ol. 40. No. 266. SEPTEMBER 1870. Published the First Day of every Month.-Price 2s. 6d. THE LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE, AND JOURNAL OF SCIENCE. Being a Continuation of Tilloch's Philosophical Magazine,' CONDUCTED BY SIR ROBERT KANE, LL.D. F.R.S. M.R.I.A. F.C.S. AND WILLIAM FRANCIS, PH.D. F.L.S. F.R.A.S. F.C.S. FOURTH SERIES. N° 266.-SEPTEMBER 1870. LONDON: PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET, Printers and Publishers to the University of London. Sold by Longmans, Green, Reader and Dyer; Kent and Co.; Simpkin, Marshall and Co.; Whittaker and Co.; and H. Baillière, London:-and by A. and C. Black, and Thomas Clark, Edinburgh; Smith and Son, Glasgow:-Hodges and Smith, Dublin:-Putnam, New York:-and Asher and Co., Berlin. BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. The FORTIETH ANNUAL MEETING of this Association will be held in LIVERPOOL, commencing on Wednesday, September 14, 1870. President-PROFESSOR HUXLEY, LL.D., F.R.S., &c. 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Local THE LAWS OF VERSE OR PRINCIPLES OF VERSIFICATION Together with an Annotated Reprint of the Inaugural Presidential Address to the Examiner in Mathematics to the University of London. ROYAL SCHOOL OF MINES. Director-Sir Roderick Impey Murchison, Bart., K.C.B., F.R.S., &c. During the Twentieth Session, 1870-71, which will commence on the 10th of October, the following COURSES of LECTURES and PRACTICAL DEMONSTRATIONS will be given : 1. Chemistry. By E. Frankland, Ph.D., F.R.S. 2. Metallurgy. By John Percy, M.D., F.R.S. 3. Natural History. By T. H. Huxley, LL D., F.R.S. 4. Mineralogy. By Warington W. Smyth, M.A., F.R.S. 6. Geology. By A. C. Ramsay, LL.D., F.R.S. 7. Applied Mechanics. By T. M. Goodeve, M.A. 8. Physics. By Frederick Guthrie, B.A., Ph.D. Instruction in Mechanical Drawing, by the Rev. J. Haythorne Edgar, M.A. The Fee for Students desirous of becoming Associates is £30 in one sum, on entrance, or two annual payments of £20, exclusive of the Laboratories. Pupils are received in the Royal College of Chemistry (the Laboratory of the School) under the direction of Dr. Frankland, and in the Metallurgical Laboratory under the direction of Dr. Percy. Tickets to separate Courses of Lectures are issued at £3 and £4 each. Officers in the Queen's Service, Her Majesty's Consuls, Acting Mining Agents and Managers, may obtain Tickets at reduced prices. Certificated Schoolmasters, Pupil Teachers, and others engaged in education, are also admitted to the Lectures at reduced fees. His Royal Highness the Prince of Wales grants two Scholarships; and several others have also been established by Government. For a Prospectus and information apply to the Registrar, Royal School of Mines, Jermyn Street, London, S.W. TRENHAM REEKS, Registrar. [ADVERTISEMENTS continued on 3rd page of Cover. THE LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE AND JOURNAL OF SCIENCE. [FOURTH SERIES.] SEPTEMBER 1870. XIX. On the Cause of the Motion of Glaciers. By JAMES CROLL, of the Geological Survey of Scotland*. The generally accepted theory proved by the Rev. Canon Moseley to be incorrect. SIN INCE the time that Professor Tyndall had shown that all the phenomena formerly attributed by Professor Forbes to plasticity could be explained upon the principle of regelation, discovered by Faraday, the viscous theory of glacier-motion has been pretty generally given up. The ice of a glacier is now almost universally believed to be, not a soft plastic substance, but a substance hard, brittle, and unyielding. The power that the glacier has of accommodating itself to the inequalities of its bed without losing its apparent continuity is referred to the property of regelation possessed by ice. All this is now plain; but what is it that impels the glacier forward is still a question under discussion. Various theories have been propounded regarding the cause of the descent of glaciers, all of which have been abandoned with the exception of that which attributes their descent to gravitation. But as the ice of the glacier descends with a differential motion, we have not only to explain what causes the glacier to slide on its bed, but also what displaces the particles of the ice over one another and alongside one another. What, then, is the force which shears the ice? The answer generally given is that gravitation alone is the force which does this; or, in other words, the mere weight of the ice is sufficient to overcome its cohesive force and to displace the particles over one another, * Communicated by the Author. Phil. Mag. S. 4. Vol. 40. No. 266. Sept. 1870. M The Rev. Canon Moseley has lately investigated this point, and has found that the amount of work performed on a glacier (assuming, of course, that the ice shears in the solid state) during its descent through a given space is enormously greater than the work of the weight of the glacier descending through that space. He has determined the amount of work performed by gravitation in the descent of a glacier, and the amount of internal work performed on the ice during the