Part 25 (1/2)
[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 furnis.h.i.+ng 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.