J 



WATER AS A MECHANICAL AGENT. 245 



glacier along the under-glacier surface ; (/) a local melting of ice, as a 

 consequence of the accumulation of pressure, diminishLng thereby resistance 

 and facilitating motion. 



(a) Fragility of ice. — Reaching a place of steep descent, great transverse 

 blocks of a glacier drop in succession. Rounding a projecting angle in a 

 valley, the ice is compressed at the side of the short turn, and drawn out on 

 the opposite side, and in the latter great crevasses are opened. Forbes men- 

 tions one chasm 500 feet wide extending across the Mer de Glace. Passing 

 narrows, the quickened motion causes irregular cross-fractures often in great 

 numbers. Flowing along a valley the resistance of the 

 sides {gg', gg', Fig. 213), together with the more rapid flow ^^g 



at the center, makes crevasses (cc') pointing obliquely up 

 stream at angles of about 45°. The direction of the pull -'' / 

 tending to produce the fractures (or that of greatest ten- y 

 sion) is oblique toward the center down stream. Hopkins >(V^ 

 has shown that this pull (pp') is strongest theoreticall}^ '^■/'~- 

 when it makes an angle of 45° with the sides of the glacier, /^p" p' 



and therefore the crevasses are at 45° with the sides up Vi 



9 Q' 



stream. This angle would be modified by the form of the ^ 



bottom, and by its pitch. 



After being extensively broken up, the glacier, on reaching a broader por- 

 tion of the valley, of gentler pitch, becomes again solid by a general welding 

 of the pieces. The welding process, called by Faraday regelation, requires 

 only pressure, and takes place whether the surfaces are moist or dry. (Hunger- 

 ford, 1882.) If a block of ice is supported at its two ends, and a fine wire 

 is passed around it at middle and weighted below, the wire will slowly melt 

 its way through ; but when the cut is completed, the mass will be as solid as 

 at the outset, regelation having gone forward above the wire. The multi- 

 tudes of fractures made in a glacier on steep slopes hence disappear below 

 where the motion is slow and the ice feels the pressure from above. 



(&) Permeating tvater. — As already explained, the summer's heat pro- 

 duces water over a glacier, and through all its crevasses and smallest crevices, 

 especially during the daytime. At night, when the source of heat is with- 

 drawn, there may be much refreezing ; but the days in summer are much 

 longer than the nights. The chief source of the water largely fails in winter, 

 and hence the difference in the summer and winter rates of movement. The 

 melting from local pressure is an addition to the amount of water, and just 

 where needed to meet special resistance. The pressure of one atmosphere 

 "lowers the melting-point of water 0-0042° F. 



(c) Plasticity. — Ice may be made by pressure to copy a seal, like wax ; or 

 by forcing it through holes to take the form of a cylinder. Kane mentions, 

 in his Arctic Explorations, the case of a table of ice, eight feet thick and 20 

 or more wide, supported only at the sides, which, in two months, had the 

 center depressed by gravity five feet. The temperature during the interval 

 was many degrees below 32° F. Guyot concluded, from the flow of the 



