312 Dv T. Thomson on the Climate and 



district of Sikkim, which is on that account tho most rainy part of 

 the whole range of the Himalaya, for, on the one hand, the more 

 eastern parts of the chain are protected by the Khasya range, and, 

 on the other, the more westerly parts are more distant from the 

 source of moisture, and, therefore, receive a less share of it. The 

 interception of the moisture from the province of Bootan and the in- 

 dependent states north of Assam, by the Khasya range, has this 

 curious effect, that the lower ranges of this portion of the Himalaya 

 are dry and arid, while above 7000 feet, to which elevation only the 

 hills to the south attain, the climate is much more humid. 



The diminution in the amount of moisture in proceeding to the 

 westward along the Himalaya from Sikkim is extremely gradual, but 

 also so far as our at present rather limited number of observations 

 goes, very regular. The effects of the south-west or rainy monsoon 

 diminish step by step, as we advance westward, till on arriving at 

 the valley of the Indus at the western extremity of the Himalaya, it 

 ceases to be observed at all. In these most western portions of the 

 chain, very little rain falls at any season of the year, and the little 

 which does occur, falls in the spring months, and is, therefore, quite 

 independent of the regular monsoon. 



It is also worthy of note, that in the more western parts of the 

 chain, the climate is extremely dry at all periods of the year, except 

 during the monsoon or rainy season, as it is called in India, while 

 to the eastward the climate of the mountains shares to a considerable 

 extent the more equable and always moist climate of Bengal. 



The most important point of all, however, regarding the climate 

 in respect of its effects on vegetation which requires to be borne in 

 mind, is that a very great portion of the rain which falls is deposited 

 on the first range of mountains upon which the rain- wind strikes. 

 I have already pointed out that this is the case with the Khasya 

 range, and it is there strikingly illustrated by the fact, that it is only 

 on the very south side of the hills that the rain-fall is so enormous, 

 the fall twenty miles north of Churra being probably less than half 

 what it is there. 



This tendency of the rain-fall to exhaust itself very considerably 

 on the first range of mountains to which it has access, is peculiarly 

 important in a mountain chain 150 miles in width, its effect being 

 that the upper part of all the large valleys, and especially the inte- 

 rior valleys and their ramifications, are much more dry than those 

 adjacent to the plains of India. Even in the most humid part of the 

 Himalaya, in Sikkim, this difference is extremely marked, and in 

 the more dry parts to the west (the extreme east interior is not 

 known), the inner valleys are so dry that rain is scarcely ever known 

 to fall. 



In close connection with the increase of aridity, as we advance 

 from the plains of India to the interior of the mountains, I may 

 mention the increased elevation of the line of perpetual congelation, 



