1841.]

ANALOGOUS STRUCTURE IN SLATY ROCKS.

9

through strata whose planes of stratification, horizontal, elevated, undulating, or contorted, offer no obstacle or modification to the omnipotent energy which has rearranged every particle in the mass subsequent to deposition. The supposition of Professor Sedgwick, who has minutely described and considered this geological puzzle, that "crystalline or polar forces acted on the whole mass simultaneously in given directions, and with adequate power," can hardly be considered as a solution of the difficulty, until it is shown that the forces in question have so acted, and can so act. The experiment is one which the boldest philosopher would be puzzled to repeat in his laboratory; it probably requires acres for its scope, and years for its accomplishment. May it not be that Nature is performing in her icy domain a repetition of the same mysterious process, and that in another view from the one which has recently been taken, the Theory of Glaciers may lead to the true solution of geological problems?

II. FIRST LETTER on GLACIERS, addressed to
PROFESSOR JAMESON.†

Account of the First Experiments, undertaken in June 1842, to determine the Laws of Motion of the Mer de Glace of Chamouni.

COURMAYEUR, PIEDMONT, 4th July 1842.

My Dear Sir-Knowing that you will be glad to hear of my safe arrival amongst the Alps, and of my farther proceedings, I hasten to give you an account, in a few words, of what I have as yet done. Finding the season more than usually advanced, I hastened to reach Chamouni, in order to ascertain whether the Mer de Glace was as yet accessible in all its extent; and I arrived at the Montanvert on the 24th June, and

* Geological Transactions, Second Series, iii. 477.
+ Edinburgh New Philosophical Journal, October 1842.

remained there for a week. I was fortunate enough to convey all my instruments to their destination, without, I believe, injury to any one of them. The Mer de Glace, so continually visited by the curious, but so little studied, seemed to me to offer great advantages for the prosecution of the objects which I proposed to myself. At first sight it appeared to me steeper and more crevassed than I recollected it to be, and I doubted for a moment whether it was adapted for my experiments; but that doubt vanished upon closer examination; and in the course of the single week which I have been able to spend there, being favoured by most excellent weather, I have obtained results so far definite and satisfactory, that, imperfect as they necessarily are, and only the commencement of what I expect to accomplish during the remainder of the season, I will state them shortly.

You will recollect that, in my lectures on glaciers delivered last December and January, and afterwards in an article written by me in the Edinburgh Review,* I insisted on the importance of considering the mechanism of glaciers as a question of pure physics, and of obtaining precise and quantitative measures as the only basis of accurate induction. I pointed out, also, the several experiments of a critical kind which might be made; such, for instance, as the determination of the motion of the ice at different points of its length, in order to distinguish between the theories of De Saussure and De Charpentier; for, if the glacier merely slides, the velocity of all its points ought (in the simplest case) to be the same; if the glacier swells in

[For April 1842, as, for example, the following passages:—

"The mechanism of a glacier is a problem of natural philosophy, and one much more difficult and embarrassing than it has commonly been supposed" [to be].— Ed. Rev., p. 52.

"The solution of this important problem [the theory of glacier motion] would be obtained by the correct measurement, at successive periods, of the spaces between points marked on insulated boulders on the glacier; or between the heads of pegs of considerable length, stuck into the matter of the ice, and by the determination of their annual progress."-Ed. Rev., p. 77.

The lectures referred to in the text were delivered by the author in the University of Edinburgh to a numerous audience, including many men of science. They traced out mainly the same views as are embraced in the article in the Edinburgh Review, but with larger details of the author's experience of 1841.]

1842.]

PRECISION ATTAINABLE BY OBSERVATION.

11

all its mass, the velocity of the inferior part ought to be greatest. Of course, I do not now advert to the many causes which might accidentally invert this law, and which would require to be fully taken into account; still less do I mean to say that anything I have now to state can be considered as critically decisive between rival theories; but my experiments certainly do show that the kind of precision which I desired to see introduced into reasonings about this subject, is practically attainable, even in a far higher degree than I expected.

