Chemical Society-Journal for April, 1870. 8vo. Comitato Geologico d'Italia-Bolletino 3. 8vo. 1870. Cracroft, Bernard, Esq. M.R.I. (the Author)-Bank Dividend Chart. 1869-70. Dempsey, J. Maurice, Esq. (the Editor)-Our Ocean Highways: a Condensed Universal Route Book. 8vo. 1870. Editors-Academy for April, 1870. 4to. Artizan for April, 1870. 4to. Chemical News for April, 1870. 4to. Engineer for April, 1870. fol. Horological Journal for April, 1870. 8vo. Pharmaceutical Journal for April, 1870. 8vo. Photographic News for April, 1870. 4to. Practical Mechanics' Journal for April, 1870. 4to. Revue des Cours Scientifiques et Littéraires, April, 1870. Scientific Opinion for April, 1870. 4to. Franklin Institute-Journal. Nos. 529, 530. 8vo. 1870. Geological Institute, Imperial, Vienna-Jahrbuch. Band XIX. No. 4. 8vo. 1869. Verhandlungen, 1869. Nos. 14-18. 8vo. Geological Society-Quarterly Journal, No. 102. 8vo. 1870. Gladstone, J. H. Esq. Ph.D. F.R.S. M.R.I.-J. Miller, M.A. Christianum Organum. 16mo. 1870. Institut Impérial de France, Académie des Sciences-Mémoires. Tomes 29, 32, 34, 35. 4to. 1864-67. Mémoires par Divers Savants. Tomes 18, 19. 4to. 1865-68. Jones, H. Bence, M.D. F.R.S. Hon. Sec. R.I. (the Author)-Life and Letters of Faraday. 2nd edition. 2 vols. 8vo. 1870. Portrait of Professor Faraday (engraved from a Drawing by George Richmond, London Corporation Library Committee-Analytical Indexes to Vols. II. and VIII. National Education Union-Report of Congress at Manchester, Nos. 3, 4. 1869. 8vo. 1869. Photographic Society-Journal, No. 216. 8vo. 1870. Porter, J. H. Esq. (the Author)-The Sugar, Beet and Beet-root Sugar. (K 97) 8vo. 1870. Preussische Akademie der Wissenschaften-Monatsberichte, Feb. 1870. 8vo. Royal Society of London-Proceedings, No. 118. 8vo. 1870. Philosophical Transactions for 1869, Vol. CLIX. Part 2. 4to. 1870. Statistical Society-Journal, Vol. XXXIII. Part 1. 8vo. 1870. Symons, G. J. Esq. (the Author)-Symons' Monthly Meteorological Magazine, April, 1870. 8vo. Report of Rainfall Committee, 1868-69. 8vo. 1870. Williams, C. J. B. M.D. F.R.S. (the Author)-The Case of the late Earl St. Maur. (K 97) 2nd edition. 8vo. 1870. WEEKLY EVENING MEETING, Friday, May 13, 1870. SIR HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. President, The REV. HENRY MOSELEY, M.A. F.R.S. CANON OF BRISTOL AND INSTIT. IMP. SC. PARIS CORRESP. On the Descent of Glaciers. GLACIERS do not take their origin in the highest Alpine regions. It is not there that the snow chiefly falls, but on a belt girding them below. This wide belt is divided horizontally into an upper and lower part by the snow-line, at a height of from 3000 to 3300 yards. Above that line snow always lies, and rain very rarely falls; beneath the snow-line the snow disappears every summer, and rains are abundant. It is from this belt about the snow-line that the glaciers are seen emerging. They lie like huge slugs along the descending valleys, swelling themselves out to fill their channels where they are wide, and thinning themselves to pass through the gorges and narrow places in them. They seldom come down to a lower level than 1100 yards. Between this level where they end and the snow-line, 3100 yards high, where they begin, they traverse sometimes a very long space-lying for the most part at a low pitch. The resemblance to a huge mollusk is kept up in this, that they move with a strange slow motion, not altogether unlike that of such an animal. The parallel will be complete if we conceive the mollusk to have its tail continually renewed as it withdraws it from under the snow-line, and its head continually melted away as it thrusts it forward below the level of from 1000 to 1400 yards. If we further imagine the steep sides of the valley through which the glacier descends to have similar but smaller glaciers crawling down them to the principal glacier, we shall understand what is meant by tributary or secondary glaciers, which are thus placed in regard to the principal ones; having a far greater pitch or slope than they, and flowing into them like tributary streams to a river. The slope of a principal glacier is often as little as 3°, and yet it may move with a velocity of 24 inches a day. The slope of a tributary glacier is sometimes 50°, and it may not advance more than 4 or 5 inches a day at the most.