This furnished the first accurate data on record concerning the average movement of the glacier during the greater part of one year.  In 1842 I caused a trigonometric survey of the whole glacier of the Aar to be made, and several lines across its whole width were staked and determined with reference to the sides of the valley;[B] for a number of successive years the survey was repeated, and furnished the numerous data concerning the motion of the glacier which I have published.  I shall probably never have an opportunity of repeating these experiments, and examining anew the condition of the glacier of the Aar; but as all the measurements were taken with reference to certain fixed points recorded upon the map mentioned in the note, it would be easy to renew them over the same locality, and to make a direct comparison with my first results after an interval of a quarter of a century.  Such a comparison would be very valuable to science, as showing any change in the condition of the glacier, its rate of motion, etc., since the time my survey was made.

These observations not only determined the fact of the motion of the glacier itself, as well as the inequality of its motion in different parts, but explained also a variety of phenomena indirectly connected with it.  Among these were the position and direction of the crevasses, those gaping fissures of unknown depths, sometimes a mile or more in length, and often measuring several hundred feet in width, the terror, not only of the ordinary traveller, but of the most experienced mountaineers.  There is a variety of such crevasses upon the glacier, but the most numerous and dangerous are the transverse and lateral ones.  The transverse ones were readily accounted for after the motion of the glacier was admitted; they must take place, whenever, the glacier advancing over inequalities or steeper parts of its bed, the tension of the mass was so great that the cohesion of the particles was overcome, and the ice consequently rent apart.  This would be especially the case wherever some steep angle in the bottom over which it moved presented an obstacle to the even advance of the mass.  But the position of the lateral ones was not so easily understood.  They are especially apt to occur wherever a promontory of rock juts out into the glacier; and when fresh, they usually slant obliquely upward, trending from the prominent wall toward the head of the glacier, while, when old, on the contrary, they turn downward, so that the crevasses around such a promontory are often arranged in the shape of a spread fan, diverging from it in different directions.  When the movement of the glacier was fully understood, however, it became evident, that, in its effort to force itself around the promontory, the ice was violently torn apart, and that the rent must take place in a direction at right angles with that in which the mass was moving.  If the mass be moving inward and downward, the direction of the rent must be obliquely upward.  As now the mass continues to advance, the crevasses must advance with it; and as it moves more rapidly toward the middle than on the margins, that end of the crevasse which is farthest removed from the projecting rock must move more rapidly also; the consequence is, that all the older lateral crevasses, after a certain time, point downward, while the fresh ones point upward.