1848.]

CURIOUS ANALOGIES IN PLANED IRON.

219

and feasibility of my anticipation. There is no difficulty in determining the exact line of pressure, for it is obviously that in which the tool is made to act, or it is mathematically parallel to the flat side of the shaving itself, if we suppose it straightened. (Fig. 7.)* In one of the specimens now before me, the planes of detrusion, or frontal dip, make an angle, as nearly as can be estimated, of seventy degrees, with the base or line of pressure. From the fibrous appearance of the whole mass, I have little doubt that it is traversed by numberless fissures or flaws parallel to the planes of actual sliding, flaws which might probably be made evident by immersing the whole in dilute acid.

Time does not allow me to add more. Some may consider these approximations and analogies trifling, but I persuade myself that you will not do so, being well aware how much has resulted in the progress of science from the patient study of minute facts not obviously related to one another. It is some pleasure to me to persuade myself that my speculations upon the cause of the motion of glaciers have had some slight influence in drawing attention to the loose manner in which bodies have hitherto been classified as solid and fluid, rigid, flexible, or plastic. On the one hand, attention is directed to the way in which stress or strain is exerted upon masses, and modified by their internal constitution in a way which no theory not embracing an expression of that constitution founded on experience can possibly represent. On the other hand, the imperfect views which practical men have entertained as to the manner in which intense strains affect materials of certain kinds and in certain forms, are apparently about to undergo a considerable revolution. I remain, my dear Sir, yours very truly.

EDINBURGH, 2d December 1848.

* [The analogy of this figure to fig. 7 of Plate I., representing the effect of vertical detrusion on a lava stream of Mount Etna, is remarkable.]

XIX. SIXTEENTH LETTER ON GLACIERS. Addressed to PROFESSOR JAMESON.*

Observations on the Movement of the Mer de Glace down to 1850.

Observations by Balmat, at different seasons, in continuation of those formerly detailed. On the gradual passage of Ice into the Fluid State; observations of M. Person. Notice of an undescribed Pass of the Alps, from Chamouni to Orsières by the Glaciers of Tour and Salena.

My dear Sir-Having had the good fortune once more to spend a few (though very few) days amongst the glaciers of Chamouni last summer, I avail myself of your kind permission to carry forward the account of my observations, which has now, for a period of eight years, been regularly communicated to the readers of your Journal. As my stay was limited by imperative engagements to little more than a week, I was prevented from undertaking a continuous series of observations on the movement of the ice. I was fortunate, however, in obtaining materials for the correction and extension of certain parts of my Map of the Mer de Glace, which were deficient in my former observations, especially as to the exact form of the basin of the great Glacier du Géant, which I had only visited once before, on occasion of the passage of the Col of that name in 1842. This year I traversed again all the difficult part of that glacier, and took angles with the theodolite from the upper part of the basin, immediately under the Aiguille du Géant. But as these observations can have little interest until reduced into the form of a corrected edition of the Map, I shall say nothing of them here.t

It will be recollected by some of your readers that a remarkable stone called "La Pierre platte," was one of the

* Edinburgh New Philosophical Journal for January 1851.

These corrections were introduced into the new edition of my Map of the Mer de Glace (including also the Glacier of Bossons), accompanying the small abridgment of my "Travels," published in 1855 by Messrs. Black under the title of "Tour of Mont Blanc and of Monte Rosa."]

Lying on the surface of the Glacier de Léchaud (in the upper part of the Mer de Glace), and carried along by the motion of the ice. It is marked C in my Map of the Glaciers.

1850.]

STATIONS C AND V ON THE MER DE GLACE.