For example:-The motion of glaciers by the measurement of the distance of blocks upon its surface from a fixed point, from one year to another, has marked indubitably the annual progress of the ice. I do not know that any one has attempted to perform the measurement in a manner which could lead to any certain conclusion respecting the motion of the ice at one season compared with another, or from month to month; still less has any one been able to state, with precision, whether the glacier moves by starts and irregularly (as we should certainly expect on the sliding theory), or uniformly and evenly; and if so, whether it moves only at one part of the twenty-four hours, and stands still during the remainder (as we should expect on the dilatation theory, as commonly expounded). Now, I have already been able—

1st. To show and measure the glacier motion, not only from day to day, but from hour to hour; so that I can tell nearly what o'clock it is by the glacier index. That you may have an idea of the coincidence which these experiments present, I give you the longitudinal motion of a point on the Mer de Glace during four consecutive days.

15.2 inches.

16.3 inches. 17.5 inches. 17.4 inches.

2d. This motion, evidently incompatible with sudden starts, takes place in the glacier as a whole, undisturbed by the most enormous dislocations of its surface, for these measures were taken where the glacier was excessively crevassed.

3d. This motion goes on day and night, and if not with

absolute uniformity, at least without any considerable anomaly. On the 28-29th June the motion

from 6 P.M. to 6 A.M. was 8.0 inches,

seeming to show a greater motion during the day.

4th. In the particular case of the Mer de Glace, the higher part (the Glacier de Lechaud) moves slower than the lower part near the Montanvert in the proportion of 3 to 5.

5th. The central part of the glacier moves faster than the edges in a very considerable proportion; quite contrary to the opinion generally entertained.

There cannot be a doubt of the accuracy of these results within the limits in which the experiment has been made. They prove how completely problems of a purely physical character admit of accurate investigation; and when a larger induction shall have freed the results from local errors, it is evident that we shall have the solid foundations of a theory. My wish to see the total eclipse of the sun on the 8th, has brought me to the south side of the Alps sooner than I could have wished; but I have now fixed so many points on the Mer de Glace, that, on my return thither, I shall be able to obtain more comprehensive results. But what is most important in

the whole matter is this-that an observer furnished with the proper instruments and methods may, by spending a few days on a glacier, determine at any particular season the amount of its motion at all the essential points, within the limits which any glacier theory can require.

1842.]

LAWS OF MOTION OF THE MER DE GLACE.

13

III. SECOND LETTER on GLACIERS, addressed to
PROFESSOR JAMESON.*

The Laws of Motion of the Mer de Glace farther stated.

CHAMOUNI, 10th August 1842.

My Dear Sir-Since I last wrote to you on the 4th July from Courmayeur, I have examined, in detail, the two principal glaciers of the Allée Blanche; and having re-crossed the Alps from Courmayeur by the Col du Geant, where I had the satisfaction of still finding the remains of Saussure's Cabane of 1788, I have pursued for a fortnight my experiments on the motion. of the Mer de Glace. Being composed, as you know, of several tributaries which are in some degree independent, and presenting also a considerable variety of surface, this glacier seems as proper as any for detailed experiments, such as those which I am attempting. Being about to quit this place on a tour to Monte-Rosa and the glaciers east of the Great St. Bernard, I wish to explain to you now in what respect my observations differ from those formerly undertaken on the glaciers, and to mention a few results, which, of course, being as yet only partial, ought not to be considered as altogether decisive of the truth or falsehood of any theory; still I believe it will be admitted that the facts established in my last (and which farther experience has confirmed), militate strongly against some of the received opinions as to the cause of glacier motion.

You are aware that, in my lectures on glaciers in December and January last, and in an article in the Edinburgh Review for April, I insisted, and so far as I know it was for the first time, on the importance of considering the glacier theory as a branch of mechanical physics, by which I mean that the cause

* Edinburgh New Philosophical Journal, October 1852. [The coincidence of a few expressions with those in the First Letter will be pardoned when it is recollected that these letters were printed precisely as written from the scene of the observations described, and that any comparison of one letter with another was impossible.]