descent; and has found that, in respect to a glacier of the same uniform rectangular section and slope as the Mer de Glace at Les Ponts, and moving with the same uniform velocity, the aggregate work of the resistances which oppose themselves to its descent in a given time is about thirty-four times the work of the weight in the same time; consequently it is physically impossible that the mere weight alone of the glacier can be the cause of its descent. The impression left on my mind after reading Canon Moseley's memoir in the Proceedings of the Royal Society for January 1869 was that, unless some very serious error could be pointed out in the mathematical part of his investigation, it would be hopeless to attempt to overturn his general conclusion as regards the received theory of the cause of the descent of glaciers, by searching for errors in the experimental data on which the conclusion rests. Had the result been that the actual shearingforce of ice is by twice, thrice, four times, or even five times too great to allow of a glacier shearing by its own weight, one might then hope that, by some more accurate method of determining the unit of shear than that adopted by Canon Moseley, his objection to the received theory of glacier-motion might be met; but when the unit of shear is found to be not simply by three times, four times, or even five times, but actually by thirty, forty, or fifty times too great, all our hopes of overturning his conclusion by searching for errors in this direction vanish, even although there are some points connected with his unit of shear that are not very satisfactory. The ice of a glacier is in the hard, solid, and crystalline state. This is now generally admitted. Then, if the particles of the ice shear in this state, Canon Moseley's calculations show that the glacier cannot possibly descend by its weight only, as is generally supposed; and the generally received theory of glacier-motion must therefore be abandoned. I can perceive no way from this conclusion. of escape I presume that few who have given much thought to the subject of glacier-motion have not had some slight misgivings in regard to the commonly received theory. There are some facts which I never could harmonize with this theory. For example, boulder-clay is a far looser substance than ice; its shearing-force must be very much less than that of ice; yet immense masses of boulder-clay will lie immoveable for ages on the slope of a hill so steep that one can hardly venture to climb it, while a glacier will come crawling down a valley which by the eye we could hardly detect to be actually off the level. Again, a glacier moves faster during the day than during the night, and about twice as fast during summer as during winter. Professor Forbes, for example, found that the Glacier des Bos near its lower extremity moved sometimes in December only 11.5 inches daily, while during the month of July its rate of motion sometimes reached 52.1 inches per day. Why such a difference in the rate of motion between day and night, summer and winter? The glacier is not heavier during the day than it is during the night, or during the summer than it is during the winter; neither is the shearing-force of the great mass of the ice of a glacier sensibly less during the day than during the night, or during the summer than during the winter; for the temperature of the great mass of the ice does not sensibly vary with the seasons. Then, if this is the case, gravitation ought to be as able to move a glacier during the night as during the day, or during the winter as during the summer. At any rate, if there should be any difference it ought to be but trifling. It is true that, owing to the melting of the ice, the crevices of the glacier are more gorged with water during summer than during winter; and this, as Professor Forbes maintains*, may tend to make the glacier move faster during the former season than during the latter. But the advocates of the regelation theory cannot conclude, with Professor Forbes, that the water favours the motion of the glacier by making the ice more soft and plastic. The melting of the ice, according to the regelation theory, cannot very materially aid the motion of the glacier. The fact that the rate of motion of a glacier depends upon the amount of heat that the ice is receiving shows that heat in some way or other stands related as a cause to the motion of the glacier. But the point under consideration is, If the ice of a glacier shears in the solid state, as is generally supposed, has Canon Moseley proved that a glacier cannot descend by its weight only? I have carefully read the interesting memoirs by Mr. Mathews and Mr. Ball in reply to Canon Moseley; and although I agree with the most of their remarks regarding the unsatisfactory nature of Mr. Moseley's own theory of glacier-motion, yet I am unable to perceive that any thing which they have advanced materially affects his general conclusion as regards the commonly received theory. If the ice of a glacier shears, nothing which I have yet Occasional Papers, pp. 166, 223, |