88 CLIMATE AND VEGETATION OF 



the wind still ascends ; it is, in short, an ascending warm moist 

 current, whatever course be pursued by the valleys it follows. 



The sides of each valley are iience equally supplied with 

 moisture, though local circumstances render the soil on one or the 

 other flank more or less humid, and favourable to a luxuriant 

 vegetation : such ditferencesare a drier soil on the north side, with 

 a too free exposure to the sun at low elevations, where its rays, 

 however transient, rapidly dry the groimd, and where the rains, 

 though very heavy, are of sliorter duration, and owing to the 

 capacity of the heated air for retaining moisture day fogs are 

 comparatively rare. In the northern parts of Sikkim, again, some 

 of the lateral valleys are so placed that the moist wind strikes 

 the side facing the south and keeps it very humid, whilst the 

 returning cold current from the neighbouring Tibetan mountains 

 imjjinges against the side facing the north, which is hence nearly 

 bare of vegetation. An infinite number of local peculiarities 

 will suggest themselves to any one conversant with physical 

 geography, as causing unequal local distribution of light, heat, 

 and moisture in the different valleys of so irregular a country 

 — the amount of slope, and its power of retaining moisture 

 and soil ; the composition and hardness of the rocks ; their dip 

 and strike ; the protection of some valleys by lofty snowed ridges, 

 and the free southern exposures of others at great elevations. 



One other peculiarity deserves especial attention, which is, the 

 position of the great masses of perpetual snow. A reference to 

 the woodcut will show that the same circumstances which affect 

 the distribution of moisture and vegetation, determine the posi- 

 tion, amount, and duration of the snow. The principal fall will 

 occur, as before sliown, where the meridional range first attains 

 a sufficiently y^reat elevation, and the air becomes consequently 

 cooled below 32° ; this is at a little above 14,000 feet, sporadic 

 falls occurring even in summer at that elevation : this snow, 

 however, melts immediately, as do the copious winter falls in 

 the summer months. As the depth of rain-fall diminishes in 

 advancing north to the higher parts of tiie meridional ranges, so 

 does the snow-fall. Its permanence, again, depends on — 1. the 

 depth of the accumulation ; 2. the mean temperature of the 

 spot; .3. the melting power of the sun's rays ; 4. the prevalence 

 and strength of evaporating winds. Now at 14,000 feet, though 

 the accumulation is immense, the amount melted by the sun's 

 rays is trifling, and there are no evaporating winds ; but the 

 mean temperature is so high, and tlie corroding powers of the 

 rain (which falls abundantly throughout summer) and of the 

 warm and humid ascending currents are so great, that the snow 

 is not perennial. At 15,000 feet, again, it becomes perennial, 

 and its permanence at this low elevation (at P) is much favoured 



