Da. Thomson on the Climate arid Vegetation of the Himalaya. 195 



After these few words on the physical structure of the mountains, the 

 vegetation of which it is my wish briefly to describe, it will still be ne- 

 cessary to devote a few minutes to the subject of climate and humidity, 

 before I can proceed to my proper subject. 



Situated in the most southern part of the temperate zone, and bound- 

 ing on the north a great peninsula, which extends far into the torrid zone, 

 the base of the Himalaya to the south possesses an almost tropical cli- 

 mate, tempered however when the sun is on the tropic of Capricorn by a 

 moderately cool winter, and variously modified in different parts of the 

 chain by the degree of humidity, a most important matter to be taken 

 into consideration in every question connected with the phenomena of 

 vegetable life. 



The source of humidity in the Himalaya is almost entirely the Bay of 

 Bengal, which is situated about 5 degrees to the south of the eastern 

 extremity of the chain ; and the wind which carries the humid atmosphere 

 along the chain, is that which is known to nautical meteorologists as the 

 south-west monsoon, a wind which begins to blow in the open sea about 

 the month of April, but whose effects are not felt in the far interior before 

 the month of June. This wind, though constant in its direction at sea, 

 is not so in its inland course; at the head of the Bay of Bengal it is 

 almost a south wind; it blows from the sea nearly due north towards the 

 Himalaya, striking in its course upon the low chain of the Khasya hills, 

 whose maximum elevation is scarcely 7,000 feet. 



Upon this range the first force of the monsoon is expended, and the 

 annual fall of rain at Churra Poonjee, elevated 4,000 feet on its southern 

 slope, amounts to about 500 inches. This range, which has its origin 

 among the mountain ranges of the south of China and north of Burmah, 

 lies to the south of the Burrampooter, and following the course of that 

 river, terminates in the concavity of its great bend, where it turns down 

 toward the sea. The Khasya mountains do not therefore entirely run 

 across the Bay of Bengal, so as to intercept the force of the monsoon from 

 the whole of the Himalaya, a part of which wind, laden to saturation with 

 moisture at a temperature of nearly 90° F., blows due north from the Bay 

 of Bengal upon the district of Sikkim, which is on that account the 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 independent 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 7,000 feet, to which elevation only the hills to the south attain, 

 the climate is very much more humid. 



The diminution in the amount of moisture in proceeding to the west- 

 ward along the Himalaya from Sikkim is extremely gradual, but also so 

 far as our at present rather limited number of observations goes, very 



