Part 6

1. _That opposite the Montanvert the eastern half of the Mer de Glace moves more rapidly than the western half._

2. _That opposite_ les Fonts _the western half of the glacier moves more rapidly than the eastern half._

3. _That opposite Trélaporte the eastern half of the glacier again moves more rapidly than the western half._

4. _That these changes in the place of greatest motion are determined by the flexures of the valley through which the Mer de Glace moves._

§ 25. _New Law of Glacier Motion._

185. Let us express these facts in another way. Supposing the points of swiftest motion for a very great number of lines crossing the Mer de Glace to be determined; the line joining all those points together is what mathematicians would call the _locus_ of the point of swiftest motion.

186. At Trélaporte this line would lie east of the centre; at the _Ponts_ it would lie west of the centre; hence in passing from Trélaporte to the _Ponts_ it would cross the centre. But at the Montanvert it would again lie east of the centre; hence between the _Ponts_ and the Montanvert the centre must be crossed a second time. If there were further sinuosities upon the Mer de Glace there would be further crossings of the axis of the glacier.

187. The points on the axis which mark the transition from eastern to western bending, and the reverse, may be called _points of contrary flexure_.

188. Now what is true of the Mer de Glace is true of all other glaciers moving through sinuous valleys; so that the facts established in the Mer de Glace may be expanded into the following general law of glacier motion:--

_When a glacier moves through a sinuous valley, the locus of the points of maximum motion does not coincide with the centre of the glacier, but, on the contrary, always lies on the convex side of the central line. The locus is therefore a curved line more deeply sinuous than the valley itself, and crosses the axis of the glacier at each point of contrary flexure._

189. The dotted line on the Outline Plan (page 68) represents the locus of the point of maximum motion, the firm line marking the centre of the glacier.

190. Substituting the word _river_ for _glacier_, this law is also true. The motion of the water is ruled by precisely the same conditions as the motion of the ice.

191. Let us now apply our law to the explanation of a difficulty. Turning to the careful measurements executed by M. Agassiz on the glacier of the Unteraar, we notice in the discussion of these measurements a section of the "Système glaciaire" devoted to the "Migrations of the Centre." It is here shown that the middle of the Unteraar glacier is not always the point of swiftest motion. This fact has hitherto remained without explanation; but a glance at the Unteraar valley, or at the map of the valley, shows the enigma to be an illustration of the law which we have just established on the Mer de Glace.

§ 26. _Motion of Axis of Mer de Glace._

192. We have now measured the rate of motion of five different lines across the trunk of the Mer de Glace. Do they all move alike? No. Like a river, a glacier at different places moves at different rates. Comparing together the points of maximum motion of all five lines, we have this result:

MOTION OF MER DE GLACE.

At Trélaporte 20 inches a day. At _les Ponts_ 23 " " Above the Montanvert 26 " " At the Montanvert 34 " " Below the Montanvert 33[C] " "

[C] This is probably under the mark. I think it likely that the swiftest motion of this portion of the Mer de Glace in 1857 amounted to a yard in twenty-four hours.

193. There is thus an increase of rapidity as we descend the glacier from Trélaporte to the Montanvert; the maximum, motion at the Montanvert being fourteen inches a day greater than at Trélaporte.

§ 27. _Motion of Tributary Glaciers._

194. So much for the trunk glacier; let us now investigate the branches, permitting, as we have hitherto done, reflection on known facts to precede our attempts to discover unknown ones.

195. As we stood upon our "cleft station," whence we had so capital a view of the Mer de Glace, we were struck by the fact that some of the tributaries of the glacier were wider than the glacier itself. Supposing water to be substituted for the ice, how do you suppose it would behave? You would doubtless conclude that the motion down the broad and slightly-inclined valleys of the Géant and the Léchaud would be comparatively slow, but that the water would force itself with increased rapidity through the "narrows" of Trélaporte. Let us test this notion as applied to the ice.

196. Planting our theodolite in the shadow of Mont Tacul, and choosing a suitable point at the opposite side of the Glacier du Géant, we fix on July 29 a series of ten stakes across the glacier. The motion of this line in twenty-four hours was as follows:--

MOTION OF GLACIER DU GÉANT.

Sixth Line: H H' upon Sketch.

Stake 1 2 3 4 5 6 7 8 9 10 Inches 11 10 12 13 12 13 11 10 9 5

197. Our conjecture is fully verified. The maximum motion here is seven inches a day less than that of the Mer de Glace at Trélaporte (192).

198. And now for the Léchaud branch. On August 1 we fix ten stakes across this glacier above the point where it is joined by the Talèfre. Measured on August 3, and reduced to twenty-four hours, the motion was found to be--

MOTION OF GLACIER DE LÉCHAUD.

Seventh Line: K K' upon Sketch.

