateau. Captaii 

 Gerard distinguishes between the summits that rise in the middle o 

 the plateau, where he states the elevation of the snow-line to be b( 

 *ween 18,000 and 19,000 feet, and the northern slopes of the chain o 

 »he Himalaya, which border on the defile of the Sutledge, and can ri 

 diate but little heat, owing to the deep ravines with which they ai ^ 

 intersected. The elevation of the village of Tangno is given at onl » 

 9300 feet, while that of the plateau surrounding the sacred lake of Mj. 

 nasa is 17,000 feet. Captain Geraixl finds the snow^-line 500 feet lowt >. 

 on the northern slopes, where the chain of the Himalaya is broke i 

 through, than toward the southern declivities facing Hindostan, and h • 

 '.here estimates the line of perpetual snow at 15,000 feet. The moi 

 striking difierences are presented between the vegetation on the Thil 

 etian plateau and that characteristic of the southern slopes ot the Hin 

 alaya. On the latter the cultivation of grain is arrested at 9974 fee. 

 and even there the corn has often to be cut when the blades are sti., 

 green. The extreme limit of forests of tali oaks and deodars is 11,900 

 feet ; that of dwarf birches, 12,983 feet. On the plains. Captain Gerard 

 found pastures up to the height of 17,000 feet; the cereals will grow ui 

 14,100 feet, or even at 18,540 feet; birches with tall stems at 14,100 

 feet, and copse or brush wood applicable for fuel is found at an elevti 

 tioa of upward of 17,000 feet, that is to say, 1280 feet above the lowej 

 limits of the snow-line at the equator, in the province of Quito. It is- 



