330 PHYSICAL GEOGRAPHY. 



parisons with other mountain chains ; but on the northern 

 declivity, under the influence of the high lands of Thibet; 

 whose mean elevation appears to be about 10800 French 

 feet (11510 English), the snow limit attains a height of 

 15600 French feet (16630 English). This phenomenon 

 was long contested both in Europe and in India, and 

 I have developed my views respecting its causes on several 

 occasions since the year 1820 : ( 403 ) it is interesting in 

 another point of view besides the purely physical one, 

 for it has exercised an important influence on the mode 

 of life of numerous tribes ; meteorological processes in the 

 atmosphere either favour or forbid an agricultural or a pas- 

 toral life to the inhabitants of extensive tracts of continent. 

 As the quantity of moisture in the atmosphere increases 

 with the temperature, this element, so important for the whole 

 organic creation, varies with the hour of the day, the season 

 of the year, and the degree of latitude and of elevation. Our 

 knowledge of the hygrometric relations of the atmosphere 

 has been materially argumented of late years by the method 

 now so generally and extensively employed, of determining 

 the relative quantity of vapour, or the condition of moisture 

 of the atmosphere, by means of the difference of the dew 

 point and of the temperature of the air, according to the ideas 

 of Dalton and of Daniell, and by the use of the wet bulb 

 thermometer. Temperature, atmospheric pressure, and the 

 direction of the wind, have all a most intimate relation to 

 the atmospheric moisture so essential to organic life, 

 influence, however, of humidity on organic life, is less 

 consequence of the quantity of vapour held in solution und< 

 different zones, than of the natur* and frequency of the 

 aqueous precipitations which refresh \>M ground in the form of 