* Masses of rock of different sizes, from huge boulders to stones, are constantly broken by the frost from the sides of the valley of the glacier, and are carried slowly down on its *The motion of the Glunberg, a tributary of the Aar glacier, inclined at 30° to 50°, was found by Desor to be 22 mètres a-year, while that of the Aar glacier, inclined at 4°, was 77 mètres. back to the level where its head melts away, and there are deposited. These are called moraines. They lie along the course of the glacier in ridges protecting the ice beneath them from the sun's rays. That ice does not therefore melt as the rest of the ice does, and so it forms a ridge of ice. A moraine is therefore a ridge of stones standing on a ridge of ice. The descent of a glacier is not a descent of the whole together, or bodily like that of a block of stone. There is an internal descent of every particle in the glacier over and alongside of every other particle. If a plane section be imagined to be made across it, the particles of ice passing through that cross-section at any given time must be conceived to be all moving at different rates so as to be sliding over and beside one another; the particles at the surface moving faster than those below, and the particles near the centre moving faster than those at a distance from it, exactly as the particles of a stream of water move. The existence of this differential motion is strikingly seen in what is called the veined structure of glaciers. This veined structure appears first to have been described by M. Guyot, in the year 1838. The following is his account of it, as he saw it on the glacier of Gries, which I translate from the work of M. Huber : "I saw under my feet the entire surface of the glacier covered with furrows an inch or two wide cut in snowy ice, and separated from one another by ridges of harder and more transparent ice. It was evident that the mass of the glacier was here formed of these two different kinds of ice the former (that of the furrows) was white and melted more rapidly; the other (that of the ridges) was more perfect, crystallized, transparent, and hard. The unequal resistance to melting of these two kinds of ice was the obvious cause of these depressions and elevations. After having followed them for several hundreds of mètres I reached a crevasse 20 or 30 feet wide, which, cutting the furrows at right angles, exhibited down to the depth of 30 or 40 feet an admirable transverse section of this structure. As far as my eye could reach I saw the mass of the glacier composed of layers of the [opaque] white ice separated from one another by layers of the transparent ice, the whole forming a mass as regularly stratified as certain calcareous rocks." This is the veined structure of glaciers. [Canon Moseley's remarks on the differential motion of glaciers, with special reference to the observations and experiments of Forbes and Tyndall, illustrated by diagrams, will be found in the 'Philosophical Magazine' for April, 1870.] The effect of the differential motion in separating and distributing the parts of a glacier has been shown in various ways. The remains of the guides lost in 1820 in Dr. Hamel's attempt to ascend Mont Blanc were found imbedded in the ice of the Glacier des Bossons in 1863. "The men and their things were torn to pieces and widely separated, many feet. All around them the ice was covered in every direction for 20 or 30 feet with the hair of one knapsack, spread over an area three or four hundred times greater than that of the knapsack. This," says Mr. Cowell, from whose paper read before the Alpine Club in April, 1864, I have made the quotation, "is not an isolated example of the scattering that takes place in or on a glacier; for I have myself seen on the Glacier of Theodule the remains of the syndic of Val Tournache scattered over a space of several acres." Whatever the force may be which causes the descent of glaciers, it must be chiefly expended in this constant displacement of the particles of the ice over one another and alongside one another, of which the veined structure affords the evidence, and to which is opposed everywhere that force of resistance which is called shearing force. By the property of ice called regelation, when any surface of ice so sheared is brought into contact with another similar surface it unites with it, so as to form of the two one continuous mass. This may be shown by experiment. If a cylinder of ice be placed in an apparatus suitable for the purpose and slowly sheared partly across, the new surfaces continually being brought in contact with one another in the act of shearing will not present the slightest appearance of separation, the ice being there as continuous as elsewhere, its molecules entering into precisely the same relations to one another in their new positions as they did in the positions out of which they have been sheared. Thus a slow displacement of shearing, by which different similar surfaces of ice in a glacier were continually being brought into presence and contact with one another, would exhibit all the phenomena of the motion of glacier ice. The