60 M. Ch. Martins on the 



cessant progression did not counterbalance this effect. A 

 kind of equilibrium is thus established between the summer 

 melting on the one hand, and the annual progression on the 

 other. If the season be warm and dry, the fusion predomi- 

 nates, and the glacier recedes ; if the summer be cold and 

 rainy, the progression more than compensates the effects of 

 the fusion, and the glacier advances. 



We now understand what are the influences which assign 

 to glaciers a mean limit, around which they may oscillate 

 without ever surpassing it. It is less easy to give a reason 

 for certain glaciers descending into inhabited valleys, while 

 others remain suspended on the sides of the highest moun- 

 tains. These differences are owing to the size and height of 

 the amphitheatres or cirques which feed these glaciers. The 

 more extensive and elevated these cirques are, the more con- 

 siderable will be the quantity of snow which accumulates on 

 them, and the more also will the outshoots from the fields of 

 snow descend into the low valleys, and regain, so to speak, 

 the ground which had been lost by the annual melting. It is 

 in this way that the glacier of the Bossons, whose source is 

 in the great plateau of Mont Blanc, a vast amphitheatre, 

 situate nearly 4000 metres above the sea, descends to 1040 

 metres, and advances to the midst of inhabited places, or- 

 chards, and cultivated fields. The glaciers of Aletsch, Viesch, 

 Grindelwald, and Zermatt, are in the same state. Every year, 

 the astonished traveller may see golden harvests by the side 

 of the glacier of Brenva, which descends the southern face of 

 Mont Blanc. The influence of the size and elevation of the 

 amphitheatres even counterbalances, according to M. Desor's 

 remark, that of exposure ; and explains the surprising fact, 

 that the longest and most important glaciers of the Bernese 

 Alps are found on the southern declivity of the chain. 



"We have seen that these glaciers have a progressive move- 

 ment which draws them towards the plain. What are the 

 laws which regulate this movement ? The investigation of 

 these laws has continually occupied the attention of all the 

 natural philosophers who have devoted themselves to this 

 subject, yet they have not hitherto deduced the cause of this 

 advance from the whole of the singular phenomena which 



