Icebergs commonly float on a base which is larger in extent than the upper surface. Hence the proportion of ice appearing above water is seldom less in elevation than one-seventh of the whole thickness; and when the summit is conical, the elevation above water is frequently one-fourth of the whole depth of the berg. Perhaps the most general form of icebergs is with one high perpendicular side, the opposite side very low, and the intermediate surface forming a gradual slope. When of such a form, captain Ross found that the higher end was generally to windward. Some icebergs have regular flat surfaces, but most usually they have different acute summits, and occasionally exhibit the most fantastic shapes. Some have been seen that were completely perforated, or containing prodigious caverns, or having many clefts or cracks in the most elevated parts, so as to give the appearance of several distinct spires. On some icebergs, where there are hollows, a great quantity of snow accumulates; others are smooth and naked. The naked sides are often filled with conchoidal excavations, of various magnitudes; sometimes with hollows the size of the finger, and as regular as if formed by art. On some bergs, pools of water occur stagnant; on others, large streams are seen oozing through crevices into the sea. In a high sea, the waves break against them as against a rock; and, in calm weather, where there is a swell, the noise made by their rising and falling is tremendous. When icebergs are aground, or when there is a superficial current running to leeward, the motion of other ice past them is so great that they appear to be moving to windward. Fields of ice, of considerable thickness, meeting a berg under such circumstances, are sometimes completely ripped up and divided through the middle. Icebergs, when acted on by the sun, or by a temperate atmosphere, become hollow and fragile. Large pieces are then liable to be broken off, and fall into the sea with a terrible crash, which, in some places, produces an echo in the neighbouring mountains. When this circumstance, called _calving_, takes place, the iceberg loses its equilibrium, sometimes turns on one side, and is occasionally inverted. The sea is thereby put into commotion, fields of ice in the vicinity are broken up, the waves extend, and the noise is heard to the distance of several miles; and sometimes the rolling motion of the berg not ceasing, other pieces get loosened and detached, till the whole mass falls asunder like a wreck.