what we commonly call slosh, which, upon freezing, becomes at once compact ice; or, the water sinking rapidly, the lower layers only may be soaked, while the upper portion remains comparatively dry.  But, under all these various circumstances, frost will transform the crystalline snow into more or less compact ice, the mass of which will be composed of an infinite number of aggregated snow-particles, very unequal in regularity of outline, and cemented by ice of another kind, derived from the freezing of the infiltrated moisture, the whole being interspersed with air.  Let the temperature rise, and such a mass, rigid before, will resolve itself again into disconnected ice-particles, like grains more or less rounded.  The process may be repeated till the whole mass is transformed into very compact, almost uniformly transparent and blue ice, broken only by the intervening air-bubbles.  Such a mass of ice, when exposed to a temperature sufficiently high to dissolve it, does not melt from the surface and disappear by a gradual diminution of its bulk, like pond-ice, but crumbles into its original granular fragments, each one of which melts separately.  This accounts for the sudden disappearance icebergs, which, instead of slowly dissolving into the ocean, are often seen to fall to pieces and vanish at once.

Ice of this kind may be seen forming every winter on our sidewalks, on the edge of the little ditches which drain them, or on the summits of broad gateposts when capped with snow.  Of such ice glaciers are composed; but, in the glacier, another element comes in which we have not considered as yet,—­that of immense pressure in consequence of the vast accumulations of snow within circumscribed spaces.  We see the same effects produced on a small scale, when snow is transformed into a snowball between the hands.  Every boy who balls a mass of snow in his hands illustrates one side of glacial phenomena.  Loose snow, light and porous, and pure white from the amount of air contained in it, is in this way presently converted into hard, compact, almost transparent ice.  This change will take place sooner, if the snow be damp at first,—­but if dry, the action of the hand will presently produce moisture enough to complete the process.  In this case, mere pressure produces the same effect which, in the cases we have been considering above, was brought about by alternate thawing and freezing,—­only that in the latter the ice is distinctly granular, instead of being uniform throughout, as when formed under pressure.  In the glaciers we have the two processes combined.  But the investigators of glacial phenomena have considered too exclusively one or the other:  some of them attributing glacial motion wholly to the dilatation produced by the freezing of infiltrated moisture in the mass of snow; others accounting for it entirely by weight and pressure.  There is yet a third class, who, disregarding the real properties of ice, would have us believe, that, because tar, for instance, is viscid when it moves, therefore ice is viscid because it moves.  We shall see hereafter that the phenomena exhibited in the onward movement of glaciers are far more diversified than has generally been supposed.