lated weight of snow above and behind. Now, neither of these things happen; the glacier moves on day and night, or from day to day, with a continuous regulated motion, which, I feel certain, could not take place were the sliding theory true.

But if possible, still stronger, as well as more multiplied, objections are to be found to the theory of dilatation, and I trust I shall not be accused of levity in thus, as it were, in a few lines, dismissing a theory which has so much primâ facie plausibility to recommend it, and which has been maintained with so much ingenuity by men such as Scheuchzer, De Charpentier, and Agassiz. It is essential to the aim of this letter, that I state briefly the grounds of the conclusions at which I have arrived, whilst it is equally essential that my observations should be confined within small compass. In another place I shall give them all the development that may be requisite.

Summarily, then (1.) The motion of the glacier, in its several parts, does not appear to follow the law which the dilatation theory would require. It has been shewn (Ed. Rev., April 1842, p. 77) that the motion ought to vanish near the origin of the glacier, and increase continually towards its lower extremity. I have found the motion of the higher part of the Mer de Glace to differ sometimes very little from that several leagues farther down; whilst in the middle, owing to the expansion of the glacier in breadth, its march was slower than in either of the other parts. (2.) Whilst I admit that the glacier is, during summer, infiltrated with water in all or most of its thickness (a point on which I had last year great doubts), I feel quite confident that, during some months of the year during which the glacier is in most rapid motion, no congelation takes place in the mass of the ice beyond a depth of a very few inches, much less during the cold of each night, and least of all, at all times, as appears to be now the opinion held upon the subject. Whilst I say that I am confident of this, I will state one proof. Less than ten days since I traversed the Mer de Glace up to the higher part of the Glacier de Lechaud, whilst it was covered with snow to a depth of six inches at

1842.]

DILATATION THEORY REFUTED.

33

Montanvert, and three times as much in the higher part. It was snowing at the time, and for a week the glacier had been in the same state nearly, the thermometer having fallen in the meanwhile to 20° Fahr. Yet I had abundant evidence that the effect of the frost had not penetrated farther into the ice than it might be expected to have done into the earth under the same circumstances. All the superficial rills were indeed frozen over; there were no cascades in the "moulins;" all was as still as it could be in mid-winter; yet even on the Glacier de Lechaud, my wooden poles, sunk to a depth of less than a foot in the ice, were quite wet, literally standing in water, and consequently unfrozen to the walls; and in the hollows beneath the stones of the moraines, by breaking the crust of ice, pools of unfrozen water might be found almost on the surface. Is it possible, then, that the mere passing chill of a summer night, or the mere cold of the ice itself at all times, can produce the congelation which has been so much insisted on?

But (3.) What was the effect of the congelation, trifling as it was, upon the motion of the glacier? So sharp and sudden a cold succeeding summer weather, must inevitably, it seems to me, were this theory true, have produced an instantaneous acceleration of the mean motion of the glacier. But the contrary was the fact; the diurnal motion fell rather short of its previous value, and [no sooner was] the severe weather past, and the little congelation which had taken place thawed, and the snow reduced to water, than the glacier, saturated in all its pores, resumed its march nearly as in the height of summer.

(4.) It has been inferred from the dilatation theory, that whilst the surface of the glacier continually wastes, it [is] at the same time heaved bodily upwards from beneath, so that its absolute level is unchanged. My experiments, as well as the most ordinary observation (as has been already remarked), disprove this hypothesis. I find that, between the 26th June and the 16th September, the surface of the ice near the side of the Mer de Glace had lowered absolutely TWENTY-FIVE feet 1.5 inches, and the centre had undoubtedly fallen more. The

D

observation of the waste of the surface by the stick sunk to a determinate depth in a hole, is and gives results below the truth.