Stake 1 2 3 4 5 6 7 8 9 10 Inches 5 8 10 9 9 8 6 9 7 6

199. Here our conjecture is still further verified, the rate of motion being even less than that of the Glacier du Géant.

§ 28. _Motion of Top and Bottom of Glacier._

200. We have here the most ample and varied evidence that the sides of a glacier, like those of a river, are retarded by friction against its boundaries. But the likeness does not end here. The motion of a river is retarded by the friction against its bed. Two observers, viz. Prof. Forbes and M. Charles Martins, concur in showing the same to be the case with a glacier. The observations of both have been 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, p. 83) a wall of ice about 150 feet high has already attracted our attention. Bending round to join the Léchaud the Glacier du Géant is here drawn away from the mountain side, and exposes a fine section. We try to measure it top, bottom, and middle, and are defeated twice over. We try it a third time and succeed. A stake is fixed 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 fixed at some risk of boulders falling upon us from above; but by skill and caution we succeed in measuring the motions of all three. For 24 hours the motions are:--

Top stake 6 inches. Middle stake 4½ " Bottom stake 2⅔ "

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 these labours and measurements have given us, let us once more climb to our station beside the Cleft under the Aiguille de Charmoz. At our first visit we saw the medial moraines of the glacier, but we knew nothing about their cause. We now know that they mark upon the trunk its tributary glaciers. Cast your eye, then, first upon the Glacier du Géant; realise its width in its own valley, and see how much it is narrowed at Trélaporte. The broad ice-stream of the Léchaud is still more surprising, being squeezed upon the Mer de Glace to a narrow white band between its bounding moraines. The Talèfre 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 Trélaporte.

204. We find the width of the Glacier du Géant to be 5,155 links, or 1,134 yards.

205. The width of the Glacier de Léchaud we find to be 3,725 links, or 825 yards.

206. The width of the Talèfre we find to be 2,900 links, or 638 yards.

207. The sum of the widths of the three branch glaciers is therefore 2,597 yards.

208. At Trélaporte 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 de Léchaud, the facts are still more astonishing. Previous to its junction with the Talèfre, this glacier has a width of 825 yards; in passing through the jaws of the granite vice at Trélaporte, its width is reduced to eighty-eight yards, or in round numbers to one-tenth of its previous width. (Look to the sketch on the next page.)

210. Are we to understand by this that the ice of the Léchaud is squeezed to one-tenth of its former _volume_? By no means. It is mainly a change of _form_, not of volume, that occurs at Trélaporte. Previous to its compression, the glacier resembles a plate of ice _lying flat_ upon its bed. After its compression, it resembles a plate _fixed upon its edge_. The squeezing, doubtless, has deepened the ice.

§ 30. _Longitudinal Compression of a Glacier._

211. The ice is forced through the gorge at Trélaporte by a pressure from behind; in fact the Glacier du Géant, immediately above Trélaporte, represents a piston or a plug which drives the ice through the gorge. What effect must this pressure have upon the plug itself? Reasoning alone renders it probable that the pressure will shorten the plug; that the lower part of the Glacier du Géant will to some extent yield to the pressure from behind.

212. Let us test this notion. About three-quarters of a mile above the Tacul, and on the mountain slope to the left as we ascend, we observe a patch of verdure. Thither we climb; there we plant our theodolite, and set out across the Glacier du Géant, a line, which we will call line No. 1 (F F' upon sketch, p. 68).

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 near the Tacul, we set out a third line, No. 3 (H H' upon sketch), across the glacier.

215. The daily motion of the centres of these three lines is as follows:--

Inches Distances asunder No. 1 50·55 } } 545 yards. No. 2 15·43 } } 487 " No. 3 12·75 }

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 of about eight inches a day.

217. A river descending the Valley du Géant would behave in substantially the same fashion. It would have its motion on approaching Trélaporte diminished, and it would pour through the defile with a velocity greater than that of the water behind.

§ 31. _Sliding and Flowing. Hard Ice and Soft Ice._

218. We have thus far confined ourselves to the measurement and discussion of glacier motion; but in our excursions we have noticed many things besides. Here and there, where the ice has retreated from the mountain side, we have seen the rocks fluted, scored, and polished; thus proving that the ice had slidden over them and ground them down. At the source of the Arveiron we noticed the water rushing from beneath the glacier charged with fine matter. All glacier rivers are similarly charged. The Rhone carries its load of matter into the Lake of Geneva; the rush of the river is here arrested, the matter subsides, and the Rhone quits the lake clear and blue. The Lake of Geneva, and many other Swiss lakes, are in part filled up with this matter, and will, in all probability, finally be obliterated by it.

219. One portion of the motion of a glacier is due to this bodily sliding of the mass over its bed.

220. We have seen in our journeys over the glacier streams formed by the melting of the ice, and escaping through cracks and _crevasses_ to the bed of the glacier. The fine matter ground down is thus washed away; the bed is kept lubricated, and the sliding of the ice rendered more easy than it would otherwise be.

