Part 25 (2/2)
[Sidenote: VISCOUS THEORY;--WHAT IS IT?]
What then is the meaning of viscosity or viscidity? I have heard it defined by men of high culture as ”gluey tenacity;” and such tenacity they once supposed a glacier to possess. If we dip a spoon into treacle, honey, or tar, we can draw the substance out into filaments, and the same may be done with melted caoutchouc or lava. All these substances are viscous, and all of them have been chosen to ill.u.s.trate the physical property in virtue of which a glacier moves. Viscosity then consists in the power of being drawn out when subjected to a force of tension, the substance, after stretching, being in a state of molecular equilibrium, or, in other words, devoid of that elasticity which would restore it to its original form. This certainly was the idea attached to Professor Forbes's words by some of his most strenuous supporters, and also by eminent men who have never taken part in any controversy on the subject.
Mr. Darwin, for example, speaks of felspathic rocks being ”stretched”
while flowing slowly onwards in a pasty condition, in precisely the same manner as Professor Forbes believes that the ice of moving glaciers is stretched and fissured; and Professor Forbes himself quotes these words of Mr. Darwin as ill.u.s.trative of his theory.[B]
The question now before us is,--Does a glacier exhibit that power of yielding to a force of tension which would ent.i.tle its ice to be regarded as a viscous substance?
[Sidenote: THEORY TESTED.]
With a view to the solution of this question Mr. Hirst took for me the inclinations of the Mer de Glace and all its tributaries in 1857; the effect of a change of inclination being always noted. I will select from those measurements a few which bear more specially upon the subject now under consideration, commencing with the Glacier des Bois, down which the ice moves in that state of wild dislocation already described. The inclination of the glacier above this cascade is 5 10', and that of the cascade itself is 22 20', the change of inclination being therefore 17 10'.
[Ill.u.s.tration: Fig. 22. Inclinations of ice cascasde of the Glacier des Bois.]
In Fig. 22 I have protracted the inclination of the cascade and of the glacier above it; the line A B representing the former and B C the latter. Now a stream of molten lava, of treacle, or tar, would, in virtue of its viscosity, be able to flow over the brow at B without breaking across; but this is not the case with the glacier; it is so smashed and riven in crossing this brow, that, to use the words of Professor Forbes himself, ”it pours into the valley beneath in a cascade of icy fragments.”
[Sidenote: INCLINATIONS OF THE MER DE GLACE.]
But this reasoning will appear much stronger when we revert to other slopes upon the Mer de Glace. For example, its inclination above l'Angle is 4, and it afterwards descends a slope of 9 25', the change of inclination being 5 25'. If we protract these inclinations to scale, we have the line A B, Fig. 23, representing the steeper slope, and B C that of the glacier above it. One would surely think that a viscous body could cross the brow B without transverse fracture, but this the glacier cannot do, and Professor Forbes himself p.r.o.nounces this portion of the Mer de Glace impa.s.sable. Indeed it was the profound creva.s.ses here formed which placed me in a difficulty already referred to. Higher up again, the glacier is broken on pa.s.sing from a slope of 3 10' to one of 5. Such observations show how differently const.i.tuted a glacier is from a stream of lava in a ”pasty condition,” or of treacle, honey, tar, or melted caoutchouc, to all which it has been compared. In the next section I shall endeavour to explain the origin of the creva.s.ses, and shall afterwards make a few additional remarks on the alleged viscosity of ice.
[Ill.u.s.tration: Fig. 23. Inclinations of Mer de Glace above l'Angle.]
FOOTNOTES:
[A] ”Mr. Hopkins,” writes Professor Forbes, ”has done me the honour, in the memoirs before alluded to, to mention with approbation my observations and experiments on the subject of glaciers. He has been more sparing either in praise or criticism of the theory which I have founded upon them. Had Mr. Hopkins,” &c.--_Eighth Letter_; 'Occ.
Papers,' p. 66.
[B] 'Occ. Papers,' p. 92.
THE CREVa.s.sES.
(17.)
[Sidenote: CREVa.s.sES CAUSED BY THE MOTION.]
Having made ourselves acquainted with the motion of the glacier, we are prepared to examine those rents, fissures, chasms, or, as they are most usually called, _Creva.s.ses_, by which all glaciers are more or less intersected. They result from the motion of the glacier, and the laws of their formation are deduced immediately from those of the motion. The creva.s.ses are sometimes very deep and numerous, and apparently without law or order in their distribution. They cut the ice into long ridges, and break these ridges transversely into prisms; these prisms gradually waste away, a.s.suming, according to the accidents of their melting, the most fantastic forms. I have seen them like the mutilated statuary of an ancient temple, like the crescent moon, like huge birds with outstretched wings, like the claws of lobsters, and like antlered deer.
Such fantastic sculpture is often to be found on the ice cascades, where the riven glacier has piled vast blocks on vaster pedestals, and presented them to the wasting action of sun and air. In Fig. 24 I have given a sketch of a ma.s.s of ice of this character, which stood in 1859 on the dislocated slope of the Glacier des Bois.
[Sidenote: FANTASTIC ICE-Ma.s.sES.]
[Ill.u.s.tration: Fig. 24. Fantastic Ma.s.s of ice.]
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