232 HELMHOLTZ 



so, and these are the places where men, and even chamois 

 are so often lost. These dangers may readily be guarded 

 against if two or three men are roped together at intervals 

 of ten or twelve feet. If then one of them falls into a 

 crevasse, the two others can hold him, and draw him out 

 again. 



In some places the crevasses may be entered, especially 

 at the lower end of a glacier. In the well-known glaciers 

 of Grindelwald, Rosenlaui, and other places, this is facili- 

 tated by cutting steps and arranging wooden planks. Then 

 any one who does not fear the perpetually trickling water 

 may explore these crevasses, and admire the wonderfully 

 transparent and pure crystal walls of these caverns. The 

 beautiful blue colour which they exhibit is the natural 

 colour of perfectly pure water ; liquid water as well as ice is 

 blue, though to an extremely small extent, so that the colour 

 is only visible in layers of from ten to twelve feet in thick- 

 ness. The water of the Lake of Geneva and of the Lago di 

 Garda exhibits the same splendid colour as ice. 



The glaciers are not everywhere crevassed; in places 

 where the ice meets with an obstacle, and in the middle of 

 great glacier currents the motion of which is uniform, the 

 surface is perfectly coherent. 



FIG. 1 08 represents one of the more level parts of the Mer 

 de Glace at the Montanvert, the little house of which is seen 

 in the background. The Gries Glacier, where it forms the 

 height of the pass from the Upper Rhone valley to the Tosa 

 valley, may even be crossed on horseback. We find the 

 greatest disturbance of the surface of the glacier in those 

 places where it passes from a slightly inclined part of its 

 bed to one where the slope is steeper. The ice is there torn 

 in all directions into a quantity of detached blocks, which by - 

 melting are usually changed into wonderfully shaped sharp 

 ridges and pyramids, and from time to time fall into the 

 interjacent crevasses with a loud rumbling noise. Seen from 

 a distance such a place appears like a wild frozen waterfall, 

 and is therefore called a cascade ; such a cascade is seen in 

 the Glacier du Talefre at 1, another is seen in the Glacier 

 du Geant at g, FIG. no, while a third forms the lower end ol 

 the Mer de Glace. The latter, already mentioned as the 



