Land ice, of which both the ice fields of the Arctics and the glaciers consist, is produced by slow and gradual transformation of snow into ice; and though the ice thus formed may eventually be as clear and transparent as the present pond or river ice, its structure is, nevertheless, entirely distinct. We may compare these different processes during any moderately cold winter in the ponds and snow meadows immediately about us. We need not join an Arctic exploring expedition, or even undertake a more tempting trip to the Alps, in order to investigate these phenomena ourselves, if we have any curiosity to do so. The first warm day after a thick fall of light, dry snow, such as occurs in the coldest of our winter weather, is sufficient to melt its surface. As this snow is porous, the water readily penetrates it, having also a tendency to sink by its own weight, so that the whole mass becomes more or less filled with moisture in the course of the day. During the lower temperature of the night, however, the water is frozen again, and the snow is now filled with new ice particles. Let this process be continued long enough and the mass of snow is changed into a kind of ice gravel, or, if the grains adhere together, to something like what we call pudding-stone, allowing, of course, for the difference of material; the snow, which has been rendered cohesive by the process of partial melting and regelation, holding the ice globules together, just as the loose materials of the pudding-stone are held together by the cement which unites them. Within this mass air is intercepted and held inclosed between the particles of ice. The process by which snowflakes or snow crystals are transformed into grains of ice, more or less compact, is easily understood. It is the result of a partial thawing under a temperature maintained very nearly at thirty-two degrees, falling sometimes a little below and then rising a little above the freezing-point, and thus producing constant alternations of freezing and thawing in the same mass of snow. This process amounts to a kind of kneading of the snow, and when combined with the cohesion among the particles more closely held together in one snowflake, it produces granular ice. Of course, the change takes place gradually, and is unequal in its progress at different depths in the same bed of fallen snow. It depends greatly on the amount of moisture infiltrating the mass, whether derived from the melting of its own surface, or from the accumulation of dew, or the falling of rain or mist upon it.