CHAPTER VII.MEASUREMENT OF GLACIERS.
Aswe have seen, the glacier does not accommodate itself to the bed in which it rests very readily. The substance, though possessing a sort of ductility, is not sufficiently plastic to mould itself with much rapidity. A sudden squall will snap a twig, or uproot a tree which a gale would only bend. In like manner, the pressure upon the ice becomes in places too great, and, unable to bear it, the ice snaps. Professor Tyndall has said—and this is borne out by my own experience—“By pressure ice can be moulded to any shape, while the same ice snaps sharply asunder if subjected to tension.” Hence arise the great crevasses which so much embarrassed the captain and myself while crossing over the glacier of Sermitsialik. These cracks are therefore continually occurring. Beginning as a loud and alarming peal, they become in the end a crash. This particularly happens when the glacier, flowing over a quick descent, moves rapidly.
In this manner are opened those crevasses already mentioned as running inward and upward from the shore, and pursuing a general parallelism. The force is not that of pressure, but of tension. The sides hang more or less upon the shore, while the centre moves on, and thus the ice is drawn apart. The shore-ice thus lags behind, and the total amount of that lagging is told in the aggregate width of the crevasses.
I have mentioned that the rate of progress of the centre of the glacier of Sermitsialik must have been about seveninches daily. Others move at the rate of five. The process for this determination is simple enough. I have performed it many times, and may thus briefly describe it.
After climbing to the surface of the glacier, I staked off a base-line near the centre, and parallel with its sides. I then set up a theodolite, first at one, then at the other end of this base-line, and, having connected it by careful angular measurement with some well-defined object (readily again recognized) on the land, at the side of the glacier, the work was done. After an interval of weeks or months, the process was repeated, and then a very simple trigonometrical computation revealed the fact that the base-line was moving down the valley, and the rate was determined to a small fraction.
The unequal movement between the centre and the sides was shown by quite another method. A line of stakes was set up across the glacier from shore to shore. After a few weeks this line became a slight curve; after a few months the curve was very perceptible. Could the observation have been continued through years, it would have been like a well-bent bow, or the letter U.
Such measurements as these I once made upon a glacier at Port Foulke, North Greenland. The top of the glacier was reached, after much difficulty, by cutting steps with an axe. The cracks were not, however, found to be so wide or so numerous as those of the glacier of Sermitsialik, but they were sufficient to inspire me with a lively sense of the danger of such exploration. When my first experiments were made, there was a strong wind howling down from themer de glace, bringing with it sharp, cutting snow-drifts. The brass instrument which I used froze to the eye, and had to be covered with buckskin. The moisture of the breath condensed upon the lenses, and I had to breathe through a tube while observing. The men whocarried the chain “scorched” their fingers with the cold metal. Under such circumstances the pursuit of science becomes a species of martyrdom, and is therefore not much in favor.
In the movement shown by these measurements we have conclusively exhibited the likeness of the glacier to a river, and the assertion that it is but a flowing river of ice is fully borne out and proved by observation with mathematical nicety. In my own experiments upon the Greenland glaciers I have but followed the example and practised the methods of Professors Agassiz, Forbes, Tyndall, and others, not forgetting the experts of the Alpine Club, whose explorations of the glaciers of the Alps have latterly become so familiar to the public.