No. III. EXTRACTS from LETTERS to PROFESSOR FORBES, on No. IV. ACCOUNT OF AN EXPERIMENT ON STOCKHOLM PITCH, CONFIRMING THE VISCOUS THEORY OF GLACIERS. In No. V. EXTRACTS from a LETTER from E. BLACKWELL, Esq., containing Observations on the Movement of Gla- * These articles ought chronologically to have preceded the two first of the ERRATA. Page 80, line 6 from bottom, for Plate II. read Plate I. 119, line 1, for XIV. read XIII. PREFATORY NOTE ON THE RECENT PROGRESS AND PRESENT ASPECT OF THE THEORY OF GLACIERS. * IN 1850, Mr. Faraday delivered a lecture at the Royal Institution on certain properties of water, and more especially of water in the act of freezing. This lecture was never (I believe) published by authority. But an abstract of it appeared in the Athenæum Journal for June 15, 1850, and also in the Literary Gazette. In this brief and imperfect summary of what must evidently have been an interesting and suggestive discourse, it is stated, that if a film of water be enclosed between two plates of ice, even at a thawing temperature, the film of water is frozen, and the plates of ice cohere; and also that damp snow becomes, by the same process, compacted into a snow-ball, which will not occur if the snow is dry and hard frozen. These facts appear to have excited little notice, until attention was called to them by Dr. Tyndall in a lecture, also delivered at the Royal Institution, on the 23d January 1857. He gave to the phenomenon the name of regelation. He applied it to explain the observation, that portions of ice crushed in a mould under Bramah's press may assume new and compact forms with* Reprinted in the Second Appendix to the present volume. b out showing any trace of flaws; this he attributed to the "regelation" of the water in the crevices. Mr. James Thomson and his brother, Professor William Thomson of Glasgow,* however, ascribed this consolidation to the effect of intense pressure, causing simultaneous liquefaction, which commences at every point of the interior of the ice to which the pressure extends (according to a previous discovery made by them to that effect), and to its subsequent solidification when the pressure is removed. Dr. Tyndall soon applied his experiments on the consolidation or moulding of ice, and his adaptation to them of Mr. Faraday's fact of "regelation," to the explanation of the veined structure and movement of glaciers, which certain previous speculations of Mr. Sorby and himself about "planes of cleavage" had brought under his notice. Thus it will be seen how the theory of glaciers became anew, in 1857, a matter of attention to men of science; and, considering the activity and ingenuity of those engaged in its study, the received doctrines were not likely to be adopted without being first thoroughly canvassed. My theory, among others, was discussed, and I congratulated myself upon the examination which it was likely to receive upon its intrinsic merits. The fact that ice can be moulded under pressure, even in hand specimens, so as completely to recover its continuity under a changed form, was an argument in favour of my interpretation of the similar fact occurring in glaciers on a great scale, which appeared to me likely to remove some natural prepossessions, as well as to throw light on the precise relations of water and ice near the freezing-point of the former or thawingpoint of the latter, to which in my writings I had repeatedly * Proceedings of the Royal Society of London, 7th May 1857, 23d February and 22d April 1858. referred. BEHAVIOUR OF ICE UNDER BRAMAH'S PRESS. XV This practical argument was the more acceptable, because the absence of such a power of being moulded under intense, rapid pressure had been urged as an objection to my theory by MM. Schlagintweit, in their work on this subject.* The fact is, that the confining of the ice by lateral compression, whether in the great experiment of nature (in glaciers), or on the small scale, is, generally speaking, requisite to its success. I had, however, somewhat underrated the difficulties which my opinions had to contend with. The new generation of thinkers, whose powers of investigation were now first to be exercised on the theory of glaciers, had to review and discuss all the preliminary objections which fifteen years before had furnished the weapons of the opponents of "plasticity" as a property of ice on the great scale. Having said all that I could urge on that subject, I had left my case with a calm and reasonable confidence that Time would be the ablest advocate of my cause. I never replied to MM. Schlagintweit's appeal to the evidence derived from the pulverization of ice under Bramah's press,† the reply being the very same as I had already made several times to the popular argument derived from the fragility of ice. It appeared to me that the difficulties felt by Dr. Tyndall and Mr. Huxley, in admitting my theory, even after the ingenious experiments of the former had demonstrated on the small scale the moulding power of ice, which I had long before asserted to be unquestionably true on the large scale, were also such as a longer familiarity with the subject, and perhaps a more * Untersuchungen über die Physikalische Geographie der Alpen, pp. 24, 122. In this case the most extravagant distortion was sought to be produced in a few moments of time. Whilst in the glacier, an almost inappreciable distortion (for small areas or hand specimens) is produced in periods of many days or weeks. Very probably also, MM. Schlagintweit operated at temperatures considerably below the freezing-point, otherwise they could hardly fail to have obtained the same results as Dr. Tyndall. deliberate consideration of the whole of my views respecting it, would materially modify. I hope I may be allowed to say that the event has proved, partially at least, that I judged rightly. It was natural that the author of so interesting an experiment as the moulding of ice at a fusing temperature under Bramah's press, should see in it the germ of a new theory. It is not less natural that I, who rather hoped for than expected such a palpable illustration of my opinion, should see in it, not a new explanation of the phenomena of glaciers, but a new proof that the explanation which I had advanced was correct. These are points which naturally fall to be decided by those of the scientific public who contemplate the question from an impartial point of view. If the aspect in which I regard it be the more correct-if the conclusions of Dr. Tyndall are rather confirmatory than subversive of my own-the result of the discussion will be one more affecting personal credit than scientific truth. If it be found that the limited plasticity of ice, which, when ice is exposed in the glacier to a peculiarly violent strain, necessitates the formation of an infinity of minute rents, is really a part of my theory :-that it also embraces the substitution of the finite sliding of the internally bruised surfaces over one another under the same circumstances, still producing a quasi-fluid character in the motion of the whole :-if it be granted, moreover, that the reconsolidation of the bruised glacial substance into a coherent whole may be effected by pressure alone acting upon granular snow or upon ice softened by imminent thaw into a condition more plastic than ice of low temperature, and that the terms "bruising and re-attachment," "incipient fissures reunited by time and cohesion," were equivalent in 1846 to the phrase "fracture and regelation" applied * The following are specimens of the phraseology used by me in that year, or previously, with reference to the pages where they will be found in the present |