The Glaciers of the Alps / Being a narrative of excursions and ascents, an account of the origin and phenomena of glaciers and an exposition of the physical principles to which they are related

by John Tyndall

The formal statement of this theory is given in the following words:—"A glacier is an imperfect fluid, or viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts." The consistency of the glacier is illustrated by reference to treacle, honey, and tar, and the theory thus enunciated and exemplified is called the 'Viscous Theory.'

It has been the subject of much discussion, and great differences of opinion are still entertained regarding it. Able and sincere men take opposite sides; and the extraordinary number of Reviews which have appeared upon the subject during the last two years show the interest which the intellectual public of England take in the question. The chief differences of opinion turn upon the inquiry as to what Professor Forbes really meant when he propounded the viscous theory; some affirm one thing, some another, and, singularly enough, these differences continue, though the author of the theory has at various times published expositions of his views.

"FACTS AND PRINCIPLES."

The differences referred to arise from the circumstances that a sufficient distinction has not been observed between facts and principles, and that the viscous theory has assumed various forms since its first promulgation. It has been stated to me that the theory of Professor Forbes is "the congeries of facts" which he has discovered. But it is quite evident that no recognition, however ample, of these facts would be altogether satisfactory to Professor Forbes himself. He claims recognition of his theory,[A] and no writer with whom I am acquainted makes such frequent use of the term. What then can the viscous theory mean apart from the facts? I interpret it as furnishing the principle from which the facts follow as physical consequences—that the glacier moves as a river because the ice is viscous. In this sense only can Professor Forbes's views be called a theory; in any other, his experiments are mere illustrations of the facts of glacier motion, which do not carry us a hair's breadth towards their physical cause.

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 illustrate 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 illustrative 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 entitle its ice to be regarded as a viscous substance?

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'.