Glaciers. The Ice of glaciers is entirely different from that of sea and river water, and is not formed in layers, but consistsof little grains of congealed snow; Hence though perfectly clear, and often smooth on the surface, it is not transparent. As glaciers in some positions, and in hot summers, decrease, they often also increase for a number of years, so as to render a valley uninhabitable. Their increase is caused partly by alternately thawing and freezing, their case by the mountain rivers, which often flow under them and thus form an arch of ice over the torrent. In the Tyrol, Switzerland, Piedmont, and Savoy, the glaciers are so numerous, that they have been calculated to form altogether a superficial extent of 1484 square miles.