CHANGES OF THE SNOW LINE. 47 



remarkable is, that there exists between the northern and the 

 southern slopes of the mountain the same difference as between 

 the southern and northern hemispheres : the line of the eternal 

 snows descends much lower on the northern than on the 

 southern slope, in the same manner as, in the southern hemis- 

 phere, the polar ice advances much nearer the Equator than in 

 the northern. 



Such is the general view-point which we must adopt for the 

 clearer comprehension of the details of observation. Of course, 

 when speaking of the limit of the eternal snows, we refer only 

 to the lower limit, that is to say, to the greatest elevation 

 attained by the snow-line in the course of a single year. As 

 for the upper limit, it entirely escapes us ; for the summits of 

 the loftiest mountains do not reach the atmospheric strata 

 which, by virtue of their refraction, cannot contain any 

 vesicular, aqueous, or condensable vapour. 



The line of eternal snow which, at the poles , is found on the 

 level of the ground, gradually rises as we approach the torrid 

 zone, where it attains its maximum of elevation, from 13,000 

 to 17,000 feet. This phenomenon does not exclusively depend 

 upon the geographical latitude, nor on the mean annual 

 temperature of the locality : it is the result of an aggregate of 

 diverse circumstances which we have not the space here to 

 enumerate and discuss. We shall content ourselves with 

 placing before the reader a table which will show the remark- 

 able differences existing in the height of the perpetual snow* 

 line in various places. 



