Part 5

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

165. You have already assured yourself by actual contact that the body of the glacier is real ice, and you may have read that glaciers move; but the actual observation of the motion of a body apparently so rigid is strangely interesting. And not only does the ice move bodily, but one part of it moves past another; the rate of motion augmenting gradually from 12 inches a day at the side to 33 inches a day at a distance from the side. This quicker movement of the central ice of glaciers had been already observed by Agassiz and Forbes; we verify their results, and now proceed to something new. Crossing the Glacier du Géant, which occupies more than half the valley, we find that our line of stakes is not yet at an end. The 10th stake stands on the part of the ice which comes from the Talèfre.

166. Now the motion of the sides is slow, because of the friction of the ice against its boundaries; but then one would think that midway between the boundaries, where the friction of the sides is least, the motion ought to be greatest. This is clearly not the case; for though the 10th stake is nearer than the 9th to the eastern or _Chapeau_ side of the valley, the 10th stake surpasses the 9th by 6 inches a day.

167. Here we have something to think of; but before a natural philosopher can think with comfort he must be perfectly sure of his facts. The foregoing line ran across the glacier a little below the Montanvert. We will run another line across a little way above the hotel. On July 18 we set out this line, and to multiply our chances of discovery we place along it 31 stakes. On the subsequent day five of these were found unfit for use; but here are the distances passed over by the remaining six-and-twenty in 24 hours.

Second Line: B B' upon the Sketch.

West Stake 2 3 4 5 6 7 8 9 10 11 12 13 Inches 11 12 15 15 16 17 18 19 20 20 21 21 Stake 15 16 17 18 19 20 21 22 23 24 25 26 Inches 23 23 23 21 23 21 25 22 22 23 25 26 East

168. Look at these numbers. The first broad fact they reveal is the advance in the rate of motion from first to last. There are however some irregularities; from 23 inches at the 17th stake we fall to 21 inches at the 18th; from 23 inches at the 19th we fall to 21 inches at the 20th; from 25 inches at the 21st we fall to 22 inches at the 22nd and 23rd; but notwithstanding these small ups and downs, the general advance of the rate of motion is manifest. Now there may have been some slight displacement of the stakes by melting, sufficient to account for these small deviations from uniformity in the increase of the motion. But another solution is also possible. We shall afterwards learn that the glacier is retarded not only by its sides but by its bed; that the upper portions of the ice slide over the lower ones. Now if the bed of the Mer de Glace should have eminences here and there rising sufficiently near to the surface to retard the motion of the surface, they might produce the small irregularities noticed above.

169. We note particularly, while upon the ice, that the 26th stake, like the 10th stake in our last line, stands much nearer to the eastern than to the western side of the glacier; the measurements, therefore, offer a further proof that the centre of this portion of the glacier is not the place of swiftest motion.

§ 23. _Unequal Motion of the two Sides of the Mer de Glace._

170. But in neither the first line nor the second were we able to push our measurements quite across the glacier. Why? In attempting to do one thing we are often taught another, and thus in science, if we are only steadfast in our work, our very defeats are converted into means of instruction. We at first planted our theodolite on the lateral moraine of the Mer de Glace, expecting to be able to command the glacier from side to side. But we are now undeceived; the centre of the glacier proves to be higher than its sides, and from our last two positions the view of the ice near the opposite side of the glacier was intercepted by the elevation at the centre. The mountain slopes, in fact, are warm in summer, and they melt the ice nearest to them, thus causing a fall from the centre to the sides.

171. But yonder on the heights at the other side of the glacier we see a likely place for our theodolite. We cross the glacier and plant our instrument in a position from which we sweep the glacier from side to side. Our first line was below the Montanvert, our second line above it; this third line is exactly opposite the Montanvert; in fact, the mark on which we have fixed the fibre-cross of the theodolite is a corner of one of the windows of the little inn. Along this line we fix twelve stakes on July 20. On the 21st one of them had fallen; but the velocities of the remaining eleven in 24 hours were found to be as follows:--

Third Line: C C' upon the Sketch.

