108 



THE FORMS OF WATER 



not end hero. The motion of a river Is re- 

 tarded by the friction against its bed. Two 

 observers, viz., Professor Forbes and M. 

 Charles Martins, concur in showing the same 

 to be the cast? with a glacier. The obser- 

 vations of both have k j en objected to ; hence 

 it is all the more incumbent on us to seek for 

 decisive evidence. 



201. At the Tacul (near the point a upon 

 the sketch plan, Fig. 5) a wall of ice about 

 150 feet high has already attracted our atten- 

 tion. Bending round to joia the Lechaud the 

 G lacier du Geaut is here drawn away from 

 the mountain side and exposes a line section. 

 We try to measure it top, bottom, and mid- 

 dle, and are defeated twice over. ^Vu try it a 

 third time and succeed. A. stake \j fixer! at 

 the summit of the ice-precipice, another 

 at 4 feet from the bottom, and a third at 35 

 feet above the bottom. These lower stakes 

 are tixed at some ri.sk of boulders falling 

 upon us from above ; but by skill and CMI- 

 tion we succeed in measuring the motions 

 of all three. For 2 1 hours the motions aro : 



Top *!ake (5 .relics. 



MiJuh! sU'.;u 4! 



B >;,io.u 3(Ako *'& '' 



202. The retarding influence of the bed of 

 the glacier is reduced to demonstration by 

 these measurements. The bottom does not 

 move with half the velocity of the surface. 



29. LATERAL COMPRESSION OF A GLACIER. 



203. Furnished with the knowledge which 

 those labors and measurements have given 

 us, let us once more climb to our station be- 

 side the Cleft, under the Aiguille de Char- 

 moz. At our first visit we saw the medial 

 moraines of the glacier, but we knew noth- 

 ing about their cause. We now know that 

 they mark upon the trunk its tributary gla- 

 ciers. Cast your eye, then, first upon the 

 Glacier du Geant ; realize its widtii in its 

 own valley, and see how much it is narrowed 

 at Treiaporte. The broad ice-stream of the 

 Lechaud is still more surprising, being 

 squeezed upon the Mer de Glace to a narrow 

 white band between its bounding moraines. 

 The Talefre undergoes similar compression, 

 Let us now descend, shake out our chain, 

 measure, and express in numbers the width 

 of the tributaries, and the actual amount of 

 compression suffered at Tie'aportc. 



204. We find the width of the Glacier du 

 G&mt to be 5155 links, or 1134 yards. 



205. The width of the Glacier de LSchaud 

 we find to be 3725 links, or 825 yards. 



200. The width of the Tatefre we find to 

 be 2900 links, or 638 yards. 



207. The sum of the widths of the three 

 branch glaciers is therefore 2597 yards. 



208. At Treiaporte these three branches 

 are forced through a gorge 893 yards wide, 

 or one third of their previous width, at the 

 rate of twenty inches a day. 



209. If we limit our view to the Glacier dc 

 Lchuud, the facts are still more astonishing 

 Previous to its junction with the Talefre^ 

 this glacier has a width of 825 yards ; in 

 passing through the jaws of the granite vise 



at Treiaporte, its width is reduced to eighty- 

 eight yards, or in round numbers to one tenth 

 of its previous width. (Look to the sketch 

 on page 9.) 



210. Aro we to understand by this that the 

 ice of the Lechaud is squeezed to one tenth 

 of its former volume? By no means. It is 

 mainly a change offonn, not of volume, Unit 

 occurs at Treiaporte. Previous to its com- 

 pression, the glacier resembles a plate of ice 

 lying fiat upon its bed. After its compres- 

 sion, it resc-mblcs a pl.ue faced upon its edge. 

 The squeezing, doubtless, has deepened the 

 ice. 



CO. LONGITUDINAL COMPRESSION OF A 

 GLACIER. 



211. The icj is forced through the gorge at 

 Tn'laporte by a piessurc from behind; in 

 fact i he Glacier du Geant, immediately above 

 Treiaporte. represents a piston or a plug 

 which drives the ice through the gorge. 

 What t'lfect must this pressure have upon 

 the plug itself V Reasoning alone renders it 

 probable that the pressure will shorten the 

 plug ; that the lower part of the Glacier du 

 Geaut will to some extent yield to the pres- 

 sure from behind. 



212. Let us test this notim. About three 

 quarters of a mile above tl-e Taeul, and en 

 the mountain-slope to the le f 'L as we ascend. 

 we observe a patch of verdi/rc. Thither wo 

 climb ; there we plant our theodolite, and set 

 out across the Glacier du GeV.ut, aline, which 

 we will call lino No. 1 (F F' upon sketch, 

 Fig. G.) 



213. About a quarter of a mile lower down 

 we find a practicable couloir on the mountain- 

 side ; we ascend it, reach a suitable platform, 

 plant our instrument, and set out a second 

 line, No. 2 (G G' upon sketch). We must 

 hasten our work here, for along this couloir 

 stones are discharged from a- small glacier 

 which rests upon the slope of Mont Tacul. 



214. Still lower down by another quarter 

 of a mile, which brings us ner.r the Tacul, 

 we set out a third line, No. 3 (II H' upon 

 sketch), across the glacier. 



215. The daily motion of tho centres o-f 

 these three lineaSs as follows : 



Inches. Distances asunder. 



No. 1 20-55 i 



No. 2 13-43 f ^ >' ards - 



437 



No. 3 12-75 f- 



216. The first line here moves five inches 

 a day more than the second ; and the second 

 nearly three inches a day more than the 

 third. The reasoning is therefore confirmed. 

 The ice-plug, which is in round numbers one 

 thousand yards long, is shortened by the 

 pressure exerted on its front at the rate cf 

 about eight inches a day. 



217. A river descending the Valley du 

 Geant would behave in substantially the samo 

 fashion. It would have its motion on ap- 

 proaching Treiaporte diminished, and it 

 would pour through the defile with a velocity 

 greater than that of the water behind^' 



