Part 11

144. The same observation proves to demonstration that it is the ice near the bottom of the higher névé that becomes the surface-ice of the glacier near its end. The waste of the surface below the snow-line brings the deeper portions of the ice more and more to the light of day.

145. There are numerous obvious indications of the existence of glacier motion, though it is too slow to catch the eye at once. The crevasses change within certain limits from year to year, and sometimes from month to month; and this could not be if the ice did not move. Rocks and stones also are observed, which have been plainly torn from the mountain sides. Blocks seen to fall from particular points are afterwards observed lower down. On the moraines rocks are found of a totally different mineralogical character from those composing the mountains right and left; and in all such cases strata of the same character are found bordering the glacier higher up. Hence the conclusion that the foreign boulders have been floated down by the ice. Further, the ends or "snouts" of many glaciers act like ploughshares on the land in front of them, overturning with slow but merciless energy huts and chalets that stand in their way. Facts like these have been long known to the inhabitants of the High Alps, who were thus made acquainted in a vague and general way with the motion of the glaciers.

§ 19. The Motion of Glaciers. Measurements by Hugi and Agassiz. Drifting of Huts on the Ice.

146. But the growth of knowledge is from vagueness towards precision, and exact determinations of the rate of glacier motion were soon desired. With reference to such measurements one glacier in the Bernese Oberland will remain forever memorable. From the little town of Meyringen in Switzerland you proceed up the valley of Hasli, past the celebrated waterfall of Handeck, where the river Aar plunges into a chasm more than 200 feet deep. You approach the Grimsel Pass, but instead of crossing it you turn to the right and follow the course of the Aar upwards. Like the Rhone and the Arveiron, you find the Aar issuing from a glacier.

147. Get upon the ice, or rather upon the deep moraine shingle which covers the ice, and walk upwards. It is hard walking, but after some time you get clear of the rubbish, and on to a wide glacier with a great medial moraine running along its back. This moraine is formed by the junction of two branch glaciers, the Lauteraar and the Finsteraar, which unite at a promontory called the Abschwung to form the trunk glacier of the Unteraar.

148. On this great medial moraine in 1827 an intrepid and enthusiastic Swiss professor, Hugi, of Solothurm (French Soleure), built a hut with a view to observations upon the glacier. His hut moved, and he measured its motion. In the three years--from 1827 to 1830--it had moved 330 feet downwards. In 1836 it had moved 2,354 feet; and in 1841 M. Agassiz found it 4,712 feet below its first position.

149. In 1840, M. Agassiz himself and some bold companions took shelter under a great overhanging slab of rock on the same moraine, to which they added side walls and other means of protection. And because he and his comrades came from Neufchâtel, the hut was called long afterwards the "Hôtel des Neuchâtelois." Two years subsequent to its erection M. Agassiz found that the "hotel" had moved 486 feet downwards.

§ 20. Precise Measurements of Agassiz and Forbes. Motion of a Glacier proved to resemble the Motion of a River.

150. We now approach an epoch in the scientific history of glaciers. Had the first observers been practically acquainted with the instruments of precision used in surveying, accurate measurements of the motion of glaciers would probably have been earlier executed. We are now on the point of seeing such instruments introduced almost simultaneously by M. Agassiz on the glacier of the Unteraar, and by Professor Forbes on the Mer de Glace. Attempts had been made by M. Escher de la Linth to determine the motion of a series of wooden stakes driven into the Aletsch glacier, but the melting was so rapid that the stakes soon fell. To remedy this, M. Agassiz in 1841 undertook the great labour of carrying boring tools to his "hotel," and piercing the Unteraar glacier at six different places to a depth of ten feet, in a straight line across the glacier. Into the holes six piles were so firmly driven that they remained in the glacier for a year, and in 1842 the displacements of all six were determined. They were found to be 160 feet, 225 feet, 269 feet, 245 feet, 210 feet, and 125 feet, respectively.

151. A great step is here gained. You notice that the middle numbers are the largest. They correspond to the central portion of the glacier. Hence, these measurements conclusively establish, not only the fact of glacier motion, but that the centre of a glacier, like that of a river, moves more rapidly than the sides.

152. With the aid of trained engineers M. Agassiz followed up these measurements in subsequent years. His researches are recorded in a work entitled "Système glaciaire," which is accompanied by a very noble Atlas of the Glacier of the Unteraar, published in 1847.

153. These determinations were made by means of a theodolite, of which I will give you some notion immediately. The same instrument was employed the same year by the late Professor Forbes upon the Mer de Glace. He established independently the greater central motion. He showed, moreover, that it is not necessary to wait a year, or even a week to determine the motion of a glacier; with a correctly-adjusted theodolite he was able to determine the motion of various points of the Mer de Glace from day to day. He affirmed, and with truth, that the motion of the glacier might be determined from hour to hour. We shall prove this farther on (162). Professor Forbes also triangulated the Mer de Glace, and laid down an excellent map of it. His first observations and his survey are recorded in a celebrated book published in 1843, and entitled "Travels in the Alps."

154. These observations were also followed up in subsequent years, the results being recorded in a series of detached letters and essays of great interest. These were subsequently collected in a volume entitled "Occasional Papers on the Theory of Glaciers," published in 1859. The labours of Agassiz and Forbes are the two chief sources of our knowledge of glacier phenomena.

§ 21. The Theodolite and its Use. Our own Measurements.

