The ice under the moraines cannot be assailed in its depths by the solar heat, because this heat becomes obscure before it reaches the ice, and as such it lacks the power of penetrating the substance. It is also communicated in great part by way of contact instead of by radiation. A thin film at the surface of the moraine-ice engages all the heat that acts upon it, its deeper portions remaining intact and transparent.
GLACIER MOTION.
PRELIMINARY.
(9.)
NÉVÉ AND GLACIER.
Though a glacier is really composed of two portions, one above and the other below the snow-line, the term glacier is usually restricted to the latter, while the French term névé is applied to the former. It is manifest that the snow which falls upon the glacier proper can contribute nothing to its growth or permanence; for every summer is not only competent to abolish the accumulations of the foregoing winter, but to do a great deal more. During each summer indeed a considerable quantity of the ice below the snow-line is reduced to water; so that, if the waste were not in some way supplied, it is manifest that in a few years the lower portion of the glacier must entirely disappear. The end of the Mer de Glace, for example, could never year after year thrust itself into the valley of Chamouni, were there not some agency by which its manifest waste is made good. This agency is the motion of the glacier.
To those unacquainted with the fact of their motion, but who have stood upon these vast accumulations of ice, and noticed their apparent fixity and rigidity, the assertion that a glacier moves must appear in the highest degree startling and incredible. They would naturally share the doubts of a certain professor of Tübingen, who, after a visit to the glaciers of Switzerland, went home and wrote a book flatly denying the possibility of their motion. But reflection comes to the aid of sense, and qualifies first impressions. We ask ourselves how is the permanence of the glacier secured? How are the moraines to be accounted for? Whence come the blocks which we often find at the terminus of a glacier, and which we know belong to distant mountains? The necessity of motion to produce these results becomes more and more apparent, until at length we resort to actual experiment. We take two fixed points at opposite sides of the glacier, so that a block of stone which rests upon the ice may be in the straight line which unites the points; and we soon find that the block quits the line, and is borne downwards by the glacier. We may well realize the interest of the man who first engaged in this experiment, and the pleasure which he felt on finding that the block moved; for even now, after hundreds of observations on the motion of glaciers have been made, the actual observance of this motion for the first time is always accompanied by a thrill of delight. Such pleasure the direct perception of natural truth always imparts. Like Antæus we touch our mother, and are refreshed by the contact.
HUGI'S MEASUREMENTS.
The fact of glacier-motion has been known for an indefinite time to the inhabitants of the mountains; but the first who made quantitative observations of the motion was Hugi. He found that from 1827 to 1830 his cabin upon the glacier of the Aar had moved 100 mètres, or about 110 yards, downwards; in 1836 it had moved 714 mètres; and in 1841 M. Agassiz found it at a distance of 1,428 mètres from its first position. This is equivalent in round numbers to an average velocity of 100 mètres a year. In 1840 M. Agassiz fixed the position of the rock known as the Hôtel des Neufchâtelois; and on the 5th of September, 1841, he found that it had moved 213 feet downward. Between this date and September, 1842, the rock moved 273 feet, thus accomplishing a distance of 486 feet in two years.
But much uncertainty prevailed regarding the motion of the boulders, for they sometimes rolled upon the glacier, and hence it was resolved to use stakes of wood driven into the ice. In the month of July, 1841, M. Escher de la Linth fixed a system of stakes, every two of which were separated from each other by a distance of 100 mètres, across the great Aletsch glacier. A considerable number of other stakes were fixed along the glacier, the longitudinal separation being also 100 mètres. On the 8th of July the stakes stood at a depth of about three feet in the ice. On the 16th of August he returned to the glacier. Almost all the stakes had fallen, and no trace, even of the holes in which they had been sunk, remained. M. Agassiz was equally unsuccessful on the glacier of the Aar. It must therefore be borne in mind, that, previous to the introduction of the facile modes of measurement which we now employ, severe labour and frequent disappointment had taught observers the true conditions of success.