In his initial explanation for the observed motions of a solid sheet subject to the combined influences of small down-slope gravity forces and the alternations of temperature, Moseley was to open up the possibility of a similar action at work in the motion of glaciers. The occurrence and causes of glacial motion were exercising many minds in the middle decades of the 19th C. Indeed, the possibility that such a thermal ratchet could be at work in the motion of glaciers may well have encouraged Moseley to again dust off his observations made more than a decade earlier (5) and explain them in the context of the theoretical model outlined in Reference (6). In his writing on glaciers however, he was careful to avoid his earlier suggestion that the precise thermal ratchet model of Reference (5) could somehow provide a complete explanation of the motion of glaciers. This may well have been the result of some fairly strident critical remarks from Forbes (7) whose views Moseley clearly respected and whose careful and detailed field observations Moseley had so heavily relied upon. His initial contribution to the later debate on glacial movement was, perhaps confusingly, entitled “on the mechanical possibility of descent of glaciers by their weight only” (8). This paper was read before the Royal Society on the 7th January, 1869. When the analytical component of this paper was also published, later that same year, its title, more accurately reflecting the papers conclusions, had become “on the mechanical impossibility of descent of glaciers by their weight only” (9).
The substance of this pair of publications was to show that gravity acting on its own could not be providing a motivating force sufficient to overcome the various resistances opposing the motion. His theoretical model envisaged a highly idealised form of glacial channel. The glacier was taken to be straight over its entire length, with a uniform rectangular cross-section that provided a constant resistance to the shearing action of the ice. It was further assumed that the ice was subjected to a uniform gravitational force throughout its length, which, under all these circumstances would result in a rate of movement that was also constant over its length. The angle of incline was taken to be constant at 4o 52’ to reproduce the conditions at the location called Tacul on the Mer de Glace, where Tyndall (10) had not long before recorded the characteristics of the glacial motions. Moseley fully appreciated the limitations of his model in respect of more realistic glacial circumstances. However, he reasoned with what appears to have been sound logic that his idealisations would if anything overestimate the powers of gravity acting on the ice to cause motion. Even so, Moseley calculated the shear stress required to provide the resistance to the gravitational motion was just a little more than 1.3 lb/in2 compared with typical shear strength of ice found, in experiments he performed, to be 75 lb/in2. These and other very simple illustrative calculations led Moseley to conclude that “it results from this investigation that the weight of the glacier is insufficient to account for its descent; that it is necessary to conceive, in addition to its weight, the operation of some other and much greater force”.
Moseley was aware of the many contemporaneous reports that glaciers generally experienced the greater part of their forward motion through material distortion rather than basal sliding. This had been recorded during Tyndall’s observations on the Mer de Glace (10). He incorporated into his simple theoretical model allowance for differential strain rates across the width and through the depth of the glacial ice. He reasoned that the relatively small component of motion attributable to base and wall sliding meant that the shear resistance between the ice and the glacial valley was generally higher than that of the ice itself. And he further concluded that the force needed, additional to the weight, “must also be such as would produce those internal molecular displacements and those strains which are observed actually to take place in glacier ice, and must therefore be present to every part of the glacier as its weight is, but more than 30 times as great". The figure of 30 to 40 times as great as the force resulting from the weight alone keeps reappearing in the subsequent discussion and yet the limited numerical examples provided would suggest that the figure should have been more like 50 to 60.
That Moseley considered heat from solar radiation might provide the missing source of energy was made very clear in reference (6). He calculates the force additional to its weight would over 24 hours be required to do extra work of 7706513 ft.lbs (it is curious that even at a time when the arithmetic must have been tedious, a seemingly spurious 7 significant figures are so often used!). This is said to be equivalent to 61.76 ft.lbs of work for every square inch of glacial surface, which is “equivalent to 0.0635 heat-units, or to the heat necessary to raise 0.0635 lbs. of water by one degree Fahrenheit. This amount of heat passing into the mass of the glacier per square inch per day, and reconverted into mechanical work there, would be sufficient, together with its weight, to bring the glacier down.” Moseley does not elaborate upon the form that he considers this conversion to mechanical energy might take, but given the prior publications (5,6) it might be inferred that he had in mind the mechanical effects of expansion and contraction. We will probably never know, since the further paper promised (at pp139 and 145) in Moseley’s response (18) to the subsequent criticisms of his papers (5,6,8,9) was never it seems produced. Moseley died in January, 1872, just a few months after his last and very lucid response (18) to the many criticisms that his work was to attract.
So what were these criticisms and how did Moseley respond? These questions I will take up in my next posting.
References used in this blog:
(5) Moseley, Henry. “On the Descent of Glaciers”, Proc Roy Soc, 7, 1855, 333-342.
(6) Moseley, Henry. “On the Descent of a Solid Body on an Inclined Plane when subjected to alternations of Temperature”, Phil Mag, S.4, 38, August 1869, 99-118.
(7) Forbes, J. D. “Remarks on the Rev H Moseley’s Theory of the Descent of Glaciers”, Proc Roy Soc., vii, June 7, 1855, 411-417.
(8) Moseley, Henry. “On the Mechanical Possibility of the Descent of Glaciers by their Weight only”. Received, Oct. 1868, read before Royal Society, 7 Jan., 1869, reported Phil. Mag., 37, 229-235. Proc. Roy. Soc., xvii, Jan., 1869, pp202-207.
(9) Moseley, Henry. “On the Mechanical Impossibility of the Descent of Glaciers by their Weight only”, Phil Mag., 37, no 208, May, 1869, 363-370.
(10) Tyndall, “Surface Velocity Measurements of the Mer de Glace”, Phil. Trans. Royal Society, cxlix, part 1, 265-266.
(11) Benn, D. “The Theory of Glacial Motion”, http://www.edu/~smithch/wallace/S184.htm
(12) Wallace, A R. “The Theory of Glacial Motion”, Phil. Mag., February, 1871.
(13) Croll, James. “On the Physical Cause of the Motion of Glaciers”, discussion of Moseley’s papers references (5,8), Phil. Mag., March, 1869, 201-206.
(14) Mathews, W. “Mechanical Properties of Ice and their Relation to Glacier Motion”, Alpine Journal, Feb., 1870 and Nature, March 24, 1870, 534-535.
(15) Ball, J. “On the Cause of the Descent of Glaciers”, Phil. Mag., July, 1870, 1-10.
(16) Croll, James. “On the Cause of the Motion of Glaciers”, Phil. Mag., September, 1870, 153-170.
(17) Croll, James G A. “Comparisons of the Stresses in Glaciers when subject to Gravity and Thermal Loading”, to be submitted to J of Glaciology, 2005.
(18) Moseley, Henry. Reply to discussion of References (5,8). Phil. Mag., 42, no 278, August, 1871, 138-149.
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