155. My object thus far is attained. I have given you proofs of glacier motion, and a historic account of its measurement. And now we must try to add a little to the knowledge of glaciers by our own labours on the ice. Resolution must not be wanting at the commencement of our work, nor steadfast patience during its prosecution. Look then at this theodolite; it consists mainly of a telescope and a graduated circle, the telescope capable of motion up and down, and the circle, carrying the telescope along with it, capable of motion right and left. When desired to make the motion exceedingly fine and minute, suitable screws, called tangent screws, are employed. The instrument is supported by three legs, movable, but firm when properly planted.

156. Two spirit-levels are fixed at right angles to each other on the circle just referred to. Practice enables one to take hold of the legs of the instrument, and so to fix them that the circle shall be nearly horizontal. By means of four levelling screws we render it accurately horizontal. Exactly under the centre of the instrument is a small hook from which a plummet is suspended; the point of the bob just touches a rock on which we make a mark; or if the earth be soft underneath, we drive a stake into it exactly under the plummet. By re-suspending the plummet at any future time we can find to a hairbreadth the position occupied by the instrument to-day.

157. Look through the telescope; you see it crossed by two fibres of the finest spider's thread. In actual work we first direct the telescope across the glacier, until the intersection of the two fibres accurately covers some well-defined point of rock or tree at the other side of the valley. This, our fixed standard, we sketch with its surroundings in a note-book, so as to be able immediately to recognise it on our return to this place. Imagine a straight line drawn from the centre of the telescope to this point, and that this line is permitted to drop straight down upon the glacier, every point of it falling as a stone would fall; along such a line we have now to fix a series of stakes.

158. A trained assistant is already upon the glacier. He erects his staff and stands behind it; the telescope is lowered without swerving to the right or to the left; in mathematical language it remains in the same vertical plane. The crossed fibres of the telescope probably strike the ice a little away from the staff of the assistant; by a wave of the arm he moves right or left; he may move too much, so we wave him back again. After a trial or two he knows whether he is near the proper point, and if so makes his motions small. He soon exactly strikes the point covered by the intersection of the fibres. A signal is made which tells him that he is right; he pierces the ice with an auger and drives in a stake. He then goes forward, and in precisely the same manner takes up another point. After one or two stakes have been driven in, the assistant is able to take up the other points very rapidly. Any requisite number of stakes may thus be fixed in a straight line across the glacier.

159. Next morning we measure the motion of all the stakes. The theodolite is mounted in its former position and carefully levelled. The telescope is directed first upon the standard point at the opposite side of the valley, being moved by a tangent screw until the intersection of the spider's threads accurately covers the point. The telescope is then lowered to the first stake, beside which our trained assistant is already standing. He is provided with a staff with feet and inches marked on it. A glance shows us the stake has moved down. By our signals the assistant recovers the point from which we started yesterday, and then determines the distance from this point to the stake. It is, say, 6 inches; through this distance, therefore, the stake has moved.

160. We are careful to note the hour and minute at which each stake is driven in, and the hour and the minute when its distance from its first position is measured; this enables us to calculate the accurate daily motion of the point in question. The distances through which all the other points have moved are determined in precisely the same way.

161. Thus we shall proceed to work, first making clear to our minds what is to be done, and then making sure that it shall be accurately done. To give our work reality, I will here record the actual measurements executed, and the actual thoughts suggested, on the Mer de Glace in 1857. The only unreality that I would ask you to allow, is that you and I are supposed to be making the observations together. The labour of measuring was undertaken for the most part by Mr. Hirst.

§ 22. Motion of the Mer de Glace.

162. On July 14, then, we find ourselves at the end of the Glacier des Bois, not far from the source of the Arveiron. We direct our telescope across the glacier, and fix the intersection of its spider's threads accurately upon the edge of a pinnacle of ice. We leave the instrument untouched, looking through it from hour to hour. The edge of ice moves slowly, but plainly, past the fibres, and at the end of three hours we assure ourselves that the motion has amounted to several inches. While standing near the vault of the Arveiron, and talking about going into it, its roof gives way, and falls with the sound of thunder. It is not, therefore, without reason that I warned you against entering these vaults in summer.

163. We ascend to the Montanvert Inn, fix on it as a residence, and then descend to the lateral moraine of the glacier a little below the inn. Here we erect our theodolite, and mark its exact position by a plummet. We must first make sure that our line is perpendicular, or nearly so, to the axis or middle line of the glacier. Our instructed assistant lays down a long staff in the direction of the axis, assuring himself, by looking up and down, that it is the true direction. With another staff in his hand, pointed towards our theodolite, he shifts his position until the second staff is perpendicular to the first. Here he gives us a signal. We direct our telescope upon him, and then gradually raising its end in a vertical plane we find, and note by sketching, a standard point at the other side of the glacier. This point known, and our plummet mark known, we can on any future day find our line. (To render the measurements more intelligible, I append on the next page an outline diagram of the Mer de Glace, and of its tributaries.)

164. Along the line just described ten stakes were set on July 17, 1857. Their displacements were measured on the following day. Two of them had fallen, but here are the distances passed over by the eight remaining ones in twenty-four hours.

DAILY MOTION OF THE MER DE GLACE.

First Line: A A' upon the Sketch.

East West Stake 1 2 3 4 5 7 9 10 Inches 12 17 23 26 25 26 27 33