After his defeat upon the Aletsch, M. Escher joined MM. Agassiz and Desor on the Aar glacier, where, between the 31st of August and the 5th of September, they fixed in concert the positions of a series of blocks upon the ice, with the view of measuring their displacements the following year.
AGASSIZ'S MEASUREMENTS.
Another observation of great importance was also commenced in 1841. Warned by previous failures, M. Agassiz had iron boring-rods carried up the glacier, with which he pierced the ice at six places to a depth of ten feet, and at each place drove a wooden pile into the ice. These six stations were in the same straight line across the glacier; three of them standing upon the Finsteraar and three on the Lauteraar tributary. About this time also M. Agassiz conceived the idea of having the displacements measured the year following with precise instruments, and also of having constructed, by a professional engineer, a map of the entire glacier, on which all its visible "accidents" should be drawn according to scale. This excellent work was afterwards executed by M. Wild, now Professor of Geodesy and Topography in the Polytechnic School of Zürich, and it is published as a separate atlas in connexion with M. Agassiz's 'Système Glaciaire.'
PROF. J. D. FORBES INVITED.
M. Agassiz is a naturalist, and he appears to have devoted but little attention to the study of physics. At all events, the physical portions of his writings appear to me to be very often defective. It was probably his own consciousness of this deficiency that led him to invoke the advice of Arago and others previous to setting out upon his excursions. It was also his desire "to see a philosopher so justly celebrated occupy himself with the subject," which induced him to invite Prof. J. D. Forbes of Edinburgh to be his guest upon the Aar glacier in 1841. On the 8th of August they met at the Grimsel Hospice, and for three weeks afterwards they were engaged together daily upon the ice, sharing at night the shelter of the same rude roof. It is in reference to this visit that Prof. Forbes writes thus at page 38 of the 'Travels in the Alps':—"Far from being ready to admit, as my sanguine companions wished me to do in 1841, that the theory of glaciers was complete, and the cause of their motion certain, after patiently hearing all they had to say and reserving my opinion, I drew the conclusion that no theory which I had then heard of could account for the few facts admitted on all hands." In 1842 Prof. Forbes repaired, as early as the state of the snow permitted, to the Mer de Glace; he worked there, in the first instance, for a week, and afterwards crossed over to Courmayeur to witness a solar eclipse. The result of his week's observations was immediately communicated to Prof. Jameson, then editor of the 'Edinburgh New Philosophical Journal.'
CENTRE MOVES QUICKEST.
In that letter he announces the fact, but gives no details of the measurement, that "the central part of the glacier moves faster than the edges in a very considerable proportion; quite contrary to the opinion generally entertained." He also announced at the same time the continuous hourly advance of the glacier. This letter bears the date, "Courmayeur, Piedmont, 4th July," but it was not published until the month of October following.
Meanwhile M. Agassiz, in company with M. Wild, returned to complete his experiment upon the glacier of the Aar. On the 20th of July, 1842, the displacements of the six piles which he had planted the year before were determined by means of a theodolite. Of the three upon the Finsteraar affluent, that nearest the side had moved 160 feet, the next 225 feet, while that nearest to the centre had moved 269 feet. Of those on the Lauteraar, that nearest the side had moved 125 feet, the next 210 feet, and that nearest the centre 246 feet. These observations were perfectly conclusive as to the quicker motion of the centre: they embrace a year's motion; and the magnitude of the displacements, causing errors of inches, which might seriously affect small displacements, to vanish, justifies us in ranking this experiment with the most satisfactory of the kind that have ever been made. The results were communicated to Arago in a letter dated from the glacier of the Aar, on the 1st of August, 1842; they were laid before the Academy of Sciences on the 29th of August, 1842, and are published in the 'Comptes Rendus' of the same date.
The facts, then, so far as I have been able to collect them, are as follows:—M. Agassiz commenced his experiment about ten months before Professor Forbes, and the results of his measurements, with quantities stated, were communicated to the French Academy about two months prior to the publication of the letter of Professor Forbes in the 'Edinburgh Philosophical Journal.' But the latter communication, announcing in general terms the fact of the speedier central motion, was dated from Courmayeur twenty-seven days before the date of M. Agassiz's letter from the glacier of the Aar.
STATE OF THE QUESTION.