[J] 'Theorie,' p. 96.

[K] 'Occ. Pap.,' p. 74.

[L] In all that has been written upon glaciers in this country the above pa.s.sages from the writings of Rendu are unquoted; and many who mingled very warmly in the discussions of the subject were, until quite recently, ignorant of their existence. I was long in this condition myself, for I never supposed that pa.s.sages which bear so directly upon a point so much discussed, and of such cardinal import, could have been overlooked; or that the task of calling attention to them should devolve upon myself nearly twenty years after their publication. Now that they are discovered, I conceive no difference of opinion can exist as to the propriety of placing them in their true position.

(15.)

The measurements of Aga.s.siz and Forbes completely verify the antic.i.p.ations of Rendu; but no writer with whom I am acquainted has added anything essential to the Bishop's statements as to the ident.i.ty of glacier and liquid motion. He laid down the conditions of the problem with perfect clearness, and, as regards the distribution of merit, the point to be decided is the relative importance of his idea, and of the measurements which were subsequently made.

[Sidenote: OBSERVATIONS OF FORBES.]

The observations on which Professor Forbes based the a.n.a.logy between a glacier and a river are the following:--In 1842 he fixed four marks upon the Mer de Glace a little below the Montanvert, the first of which was 100 yards distant from the side of the glacier, while the last was at the centre "or a little beyond it." The relative velocity of these four points was found to be

1.000 1.332 1.356 1.367.

The first observations were made upon two of these points, two others being subsequently added. Professor Forbes also determined the velocity of two points on the Glacier du Geant, and found the ratio of motion, in the first instance, to be as 14 to 32. Subsequent measurements, however, showed the ratio to be as 14 to 18, the larger motion belonging to the station nearest to the centre of the glacier. These are the only measurements which I can find in his large work that establish the swifter motion of the centre of the glacier; and in these cases the velocity of the centre is compared with that of _one side_ only. In no instance that I am aware of, either in 1842 or subsequent years, did Professor Forbes extend his measurements quite across a glacier; and as regards completeness in this respect, no observations. .h.i.therto made can at all compare with those executed at the instance of Aga.s.siz upon the glacier of the Aar.

In 1844 Professor Forbes made a series of interesting experiments on a portion of the Mer de Glace near l'Angle. He divided a length of 90 feet into 45 equal s.p.a.ces, and fixed pins at the end of each. His theodolite was placed upon the ice, and in seventeen days he found that the ice 90 feet nearer the centre than the theodolite had moved 26 inches past the latter. These measurements were undertaken for a special object, and completely answered the end for which they were intended.

In 1846 Professor Forbes made another important observation. Fixing three stakes at the heights of 8, 54, and 143 feet above the bed of the glacier, he found that in five days they moved respectively 2.87, 4.18, and 4.66 feet. The stake nearest the bed moved most slowly, thus showing that the ice is r.e.t.a.r.ded by friction. This result was subsequently verified by the measurements of M. Martins, and by my own.

If we add to the above an observation made during a short visit to the Aletsch glacier in 1844, which showed its lateral r.e.t.a.r.dation, I believe we have before us the whole of the measurements executed by Professor Forbes, which show the a.n.a.logy between the motion of a glacier and that of a viscous body.

[Sidenote: MEASUREMENTS OF AGa.s.sIZ.]

Ill.u.s.trative of the same point, we have the elaborate and extensive series of measurements executed by M. Wild under the direction of M.

Aga.s.siz upon the glacier of the Aar in 1842, 1843, 1844, and 1845, which exhibit on a grand scale, and in the most conclusive manner, the character of the motion of this glacier; and also show, on close examination, an a.n.a.logy with fluid motion which neither M. Aga.s.siz nor Professor Forbes suspected. The former philosopher publishes a section in his 'Systeme Glaciaire,' ent.i.tled 'Migrations of the Centre;' in which he shows that the middle of the glacier is not always the point of swiftest motion. The detection of this fact demonstrates the attention devoted by M. Aga.s.siz to the discussion of his observations, but he gives no clue to the cause of the variation. On inspecting the shape of the valley through which the Aar glacier moves, I find that these "migrations" follow the law established in 1857 upon the Mer de Glace, and enunciated at page 286.

To sum up this part of the question:--The _idea_ of semi-fluid motion belongs entirely to Rendu; the _proof_ of the quicker central flow belongs in part to Rendu, but almost wholly to Aga.s.siz and Forbes; the proof of the r.e.t.a.r.dation of the bed belongs to Forbes alone; while the discovery of the locus of the point of maximum motion belongs, I suppose, to me.

FORBES'S THEORY.

(16.)

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 ill.u.s.trated 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.

[Sidenote: "FACTS AND PRINCIPLES."]

The differences referred to arise from the circ.u.mstances that a sufficient distinction has not been observed between _facts_ and _principles_, and that the viscous theory has a.s.sumed 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 ill.u.s.trations of the facts of glacier motion, which do not carry us a hair's breadth towards their physical cause.

[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.]