4 Professor Forbes' Account of his recent 



Another mode of considering the successive conditions of a 

 certain portion of the glacier, will lead also to the admission 

 of the ever-varying state of its aggregation and subdivision. 

 In a glacier, like the Mer de Glace of Chamouni, which pre- 

 sents a great many and well-marked " accidents'' of surface 

 in its different parts, it is yet perfectly well known, that, though 

 continually moving and changing, the distribution of these 

 " accidents" is sensibly invariable. Every year, and year af- 

 ter year, the water courses follow the same lines of direction, 

 — their streams are precipitated into the heart of the glacier 

 by vertical funnels called " moulins ;" at the very same 

 points, the fissures, though forming very different angles with 

 the axis or sides of the glacier at different points of its length, 

 opposite the same point are always similarly disposed, — the 

 same parts of the glacier, relatively to fixed rocks, are every 

 year passable, and the same parts are traversed by innume- 

 rable fissures. Yet the solid ice of one year is the fissured 

 ice of the next, and the very ice which this year forms the 

 walls of a " moulin," will next year be some hundred feet far- 

 ther forward and without perforation, whilst the cascade re- 

 mains immovable, or sensibly so, with reference to fixed ob- 

 jects around. All these facts, attested by long and invariable 

 experience, prove that the ice of the glaciers is insensibly and 

 continually moulding itself under the influence of external 

 circumstances, of which the principal, be it remarked, is its 

 own weight affecting its figure, in connection with the sur- 

 faces over which it passes, and between which it struggles on- 

 wards. It is, in this respect, absolutely comparable to the 

 water of a river, which has here its deep pools, here its con- 

 stant eddy, continually changing in substance, yet ever the 

 same in form. 



With reference to the yet more essential modifications of 

 structure, I mean the veined structure which I formerly de- 

 scribed ; I shewed in my last letter, that it is equally muta- 

 ble and subjected to the momentary conditions of external re- 

 straint ; and, that far from being an original structure in the 

 higher part of the glacier, variously modified in its subsequent 

 course, but never annihilated, it owes its existence at any 

 moment to the conditions of varying velocity indifferent parts 



