2 Professor Forbes' Account of his recent 



or indeed any glacier is easily accessible, or sufficiently free 

 from snow for accurate observations, — having also, especially 

 during the month of September, observed it under every cir- 

 cumstance of weather and a great range of atmospheric tem- 

 perature, I believe that I have obtained the chief data neces- 

 sary for basing a theory of its motion, upon sound mechanical 

 principles. The changes which I have witnessed upon its 

 surface, during the period of above three months during which I 

 have studied it, are so great and remarkable, and in some re- 

 spects so unexpected, as to be of capital importance in any 

 theory which may be proposed. 



I was very greatly struck with the change, in the general 

 appearance of the glacier during my absence, from the 10th 

 August to the 10th September. I left it comparatively high 

 and tumid in the centre, at no great depth below the arrete of 

 its natural boundary, the moraine by its side ; and fissured by 

 crevasses, deep and rather narrow, with well-defined vertical 

 walls. — On my return, the icy mass had most visibly sunk in 

 its bed ; it seemed to me to have a wasted, cadaverous look ; 

 the moraines protruded far higher than before from its sides ; 

 and the ice itself clinging to the moraine at a considerable 

 height above its general level, was covered by the fallen masses 

 of stone and gravel which had rolled down the inclined plane 

 formed by this central subsidence. The whole resembled 

 somewhat the Wye, or some of those narrow tidal rivers 

 whose muddy banks are left exposed by the retreat of the ocean. 

 That this subsidence was in a good measure occasioned by the 

 melting of the ice in contact with the bottom of the valley 

 in which it lies, and by the falling together of the parts in a 

 soft and yielding state, owing to a complete infiltration of the 

 whole mass with water during the warm season of the year, 

 was proved by a variety of circumstances which I shall not 

 stop to detail. I may mention however, that the crevasses were 

 wider but less deep and regular, — excessively degraded on the 

 side to which the mid-day sun had free access, and in many 

 places where several crevasses nearly joined, the icy partitions 

 had sunk gradually towards a level, and thus rendered the 

 fissured parts of the glacier more easily traversed than at an 

 earlier part of the season. It is plain, too, that the fact of the 



