1844.]

EXPLANATION OF THE VEINED STRUCTURE.

57

in the last chapter of my book of Travels; but not having it by me, I cannot refer you to the particular passages. The point in question is undoubtedly the least obvious and most difficult part of the theory, but as I have no doubt of its exactness, it will have a proportionate weight in deciding in its favour the opinion of persons accustomed to mechanical theories. It would be difficult to bring it home to the apprehension of ordinary readers; and, for this reason, I have dwelt upon it, perhaps, too shortly in the chapter alluded to.

You will readily admit, that if I shall demonstrate separate reasons for the existence of each of the structures figured above (the first a plan, the second a section), the result will be the spoon-shaped structure which I have shewn to exist in glaciers.

(1.) The tearing asunder of the particles of the glacier owing to the friction of the sides is nearly but not quite, parallel to the sides; for this reason, that the lines of greatest strain are determined, not merely by the force of gravitation which urges the particles forwards, but there is a drag towards the centre of the stream, in consequence of the greater velocity there.

Or view the matter thus-the movement of the ice stream (considered just now solely as respects its surface), is effected against a varying resistance. The line of particles in the direction a a present a greater force of opposition to the movement of the particle a, than the line of particles bẞ present to the movement of b. This is owing to the lateral friction acting more powerfully in retarding the first than the second; consequently the virtual wall of the glacier, or plane of complete

resistance, will be no longer A B, but inclined (for the particle a) in the direction A'B'.

If this reasoning require support from experiment, it is easily had. I have described, in a foot-note to my last chapter, the experiment of dusting powder upon a moving viscous stream; and our friend Heath has now a specimen of the result, shewing the lines of separation in the direction I have stated. The same is remarkably shown in the case of a stream of water, for instance, a mill-race. Although the movement of the water, as shown by floating bodies, is exceedingly nearly (for small velocities, sensibly) parallel to the sides, yet the variation of speed from the side to the centre of the stream occasions a ripple or molecular discontinuity, which inclines forward from the sides to the centre of the stream at an angle with the axis, depending on the ratio of the central and lateral velocities. The veined structure of the ice corresponds to the ripple of the water, a molecular discontinuity whose measure is not comparable to the actual velocity of the ice; and, therefore, the general movement of the glacier, as indicated by the moraines, remains sensibly parallel to the sides.*

(2.) If I have explained myself distinctly as respects the

fissures produced by lateral friction, there will be little difficulty

I have lately identified completely the planes of separation in the lava streams of Etna, which correspond perfectly to those of the glacier, being nearly vertical at the sides, and directed slightly towards the centre of the stream.

1844.] SURFACES OF DISCONTINUITY IN A VERTICAL PLANE.

59

in applying the same reasoning to the resistance of the frontal dip, exhibited in the second figure of this letter. When a fluid, or semi-fluid, is very viscous, there is a great resistance to its onward motion in the direction which gravity and the fall of the bed prescribe. Let L M be the surface, N O the bed of a glacier; then the resolved force is usually considered as acting on the particles m n, in the directions m m', n n', parallel to the bed. But if we reflect that, owing to the length of the glacier, and the toughness or consistency in its mass, the resistance of the line of particles nv is enormous, the plane of complete resistance N O will virtually be twisted in the direction N'O', and the particle tends to be thrust forwards and upwards, which will evidently produce the frontal dip.

(3.) But there is a peculiarity in the vertical plane which did not exist in the horizontal one. In the case we first considered, the veined structure exists almost entirely in the neighbourhood of the sides of the glacier, and is lost towards its centre, being due to the influence of friction, which varies with the distance from the side; the central part, e f g h (fig. 11), moving nearly uniformly, would cease to exhibit a linear arrangement. The completion of the curve is due to the influence of the curvilinear bottom, combined with the opposing mass of the glacier in front; and this

L

move onward in the direction in which the effective pressure is greatest; and it is plain, that, owing to the diminishing relation between the weight of the superincumbent particles and

SEVENTH LETTER ON GLACIERS.

60

the frontal resistance, the direction in which tend to slide over one another, or to produce re verticality at the surface, and on the whole wil to produce lines of discontinuity, such as N M. (4.) Considering the glacier at different po

a

Fig. 16.

it is evident, by similar reasoning, that near névé a the frontal dip will be all but vertical, 1 horizontal resistance is enormous; whilst at where it tends to vanish, the shells will tend to the bed. It is needless to add, that the relati the particles over one another, producing disco to be confounded with their absolute motion exactly as under head (1.) I must, however, the tendency of any particle due to the hydrost be to describe ultimately the whole curve N glacier, this may account for some of the facts, which indicate a tendency in the ice to expe in it, as well as the convexity of the glacier at remarkable rise of surface during winter.

Lastly, The ablation of the surface of the descent from a to b (fig. 16) will tend continu observed elongated forms of the superficial band shells of structure obliquely.

1844.]

GREAT GLACIER OF ALETSCH.

61

IX. EIGHTH LETTER on GLACIERS,* addressed to PROFESSOR JAMESON by PROFESSOR FORBES.

Observations on the Motion of the Glacier of Aletsch-Experiments on the Plasticity of the Ice of the Mer de Glace, by Observing the Distortion of a Limited Space of Ice devoid of Crevasses-Movement of a Glacier of the Second Order 8000 Feet above the Sea.

GENEVA, 30th August 1844.

My Dear Sir-The theory of glaciers has now reached that point when it can only receive some material addition by the multiplication of accurate measurements; and these measurements must be conducted in the manner which will best discriminate between rival hypotheses, and, if possible, yield direct instead of indirect proofs of each fundamental fact assumed. In my former letters I have insisted sufficiently upon the importance of the results which a system of nice measurement has introduced into this branch of science, and their value to the theorist who afterwards wishes to put numerical for unknown quantities in his investigations; I also showed that there is a continuity and approximate constancy in the motions of glaciers, which permits us to obtain, with certain precautions, in a few days, better results than any one had previously acquired during the lapse of months or years. I have now to announce to you that I have pushed these measurements to a still greater degree of minuteness, and with results which show that the methods I have employed are trustworthy, and are able to afford the direct solution of questions which at first appeared to admit of only indirect or inductive proof.

Of this class, by far the most important appeared to be the manner in which the glacier alters its form in such a way, and to such a degree, as to suffer its central portion to descend towards the valley with double or treble the velocity of its lateral parts. Such, for instance, I have found to be the case in the middle region of the great glacier of Aletsch, where its inclination is small (about 4o), and where the continuity of the

* Edinburgh New Philosophical Journal, October 1844.