Page:Popular Science Monthly Volume 7.djvu/477

Rh melting and freezing which go on through all but the coldest weather of the year. Whether it is the pressure of the névé, or the irresistible expansion caused by the action of cold upon the water pervading its great mass, that drives off the glacier, is a question respecting which the most intelligent observers are by no means unanimous. Against the theory of weight as the motive power is urged the fact that occasionally the glaciers are not strictly adjacent to the superior snowfield, but separated from it by an intervening space of bare rock. When once it has emerged from the névé, the glacier becomes a stream of ice chiefly distinguished from a fluid river by the greater sluggishness of its current. How, it may be asked, can a mass of solid ice move in a fixed channel? The question was long unanswered. Indeed, it was only slowly that the truth forced itself upon the scientific world that it does actually move at all. And, the fact being conceded, the explanation is still not altogether easy. Prof. Sexe imagines that the plastic character of the glacier, as he has observed it in the neighborhood of Justedal, resides in the ease with which the ice fractures and the equal facility with which it reunites when fragments are brought together. Thus it is that, under the immense weight of the glacier, the glassy material of which it is composed is rent when brought into contact with some solid rock standing in its bed, and that the parted streams become one again as soon as the obstacle in their way is passed. So also it is that longitudinal fissures regularly form in the lower part of the glacier of Boium, when the glacier reaches a point where it can expand in the less contracted valley, while transverse fissures open in the glacier of Suphelle at a place where the inclination suddenly becomes more considerable than it was at first, and close up as soon as the slope is again a gentle one. M. de Seue, on the other hand, emphasizes the peculiar constitution of the ice of glaciers, that is, the ice which is formed by the compression and metamorphosis of snow "The ice of the glacier," he says, "is, as already remarked, composed of distinct particles. From a piece of this ice you can, as a general thing, easily remove the particles, one after the other, without injuring the surrounding ones; and, if you should find a particle which cannot be taken out without afflicting the rest, you will still notice that you can move it a little relatively to the others without harming them. Take a piece of the ice of the glaciers of a convenient size, and, in trying (so gently that it does not break) to twist or bend it, you will notice at once that there is a little changeableness in the minute portions of which it is composed."

Both M. de Seue and Prof. Sexe reject the theory of expansion as failing to account for the phenomenon of the glacier's progression, and both virtually agree in ascribing that progression to the combined influence of the enormous pressure exerted by the glacier's weight and the melting produced by the air. Unfortunately for the former