Asks THH question on flow of glaciers after ice has been fractured and fragmented.
CD had to leave Royal Society lecture [joint paper by THH and J. Tyndall, "On the structure and motions of glaciers", Philos. Trans. R. Soc. Lond. 147 (1857): 327–46] before the end because of headache.
Down Bromley Kent
My dear Huxley
I was most deeply interested by the lecture at the Royal, & was much vexed at being forced to retreat from my head aching so, & a pretty night I had afterwards! It is a horrid shame to trouble you, but I can hardly get subject out of my head, & am so very curious on one point, that I must beg for an answer on a point which I daresay came into the tail of the paper.— The point is, do you suppose if the fragments of ice did not after their innumerable fractures become united together by freezing; would the mass flow on? I suppose not, judging from the high angle of a talus of broken rocks though of course rock is far less brittle than ice. I presume the downward pressure could not be communicated through a fragmentary mass, just as you cannot push out an inch of sand in a gun barrel, though you could easily push out a piece of loose sandstone—or again, I suppose Tyndall would have found it far more difficult with his screw to have pushed out granular salt out of his semi-ring-model, than the piece of solid ice —or again I presume a scaffold pole would slide lengthways down a slope, down which a pile of fragments of wood would not slide.— Do tell me whether this is correct, for it seems to me quite beautiful if the freezing of the brittle ice, accounts for its filling all inequalities, its apparent solidity, & its flowing motion.—
By the way Tyndall ought to explain for ignoramuses how he knows that the ice freezes together & not merely coheres, like two pieces of wet glass— I do not understand how the non-coherence of snow or ice under a freezing temperature explains this; for it would be dry & so would not cohere; nor could I understand how he distinguishes the pillars or pinnacles (if I understand rightly) by which the pieces of Wenham ice were united, from preexisting ice thawed away. But I daresay all this is made clear in paper. The point which I want to know is the previous one, whether or not the freezing of the fragments together & the consequent easy transmission of force in one direction, is the cause of the flowing or sliding movement of the glaciers.—
I never heard a more interesting paper than the part, which I did hear.— Forgive me for being so troublesome
Ever yours | C. Darwin
- f1 2041.f1Dated by CD's reference to Tyndall and Huxley 1857 (see n. 2, below).
- f2 2041.f2Huxley and John Tyndall's paper ‘On the structure and motion of glaciers’ (Tyndall and Huxley 1857) had been read to the Royal Society of London on 15 January 1857. The paper, which challenged the viscous theory of glaciers proposed in 1843 by James David Forbes, contained observations made by the two during a trip to Switzerland in August 1856 and the results of subsequent experiments conducted by Tyndall.
- f3 2041.f3Assuming ice to be a hard, brittle substance rather than a viscous one, Tyndall suggested that under pressure the ice would break into fragments and that these would subsequently re-unite when they became moist through melting, a process he called ‘regelation’ on the suggestion of Joseph Dalton Hooker (see Rowlinson 1971, p. 192). Regelation explained how the form of the glacier could be moulded under pressure such that it appeared to flow as if it were viscous.
- f4 2041.f4Tyndall had used a hydraulic press in his demonstrations to show how ice could be moulded into any shape (Tyndall and Huxley 1857, p. 329–31).
- f5 2041.f5Tyndall had observed that ice blocks in a hot shop-window froze together despite the fact that the blocks themselves were melting (Tyndall and Huxley 1857, p. 329). Wenham ice was imported, mainly from Norway, by the Wenham Ice Company of the Strand, London.
- f6 2041.f6Tyndall and Huxley were not so much interested in the ‘cause’ of glacial flow, which, like Forbes, they ascribed to the weight of the glacier: ‘the glacial valley is a mould through which the ice is pressed by its own gravity’ (Tyndall and Huxley 1857, pp. 332–3). Instead, they were concerned with how the pressure was communicated through the body of the glacier, whether by virtue of its true viscosity, as Forbes believed, or by the process of regelation in the fragmentary mass giving the appearance of viscous flow (p. 333). See also letters to T. H. Huxley, 3 February , and to John Tyndall, 4 February .