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of the intense pressure with which the whole mass of the glacier is urged against the side of the valley; but as the glacier is finally compelled to move in the direction of the declivity from E towards D, a longitudinal tearing force arises parallel to E D, and the motion is facilitated by the formation of the veined structure already mentioned, for of longitudinal crevasses there are absolutely none. Such crevasses as there are, are transverse to the blue bands, shewing the usual direction of the tearing force due to the lateral friction. But they are small and isolated.

Now, in this case, the development of the structure is evidently due to the projection at D, in contact with which it terminates; and though the whole glacier is more or less structured in the manner described and figured in my former work, this remarkable development gradually ceases [in an upward direction] about E, where the rush of the ice-current past the promontory B has ceased to exert so palpable an effect. I can positively state that no such unusual development of the structure occurred opposite to B in 1842, and for this reason, that the glacier lying then more in the trough of the valley, not being violently pressed against the promontory B, and embayed in the height * C, the friction and longitudinal tearing force was incomparably less. I wish I could convey any adequate idea of the beauty of the ice for an extent of some hundred feet in length, and for a comparatively trifling breadth between D and E. It resembled the greenish-veined marble called by the Italians Cipollino, when of the highest perfection, and polished or wetted; and it was impossible to resist the wish to carry off slabs, and to perpetuate it by hand specimens in cabinets.

I did not, however, content myself with arguing the relative motions of the parts of the ice and embaying of the rest, from the mere configuration of the ground. But I made the following experiments, which proved that my first belief was

correct.

The instrument [theodolite] being stationed at B [Plate VIII. fig. 4], and a transverse visual line established with reference

* [Evidently a misprint for bight, which is the word used in my original notes.]

to an object on the farther side of the glacier, two vertical holes were made at (1) and (2), the first on the nearer, the second on the farther side of the remarkable veined structure already described. If that structure was occasioned, as I suspected, by the rapidity with which the farther portion of the ice was moved past the nearer portion, such difference of velocity might be expected to be observed in a marked manner. The mark (1) was about 50 feet from the nearest edge of the glacier, and 103 feet below the station B. The mark (2) was about 170 feet farther and 60 feet higher. An approximate section of this part of the glacier, together with the measurements on which it is founded, is given in Plate VIII. fig. 4, and will serve to compare the state of matters at any future time. The following table gives the result of the motions for two days:

consequently the velocity of motion had increased in the space of 170 feet traversed by the veined structure, in the ratio of no less than 29 to 10.

I also examined the condition of motion of the embayed ice in the position C on the ground-plan, of which a section (on a scale much larger than the last) is given in fig. 3. It is made through the visual line, or in the direction C, C 2 of the plan [p. 182]. Two pins were fixed at C1 by excavating a niche in the nearly vertical face of the ice; one was placed vertically, the other horizontally. At C 2, 40 feet higher and 69 feet more distant, a vertical pin was placed. The diagonal from C to C 1 was also accurately measured with a line. After more than 24 hours' interval, it was found that the two marks at C1 had not moved towards the right hand, or in the direction of motion of the glacier by the smallest perceptible quantity; and that [the]

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ANALOGY FROM THE RIVER RHONE.

185

mark C2 had advanced by only a small fraction of an inch (0-2); but on the other hand the vertical pin at C 1 had approached the station C by two inches; shewing that the motion is here entirely directed outwards and upwards against the bank at C, exactly as an hydrostatic pressure acting on a plastic mass would occasion. The result was the more interesting because it was altogether unexpected. It had not occurred to me that the embaying of the ice could be so complete as to leave no appreciable effect due to the drag of the central ice in the direction of the declivity.

