bottom force out a crease, or two or three successive parallel creases, on either side; and, I daresay, you recollect that one effect of the great land-slip at Lyme, some years ago, was to elevate a ridge of shingle above the level of the sea at some distance from the shore. A succession of equidistant wrinkles will be formed whenever different parts of a plastic body are subjected in succession to a pressure violent enough to produce detrusion, or sliding amongst the particles. Thus, if a very viscid fluid is poured in a gradual stream upon a flat surface, so that it may spread uniformly, a succession of circular creases is formed in consequence of the hydrostatic pressure from the heaped-up centre becoming sufficient to overcome, for a moment, the viscosity at a certain distance from the centre; as the circumference rises in a crease, the centre falls, the central pressure suddenly diminishes, and if the stream continue to be poured uniformly upon the centre, although the circles will expand slightly, it will not be until a sufficient head is again raised to overcome suddenly the viscosity of the fluid that a new wrinkle is formed in exactly the same relative position as the first.* Treacle, mortar, tar, and similar bodies, usually present such creases when poured out. What has now been said of a viscid mass spreading uniformly from a centre applies equally to one confined in a trough with parallel sides, if constantly fed from one end. A succession of waves are thus formed, as in Mr. Milward's mud-slides, or as in a glacier. They become confounded or not at a distance from the origin; that depends entirely on the rate of motion of the stream at different points, which again depends chiefly upon the declivity of its bed. These wrinkles or creases, then, do occur at regular intervals, even in bodies perfectly homogeneous, and, under external circumstances, perfectly uniform. The intervals of such waves depend, in these cases, solely upon the physical qualities of tenacity, specific gravity, etc., of the body, and the more or less * The steps of this process are attempted to be illustrated by the curves in Plate IX. fig. 4. 1848.] ORIGIN OF THE WRINKLES ON THE GLACIER. 217 ample stream which furnishes it. We perceive here nothing like an annual recurrence; and this circumstance at first puzzled me, the intervals between the dirt-bands of the Mer de Glace (which are evidently the same with the wrinkles) being, as observed by me at the very time of their first discovery, so nearly consistent with what I supposed to be the annual motion of the ice stream, and which was afterwards confirmed by direct experiment, as scarcely to allow us to suppose the coincidence fortuitous. But an easy experiment establishes the analogy perfectly. If the stream of plastic matter already supposed be not uniformly supplied, but arrive in gushes, every such overflow, by the rapid rise of the head, throws off a wrinkle in the most regular manner. So that, for example, on examining those plaster models, formerly repeatedly described by me, in which cupfuls of white and of blue plaster of Paris were alternately poured down an inclined channel, each separate flow was found to constitute a wave or crease. In a glacier, especially in its higher regions, the difference of summer and of winter velocity is sufficient to produce what may be called (relatively) a gush; and I suppose that the wrinkles are formed in most glaciers at the foot of the steeps of the névé (as Mr. Milward also believes), where a pressure á tergo is produced by the heat of the short summer, sufficient to overcome the incalculable resistance which a mass of half-frozen snow, hundreds of feet thick and hundreds of yards wide, presents, to be squeezed and moulded, after the manner of a semifluid, into a convex wrinkle. Of the fact there is no doubt. Each wrinkle, then, is nothing else than a local swelling, such as those figured by M. Collin, taking place at the moment when the upward and forward force due to the quasihydrostatic pressure of the mass becomes insupportable, and gives rise to the forced separation of the cohering substance by countless fissures, constituting the frontal dip of the veined structure of the glacier, whose position, taken in connection with the wrinkles, is shewn in Plate IX. fig. 5. I farther beg leave to direct your attention to a very In curious illustration of these views which I lately noticed. the mechanical turning or planing of malleable iron, the spiral shavings have a structure which is truly remarkable, and shews convincingly that the effect of a steady pressure upon a semisolid or plastic body, is really such as to produce not merely wrinkles or creases on the surface, in the usual wave-like form, advanced in the centre, and withdrawn or retarded at the sides; but that the shaving has its particles squeezed upwards and forwards, as I have maintained that the mass of ice is, in consequence of the intense frontal resistance, and when the tenacity of the metal is pushed to its utmost limit of endurance, detrusion takes place at intervals sensibly equal, as in one of the specimens herewith sent,-being nothing else than the wrinkles exaggerated, and the bruise producing the veined structure pushed to an actual separation. (See Plate IX. fig. 6). These specimens (and such may be found in the workshop of almost any machine-maker)* have the higher degree of interest, because the surface of detrusion makes so very large an angle with the line of pressure. This process of heaping