Russel Wallace : Alfred Russell Wallace (sic)
The other paper is a criticism of the Rev. Canon Moseley's supposed proof that glaciers do not descend by the force of gravity, and of the arguments of Messrs. Ball and Matthews on the other side. It will be remembered, that Canon Moseley determined by experiment the "shearing" force of ice, that is, the force required to fracture it by parallel pressure. A plug of ice of known cross-section is fitted into a hole through two smooth boards, and the force required to break the ice by sliding the boards over each other is the "shearing" force. Increasing this in proportion to the dimensions of a glacier, or of any large portion of one, it was calculated that the force required to cause the different parts of a glacier to slide over each other (as they must do in descending a valley of constantly varying form and size) was at least thirty times greater than the force of gravity on a slope such as glaciers easily descend. Canon Moseley came to the conclusion that expansion and contraction of the ice by heat and cold was the moving power; and the fact that the glaciers move slower by night than by day, and in winter than in summer, was supposed to prove conclusively that heat is the cause of motion.
Mr. Croll believes that Canon Moseley has demonstrated that gravity alone does not cause glaciers to descend, but he completely demolishes the theory of contraction and expansion. He admits that heat aids the motion, but maintains that it does so by acting on the molecules of [[p. 310]] the ice, which it loosens momentarily from their mutual cohesion, and allows to be re-arranged under the influence of gravity. Heat, he says, is the condition, gravity the cause of the motion which takes place, molecule by molecule rather than in masses. It seems very doubtful, however, if this theory is more tenable than the one it is intended to supersede. If heat entering the glacier loosens the molecules in its passage and enables them to move insensibly into new positions, it is difficult to understand what causes the numerous longitudinal and transverse fissures of a glacier, the production of which is often attended by loud reports, and which indicate movements of masses, not of molecules. And how could molecular motion lead to that heavy grinding of the ice over its bed, which scores and wears down the hardest rocks, and whitens great rivers with the finely triturated mud?
None of the opponents of Canon Moseley have noticed what seems to the present writer to be a radical fallacy in his argument about "shearing force." He assumes that, whatever the bulk or weight of the glacier, or of any portion of it to which the formula of the shearing force may be applied, the whole mass shears at once by the action of gravity on the same mass, and does not recognise the possibility of one portion of a glacier acting by its weight to shear another and much smaller portion. But this must inevitably occur; for, owing to the excessive irregularity of the bed in which every glacier moves, the mass must be every where in varying states of tension and compression, and must contain at each instant certain lines and planes of least resistance, the extent of which lines and surfaces may be very small compared with the dimensions of the glacier itself. At any moment, therefore, the whole descending weight of a portion of the glacier containing perhaps thousands of cubic yards of ice, may act so as to cause the shearing of a few superficial feet where the tension is greatest. This being effected, a partial equilibrium is produced there; but the points or surfaces of greatest tension are shifted, and another small shear or fracture occurs; and by this process and the continued regelation of fractured surfaces brought into contact, it may easily be seen that the glacier as a whole would be gradually moulded to its bed, which it would descend as surely as if it were a viscous mass. Another source of motion not taken into account either by Canon Moseley or Mr. Croll is the irregular melting away of the under surface of the glacier by terrestrial heat, which would often form unsupported hollows till a fracture occurred, and every such fracture must result in a downward motion of a portion of the glacier. The observed difference of the rate of motion between winter and summer, day and night, is more probably due to the different quantities of water which descend the crevasses into the bed of the glacier at those periods, than to any direct action of the heat. It is well known that in the higher portions of a glacier the supply of water from melting snow diminishes during the night, as it does in a still greater degree during the winter; and the large quantity of water that flows beneath every glacier in the summer must greatly assist its motion, both by melting away its lower surface, and by, to some extent, buoying it up.
Mr. Matthews's important experiment of the bar of ice which gradually curved by its own weight, should be tried again in an atmosphere kept at the freezing point. This would settle the question whether heat is an essential condition for the curvature or motion of ice by gravitation; but so far as the facts lead us at present, the arguments of Canon Moseley and Mr. Croll by no means prove that glaciers do not descend by the force of gravity alone.
[The publication of this article has been delayed. It was in our hands before the appearance of Mr. Ball's paper in the Philosophical Magazine for February, where a view almost identical with Mr. Wallace's is ably advocated.--Ed.]
1"On Ocean Currents." By James Croll, of the Geological Survey of Scotland (3 parts). "On the Cause of the Motion of Glaciers." By the same author. (Extracted from the Philosophical Magazine of 1870.)
The debate on the causes of glacier motion lasted for many years during the 19th Century. Major sticking points were (1) the fact that ice appears to be a rigid, brittle material, yet the large-scale motion of glaciers requires that the ice flows as a continuous mass; and (2) a mass under a constant force (gravity) should be expected to accelerate downhill (as all falling objects do) whereas glaciers exhibit periodic, often annual, fluctuations in speed. Wallace's note covers a range of issues connected with these problems. He correctly dismisses some of the fallacies current at the time, giving reasons why ice motion cannot be due to freeze-thaw action or "molecular adjustment." His own preferred mechanism, of localised fracture and regelation (healing of fractures by refreezing) is closer to the mark, but still not complete. In fact, the correct mechanisms of ice flow--involving visco-plastic deformation in response to gravity--had been recognised by James David Forbes some years previously, although serious problems remained with ice flow theory that were not really addressed until the 1950s.
Where Wallace does hit true, however, is his recognition of basal sliding as the main mechanism by which Alpine glaciers move. His comments that meltwater reaching the bed will encourage motion by "melting away its lower surface, and . . . to some extent, buoying it up" are exactly correct. This is indeed the reason why Alpine glaciers flow more rapidly during the day and in summer, when meltwater fluxes are highest. Confirmation of this fact had to await the development of drilling equipment, allowing the real-time measurement of subglacial water pressures simultaneously with accurate velocity measurement.
Wallace was not the first scientist to emphasise basal sliding (de Saussure has this honour), but he certainly had a more lucid view of the problem than many of his contemporaries. Forbes, an otherwise perceptive field worker and theorist, had dismissed sliding as unimportant.