[Sidenote: MODE OF MEASUREMENT.]

In setting a straight row of stakes across the glacier, our mode of proceeding was in all cases this:--The theodolite was placed on the mountain-side flanking the glacier, quite clear of the ice; and having determined the direction of a line perpendicular to the axis of the glacier, a well-defined object was sought at the opposite side of the valley as close as possible to this direction; the object being, in some cases, the sharp edge of a cliff; in others, a projecting corner of rock; and, in others, a well-defined mark on the face of the rock. This object and those around it were carefully sketched, so that on returning to the place it could be instantly recognized. On commencing a line the point of intersection of the two spiders' threads within the telescope was first fixed accurately upon the point thus chosen, and an a.s.sistant carrying a straight baton was sent upon the ice. By rough signalling he first stood near the place where the first stake was to be driven in; and the object end of the telescope was then lowered until he came within the field of view. He held his staff upright upon the ice, and, in obedience to signals, moved upwards or downwards until the point of intersection of the spiders-threads exactly hit the bottom of the baton; a concerted signal was then made, the ice was pierced with an auger to a depth of about sixteen inches, and a stake about two feet long was firmly driven into it. The a.s.sistant then advanced for some distance across the glacier; the end of the telescope was now gently raised until he and his upright staff again appeared in the field of view. He then moved as before until the bottom of his staff was struck by the point of intersection, and here a second stake was fixed in the ice. In this way the process was continued until the line of stakes was completed.

Before quitting the station, a plummet was suspended from a hook directly underneath the centre of the theodolite, and the place where the point touched the ground was distinctly marked. To measure the motion of the line of stakes, we returned to the place a day or two afterwards, and by means of the plummet were able to make the theodolite occupy the exact position which it occupied when the line was set out.

The telescope being directed upon the point at the opposite side of the valley, and gradually lowered, it was found that no single stake along the line preserved its first position: they had all shifted downwards.

The a.s.sistant was sent to the first stake; the point which it had first occupied was again determined, and its present distance from that point accurately measured. The same thing was done in the case of each stake, and thus the displacement of the whole row of stakes was ascertained.[A]

The time at which the stake was fixed, and at which its displacement was measured, being carefully noted, a simple calculation determined _the daily motion_ of the stake.

[Sidenote: THE FIRST LINE.]

Thus, on the 17th of July, 1857, we set out our first line across the Mer de Glace, at some distance below the Montanvert; on the day following we measured the progress of the stakes. The observed displacements are set down in the following table:--

First Line.--Daily Motion.

No. of stake. Inches.

West 1 moved 12-1/4 2 " 16-3/4 3 " 22-1/2 4 " ...

5 " 24-1/2 6 moved ...

7 " 26-1/4 8 " ...

9 " 28-3/4 10 " 35-1/2 East.

[Sidenote: THE CENTRE-POINT NOT THE QUICKEST.]

The theodolite in this case stood on the Montanvert side of the valley, and the stakes are numbered from this side. We see that the motion gradually augments from the 1st stake onward--the 1st stake being held back by the friction of the ice against the flanking mountain-side. The stakes 4, 6, and 8 have no motion attached to them, as an accident rendered the measurement of their displacements uncertain. But one remarkable fact is exhibited by this line; the 7th stake stood upon the _middle_ of the glacier, and we see that its motion is by no means the quickest; it is exceeded in this respect by the stakes 9 and 10.

The portion of the glacier on which the 10th stake stood was very much cut up by creva.s.ses, and, while the a.s.sistant was boring it with his auger, the ice beneath him was observed, through the telescope, to slide suddenly forward for about 4 inches. The other stakes retained their positions, so that the movement was purely local. Deducting the 4 inches thus irregularly obtained, we should have a daily motion of 31-1/2 inches for stake No. 10. The place was watched for some time, but the slipping was not repeated; and a second measurement on the succeeding day made the motion of the 10th stake 32 inches, whilst that of the centre of the glacier was only 27.

Here, then, was a fact which needed explanation; but, before attempting this, I resolved, by repeated measurements in the same locality, to place the existence of the fact beyond doubt. We therefore ascended to a point upon the old and now motionless moraine, a little above the Montanvert Hotel; and choosing, as before, a well-defined object at the opposite side of the valley, we set between it and the theodolite a row of twenty stakes across the glacier. Their motions, measured on a subsequent day, and reduced to their daily rate, gave the results set down in the following table:--

Second Line.--Daily Motion.

No. of stake. Inches.