East West Stake 1 2 3 4 5 6 7 8 9 10 11 Inches 20 23 29 30 34 28 25 25 25 18 9

172. Both the first stake and the eleventh in this series stood near the sides of the glacier. On the eastern side the motion is 20 inches, while on the western side it is only 9. It rises on the eastern side from 20 to 34 inches at the 5th stake, which we, standing upon the glacier, can see to be much nearer to the eastern than to the western side. _The united evidence of these three lines places the fact beyond doubt, that opposite the Montanvert, and for some distance above it and below it, the whole eastern side of the glacier is moving more quickly than the western side._

§ 24. _Suggestion of a new Likeness of Glacier Motion to River Motion. Conjecture tested._

173. Here we have cause for reflection, and facts are comparatively worthless if they do not provoke this exercise of the mind. It is because facts of nature are not isolated but connected, that science, to follow them, must also form a connected whole. The mind of the natural philosopher must, as it were, be a web of _thought_ corresponding in all its fibres with the web of _fact_ in nature.

174. Let us, then, ascend to a point which commands a good view of this portion of the Mer de Glace. The ice-river we see is not straight but curved, and its curvature is _from_ the Montanvert; that is to say, its convex side is east, and its concave side is west (look to the sketch). You have already pondered the fact that a glacier, _in some respects_, moves like a river. How would a river move through a curved channel? This is known. Were the ice of the Mer de Glace displaced by water, the point of swiftest motion at the Montanvert would not be the centre, but a point east of the centre. Can it be then that this "water-rock," as ice is sometimes called, acts in this respect also like water?

175. This is a thought suggested on the spot; it may or it may not be true, but the means of testing it are at hand. Looking up the glacier, we see that at _les Ponts_ it also bends, but that there its convex curvature is towards the western side of the valley (look again to the sketch). If our surmise be true, the point of swiftest motion opposite _les Ponts_ ought to lie west of the axis of the glacier.

176. Let us test this conjecture. On July 25 we fix in a line across this portion of the glacier seventeen stakes; every one of them has remained firm, and on the 26th we find the motion for 24 hours to be as follows:--

Fourth Line: D D' upon the Sketch.

East West Stake 1 2 3 4 5 8 7 8 9 10 11 12 13 14 15 Inches 7 8 13 15 16 19 20 21 21 23 23 21 22 17 15

177. Inspected by the naked eye alone, the stakes 10 and 11, where the glacier reaches its greatest motion, are seen to be considerably to the west of the axis of the glacier. Thus far we have a perfect verification of the _guess_ which prompted us to make these measurements. You will here observe that the "guesses" of science are not the work of chance, but of thoughtful pondering over antecedent facts. The guess is the "induction" facts, to be ratified or exploded by the test of subsequent experiment.

178. And though even now we have exceedingly strong reason for holding that the point of maximum velocity obeys the law of liquid motion, the strength of our conclusion will be doubled if we can show that the point shifts back to the eastern side of the axis at another place of flexure. Fortunately such a place exists opposite Trélaporte. Here the convex curvature of the valley turns again to the east. Across this portion of the glacier a line was set out on July 28, and from measurements on the 31st, the rate of motion per 24 hours was determined.

Fifth Line: E E' upon the Sketch.

West East Stake 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Inches 11 14 13 15 15 16 17 19 20 19 20 18 16 15 10

179. Here, again, the mere estimate of distances by the eye would show us that the three stakes which moved fastest, viz. the 9th, 10th, and 11th, were all to the east of the middle line of the glacier. The demonstration that the point of swiftest motion wanders to and fro across the axis, as the flexure of the valley changes, is, therefore,--shall I say complete?

180. Not yet. For if surer means are open to us we must not rest content with estimates by the eye. We have with us a surveying chain: let us shake it out and measure these lines, noting the distance of every stake from the side of the glacier. This is no easy work among the crevasses, but I confide it confidently to Mr. Hirst and you. We can afterwards compare a number of stakes on the eastern side with the same number of stakes taken at the same distances from the western side. For example, a pair of stakes, one ten yards from the eastern side and the other ten yards from the western side; another pair, one fifty yards from the eastern side and the other fifty yards from the western side, and so on, can be compared together. For the sake of easy reference, let us call the points thus compared in pairs, _equivalent points_.

181. There were five pairs of such points upon our fourth line, D D', and here are their velocities:

Eastern points; motion in inches 13 15 16 18 20 Western " " " " 15 17 22 23 23

In every case here the stake at the western side moved more rapidly than the equivalent stake at the eastern side.

182. Applying the same analysis to our fifth line, E E', we have the following series of velocities of three pairs of equivalent points:--

Eastern points; motion in inches 15 18 19 Western " " " " 13 15 17

183. Here the three points on the eastern side move more rapidly than the equivalent points on the western side.

184. It is thus proved:--