16. The specific gravity of sea-ice is 0.91, and accordingly about nine parts of a cubical block of ice are under water, while one part only rises above the surface. If, however, the ice of a floe be irregularly formed and full of bubbles, the specific gravity will be correspondingly reduced, and the volume submerged may diminish to two-thirds of the whole mass. 17. The irregularity of the forms of ice is so great, that no deduction can safely be drawn from them; cases may occur where a recently-formed ice-floe, which has been attached to old ice, is forced by its neighbour to sink under the normal level; hence the submergence of floes beneath the level of the sea is often overstated. 18. The temperature of the Arctic Sea at the surface is generally below the freezing point, and then increases slightly with the depth. Sir James Ross observed that the temperature in all oceans does not alter at great depths, and placed this constant temperature at 39° F. In summer the temperature of the atmosphere rises little above freezing point, and, according to Sir James Ross, it is still less at the South Pole, because he saw no thaw-water streaming down from the icebergs there as he did in the North. It was first observed in Forster’s days, that is about a century ago, that the salt was gradually eliminated from frozen sea-water. Of this fact Cook knew nothing; and even Sir James Ross endorses Davis’s remark that “the deep sea freezes not.” But the fact that ice is formed on the open sea, and far from the vicinity of land, was first asserted by Scoresby, and has been confirmed by all subsequent observers, though it was long disputed. 19. The crackling sound so commonly heard along the outer edge of the ice exposed to the action of the waves, is a consequence of the penetration of its pores by the sea-water, which is then immediately frozen, and disruption follows at once. But disruption on a far grander scale is due to a cause the very opposite of this, the sudden contraction and splitting of the ice, even in the great ice-fields, which is produced usually in winter by the sudden fall of the temperature. 20. When light falls on a field of pack-ice, it is reflected in the stratum of air above it, and this span of light, called the “ice-blink,” just above the horizon, warns the navigator of the impossibility of penetrating further. This phenomenon is often observed also over drift-ice, although not so intense nor so yellow in colour as over pack-ice. 21. Water spaces, on the other hand, show their presence by dark spots on the horizon, produced by the formation of clouds from ascending mists. These are the so-called “water-sky,” and faithfully indicate the “leads” beneath them. Above the larger “ice-holes,” they assume the dark colours of a thunder-sky, though they are never so strongly defined.