expansion of the interior Yet when the protrusion surface of it, even at the

itself, and even the closely adjoining surface, carefully and perfectly clean. of the ice cylinder took place, the height of an inch or two above the neck of the bottle, was smeared with red colour. In one of the specimens the definite ring of red was, as it were, transferred from its position on the internal expansion of the bottle to the surface of the comparatively narrow cylinder of ice which had been discharged through the aperture. Now this could not possibly have occurred unless the plastic substance of the ice had been forced laterally and by a converging pressure from all sides (up even to the particles in contact with the interior of the glass), so as to be forced through the contracted outlet as a tenacious fluid under its own pressure, or a plastic solid subjected to a considerable force would do under like circumstances. The Frontispiece, Plate VI fig. 1, taken from a coloured chalk sketch made at the time* from one of these experiments, will explain more distinctly my meaning. It shows the impress of the coloured ring transferred from the comparatively large surface whence it was derived to the cylinder of small diameter into which it has been compressed. Figure 2 shows the appearance of the protruded ice when partly thawed, the curved surfaces of air-bubbles indicating the graduated effect of friction as the distance varied from the glass, which appears to be consistent only with a molecular plasticity of the ice. In these experiments the slow progress of congelation of the interior water, which is the source of the intense pressure, is eminently favourable to their development, while it also bears some analogy to the extremely gradual internal movements of a glacier. Were it attempted to produce by intense pressure acting for a few minutes what we here produce in many hours, or even some days,† the effect, though perhaps externally analogous, would be deficient in the evidence of plasticity. Nov. 1858.]

* [In the winter 1846-7, if I recollect rightly.]

[Mr. Christie's experiment lasted several days. His letter, which is now before me, is dated 4th April 1846.]

1846.] ON THE DEPRESSION OF THE GLACIER SURFACE.

169

XIV. ELEVENTH LETTER ON GLACIERS.* Addressed to PROFESSOR JAMESON.

Observations on the Depression of the Glacier Surface-Ablation and Subsidence distinguished and ascertained-On the Relative Velocity of the Surface and Bottom of a Glacier.

The

MY DEAR SIR-In my Tenth Letter on Glaciers, which you did me the favour to publish lately, a question was discussed respecting the apparent depression of the surface of a glacier. I had already pointed out in the first edition of my Travels, that several causes combine to produce this depression, but that observations were wanting to distinguish them. causes then enumerated were (if I mistake not) these:-1. The actual waste or melting of the ice at its surface. 2. The subsidence of the glacier in its bed, owing to the melting of its inferior surface, whether by the heat of the earth, or that due to currents of water. 3. The effect of the drawing out of the glacier where it is in a state of distension, which tends to reduce the thickness of the mass of ice; (when a glacier is violently compressed the effect will be contrary, or an elevation will result); to which may be added the influence of the slope of the bed of the glacier, by which, as it moves forward, its absolute elevation is diminished, or the contrary if it ascends. I had also pointed out a method by which the first of these effects, or the absolute ablation of the ice (as it has been termed by M. Agassiz), might be distinguished from the other two, namely, by driving a horizontal hole into the wall of a crevasse, and observing the diminution of the thickness of the stratum of ice above it. The partial and total effects I have observed in the following manner, during the present summer, on the Mer

* Edinburgh New Philosophical Journal, October 1846. [The Tenth Letter of the Series is not reprinted, being mainly controversial, and not containing new original observations. It will be found in the same Journal for January 1846.] † Travels, 1st Edition (1843), p. 154.

de Glace of Chamouni. great depth, was selected, running in a direction transverse to the glacier. The most vertical wall (nearest A, Plate VI., fig. 1) is always the [one] least exposed to the sun, and the waste of its surface is very small, unless in the case of rain. In this wall a horizontal hole, C, was bored, to the depth of at least a foot, and was renewed from time to time. The depth at which this hole existed below the surface of the glacier was determined by stretching a string, AB, across the crevasse, and measuring by a line the vertical height from C to AB. The variation of this quantity gives the actual fusion of the surface, free from the errors mentioned in my former letter. variable, depending on the weather as well as on the place of experiment. Opposite the Montanvert, about 200 feet from the side of the glacier, during the hot weather of July and August 1846, the ablation amounted on an average to 3.62 inches per day; at a higher station between the Angle and Trelaporte (opposite station Q of the year 1844, see Eighth Letter), it was only 2.73 inches, the ice being also remarkably clean and white, and the distance from the western bank of the glacier 553 feet.

A crevasse, nearly vertical, and of no

It is, of course, very

The subsidence of the glacier in its bed, or the difference between the geometrical depression of the surface and the ablation, was very easily and most accurately obtained in the following manner:-The theodolite being placed and levelled on the ice in the neighbourhood of the place of observation (not necessarily always on the same spot), the height of the horizontal wire of the telescope above the horizontal hole pierced in the side of the crevasse was noted by directing the level upon a measuring tape divided into feet and inches [held vertically by an assistant], the ring at the extremity of which was passed over the boring instrument, which was then firmly adjusted in the horizontal hole. The reading at the telescope gave the height of the eye at the moment above the hole in question. The level was then directed against a fixed object on the moraine, where a cross had been cut in a stone as a point of departure for the