Part 21 (1/2)
[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 s.h.i.+fted 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 ” ...