Page:Popular Science Monthly Volume 7.djvu/478

462 theory—of expansion—it is difficult to see how it will explain the greater motion of the glacier in precisely that part of the year and of the day when the heat is the greatest, and the influence of expansion by freezing must necessarily be least operative.

The rapidity of the glacier's motion is much greater than we would naturally expect. By a course of very careful observations and measurements, Prof, Sexe found that, in the middle of July, 1868, the ice on the surface of the glacier of Boium, near the centre of the glacier and some distance from the lower end, moved 204$1/2$ inches (Norwegian) in 211 hours. This was about $97/100$ of an inch per hour, and, if kept up during the entire year, would have given a total motion of about 707 feet. Here the motion was considerably greater by day than by night; in the former, exceeding 1$1/20$ inch per hour, and, in the latter, being about $17/20$ only. M. de Seue, however, coming later in the same month, when the weather was less uniformly pleasant, found the average rate of progression of the same point very much diminished, viz., to less than $3/4$ of an inch per hour; and there is every reason to believe that, had these gentlemen been able to resume their observations in winter, they would have found that the motion during the cold weather is almost inappreciable. It is unnecessary to say that the difficulty of taking observations during half the year, on account of the temperature, and the glacier being covered over with deep snow, can scarcely be exaggerated.

It is not every part of the glacier which moves with even the rapidity mentioned. Toward the sides (and undoubtedly we should also find it so at the bottom, if we could get at it), the influence of friction can be detected retarding the motion. Near the lower end, also, the ice appeared to advance not more than one-third as fast as further up.

Of the numerous glaciers which M. de Seue describes as descending from the snow-field of Justedal, a number seem to be objects of great beauty. The photographic views which accompany his paper unfortunately fail to convey a very satisfactory idea of their appearance. The most considerable is the glacier of Trensbergdal, some nine miles long, and from two-thirds of a mile to nearly a mile in width. In several cases two or more glaciers meet, and, joining their contents, merge so thoroughly that they can be distinguished only by the moraines—lines of detached blocks of stone, torn from the rocks above in the downward course of the icy current, and which mark their edges even after their junction. Several are instances of what the French savants have called glaciers remaniés, the internal structure being altogether changed in consequence of their having been precipitated over ledges of rocks of considerable height. One of the smallest of the number is the glacier of Lunde, which is less than a mile in length, and only 100 feet in breadth. The interest attaching to it, however, lies in the suddenness with which it makes its spring