with Geographic Botany. — Temperature. 185 



very considerable elevation of the table lands stretching from 

 their northern roots. Immediately bordering the Himma-leh 

 range to the north are the greater and the lesser Thibet, or, 

 as M. Balbi terms them, east and west. He regards them as 

 consisting of two extensive table lands, having the surprising 

 hejght above the ocean of from 8000 to upwards of 14,000 

 feet. In addition to this elevation they contain large sandy 

 plains ; and the heating of the atmosphere from their burning 

 surface, added to the necessary allowances for elevation, will 

 fully account for the high station cultivation takes on the 

 north flank. It will be sufficient to add, that on the southern 

 side the presence of moisture, and the increased quantity of 

 rain falling over the plain of India, will of course cause a de- 

 pression of temperature unknown in the transparent and 

 heated atmosphere of Thibet. 



Valleys usually enjoy a milder climate than the adjacent 

 country, from the protection afforded them by surrounding 

 eminences. Some of those among the Himma-leh moun- 

 tains on the Indian side have a vegetation hardly to be ex- 

 pected in such situations. The vegetation of the tropics has 

 migrated into them, at elevations of 2000 feet and upwards. 

 In these valleys the advantageous circumstances are/a pro- 

 tection from destructive winds, and an exposure to the warm 

 temperature and copious rains of a tropic, or at least sub- 

 tropic, climate. Detracting causes will sometimes occur even 

 in valleys ; the stillness of the air promotes nocturnal radiation 

 from the soil, and under this heat rapidly disappears. Mr. 

 Daniel on this subject says, " I have seen a difference of thirty 

 degrees on the same night between two thermometers, one 

 placed in a valley, the other on a surrounding eminence, in 

 favour of the latter." The valleys in Switzerland are cold for 

 another reason — the sides are often so precipitous that they 

 are more properly ravines ; the sun's rays descend into them 

 only during a very small portion of the day, and the conse- 

 quence is, the snow line often sinks 2500 feet. 



Similar instances of the powerful agency of local causes 

 might be multiplied indefinitely ; I shall only give one more, 

 which places it in another point of view, where what at first 

 appears an unfavourable exposure, and what in reality it still 

 continues to be, has enlarged the range of a plant. In many of 

 the alpine valleys of Dauphine the declivities with a northern 

 exposure are covered with larch ; those, on the other hand, 

 with a southern aspect are entirely destitute of them. 



Assuming a position at the equator, it will be observed, 

 that a set of phenomena takes place simultaneously in two 



Ann. $ Mag \N. Hist. Vol.ix. O 