The precise analogy of the phenomena now described to what obtain when the motion of a stream of water is interrupted by lateral obstacles, will suggest itself to almost every one; but to take an illustration which is not imaginary, I have given, in the annexed figure, a sketch of what occurs in the course of

the river Rhone at the bridge near Sierre. The course of the current is indicated by the arrows, the pier of the bridge b embays the water at c, where it is whirled about by the tangential action of the current, but does not escape; the wavy lines, e d, indicate the ripple produced by the friction of the central past the lateral portions of the stream, the disturbance from which is propagated towards the shore in the waves f. All that is here described may be still better seen in a sluggish stream, and if it be covered with any kind of scum, it will be torn up into longitudinal shreds between e and d, exactly corresponding to the position of the icy bands. The nature of the force producing these lines of scum is also well shewn by

the rotatory motion acquired by any floating patches of foam, which is always in a direction forwards and towards the side of the stream to which it is nearest; thus, in the preceding figure, it would be in the direction of the hands of a watch. I remain, etc.

EDINBURGH, 24th October 1846.

XVI. THIRTEENTH LETTER ON GLACIERS.
Addressed to Professor JAMESON.*

Acceleration of the Surface Motion of Glaciers confirmed-Velocity of the Mer de Glace in the Summer of 1846 at Stations C, D, P, and R-Velocity of the Glacier of Talefre, near its point of discharge-Discovery of a Knapsack ten years buried in the Ice, and deduction of the motion-The Glacier of Nant Blanc visited, and its Motion determined-The Glacier of Miage revisited, and motion measured-On the Conversion of the Névé into Ice-Congelation of Infiltrated Water not necessary to produce the Veined Structure-An Attempt to explain the apparent rejection of Stones from the Glacier-Ridges of Ice in certain parts of Glaciers due to the bruising effect of intense Pressure-Three Orders of discontinuity, Ridges, Crevasses, Veined Structure.

My dear Sir-Since the completion of my Twelfth Letter, I have observed, in the Comptes Rendus for 26th October, an account of a communication made to the Academy of Sciences by M. Martins, of an experiment on the relative motion of the surface and inferior part of a glacier, on which I have also made the experiments detailed in my Eleventh Letter (dated 16th September), and published on 1st October in The New Edinburgh Philosophical Journal. My experiments, it will be recollected, were made at three points of the terminal face of the Glacier des Bois, and proved, as I had long ago anticipated, that the superficial ice has by much the most rapid motion. MM. Dollfuss and Martins arrive at the same result, establishing the desired identity with the motion of rivers; but their experiment being made, not on the terminal face, but only on the steep

*Read before the Royal Society of Edinburgh, 21st December 1846. Published in Edinburgh Philosophical Journal, January 1847.

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RELATIVE MOTIONS OF SURFACE AND BOTTOM.

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lateral face of a glacier, is open to the possible objections which I anticipated in my former letter, and which I carefully endeavoured to avoid (see p. 173 of the Eleventh Letter, and fig. 2 of Plate VI.), so that even the most scrupulous might be, if possible, satisfied. It is pleasing to me to find that the French observers corroborate my result; but I must remark that they obtained it a fortnight later, and my publication preceded theirs nearly a month.

I shall not seem to insist too much upon the conclusiveness of this observation, when it is recollected that an opponent of the Viscous Theory has virtually staked the question of the cause of glacier motion upon such an experiment (notwithstanding that I think he has attached to it an undue importance), in the following passage :—

*The claims of the two theories (the sliding theory and that of plastic motion) will undoubtedly be determined by other means. The observations required are such as shall determine, as far as possible, the motions of the upper and lower surfaces of a glacier. We may never hope to have access to the bottom of a glacier in its deeper portions, but at the extremities of glaciers the amount of sliding may easily be ascertained, as well as at many other points, probably, if sought for, along their flanks; fissures also, of considerable depth, are not unfrequently met with, in which the deviation from verticality, if it exist, might be easily determined; and though the evidence thus obtained might not afford positive demonstration with respect to the deepest portions of a glacier, still, should it all concur in showing an approximate equality in the motions of the upper and lower surfaces, every candid and impartial mind must admit, I conceive, the sliding in preference to the viscous theory; but if, on the contrary, it should be proved that the velocity of the upper bears a large ratio to that of the lower surface, the claims of the latter theory must be at once admitted."* It is very fortunate that independent observers on different glaciers should have arrived, in ignorance of each other's results, at conclusions which permit only the alternative favourable to the viscous * Mr. Hopkins, in London Philosophical Magazine, March 1845, p. 250.