221. As a skater also you know how much ice is weakened by a thaw. Before it actually melts it becomes rotten and unsafe. Test such ice with your penknife: you can dig the blade readily into it, or cut the ice with ease. Try good sound ice in the same way: you find it much more resistant. The one, indeed, resembles soft chalk; the other hard stone.

222. Now the Mer de Glace in summer is in this thawing condition. Its ice is rendered soft and yielding by the sun; its motion is thereby facilitated. We have seen that not only does the glacier slide over its bed, but that the upper layers slide over the under ones, and that the centre slides past the sides. The softer and more yielding the ice is, the more free will be this motion, and the more readily also will it be forced through a defile like Trélaporte.

223. But in winter the thaw ceases; the quantity of water reaching the bed of the glacier is diminished or entirely cut off. The ice also, to a certain depth at least, is frozen hard. These considerations would justify the opinion that in winter the glacier, if it moves at all, must move more slowly than in summer. At all events, the summer measurements give no clue to the winter motion.

224. This point merits examination. I will not, however, ask you to visit the Alps in mid-winter; but, if you allow me, I will be your deputy to the mountains, and report to you faithfully the aspect of the region and the behaviour of the ice.

§ 32. _Winter on the Mer de Glace._

225. The winter chosen is an inclement one. There is snow in London, snow in Paris, snow in Geneva; snow near Chamouni so deep that the road fences are entirely effaced. On Christmas night--nearly at mid-night--1859, your deputy reaches Chamouni.

226. The snow fell heavily on December 26; but on the 27th, during a lull in the storm, we turn out. There are with me four good guides and a porter. They tie planks to their feet to prevent them from sinking in the snow; I neglect this precaution and sink often to the waist. Four or five times during our ascent the slope cracks with an explosive sound, and the snow threatens to come down in avalanches.[D]

[D] Four years later, viz. in the spring of 1863, a mighty climber and noble guide and companion of mine, named Johann Joseph Bennen, was lost, through the cracking and subsequent slipping of snow on such a slope.

The freshly-fallen snow was in that particular condition which causes its granules to adhere, and hence every flake falling on the trees had been retained there. The laden pines presented beautiful and often fantastic forms.

227. After five hours and a half of arduous work the Montanvert was attained. We unlocked the forsaken auberge, round which the snow was reared in buttresses. I have already spoken of the complex play of crystallising forces. The frost figures on the window-panes of the auberge were wonderful: mimic shrubs and ferns wrought by the building power while hampered by the adhesion between the glass and the film in which it worked. The appearance of the glacier was very impressive; all sounds were stilled. The cascades which in summer fill the air with their music were silent, hanging from the ledges of the rocks in fluted columns of ice. The surface of the glacier was obviously higher than it had been in summer; suggesting the thought that while the winter cold maintained the lower end of the glacier jammed between its boundaries, the upper portions still moved downwards and thickened the ice. The peak of the Aiguille du Dru shook out a cloud-banner, the origin and nature of which have been already explained (84). (See _Frontispiece_.)

228. On the morning of the 28th this banner was strikingly large and grand, and reddened by the light of the rising sun, it glowed like a flame. Roses of cloud also clustered round the crests of the Grande Jorasse and hung upon the pinnacles of Charmoz. Four men, well roped together, descended to the glacier. I had trained one of them in 1857, and he was now to fix the stakes. The storm had so distributed the snow as to leave alternate lengths of the glacier bare and thickly covered. Where much snow lay great caution was required, for hidden crevasses were underneath. The men sounded with their staffs at every step. Once while looking at the party through my telescope the leader suddenly disappeared; the roof of a crevasse had given way beneath him; but the other three men promptly gathered round and lifted him out of the fissure. The true line was soon picked up by the theodolite; one by one the stakes were fixed until a series of eleven of them stood across the glacier.

229. To get higher up the valley was impracticable; the snow was too deep, and the aspect of the weather too threatening; so the theodolite was planted amid the pines a Little way below the Montanvert, whence through a vista I could see across the glacier. The men were wrapped at intervals by whirling snow-wreaths which quite hid them, and we had to take advantage of the lulls in the wind. Fitfully it came up the valley, darkening the air, catching the snow upon the glacier, and tossing it throughout its entire length into high and violently agitated clouds, separated from each other by cloudless spaces corresponding to the naked portions of the ice. In the midst of this turmoil the men continued to work. Bravely and steadfastly stake after stake was set, until at length a series of ten of them was fixed across the glacier.