221

earliest points whose position was ascertained by me in 1842. Its daily motion was watched by me during that summer,* and its annual motion was ascertained by renewed observations in 1843, 1844, 1846, and again this year. I measured the distance along the ice from the original position of the "Pierre platte" on the 27th June 1842 (ascertained by reference to fixed marks on the rocks) to its position on the 12th July 1850. and found it to be 2520 feet. But of this distance, 1212 feet had been travelled at my previous observation on the 21st July 1846, leaving 1308 feet during the last four years against 1212 in the first four. When more accurately stated and compared, the mean annual and daily motions will stand as follows:

We cannot infer, with absolute certainty, that the slight increase of velocity here noticed since 1844 is due to a change in the conditions of the glacier (although I believe that the recurrence of several snowy seasons and the very marked increase of the volume and extent of the glacier during these years would produce such an effect), because it has moved nearly half a mile from its position when first observed, and the part of the glacier on which it now lies may be subject to different accelerating and retarding causes.

'It is mentioned in my Thirteenth Letter, page 4, that I marked a fine solitary block towards the centre of the Mer de Glace, opposite "Les Ponts," with the letter V in 1846, and that I took angles for fixing its place with reference to the adjacent rocks. It was then about 760 feet distant from the west bank. I had little difficulty in recognizing the block in 1850, although it had travelled a great distance, and was considerably lower than the Moutanvert. It had preserved its parallelism to the shore, for I found it at almost the same distance from the west bank as at first; and by measuring carefully along the side of the glacier, I estimated its progress in four years, from

* Travels in the Alps of Savoy, 2d edition, p. 139, 140.

30th July 1846 to 13th July 1850, at 3255 feet. This gives, for the mean motion in 365 days, 822.8 feet, or the mean daily motion 27-05 inches, which is remarkably large. Its position is very near the point of one of the "dirt-bands," but a little nearer the western bank. It lies, however, on the band.

I shall now give the sequel of my guide Auguste Balmat's observations on the motion of the Glacier des Bois (the outlet of the Mer de Glace), and of the Glacier des Bossons, since the period to which the table in my Fourteenth Letter extends,* which will be found to embrace continuous observations, by periods of a few weeks from the 2d October 1844 to the 21st November 1845. They were continued in like manner until the 19th February 1846, when they were interrupted by Balmat's illness, which was accompanied by inflammation of the eyes. But in October of the same year they were resumed, and were continued without intermission until the end of June 1848, embracing altogether a period of nearly four years, with only eight months' intermission. It is necessary to observe that the station on the glacier of Bossons was altogether changed after the above mentioned interruption, being transferred from the west to the east side (in the same region of the glacier), and it was 340 feet from the bank. The station on the Glacier des

Bois was almost unchanged [?], and was about 280 feet from the north bank, between the Côte du Piget and the acclivity of the Chapeau. I have added a column giving the mean of the temperatures of the several periods of observation, carefully calculated from the published observations at Geneva and the great St. Bernard, on the same principle as I have fully explained in my Fourteenth Letter above referred to.† The comparisons of the temperature and the rate of motion lead to conclusions similar to those which I have drawn in that paper from the earlier observations, the general observation always holding that the acceleration in spring is in a greater proportion to the temperature than at any other season of the year, on account of

* [Borrowed from the paper in the Philosophical Transactions, reprinted at page 128 of this volume.] † [See rather p. 131 above.]

1850.]

CONFIDENCE DUE TO BALMAT'S OBSERVATIONS.

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the great influence of the melting snows in imparting fluidity to the glacier masses. I do not mean that the comparison leads always to consistent results. I do not think that the causes of the comparative acceleration of one glacier and retardation of another have yet been clearly brought out, though I conceive that accurate local observations, combined with such measurements, would gradually but surely unveil them. Nor do I mean to affirm that measurements made with so much labour and trouble, and under circumstances even of personal danger at certain seasons of the year, are irreproachable in point of accuracy. I think it even probable that oversights have occurred; but I have very strong reason for confiding in the absolute fidelity with which the observations have been made and transmitted to me. Circumstances have transpired since my last publication which increase this confidence; and I should be ungrateful if I did not once more publicly acknowledge, whilst giving to the world the sequel of observations made under such circumstances that their resumption is scarcely probable, the lasting obligations which I owe to the zeal, fidelity, and disinterestedness of my worthy though humble friend and guide.