Glaciers irfthc Alps. 81 



by the periodical return of the heat of each day, determines in 

 the upper surface a state of tension opposed to the state of the 

 inferior surface, and the inverse effect results from the recool- 

 ing which affects the upper surface during the winter and the 

 night, whilst the temperature of the other remains always the 

 same. Consequently, by this opposition, the upper fissures are 

 formed during the day and during the summer, the lower cre- 

 vices during the night and winter. Each fissure presents at 

 the beginning but a small aperture formed in the upper or lower 

 surface of the mass of ice, in a state of tension. Its enlarge- 

 ment is gradual, as the influence of the atmosphere and the 

 course of the temperature in the interior of the glacier ; it often 

 finally traverses the glacier in its whole extent, and then it opens 

 further, forming a wide and frightful fissure. There is still to 

 remark concerning the course of fissures, that in the glaciers 

 nearly horizontal and very long, such as the inferior glacier of 

 the Aar and that of Aletsch, we never find them very wide. In 

 proportion as the declivity of the glacier is steep, the fissure 

 becomes wider. This appears to depend on the more or less 

 great resistance which the glacier has to overcome in its pro- 

 gressive movement." 



We shall terminate this article by relating some considera- 

 tions of the author concerning the periods of progression and 

 retrogression which he has observed in the glaciers, a subject 

 which he proposes to study afterwards in a much more accurate 

 manner. " Each glacier of the second kind," says he, '' is, as we 

 have seen, originally a glacier of the first kind ; having arrived 

 at its second state, it advances to dissolution. When, by a series 

 of years abounding in snow, the glaciers of the first kind accu- 

 mulate in an extraordinary manner, they produce equally to- 

 wards their lower border glaciers of the second kind of great 

 magnitude. These colossal masses, more extended in all their 

 dimensions than they are in general, necessarily require a longer 

 time to dissolve ; and as their progressive movements continue, 

 they advance also much further into the inhabited valleys. On 

 the contrary, the glaciers of the first kind, which are not so 

 thick, never produce large glaciers of the second kind ; then 

 those of less ntagnitude are dissolved before they reach the 

 bottom of the valleys, and appear to contract themselves. More- 

 APllIL — JUNE 1831. F 



