Ai-i'ENDix G. RECIPROCAL EFFECTS OF VEGETATION, ETC. 439 



almost invariably to the north, or leeward of the great snowy 

 peaks, and consequently in a drier climate ; and there it will be 

 seen that these proportions are occasionally inverted ; and in Tibet 

 itself a degree of relative dryness is encountered, such as is 

 never equalled on the plains of Eastern Bengal or the Grangetic 

 delta. Whether an isolated peak rising near Calcutta, to the eleva- 

 tion of 19,000 feet, would present similar results to the above, is 

 not proven by these observations, but as the relative humidity is 

 the same at all elevations on the outermost ranges of Sikkim, 

 which attain 10,000 feet, and as these rise from the plains like steep 

 islands out of the ocean, it may be presumed that the effects of 

 elevation would be the same in both cases. 



The first effect of this humid wind is to clothe Sikkim with forests, 

 that make it moister still ; and however difficult it is to separate cause 

 from effect in such cases as those of the reciprocal action of humidity 

 on vegetation, and vegetation on humidity, it is necessary for the 

 observer to consider the one as the effect of the other. There is 

 no doubt that but for the humidity of the region, the Sikkim 

 Himalaya would not present the uniform clothing of forest that it 

 does ; and, on the other hand, that but for this vegetation, the 

 relative humidity would not be so great.* 



The great amount of relative humidity registered at 6000 to 8000 



* Balloon ascents and observations on small mountainous islands, therefore, 

 offer the best means of solving such questions : of these, the results of ballooning, 

 under Mr. Welsh's intrepid and skilful pioneering (see Phil. Trans, for 1853), have 

 proved most satisfactory ; though, from the time for observation being short, and 

 from the interference of belts of vapour, some anomalies have not been eliminated. 

 Islands again are still more exposed to local influences, which may be easily elimi- 

 nated in a long series of observations. I think that were two islands, as different 

 in their physical characters as St. Helena and Ascension, selected for comparative 

 observations, at various elevations, the laws that regulate the distribution of humidity 

 in the upper regions might be deduced without difficulty. They are advantageous 

 sites, from differing remarkably in their humidity. Owing partly to the inde- 

 structible nature of its component rock (a glassy basalt), the lower parts of Ascension 

 have never yielded to the corroding effects of the moist sea air which surrounds it; 

 which has decomposed the upper part into a deep bed of clay. Hence Ascension 

 does not support a native tree, or even shrub, two feet high. St. Helena, on 

 the other hand, which can hardly be considered more favourably situated for 

 humidity, was clothed with a redundant vegetation when discovered, and trees 

 and tree-ferns (types of humidity) still spread over its loftiest summits. Here 

 the humidity, vegetation, and mineral and mechanical composition reciprocate 

 their influences. 



