Appendix I. DISTURBING INFLUENCES. 447 



descending currents in the valleys, and to cloud or sunshine. Other 

 and still more important modifying influences are to be traced to the 

 monthly variations in the amount of humidity in the air and the 

 strength of its currents, to radiation, and to the evolution of heat 

 which accompanies condensation raising the temperature of elevated 

 regions during the rainy season. The proximity of large masses of 

 snow has not the influence I should have expected in lowering the 

 temperature of the surrounding atmosphere, partly no doubt because 

 of the more rapid condensation of vapours which it effects, and 

 partly because of the free circulation of the currents around it. 

 The difference between the temperatures of adjacent grassy and 

 naked or rocky spots, on the other hand, is very great indeed, the 

 former soon becoming powerfully heated in lofty regions where 

 the sun's rays pass through a rarefied atmosphere, and the rocks 

 especially radiating much of the heat thus accumulated, for long 

 after sunset. In various parts of my journals I have alluded to 

 other disturbing causes, which being all more or less familiar to 

 meteorologists, I need not recapitulate here. Their "combined 

 effects raise all the summer temperatures above what they should 

 theoretically be. 



In taking Calcutta as a standard of comparison, I have been 

 guided by two circumstances ; first, the necessity of selecting a spot 

 where observations were regularly and accurately made ; and secondly, 

 the being able to satisfy myself by a comparison of my instruments 

 that the results should be so far strictly comparable. 



I have allowed 1° Fahr. for every degree in latitude intervening 

 between Sikkim and Calcutta, as the probable ratio of diminution 

 of temperature. So far as my observations made in east Bengal and 

 in various parts of the Gangetic delta afford a means of solving this 

 question, this is a near approximation to the truth. The spring- 

 observations however which I have made at the foot of the Sikkim 

 Himalaya would indicate a much more rapid decrement ; the mean 

 temperature of Titalya and other parts of the plains south of the 

 forests, between March and May being certainly 6° — 9° lower than 

 Calcutta : this period however is marked by north-west and north- 

 east winds, and by a strong haze which prevents the sun's rays 

 from impinging on the soil with any effect. During the southerly 



