Oit the Downward Progress of the Glaciers of the Alps. 105 



pressure, and evaporcation, these 17 metres become reduced 

 to 5 metres of neve, or ice of neve, the density of which is 

 250 kilogrammes the cubic metre. During the three warm- 

 est months of the year, or rather the three in which there is 

 least cold, the ablation does not exceed one metre of the sur- 

 face ; there remain, therefore, four metres every year, which 

 the sun cannot melt at that altitude. If no movement took 

 place in these four metres, if the property they possess of 

 moving onwards, and gradually reaching the lower regions, 

 were taken away, and they were thus rendered immoveable, 

 four new metres would be added every year to those of preced- 

 ing years ; and even during historical times, the neves accu- 

 mulated in the upper regions would surpass the height of the 

 most elevated summits of the chain. 



Thus, by reference to facts, we obtain a proof that the 

 movement alluded to takes place on all the cols or passes 

 and most elevated summits of the Alps, wherever the heat 

 of summer is not sufficient to melt the entire snows of winter. 

 This movement, therefore, has its point of departure in the 

 circuses, passes, and summits; it commences at the upper limit 

 of all the basins of the glaciers, propagates and develops it- 

 self throughout their whole mass, and the whole extent of their 

 course ; very feeble at first, it gradually acquires some de- 

 gree of speed in the middle regions, and again becomes slower 

 in the lower regions. All the snows above 3000 metres being 

 destined by nature to be melted, that they may not accumu- 

 late indefinitely, they are made to descend to the lower re- 

 gions where they are exposed to a warmer sun which allows 

 them to dissolve into water. 



The alimentation and movement are, therefore, two forces, 

 one of which accumulates the snows of the higher regions in 

 a vertical direction, and the other spreads them over a greater 

 surface in a longitudinal direction. If these two forces ex- 

 isted alone in nature, nothing would arrest the glaciers in 

 their downward progress, and they would invade all the val- 

 leys of the Alps ; but a third important element is added to 

 the two others, namely, ablation ; it is proportionate to the 

 surrounding temperature, and acts in the inverse ratio of al- 

 titude. In the upper regions there is scarcely any ablation ; 