West 1 moved 7-1/2 2 " 10-3/4 3 " 12-1/4 4 " 14-1/2 5 " 16 6 " 16-3/4 7 " 17-1/2 8 " 19 9 " 19-1/2 10 " 21 11 moved 21 12 " 22-1/2 13 " 21 14 " 22-1/2 15 " 20-1/2 16 " 21-3/4 17 " 22-1/4 18 " 25-1/4 19 " ...

20 " 25-3/4 East.

[Sidenote: CORROBORATIVE MEASUREMENTS.]

As regards the r.e.t.a.r.dation of the side, we observe here the same fact as that revealed by our first line--the motion gradually augments from the first stake to the last. The stake No. 20 stood upon the dirty portion of the ice, which was derived from the Talefre tributary of the Mer de Glace, and far beyond the middle of the glacier. These measurements, therefore, corroborate that made lower down, as regards the non-coincidence of the point of swiftest motion with the centre of the glacier.

But it will be observed that the measurements do not show any r.e.t.a.r.dation of the ice at the eastern extremity of the line of stakes--the motion goes on augmenting from the first stake to the last.

The reason of this is, that in neither of the cases recorded were we able to get the line quite across the glacier; the creva.s.ses and broken ice-ridges, which intercepted the vision, compelled us to halt before we came sufficiently close to the eastern side to make its r.e.t.a.r.dation sensible. But on the 20th of July my friend Hirst sought out an elevated station on the Chapeau, or eastern side of the valley, whence he could command a view from side to side over all the humps and inequalities of the ice, the fixed point at the opposite side, upon which the telescope was directed, being the corner of a window of the Montanvert Hotel.

Along this line were placed twelve stakes, the daily motions of which were found to be as follows:--

Third Line.--Daily Motion.

No. of stake. Inches.

East 1 moved 19-1/2 2 " 22-3/4 3 " 28-3/4 4 " 30-1/4 5 " 33-3/4 6 " 28-1/4 7 moved 24-1/2 8 " 25 9 " 25 10 " 18 11 " ...

12 " 8-1/2 West.

The numbering of the stakes along this line commenced from the Chapeau-side of the glacier, and the r.e.t.a.r.dation of that side is now manifest enough; the motion gradually augmenting from 19-1/2 to 33-1/2 inches. But, comparing the velocity of the two extreme stakes, we find that the r.e.t.a.r.dation of stake 12 is much greater than that of stake 1.

Stake 5, moreover, which moved with the _maximum_ velocity, was not upon the centre of the glacier, but much nearer to the eastern than to the western side.

[Sidenote: A NEW PECULIARITY OF GLACIER MOTION.]

It was thus placed beyond doubt that the point of maximum motion of the Mer de Glace, at the place referred to, is not the centre of the glacier. But, to make a.s.surance doubly sure, I examined the comparative motion along three other lines, and found in all the same undeviating result.

This result is not only unexpected, but is quite at variance with the opinions. .h.i.therto held regarding the motion of the Mer de Glace. The reader knows that the trunk-stream is composed of three great tributaries from the Geant, the Lechaud, and the Talefre. The Glacier du Geant fills more than half of the trunk-valley, and the junction between it and its neighbours is plainly marked by the dirt upon the surface of the latter. In fact four medial moraines are crowded together on the eastern side of the glacier, and before reaching the Montanvert they have strewn their debris quite over the adjacent ice. A distinct limit is thus formed between the clean Glacier du Geant and the other dirty tributaries of the trunk-stream.

Now the eastern side of the Mer de Glace is observed on the whole to be much more fiercely torn than the western side, and this excessive creva.s.sing has been referred to _the swifter motion of the Glacier du Geant_. It has been thought that, like a powerful river, this glacier drags its more sluggish neighbours after it, and thus tears them in the manner observed. But the measurement of the foregoing three lines shows that this cannot be the true cause of the creva.s.sing. In each case the stakes which moved quickest _lay upon the dirty portion of the trunk-stream_, far to the east of the line of junction of the Glacier du Geant, which in fact moved slowest of all.

[Sidenote: LAW OF MOTION SOUGHT.]

The general view of the glacier, and of the shape of the valley which it filled, suggested to me that the a.n.a.logy with a river might perhaps make itself good beyond the limits. .h.i.therto contemplated. The valley was not straight, but sinuous. At the Montanvert the convex side of the glacier was turned eastward; at some distance higher up, near the pa.s.sages called _Les Ponts_, it was turned westward; and higher up again it was turned once more, for a long stretch, eastward. Thus between Trelaporte and the Ponts we had what is called a point of contrary flexure, and between the Ponts and the Montanvert a second point of the same kind.

[Sidenote: CONJECTURE REGARDING CHANGE OF FLEXURE.]

