Notice of Prof. Forbes' 's Travels in the Alps. 175 



er may be quite imperceptible upon the small scale, or under any but 

 an enormous pressure." A column of the body itself is the source of 

 the pressure — a column often of several hundred or thousand feet in 

 height, — the origin and termination of many of the largest of the gla- 

 ciers having not less than four thousand feet of elevation between their 

 origin and end. Were the glaciers suddenly converted into water, 

 the bottom would move with a force of forty-four millions of feet, 

 or eight thousand three hundred and thirty-three miles in twenty-four 

 hours ; whereas the velocity of a glacier is only about two feet in that 

 time, implying a pressure, however, quite sufficient to produce the effect 

 of slow descent. " A glacier is not coherent ice, but is a granular com- 

 pound of ice and water, possessing under certain circumstances, espe- 

 cially when much saturated with moisture, a rude flexibility, sensible 

 even to the hand." Glaciers do collapse, and thus choke up and close 

 their own crevasses (fissures) with their plastic substance. A glacier 

 contracts when embayed by rocks, and presses through a narrower chan- 

 nel than that by which it entered, and having passed the gorge into a 

 wide valley, it spreads itself out wider, as a viscous substance would 

 do.* The motion of a glacier resembles that of a viscous fluid ; the 

 velocity varies with the slope ; the surface, especially the middle of 

 the surface, moves fastest. 



The higher and central parts of such a mass will move nearly to- 

 gether, while the bottom and sides will be most influenced by the fric- 

 tion. The mean velocity of the entire stream is the mean between that 

 of the top and bottom. The flow will vary with the temperature ; hot 

 water runs out of an orifice more rapidly than cold. All these circum- 

 stances are imitated by the glacier. The centre of a glacier moves 

 faster than the sides ; the top than the sides and bottom. Glaciers slide 

 over their beds in consequence of the particles of ice sliding over each 

 other ; the motion of the superficial parts pulling along the inferior. 

 A glacier descending into a valley, is like a body pulled asunder and 

 stretched, and not like a body forced on by superior pressure alone. 

 " The glacier, like a stream, has its pools and its rapids. Where it is 

 embayed by rocks, it accumulates ; its declivity diminishes, and its ve- 

 locity at the same time. When it passes down a steep, or issues by a 

 narrow outlet, its velocity increases." 



" The veined structure of the ice is a consequence of the viscous theory.'''' 

 A viscous stream presents wrinkles, or curvilinear arrangements of the 

 floating matter, accompanied by a crumpling, or inequality of the sur- 

 face. The dirt bands described very minutely by Prof. Forbes, are at- 



* An opinion first brought prominently forward by M. Rendu, now Bishop of 

 Annecy. 



