Part 12

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