up by internal sliding of the parts of a semifluid mass was pointed out by me, I believe, for the first time, as applicable not only to very tenacious bodies, but even to streams no more viscid than common water. But I concluded that when the frontal resistance (due to friction and cohesion) becomes very great, the planes of least resistance may assume an inclination of 60° or more, a notion which has been treated as practically untenable by a mathematical critic of my theory, whilst he admits that it is theoretically possible. The iron shavings in question demonstrate the truth They are not so common as I supposed when I wrote this; they are principally to be found when coarse planings are made from iron of not the very best quality, and not lubricated with water. The finest iron is too plastic. On mentioning recently these observations to Mr. James Naysmith of Manchester, he stated it to me, as a fact familiar to practical men, that, in turning a cylinder three feet in circumference, the shaving, owing to frontal condensation, and the overriding of the parts, is perhaps only two and a half feet long. How perfect the analogy with what I have always maintained to be the mechanism of the glacier ! It is this thickening, amounting to one-fifth part, which compensates during winter the summer's waste. 1848.] CURIOUS ANALOGIES IN PLANED IRON. 219 and feasibility of my anticipation. There is no difficulty in determining the exact line of pressure, for it is obviously that in which the tool is made to act, or it is mathematically parallel to the flat side of the shaving itself, if we suppose it straightened. (Fig. 7.)* In one of the specimens now before me, the planes of detrusion, or frontal dip, make an angle, as nearly as can be estimated, of seventy degrees, with the base or line of pressure. From the fibrous appearance of the whole mass, I have little doubt that it is traversed by numberless fissures or flaws parallel to the planes of actual sliding, flaws which might probably be made evident by immersing the whole in dilute acid. Time does not allow me to add more. Some may consider these approximations and analogies trifling, but I persuade myself that you will not do so, being well aware how much has resulted in the progress of science from the patient study of minute facts not obviously related to one another. It is some pleasure to me to persuade myself that my speculations upon the cause of the motion of glaciers have had some slight influence in drawing attention to the loose manner in which bodies have hitherto been classified as solid and fluid, rigid, flexible, or plastic. On the one hand, attention is directed to the way in which stress or strain is exerted upon masses, and modified by their internal constitution in a way which no theory not embracing an expression of that constitution founded on experience can possibly represent. On the other hand, the imperfect views which practical men have entertained as to the manner in which intense strains affect materials of certain kinds and in certain forms, are apparently about to undergo a considerable revolution. I remain, my dear Sir, yours very truly. EDINBURGH, 2d December 1848. * [The analogy of this figure to fig. 7 of Plate I., representing the effect of vertical detrusion on a lava stream of Mount Etna, is remarkable.] XIX. SIXTEENTH LETTER ON GLACIERS. Addressed to PROFESSOR JAMESON.* Observations on the Movement of the Mer de Glace down to 1850. Observations by Balmat, at different seasons, in continuation of those formerly detailed. On the gradual passage of Ice into the Fluid State; observations of M. Person. Notice of an undescribed Pass of the Alps, from Chamouni to Orsières by the Glaciers of Tour and Salena. My dear Sir-Having had the good fortune once more to spend a few (though very few) days amongst the glaciers of Chamouni last summer, I avail myself of your kind permission to carry forward the account of my observations, which has now, for a period of eight years, been regularly communicated to the readers of your Journal. As my stay was limited by imperative engagements to little more than a week, I was prevented from undertaking a continuous series of observations on the movement of the ice. I was fortunate, however, in obtaining materials for the correction and extension of certain parts of my Map of the Mer de Glace, which were deficient in my former observations, especially as to the exact form of the basin of the great Glacier du Géant, which I had only visited once before, on occasion of the passage of the Col of that name in 1842. This year I traversed again all the difficult part of that glacier, and took angles with the theodolite from the upper part of the basin, immediately under the Aiguille du Géant. But as these observations can have little interest until reduced into the form of a corrected edition of the Map, I shall say nothing of them here.t It will be recollected by some of your readers that a remarkable stone called "La Pierre platte," was one of the * Edinburgh New Philosophical Journal for January 1851. These corrections were introduced into the new edition of my Map of the Mer de Glace (including also the Glacier of Bossons), accompanying the small abridgment of my "Travels," published in 1855 by Messrs. Black under the title of "Tour of Mont Blanc and of Monte Rosa."] Lying on the surface of the Glacier de Léchaud (in the upper part of the Mer de Glace), and carried along by the motion of the ice. It is marked C in my Map of the Glaciers. |