summits,

"The great agent which Nature employs to relieve her overladen mountains is the glaciers."*

So much has been written and so much discussion has been raised on the origin, structure, motion, transporting power, &c. of glaciers, that it is difficult to convey, in small compass, a clear idea of the more important phenomena, which may now be considered as settled and ascertained.

The origin of the glacier may be said to be in the clouds; for from thence descends the snowy shower which forms it. This snow is at first dry, and often at a temperature far below the freezing point; but the summer's sun first raises it at the surface to 32° and then melts it. The water thus formed trickles through the colder mass below, altering its character and producing a peculiar kind of ice inclosing numerous air-bubbles. This ice is distinctive of that portion of the glacier which is above the snow-line, and which is usually called by the French name of the névé, while the portion which is below the snow-line is alone denominated glacier.

It seems doubtful whether all the water which trickles through the mass during summer becomes entirely frozen during the succeeding winter; or, whether a heavy snow at the commencement of winter may not so protect and cover up the névé as to leave water still liquid at a considerable depth. Mr. Huxley is of opinion (and Professor

* Glaciers of the Alps, p. 249.

Tyndall thinks his idea quite within the range of possibility), that water cells may be transmitted from the origin of the glacier to its end, retaining their contents always liquid.

One of the elements in the formation of glacier ice is thus evident. The alternate melting and freezing, and the saturation of the snow by the water which trickles through it, give a peculiar character to the ice of the névé, which prepares it for further changes. It has yet a much greater assimilation to snow than to ice; but, as the fresh layers of each year come to add to the previous mass, it is also evident that the lower layers must be subject to steady constant pressure. This pressure is considered to be the second great agent in the production of glacier ice. "The lower portions of the névé," says Professor Tyndall,* "are removed by pressure more and more from the condition of snow; the air-bubbles which give to the névé ice its whiteness are more and more expelled; and this process, continued throughout the entire glacier, finally brings the ice to that state of magnificent transparency which we find at the termination of the glacier of Rosenlaui and elsewhere."

If we thus gain some idea of the nature of the ice of the névé, or that portion of the glacier which is above the snow-line, we naturally inquire what is the state of the ice below the snow-line, and how is it maintained. The snow which falls annually on the glacier, properly so called, is all * Glaciers of the Alps, p. 252.

melted away during summer, together with a considerable quantity of the ice itself, so that, without some fresh source of supply, the lower part of the glacier must in the course of years entirely melt away. This source of supply is found in the motion of the glacier, which brings down steadily and imperceptibly the ice of the névé to replace that of the glacier, which is also moving slowly onwards, while to all appearance it is as fixed and rigid as the mountains themselves.

The fact of this movement of the glaciers must have been evident to the peasants of the Swiss valleys, long before it had become a subject of investigation by scientific men. To persons dwelling on the spot, there are palpable evidences to prove that the great river of ice moves as surely, though far more slowly than the lesser river of water which rolls underneath it, and pours forth from the ice-cavern at its mouth.*

The first scientific explorer of the Swiss Alps appears to have been Scheuchzer, of Zurich, in 1705; and the theory by which he attempted to explain the motion of the glacier was as follows: During the summer the surface of the glacier is covered with innumerable rills and streamlets, caused by the melting of the ice and snow, and these penetrate into all the cracks and openings in the ice. Now, as water expands in the act of freezing, it may be supposed that when all these rills and collections of water become frozen by the * See Frontispiece to this chapter.

winter's cold, the glacier itself must necessarily expand. And since it is bounded by rocky walls, and unable to yield in other directions, it must consequently expand in the quarter which offers least resistance, that is, down the valley.

To this theory there are many objections, two of the most striking of which are, that the motion of glaciers is most rapid in warm weather, when the ice is melting, not in the cold weather, when the water contained in the crevices of the glacier is supposed to freeze. Also, that the water which thus filters through the ice does not, in fact, freeze at all, even in winter, beyond a certain depth below the surface.

In a work on the Snow Mountains of Switzerland, published at Berne, in 1760, Grüner endeavoured to explain the cause of glacier motion, and his idea, being taken up and enlarged upon by Saussure, became very popular for a time. It was founded on the fact that all glaciers send forth from their lower end rivers of water, more or less important, according to the size of the glacier. These rivers flow out from beneath the ice, varying in volume with the season, and attaining their greatest size and rapidity in the height of summer, when the largest amount of snow and ice are being melted. It was concluded that the greater part of this water was supplied by the melting of the under part of the glacier, owing to the heat of the earth on which it rests. The bed of the glacier being always formed by the sloping sides

of mountains, and the intervening valleys, and thus, having a sensible inclination downwards, it was reasonable to suppose that there would be a constant tendency to descend by mere weight; a tendency, however, resisted by the inequalities of the ground or rock, and by the varying form of the channel through which the glacier had to pass. These impediments were supposed to be overcome by the constant melting of the portions of ice in contact with the rock, which would enable the whole mass to slide onwards. Saussure also imagined that the hydrostatic pressure of the water itself might contribute towards the same effect. This last idea is inconsistent with known facts respecting the structure of glaciers; but the melting of the lower surface is acknowledged to be one of the means by which a glacier slides downwards.

The glacier, having its origin on the heights, and descending through ravines and valleys of different widths, curves, and inclinations, to the lower and warmer regions, has many points of resemblance to a river. The motion of the glacier, though imperceptible to us, may have its own variations in different parts of its course, just as a river has its currents, its rapids, and its quiet full flow. These variations may be caused by precisely the same circumstances which give variety to the stream, namely, the form and declivity of its bed, the depth or shallowness of the contents, the rocky, or pebbly, or earthy surface over which they flow. That this variety does really exist, and

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