Supposing a river, instead of the glacier, to sweep through this valley; _its_ point of maximum motion would not always remain central, but would deviate towards that side of the valley to which the river turned its convex boundary. Indeed the positions of towns along the banks of a navigable river are mainly determined by this circ.u.mstance. They are, in most cases, situate on the convex sides of the bends, where the rush of the water prevents silting up. Can it be then that the ice exhibits a similar deportment? that the same principle which regulates the distribution of people along the banks of the Thames is also acting with silent energy amid the glaciers of the Alps? If this be the case, the position of the point of maximum motion ought, of course, to shift with the bending of the glacier. Opposite the Ponts, for example, the point ought to be on the Glacier du Geant, and westward of the centre of the trunk-stream; while, higher up, we ought to have another change to the eastern side, in accordance with the change of flexure.

On the 25th of July a line was set out across the glacier, one of its fixed termini being a mark upon the first of the three Ponts. The motion of this line, measured on a subsequent day, and reduced to its daily rate, was found to be as follows:--

Fourth Line.--Daily Motion.

No. of stake. Inches.

East 1 moved 6-1/2 2 " 8 3 " 12-1/2 4 " 15-1/4 5 " 15-1/2 6 " 18-3/4 7 " 18-1/4 8 " 18-3/4 9 " 19-1/2 10 moved 21 11 " 20-1/2 12 " 23-1/4 13 " 23-1/4 14 " 21 15 " 22-1/4 16 " 17-1/4 17 " 15 West.

This line, like the third, was set out and numbered from the eastern side of the glacier, the theodolite occupying a position on the heights of the Echelets. A moment's inspection of the table reveals a fact different from that observed on the third line; _there_ the most easterly stake moved with more than twice the velocity of the most westerly one; _here_, on the contrary, the most westerly stake moves with more than twice the velocity of the most easterly one.

To enable me to compare the motion of the eastern and western halves of the glacier with greater strictness, my able and laborious companion undertook the task of measuring with a surveyor's chain the line just referred to; noting the pickets which had been fixed along the line, and the other remarkable objects which it intersected. The difficulty of thus directing a chain over creva.s.ses and ridges can hardly be appreciated except by those who have tried it. Nevertheless, the task was accomplished, and the width of the Mer de Glace, at this portion of its course, was found to be 863 yards, or almost exactly half a mile.

Referring to the last table, it will be seen that the two stakes numbered 12 and 13 moved with a common velocity of 23-1/4 inches per day, and that their motion is swifter than that of any of the others.

The point of swiftest motion may be taken midway between them, and this point was found by measurement to lie 233 yards _west_ of the dirt which marked the junction of the Glacier du Geant with its fellow tributaries: whereas, in the former cases, it lay a considerable distance _east_ of this limit. Its distance from the eastern side of the glacier was 601 yards, and from the western side 262 yards, being 170 yards west of the centre of the glacier.

[Sidenote: CONJECTURE TESTED.]

But the measurements enabled me to take the stakes in pairs, and to compare the velocity of a number of them which stood at certain distances from the eastern side of the valley, with an equal number which stood at the same distances from the western side. By thus arranging the points two by two, I was able to compare the motion of the entire body of the ice at the one side of the central line with that of the ice at the other side. Stake 17 stood about as far from the western side of the glacier as stake 3 did from its eastern side; 16 occupied the same relation to 4; 15, to 5; 13, to 7; and 12, to 9.

Calling each pair of points which thus stand at equal distances from the opposite sides _corresponding points_, the following little table exhibits their comparative motions:--

Numbers and Velocities of Corresponding Points on the Fourth Line.

No. Vel. No. Vel. No. Vel. No. Vel. No. Vel.

West 17 15 16 17-1/4 15 22-1/4 13 23-1/4 12 23-1/4 East 3 12-1/2 4 15-1/4 5 15-1/2 7 18-1/4 9 19-1/2

[Sidenote: WESTERN HALF MOVES QUICKEST.]

The table explains itself. We see that while stake 17, which stands _west_ of the centre, moves 15 inches, stake 3, which stands an equal distance _east_ of the centre, moves only 12-1/2 inches. Comparing every pair of the other points, we find the same to hold good; the western stake moves in each case faster than the corresponding eastern one.

Hence, _the entire western half of the Mer de Glace, at the place crossed by our fourth line, moves more quickly than the eastern half of the glacier_.

We next proceeded farther up, and tested the contrary curvature of the glacier, opposite to Trelaporte. The station chosen for this purpose was on a gra.s.sy platform of the promontory, whence, on the 28th of July, a row of stakes was fixed at right angles to the axis of the glacier.