328 PHYSICAL GEOGRAPHY. 



peculiar to the temperate and cold zones, and on which, 

 since the immortal work of Saussure, a new light has of late 

 years been thrown by Venetz and Charpentier, and by the 

 meritorious arid intrepid perseverance of Agassiz. 



We know only the lower and not the upper limit of 

 perpetual snow, for the highest mountains of the earth are 

 far from attaining to those strata of highly rarefied and ex- 

 cessively dry air, concerning which we may suppose, with 

 Bouguer, that they no longer contain vesicular vapour 

 capable of being converted into crystals of snow, and of thus 

 becoming visible. The lower limit of perpetual snow is not, 

 however, a mere function of the geographical latitude, or of 

 the mean annual temperature, nor is it even under the 

 equator as was long supposed, or even within the tropics, 

 that the snow line reaches its greatest elevation above the 

 level of the sea. The phsenomenon is a very complicated 

 one, depending generally on relations of temperature and 

 moisture, and on the peculiar shape of the mountains : if 

 these relations are subjected to a still more particular 

 analysis, which a great number of recent determinations 

 renders possible, ( 401 ) we shall recognise, as concurrent causes, 

 the differences of temperature in the different seasons of the 

 year, the direction of the prevailing winds, and whether 

 they have blown over land or sea, the degree of dryness 

 or of moisture of the upper strata of air, the absolute 

 thickness of the accumulated mass of fallen snow, the 

 relation of the height of the snow line to the total height of 

 the mountain, the position of the latter in the chain of 

 which it forms a part, the steepness of its declivities, the 

 vioinity of other summits also covered with perpetual 

 snow, the extent, position, and elevation of the plain from 



