FOOTNOTES:

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

[B] Agassiz, 'Système 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 assistants, 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.

HALF OF SUMMER MOTION.

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

No. of stake. Inches.
West 1 71/4
2 11
3 131/2
4 13
5 133/4
6 141/4
7 153/4
8 153/4
9 121/4
10 12
11 61/2 East.

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 73/4
2 91/2
3 133/4
4 16
5 16
6 153/4
7 171/2
8 161/2
9 141/2
10 14

The maximum here is an inch and three-quarters greater than that of line No. 1. The summer maximum at this portion of the glacier also exceeds that of the part intersected by line No. 1. The surface of the glacier between the two lines is in a state of tension which relieves itself by a system of transverse fissures, and thus permits of the quicker advance of the forward portion.

My desire, in making these measurements, was, in the first place, to raise the winter observations of the motion to the same degree of accuracy as that already possessed by the summer ones. Auguste Balmat had already made a series of winter observations on the Mer de Glace; but they were made in the way employed before the introduction of the theodolite by Agassiz and Forbes, and shared the unavoidable roughness of such a mode of measurement. They moreover gave us no information as to the motion of the different parts of the glacier along the same transverse line, and this, for reasons which will appear subsequently, was the point of chief interest to me.

CAUSE OF GLACIER-MOTION.
DE SAUSSURE'S THEORY.
(13.)

Perhaps the first attempt at forming a glacier-theory is that of Scheuchzer in 1705. He supposed the motion to be caused by the conversion of water into ice within the glacier; the known and almost irresistible expansion which takes place on freezing, furnishing the force which pushed the glacier downward. This idea was illustrated and developed with so much skill by M. de Charpentier, that his name has been associated with it; and it is commonly known as the Theory of Charpentier, or the Dilatation-Theory. M. Agassiz supported this theory for a time, but his own thermometric experiments show us that the body of the glacier is at a temperature of 32° Fahr.; that consequently there is no interior magazine of cold to freeze the water with which the glacier is supposed to be incessantly saturated. So that these experiments alone, if no other grounds existed, would prove the insufficiency of the theory of dilatation. I may however add, that the arguments most frequently urged against this theory deal with an assumption, which I do not think its author ever intended to make.

THE GLACIER SLIDES.

Another early surmise was that of Altmann and Grüner (1760), both of whom conjectured that the glacier slid along its bed. This theory received distinct expression from De Saussure in 1799; and has since been associated with the name of that great alpine traveller, being usually called the 'Theory of Saussure,' and sometimes the 'Sliding Theory.' It is briefly stated in these words:—

"Almost every glacier reposes upon an inclined bed, and those of any considerable size have beneath them, even in winter, currents of water which flow between the ice and the bed which supports it. It may therefore be understood that these frozen masses, drawn down the slope on which they repose, disengaged by the water from all adhesion to the bottom, sometimes even raised by this water, must glide by little and little, and descend, following the inclinations of the valleys, or of the slopes which they cover. It is this slow but continual sliding of the ice on its inclined base which carries it into the lower valleys."[A]

STRAINED INTERPRETATION.

De Saussure devoted but little time to the subject of glacier-motion; and the absence of completeness in the statement of his views, arising no doubt from this cause, has given subsequent writers occasion to affix what I cannot help thinking a strained interpretation to the sliding theory. It is alleged that he regarded a glacier as a perfectly rigid body; that he considered it to be "a mass of ice of small depth, and considerable but uniform breadth, sliding down a uniform valley, or pouring from a narrow valley into a wider one."[B] The introduction "of the smallest flexibility or plasticity" is moreover emphatically denied to him.[C]

It is by no means probable that the great author of the 'Voyages' would have subscribed to this "rigid" annotation. His theory, be it remembered, is to some extent true: the glacier moves over its bed in the manner supposed, and the rocks of Britain bear to this day the traces of these mighty sliders. De Saussure probably contented himself with a general statement of what he believed to be the substantial cause of the motion. He visited the Jardin, and saw the tributaries of the Mer de Glace turning round corners, welding themselves together, and afterwards moving through a sinuous trunk-valley; and it is scarcely credible that in the presence of such facts he would have denied all flexibility to the glacier.

The statement that he regarded a glacier to be a mass of ice of uniform width, is moreover plainly inconsistent with the following description of the glacier of Mont Dolent: "Its most elevated plateau is a great circus, surrounded by high cliffs of granite, of pyramidal forms; thence the glacier descends through a gorge, in which it is narrowed; but after having passed the gorge, it enlarges again, spreading out like a fan. Thus it has on the whole the form of a sheaf tied in the middle and dilated at its two extremities."[D]

GLACIER OF MONT DOLENT.

Curiously enough this very glacier, and these very words, are selected by M. Rendu as illustrative of the plasticity of glaciers. "Nothing," he says, "shows better the extent to which a glacier moulds itself to its locality than the form of the glacier of Mont Dolent in the Valley of Ferret;" and he adds, in connexion with the same passage, these remarkable words:—"There is a multitude of facts which would seem to necessitate the belief that the substance of glaciers enjoys a kind of ductility which permits it to mould itself to the locality which it occupies, to grow thin, to swell, and to narrow itself like a soft paste."[E]