230. Many of the stakes were fixed in the snow. They were four feet in length, and were driven in to a depth of about three feet. But that night, while listening to the wild onset of the storm, I thought it possible that the stakes and the snow which held them might be carried bodily away before the morning. The wind, however, lulled. We rose with the dawn, but the air was thick with descending snow. It was all composed of those exquisite six-petaled flowers, or six-rayed stars, which have been already figured and described (§ 9). The weather brightening, the theodolite was planted at the end of the first line. The men descended, and, trained by their previous experience, rapidly executed the measurements. The first line was completed before 11 A. M. Again the snow began to fall, filling all the air. Spangles innumerable were showered upon the heights. Contrary to expectation, the men could be seen and directed through the shower.

231. To reach the position occupied by the theodolite at the end of our second line, I had to wade breast-deep through snow which seemed as dry and soft as flour. The toil of the men upon the glacier in breaking through the snow was prodigious. But they did not flinch, and after a time the leader stood behind the farthest stake, and cried, _Nous avons fini_. I was surprised to hear him so distinctly, for falling snow had been thought very deadening to sound. The work was finished, and I struck my theodolite with a feeling of a general who had won a small battle.

232. We put the house in order, packed up, and shot by glissade down the steep slopes of _La Filia_ to the vault of the Arveiron. We found the river feeble, but not dried up. Many weeks must have elapsed since any water had been sent down from the surface of the glacier. But at the setting in of winter the fissures were in a great measure charged with water; and the Arveiron of to-day was probably due to the gradual _drainage_ of the glacier. There was now no danger of entering the vault, for the ice seemed as firm as marble. In the cavern we were bathed by blue light. The strange beauty of the place suggested magic, and put me in mind of stories about fairy caves which I had read when a boy. At the source of the Arveiron our winter visit to the Mer de Glace ends; next morning your deputy was on his way to London.

§ 33. _Winter Motion of the Mer de Glace._

233. Here are the measurements executed in the winter of 1859:--

Line No. I.

Stake 1 2 3 4 5 6 7 8 9 10 11 Inches 7 11 14 13 14 14 16 16 12 12 7

Line No. II.

Stake 1 2 3 4 5 6 7 8 9 10 Inches 8 10 14 16 16 16 18 17 15 14

234. Thus the winter motion of the Mer de Glace near the Montanvert is, in round numbers, half the summer motion.

235. As in summer, the eastern side of the glacier at this place moved quicker than the western.

§ 34. Motion of the Grindelwald and Aletsch Glaciers.

236. As regards the question of motion, to no other glacier have we devoted ourselves with such thoroughness as to the Mer de Glace; we are, however, able to add a few measurements of other celebrated glaciers. Rear the village of Grindelwald in the Bernese Oberland, there are two great ice-streams called respectively the Upper and the Lower Grindelwald glaciers, the second of which is frequently visited by travellers in the Alps. Across it on August 6, 1860, a series of twelve stakes was fixed by Mr. Vaughan Hawkins and myself. Measured on the 8th and reduced to its daily rate, the motion of these stakes was as follows:--

MOTION OF LOWER GRINDELWALD GLACIER.

Stake 1 2 3 4 5 6 7 8 9 10 11 12 Inches 18 19 20 21 21 21 22 20 19 18 17 14

237. The theodolite was here planted a little below the footway leading to the higher glacier region, and at about a mile above the end of the glacier. The measurement was rendered difficult by crevasses.

238. The largest glacier in Switzerland is the Great Aletsch, to which further reference shall subsequently be made. Across it on August 14, 1860, a series of thirty-four stakes was planted by Mr. Hawkins and me. Measured on the 16th and reduced to their daily rate, the velocities were found to be as follows:--

MOTION OF GREAT ALETSCH GLACIER.

East Stake 1 2 3 4 5 6 7 8 9 10 11 12 Inches 2 3 4 6 8 11 13 14 16 17 17 19 Stake 13 14 15 16 17 18 19 20 21 22 23 Inches 19 18 18 17 19 19 19 19 17 17 15 Stake 24 25 26 27 28 29 30 31 32 33 34 Inches 16 17 17 17 17 17 17 17 16 12 12 West

239. The maximum motion here is nineteen inches a day. Probably the eastern side of the glacier is shallow, the retardation of the bed making the motion of the eastern stakes inconsiderable. The width of the glacier here is 9,030 links, or about a mile and a furlong. The theodolite was planted high among the rocks on the western flank of the mountain, about half a mile above the Märgelin See.

§ 35. _Motion of Morteratsch Glacier._

240. Far to the east of the Oberland and in that interesting part of Switzerland known as the Ober Engadin, stands a noble group of mountains, less in height than those of the Oberland, but still of commanding elevation. The group derives its name from its most dominant peak, the Piz Bernina. To reach the place we travel by railway from Basel to Zürich, and from Zürich to Chur (French Coire), whence we pass by diligence over either the Albula pass or the Julier pass to the village of Pontresina. Here we are in the immediate neighbourhood of the Bernina mountains.