of the (ilaciem of the Alps. 113 



We have seen at the outset, that their reason of existence 

 is subject to three conditions, which consist of alimentation, 

 ablation, and movement. The two first are essentially me- 

 teorological ; the third, the dynamic force, is nothing more 

 than a property of matter. Passing over it, it only remains 

 for us to inquire what may have been the modifications that 

 have taken place in the alimentation and ablation. 



The fluctuations in the alimentation, or in the quantity of 

 snow fallen on a spot in a given time, are not directly connected 

 with a lowering of the temperature ; they do not necessarily 

 imply a vai'iation in the thermcmietrical mean. More snow 

 may fall in a medium whose temperature maintains itself be- 

 tween -1- 1° and— 12° C, than in that where the temperature 

 remains within the limits of— 12° — 24° C. We know that 

 the coldest winters are not the most snowy. 



It would not be a sinking of the temperature, therefore, 

 that we would have to regard as the origin of the super- 

 abundant alimentation of glaciers ; it would arise from a 

 more considerable evaporation in the low and warm regions, 

 because it is the hygrometrical state of the air. or the quan- 

 tity of vapour which is converted into snow on the great 

 condensers of the Alps, which is the primary cause of the 

 alimentation. 



The melting or ablation is in direct ratio with the tem- 

 perature, as the experiments of M. Agassiz on the glacier of 

 the Aar sufficiently demonstrate. If we could succeed in 

 shewing that the secular advancement of the glaciers arose 

 from a less active ablation in the present times, we might 

 thence immediately conclude, that the mean summer temper- 

 ature has sunk for some ages, but this conclusion would ap- 

 ply only to the fraction comprehending the four warmest 

 months of the year ; during the eight other months, the 

 ablation scarcely amounts to anything, because during this 

 period the glaciers are covered with a mantle of fresh snow, 

 which protects them from melting. 



From the preceding facts, we may pei'ceive that the problem 

 is very complex ; yet, if we set aside useless terms, it is only 

 in the study of meteorological phenomena that we can find 

 the solution of it ; it may even be found in two proposition;! 



VOL. XI-VII. NO. XCIII. — .JULY 1849. H 



