22. The annual evaporation from the surface of the ice, which even in winter is never entirely interrupted during the severest frost, and the destruction of ice by the action of rain and waves, are balanced, to speak generally, by its re-formation by frost. The maximum accumulation of ice takes place in spring, its minimum in the beginning of autumn. We observed in the autumn of 1873 not only the evaporation of the snow of the preceding winter, but also a vertical decrease of ice of about four feet. Evaporation is, therefore, the most potent regulator of the balance between waste and growth in the accumulation of ice; and next in importance is the drifting of its masses towards the south through all those openings by which the Polar waters mingle with the waters of lower latitudes. 23. However great the agitation of the sea may be in the open ocean, and though it may dash its waves with wild fury on the edge of the ice, within the icy girdle it is undisturbed, in consequence of the enormous weight of the superincumbent masses. It is only in the large “ice-holes,” and when the winds are very high, that the action of waves is discernible. An isolated accumulation of floes in a circular form, suffices to produce a calm interior sea, and its outer edge only encounters the beat of the ocean. 24. The ceaseless attack to which the ice is exposed on its outer edge is the cause of its excavation and undermining. Hence its centre of gravity is constantly displaced; and the overturning of its masses and its strange transformations are the consequences of this instability. The smaller the masses of the ice, the more fantastic are the shapes they assume. 25. Change of colour in the sea as we enter the ice-region is frequently, though not invariably, observed. Almost immediately on entering the ice, its normal dull green colour gives place to a deep ultramarine blue, especially in the East Greenland seas, and this colour is maintained under all changes of the weather, and is only modified by local currents. Two hundred and fifty years ago it appeared to Hudson, on the coast of Spitzbergen, that the sea, whenever it was free from ice, was green, and that its being covered with ice and its blueness of colour were intimately connected. Sir James Ross states that in both Polar oceans the colour of the sea changes in the neighbourhood of ice, and that the dull brownish colour sometimes seen near pack-ice in the Antarctic Ocean is owing to an infinite number of animalculæ. The rapid fall of the temperature of the water to the zero point is another indication that ice is near.