113 



THE FORMS OF WATER 



you less instructed than you now are, I 

 might lay a wager that the aspect of these 

 fissures would cause you to conclude that the 

 centre of the glacier is left behind by the 

 quicker motion of the sides. 



271. This indeed was the conclusion 

 drawn by M. Agassiz from this very appear- 

 ance, before he had measured the motion of 

 the sides and centre of the glacier of the 

 Unteraar. Intimately versed with the treat- 

 ment of mechanical problems, Mr. Hopkins 

 immediately deduced the obliquity of the 

 lateral crevasses from the quicker How of the 

 centre. Standing beside the glacier with 

 pencil and note-book in hand," I would at 

 once make the matter clear to you thus. 



272. Let A c, in the annexed figure, be one 

 side of Ihe glacier, and B D the other ; and 

 let the direction of motion be that indicated 

 t>y the arrow. Let s T be a transverse slice 

 of the glacier, taken straight across it, say to- 

 day. A few days or weeks hence this slice 

 wi'll have been carried -down, and because the 

 centre moves more quickly than the bides it 

 will not remain straight, but will bend into 

 the form s' T'. 



273. Supposing T i to be a small square of 

 the original slice near the side of the glacier. 

 In its new position the square will be distort- 

 ed to the lozenge-shaped figure T' i'. Fix 

 your attention upon the diagonal T i of the 

 square : in the lower position this diagonal, 

 if the ice could stretch, would be lengthened 

 to T' i'. But the ice does not stretch ; it 

 breaks, and we have a crevasse formed at 

 right angles to T' i. The mere inspection of 

 the diagram will assure you that the crevasse 

 will point obliquely upward. 



274. Along the whole side of the glacier 

 the quicker movement of the centre produces 

 a similar state of strain ; and the conse- 

 quence is that the sides are copiously cut by 

 those oblique crevasses, even at places where 

 the centre is free from. them. 



275. It is curious to see at other places the 

 transverse fissures of the centre uniting with 

 those at the sides, so as to form great curved 

 crevasses which stretch across the glacier 

 A 'rom side to side. The convexity of the 

 curve is turned upward, as mechanical prin- 

 ciples declare it ought to be. But if you 

 were ignorant of those principles, you would 

 never infer frorn the aspect of these curves 

 the quicker motion of the centre. In land- 

 slips, and in the motion of parliaily indurat- 

 ed mud, you may sometimes notice appear- 

 ances similar to those exhibited by the ice. 



41. LONGITUDINAL CREVASSES 



276. "We have thus unravelled the orlgia 

 of both transverse and marginal creva-ses. 

 But where a glacier issues from a steep and 

 narrow defile upon a comparatively level 

 plain which allows it room to expand later- 

 ally, its motion is in part arrested, and the 

 level portion has to bear the thrust of the 

 steeper portions behind. Here the line of 

 thrust is in the direction of the glacier, 

 while the direction at right angles to this is 

 one of tension. Across this latter the glacier 

 breaks, and longitudinal crevasses are formed. 



277. Examples of this kind of crevasse are 

 furnished by the lower pait of the Glacier of 

 the Rhone, when looked down upon from 

 the Gi imsel Pass, or from any commanding 

 point on the flanking mountains. 



42. CREVASSES IN RELATION TO CUIIVA- 

 TUKE OP GLACIER. 



278. One point in addition remains to bo 

 discussed, and your present knowledge will 

 enable you to master it in a moment. You 

 remember at an early period of our researches 

 that we crossed the Mer de Glace from the 

 Chapeau side to the Montanvert side. I then 

 desired you to notice that the Chapeau side 

 of the glacier was more fissured than either 

 the centre or the Montanvert side (75). Why 

 should this be so ? Knowing as we now do 

 that the Chapeau side of the glacier moves 

 more quickly than the other, that the point 

 of maximum motion does not lie on the cen- 

 tre but far east of it, we are prepared to an- 

 swer this question in a peifectly satisfactory 

 manner. 



279. Let AB and c D, in the following dia- 

 gram, represent the two curved sides of the 

 Mer de Glace at the Montanvert, and let m n 

 be a straight line across the glacier. Let o 

 be the point of maximum motion. The me- 

 chanical state of the two sides of the glacier 

 may be thus made plain. Supposing the line 

 m n to be a straight elastic string with its 

 ends fixed ; let it be grasped firmly at tho 

 point o by the finger and thumb, and drawn 

 too, keeping the distance between o' and the 

 side c D constant. Here the length, n o of 

 the string would have stretched to n o\ and 

 the length m o to m o', and you see plainly 

 that the stretching of the short line, in com- 

 parison with its length, is greater than that 

 of the long line in comparison with its length. 



