With regard to the mechanical origin of the crevasses the most vague and untenable notions had been entertained until Mr. Hopkins published his extremely valuable papers. To him, indeed, we are almost wholly indebted for our present knowledge of the subject, my own experiments upon this portion of the glacier-question being for the most part illustrations of the truth of his reasoning. To understand the fissures in their more complex aspects it is necessary that we should commence with their elements. I shall deal with the question in my own way, adhering, however, to the mechanical principles upon which Mr. Hopkins has based his exposition.
Let a b, c d, be the bounding sides of a glacier moving in the direction of the arrow; let m, n be two points upon the ice, one, m, close to the retarding side of the valley, and the other, n, at some distance from it. After a certain time, the point m will have moved downwards to m', but in consequence of the swifter movement of the parts at a distance from the sides, n will have moved in the same time to n'. Thus the line m n, instead of being at right angles to the glacier, takes up the oblique position m' n'; but to reach from m' to n' the line m n would have to stretch itself considerably; every other line that we can draw upon the ice parallel to m' n' is in a similar state of tension; or, in other words, the sides of the glacier are acted upon by an oblique pull towards the centre. Now, Mr. Hopkins has shown that the direction in which this oblique pull is strongest encloses an angle of 45° with the side of the glacier.
LINE OF GREATEST STRAIN.
What is the consequence of this? Let a b, c d, [Fig. 26], represent, as before, the sides of the glacier, moving in the direction of the arrow; let the shading lines enclose an angle of 45° with the sides. Along these lines the marginal ice suffers the greatest strain, and, consequently across these lines and at right angles to them, the ice tends to break and to form marginal crevasses. The lines, o p, o p, mark the direction of these crevasses; they are at right angles to the line of greatest strain, and hence also enclose an angle of 45° with the side of the valley, being obliquely pointed upwards.
MARGINAL AND TRANSVERSE CREVASSES.
This latter result is noteworthy; it follows from the mechanical data that the swifter motion of the centre tends to produce marginal crevasses which are inclined from the side of the glacier towards its source, and not towards its lower extremity. But when we look down upon a glacier thus crevassed, the first impression is that the sides have been dragged down, and have left the central portions behind them; indeed, it was this very appearance that led M. de Charpentier and M. Agassiz into the error of supposing that the sides of a glacier moved more quickly than its middle portions; and it was also the delusive aspect of the crevasses which led Professor Forbes to infer the slower motion of the eastern side of the Mer de Glace.
The retardation of the ice is most evident near the sides; in most cases, the ice for a considerable distance right and left of the central line moves with a sensibly uniform velocity; there is no dragging of the particles asunder by a difference of motion, and, consequently, a compact centre is perfectly compatible with fissured sides. Nothing is more common than to see a glacier with its sides deeply cut, and its central portions compact; this, indeed, is always the case where the glacier moves down a bed of uniform inclination.
But supposing that the bed is not uniform—that the valley through which the glacier moves changes its inclination abruptly, so as to compel the ice to pass over a brow; the glacier is then circumstanced like a stick which we try to break by holding its two ends and pressing it against the knee. The brow, where the bed changes its inclination, represents the knee in the case of the stick, while the weight of the glacier itself is the force that tends to break it. It breaks; and fissures are formed across the glacier, which are hence called transverse crevasses.