late Principal Forbes calculated that in the Aar glacier the shearing displacement amounts to 13ths of the whole displacement of the glacier. The sliding displacement of the whole glacier bodily on its bed is, in comparison with it, unimportant as it regards the expenditure of force requisite to bring it about. The differential motion is the great and characteristic phenomenon of the descent of glaciers; but it is that, in assigning an adequate cause for which, existing glacier theories seem to me most conspicuously to have failed. The Swiss philosopher De Saussure was the first to study the descent of glaciers with care, and wrote on it about sixty years ago. He held that glaciers slip down the slopes on which they rest by their weight, just as other bodies slip down inclined planes. This explanation is simple, and was generally accepted as long as it was thought that glaciers slipped down bodily like blocks of stone would, with an equal motion of all their particles; but when the internal motion of their particles upon one another, like that of running water, came to be discovered, and when it was found that the high-pitched tributary glaciers moved slower than the low-pitched principal ones, this theory was brought into doubt, for it was in direct contradiction to these facts. M. Rendu, the Bishop of Annecy, considered the descent of a glacier to be so like that of a fluid that it was impossible to explain it otherwise than by supposing ice actually to be a fluid and not the solid thing it seems to be. He was the founder of the celebrated viscous theory of the descent of glaciers, advocated with such remarkable energy, industry, and ability by the late Principal James Forbes, whose various works on glaciers have exhausted the whole field of observation and supply most of the facts on which the true solution of the problem, whenever it is arrived at, must be founded. When, however, at another stage of the inquiry, it came to be discovered by Faraday and Tyndall that ice, when broken up, was capable of being united again by sufficient pressure, so as to become as perfectly solid and homogeneous as it was before, it became evident that supposing a sufficient pressure to be exerted on the glacier, in the direction of its descent, to crush its substance through the contractions and gorges of its channel, and over the irregularities in its bed, it would re-form itself and solidify, and become a compact mass again as it was before, when it had passed these obstructions. This is the regelation theory. At this stage the question had assumed this new form-"If ice be a viscous fluid, according to the viscous theory, is it fluid enough to descend by its own weight; or, if it be a solid, according to the regelation theory, is it little enough solid so to descend?" If, instead of ice, a glacier were of water, it would obviously descend by its weight. The same would be true if it were of oil, or soft mud, or quicksilver, or probably of pitch; but if it were of iron, or of copper, or of lead, it would not descend by its weight only, unless, indeed, these metals were in a state of fusion. A quicksilver glacier would descend by its weight only because it shears easily; a cast-iron one would not, because it shears with difficulty. There must, therefore, exist a relation between the shearing force and the weight of a given volume of a glacier, so that it may just descend by its weight only. Now, it is possible to investigate mathematically what that relation is. I have made that investigation.* I have founded it on this wellknown law of mechanical philosophy, that "The aggregate WORK of the forces which produce the displacement of a body or a system of bodies (however related) must at least equal the aggregate work of the resistances which oppose that displacement." The resistances opposed to the displacement of a glacier are— 1stly, Those which oppose themselves to the shearing of one surface of ice over another, which is continually taking place throughout the whole mass by reason of the differential motion; 2ndly, The friction of the superimposed laminæ of ice upon one another, which is greater in the lower than the upper; 3rdly, Abrasion of the ice on the bottom and sides of the channel of the glacier. If it descends by its weight only, then the work of its weight in its descent through any distance must at least equal the sum of the works of all these resistances. It is of course impossible to represent this relation mathematically in respect to an actual glacier having a variable direction and an irregular channel and slope; but in respect to an imaginary one having a constant direction and a uniform channel and slope, it is possible. I * 'Phil. Mag.,' May, 1869. |