THE SNOW-LINE. 337 



Venetz, Charpentier, and the intrepid and persevering observer 

 Agassiz. 



We know only the lower and not the upper limit of per- 

 petual snow, for the mountains of the earth do not attain to 

 those ethereal regions of the rarified and dry strata of air in 

 which we may suppose, with Bouguer, that the vesicles of aque- 

 ous vapour are converted into crystals of ice, and thus rendered 

 perceptible to our organs of sight. The lower limit of snow is 

 not, however, a mere function of geographical latitude, or of 

 mean annual temperature ; nor is it at the equator, or even in 

 the region of the tropics, that this limit attains its greatest 

 elevation above the level of the sea. The phenomenon of 

 which we are treating is extremely complicated, depending on 

 the general relations of temperature and humidity, and on the 

 form of mountains. On submitting these relations to the test 

 of special analysis, as we may be permitted to do from the 

 number of determinations that have recently been made,* 

 we shall find that the controlling causes are the differences 

 in the temperature of different seasons of the year; the direc- 

 tion of the prevailing winds and their relations to the land 

 and sea; the degree of dryness or humidity in the upper 

 strata of the air; the absolute thickness of the accumulated 

 masses of fallen snow; the relation of the snow-line to the 

 total height of the mountain ; the relative position of the latter 

 in the chain to which it belongs, and the steepness of its 

 declivity; the vicinity of other summits likewise perpetually 

 covered with snow ; the expansion, position, and elevation of 

 the plains from which the snow-mountain rises as an isolated 

 peak, or as a portion of a chain ; whether this plain be part 

 of the sea coast, or of the interior of a continent; whether it be 

 covered with wood, or waving grass ; and whether, finally, it 

 consist of a dry and rocky soil, or of a wet and marshy 

 bottom. 



The snow-line which, under the equator in South America, 

 attains an elevation equal to that of the summit of Mont 

 Blanc in the Alps, and descends, according to recent mea- 

 surements, about 1023 feet lower towards the northern tropic 

 in the elevated plateaux of Mexico (in 19 north latitude), 



* See my table of the height of the line of perpetual snow, in both 

 hemispheres, from 71 15' N. lat. to 53 54' S. lat., in my Asie centrale, 

 t. iii. p. 360. 



Z 