I am perfectly ready to admit, with M. de C the congelation of the infiltrated water of glacie ant part of their functions; only, I conceive th once a year to any effective extent, inste continually, as he supposes. Every thing whi on the glacier, during cold weather and whe snow, confirms the idea I have always enter progress of congelation in the mass of the glacie to that of a mass of moist earth, and that, the variations of temperature can make no sensible respect to the mass of the infiltrated ice. The of winter must, however, produce a very sensi considering that the temperature of the mass 32°, it may be expected that the congelation capillary fissures in ice will, in the course of quillity, reach a great depth. I apprehend tha an annual congelation, and that its effect is glacier onwards by sliding down its bed-for of so enormous a body seems evidently to rend but (what Mr. Hopkins has very well shewn is native, and which he has used as an argumen pentier's theory) to dilate the ice in the directio ance, that is, vertically, and consequently thickness. The tendency of such a force wou to restore during the winter the thickness of ice summer; and in those winters which are les depth of ice being frozen, a less expansion wo permanent diminution of the glacier would result be more certain than the fact, so well stated by his 10th section, that the glacier does not owe the snow of avalanches, nor indeed to any sno the greater part of its surface.

In conclusion the admission of semifluid n

1844.]

MOTION OF A SEMIFLUID.

35

by the weight of the ice itself, appears to explain the chief facts of glacier-movement, viz. (1.) That it is more rapid at the centre than at the sides. (2.) For the most part, most rapid near the lower extremity of glaciers, but varying rather with the transverse section than the length. (3.) That it is more rapid in summer than in winter, in hot than in cold weather, and especially more rapid after rain, and less rapid in sudden frosts. (4.) It is farther in conformity with what we know of the plasticity of semisolids generally, especially near their point of fusion. Many examples will occur to every one of what they have observed of the plasticity of hard bodiessuch as sealing-wax, for example-exposed for a long time to a temperature far below their melting heat, and which have moulded themselves to the form of the surfaces on which they rest. (5.) When the ice is very highly fissured, it yields sensibly to the pressure of the hand, having a slight determinate play, like some kinds of limestone, well-known for this quality of flexibility. (6.) I have formerly endeavoured to shew how such a condition of semirigidity, combined with the determined movements of the glacier, accounts for the remarkable veined structure which pervades it.*

VI. FIFTH LETTER on GLACIERS, addressed to the Right Honourable EARL CATHCART.†

Motion of the Mer de Glace in the year 1842-3-Winter Movement-The Veined Structure reproduced at the foot of an Ice Cascade-Cuts the Medial Moraines Wrinkles in the Ice of the Glacier du Géant and that of Grindelwald, probably corresponding to the position of the Dirt-Bands-An Ancient Moraine at Chamouni.

ROME, 29th January 1844.

My Lord-In reply to your kind letter of the 14th December last, requesting me to communicate to the Royal Society any observations upon glaciers which I was enabled to make

* [See page 23.]

Edinburgh New Philosophical Journal, July 1844.

Society of Edinburgh.]

[Read to the Royal

during last summer, I may mention, that the state of my health was so indifferent during the finer months of the year, and the caution which it required so great, that I was quite unable to prosecute, as I had hoped, the subject of my previous inquiries in Switzerland. As, however, the journey was not quite unproductive, I will very shortly state the additional facts which I was enabled to observe, claiming from you and from the Society the indulgence which their scantiness requires.

For

At Chamouni, the most obvious consideration was to determine the actual annual motion of the ice, the partial motions of which during the summer months had been carefully ascertained by me, as stated in my former communications. this purpose, I had two marks of a permanently distinguishable kind, namely, blocks of stone lying on the surface of the ice; the one, formerly marked D 7, and referred to in my Travels by that name, situated a little lower than the position of the Montanvert; the other, marked C, or "Pierre platte," on the Glacier de Léchaud, near its junction with the Glacier du Géant. It was the former of these masses which had been approximately observed in position by my guide, Auguste Balmat, during the winter of 1842-3, with great labour and fidelity-observations which first conclusively proved the fact which I had previously suspected, although opposed to the received opinions-that the glacier moves with considerable velocity even in winter. By going to the spot with Balmat, and verifying the marks which he had from time to time made, I ascertained that his measurements, if not absolutely correct, did not admit of being materially improved, owing to the great size and repeated turning over of the block in question. His measurements between October 1842 and June 1843 have been published in the volume already cited. I had the mortification, however, to find, on the 11th September 1843, when I visited the block, that though still upon the ice, it had got shoved so near the moraine of the glacier near an angle of its course, as to be well nigh stranded; and that, in fact, since Balmat's last mark in [Travels in the Alps, 1st edition, p. 151.]

*[See page 25.]