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.

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. The 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. A crevasse, nearly vertical, and of no 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.

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

1846.]

ABLATION AND SUBSIDENCE MEASUred.

171

vertical height. The height of the eye above or below the fixed point was measured, and the sum or the difference (as the case might be) of this measure and the last gives the difference of the level of the horizontal hole in the ice, and the mark on the moraine. The following may serve as an example :—

Of course the horizontal hole may be renewed as often as convenient, all that is necessary being to ascertain the difference of level of the old and new hole.

In the preceding example (which has been selected by chance), the subsidence bears an unusually small proportion to the ablation. At the station in question, the average daily ablation in July and August was 3.62 inches, the average daily subsidence 1.63 inches. The sum of the two, or the geometrical depression of the surface 5.25 inches, whereof seven-tenths were produced by ablation, three-tenths by subsidence. These relations, together with those opposite the old station Q, are shown in one view in the following table of

The last two columns show the effects of the ablation and

subsidence in hundredth parts of the whole depression.

See Plate VI. fig. 1.

Taken from the observation of the neighbouring mark D·2.