THE SURFACE OF THE EARTH, 



176. Climate dependent on elevation. Snow Line. It has 



been explained in our Tract on Terrestrial Heat, that as we 

 ascend into the atmosphere from the level of the sea, the mean 

 temperature gradually decreases, and beyond a certain elevation, 

 it falls even below th^freezing point of water ; above this limit, 

 therefore water cannot exist in the liquid state, and must assume 

 the state of snow or ice. Such an elevation is accordingly 

 designated the limit of perpetual snow, and it is marked on all 

 lofty mountains by the limits of their snow-covered sides. This 

 boundary is accordingly called the Snow Line. 



It may be stated as a general fact, subject nevertheless to 

 some local qualification, that the elevation of the snow-line is 

 greatest at those parts of the earth, where the climate at the level 

 of the sea is hottest, and that its elevation decreases with the 

 decrease of the mean temperature of the same level. The snow- 

 line, therefore, has the greatest elevation within the tropics, and 

 decreases gradually with the increase of latitude, until at the 

 polar circles it falls to the surface. 



Since climate therefore varies, not only with the latitude but 

 with the elevation of the place above the level of the sea, it 

 follows that in mountainous regions a variety of climates will 

 prevail, depending on the elevation of the summits. If that 

 elevation exceed the limits of the snow-line, all varieties of 

 climate between that which characterises the sea level, or 

 which is natural to the plateaux on which the mountains stand, 

 and the climate of the poles are found, and consequently a 

 corresponding variety of natural productions, vegetable and 

 animal, and corresponding susceptibilities of culture prevail. 

 This is a circumstance which gives much interest to mountainous 

 countries, and often confers upon them great commercial and 

 social advantages. 



It is evident, also, that the higher the mean temperature of 

 the plateaux is upon which snow-capped mountain ranges stand, 

 the greater will be the varieties of climates exhibited between 

 their summits and their bases, and the greater consequently will 

 be also the variety of natural productions and artificial culture. 

 Hence arises the magnificent display of vegetation exhibited in 

 those ranges of the Andes and Cordilleras, and other lofty ridges 

 which intersect the torrid zone. 



177- Vegetation of the Himalayas. Although the chain of 

 the Himalayas far exceeds in elevation the Andes and Cordilleras 

 of South America, they are thus, from their geographical position, 

 being situate far beyond the limits of the torrid zone, excluded 

 from the advantages here noticed, and they present none of 

 that inexhaustible variety of phenomena by which the ridges 

 178 



