THE VEINED STRUCTURE AND THE DIFFERENTIAL MOTION.
(28.)
DIFFERENTIAL MOTION GREATEST AT EDGES.
I have now to examine briefly the explanation of the structure which refers it to differential motion—to a sliding of the particles of ice past each other, which leaves the traces of its existence in the blue veins. The fact is emphatically dwelt upon by those who hold this view, that the structure is best developed nearest to the sides of the glacier, where the differential motion is greatest. Why the differential motion is at its maximum near to the sides is easily understood. Let a b, c d, [Fig. 47], represent the two sides of a glacier, moving in the direction of the arrow, and let m a b c n be a straight line of stakes set out across the glacier to-day. Six months hence this line, by the motion of the ice downwards, will be bent to the form m a' b' c' n: this curve will not be circular, it will be flattened in the middle; the points a and c, at some distance on each side of the centre b, move in fact with nearly the same velocity as the centre itself. Not so with the sides:—a' and c' have moved considerably in advance of m and n, and hence we say that the difference of motion, or the differential motion, of the particles of ice near to the side is a maximum.
During all this time the points m a' b' c' n have been moving straight down the glacier; and hence it will be understood that the sliding of the parts past each other, or, in other words, the differential motion, is parallel to the sides of the glacier. This, indeed, is the only differential motion that experiment has ever established; and consequently, when we find the best blue veins referred to the sides of the glacier because the differential motion is there greatest, we naturally infer that the motion meant is parallel to the sides.
STRUCTURE OBLIQUE TO SIDES.
But the fact is, that this motion would not at all account for the blue veins, for they are not parallel to the sides, but oblique to them. This difficulty revealed itself after a time to those who first propounded the theory of differential motion, and caused them to modify their explanation of the structure. Differential motion is still assumed to be the cause of the veins, but now a motion is meant oblique to the sides, and it is supposed to be obtained in the following way:—Through the quicker motion of the point c' the ice between it and n becomes distended; that is to say, the line c' n is in a state of strain—there is a drag, it is said, oblique to the sides of the glacier; and it is therefore in this direction that the particles will be caused to slide past each other. Dr. Whewell, who advocates this view, thus expounds it. He supposes the case of an alpine valley filled with india-rubber which has been warmed until it has partially melted, or become viscous, and then asks, "What will now be the condition of the mass? The sides and bottom will still be held back by the friction; the middle and upper part will slide forwards, but not freely. This want of freedom in the motion (arising from the viscosity) will produce a drag towards the middle of the valley, where the motion is freest; hence the direction in which the filaments slide past each other will be obliquely directed towards the middle. The sliding will separate the mass according to such lines; and though new attachments will take place, the mass may be expected to retain the results of this separation in the traces of parallel fissures."[A] Nothing can be clearer than the image of the process thus placed before the mind's eye.
One fact of especial importance is to be borne in mind: the sliding of filaments which is thus supposed to take place oblique to the glacier has never been proved; it is wholly assumed. A moraine, it is admitted, will run parallel to the side of a glacier, or a block will move in the same direction from beginning to end, without being sensibly drawn towards the centre, but still it is supposed that the sliding of parts exists, though of a character so small as to render it insensible to measurement.
STRUCTURE CROSSES LINES OF SLIDING.