Part 23 (1/2)

[Sidenote: DESCENT OF BOULDERS.]

On the morning of the 18th there was no cloud to be seen anywhere, and the sunlight glistened brightly on the surface of the ice. We ascended to the Tacul. The spontaneous falling of the stones appeared more frequent this morning than I had ever seen it. The sun shone with unmitigated power upon the ice, producing copious liquefaction. The rustle of falling debris was incessant, and at frequent intervals the boulders leaped down the precipice, and rattled with startling energy amid the rocks at its base. I sent Simond to the top to remove the looser stones; he soon appeared, and urged the moraine-s.h.i.+ngle in showers down the precipice, upon a bevelled slope of which some blocks long continued to rest. They were out of the reach of the guide's baton, and he sought to dislodge them by sending other stones down upon them.

Some of them soon gave way, drawing a train of smaller s.h.i.+ngle after them; others required to be hit many times before they yielded, and others refused to be dislodged at all. I then cut my way up the precipice in the manner already described, fixed the stake, and descended as speedily as possible. We afterwards fixed the bottom stake, and on the 20th the displacements of all three were measured.[C] The s.p.a.ces pa.s.sed over by the respective stakes in 24 hours were found to be as follows:--

Inches.

Top stake 6.00 Middle stake 4.50 Bottom stake 2.56

[Sidenote: MOTION OF STAKES.]

The height of the precipice was 140.8 feet, but it sloped off at its upper portion. The height of the middle stake above the ground was 35 feet, and of the bottom one 4 feet. It is therefore proved by these measurements that the bottom of the ice-wall at the Tacul moves with less than half the velocity of the top; while the displacement of the intermediate stake shows how the velocity gradually increases from the bottom upwards.

FOOTNOTES:

[A] 'Edinb. Phil. Journ.,' Oct. 1846, p. 417.

[B] Aga.s.siz, 'Systeme Glaciaire,' p. 522.

[C] On this latter occasion my guide volunteered to cut the steps for me up to the pickets; and I permitted him to do so. In fact, he was at least as anxious as myself to see the measurement carried out.

WINTER MOTION OF THE MER DE GLACE.

(12.)

The winter measurements were executed in the manner already described, on the 28th and 29th of December, 1859. The theodolite was placed on the mountain's side flanking the glacier, and a well-defined object was chosen at the opposite side of the valley, so that a straight line between this object and the theodolite was approximately perpendicular to the axis of the glacier. Fixing the telescope in the first instance with its cross hairs upon the object, its end was lowered until it struck the point upon the glacier at which a stake was to be fixed.

Thanks to the intelligence of my a.s.sistants, after the fixing of the first stake they speedily took up the line at all other points, requiring very little correction to make their positions perfectly accurate. On the day following that on which the stakes were driven in, the theodolite was placed in the same position, and the distances to which the stakes had moved from their original positions were accurately determined. As already stated, the first line crossed the glacier about 80 yards above the Montanvert Hotel.

[Sidenote: HALF OF SUMMER MOTION.]

Line No. I.--Winter Motion in Twenty-four Hours.

No. of stake. Inches.

West 1 7-1/4 2 11 3 13-1/2 4 13 5 13-3/4 6 14-1/4 7 15-3/4 8 15-3/4 9 12-1/4 10 12 11 6-1/2 East.

[Sidenote: THE SAME LAW IN SUMMER AND WINTER.]

The maximum here is fifteen and three-quarters inches; the maximum summer motion of the same portion of the glacier is about thirty inches. These measurements also show that in winter, as well as in summer, the side of the glacier opposite to the Montanvert moves quicker than that adjacent to it. The stake which moved with the maximum velocity was beyond the moraine of La Noire. The second line crossed the glacier about 130 yards below the Montanvert.

Line No. II.--Winter Motion in Twenty-four Hours.

No. of stake. Inches.

1 7-3/4 2 9-1/2 3 13-3/4 4 16 5 16 6 15-3/4 7 17-1/2 8 16-1/2 9 14-1/2 10 14