12. The perpetual unrest in the Arctic Sea, which continues undiminished even in the severest winter, and the incessant change in the “leads” and “ice-holes,” are the main causes of the increase of the ice, both in its area and in its vertical depth. Were this constant movement to cease, the result would be the formation of a sheet of ice of the uniform thickness of about eight feet over the whole Polar region. 13. A layer of snow, which, like the ice itself, is at a minimum in autumn, covers the whole surface of all the ice-fields. This snow, which in winter is sometimes as hard as a rock, sometimes as fine as dust, takes, towards the end of summer, more and more the character of the glacier snow of our lofty Alpine ranges. Its grains, in a humid state, exceed the size of beans, and when in motion they make a rustling noise like sand. This granular snow is the residuum of the incomplete evaporation of what fell in the winter, and of the surface of the ice which has become “rotten” and porous. Its crystals are frequently from a third to a sixth of an inch in length, and firm ice is found even in autumn only at the depth of one or two feet. In the North of Spitzbergen, Parry observed that the surface of the ice was frequently cut up into ice-needles of more than a foot long by the drops of rain, which in summer fall upon it, and in some places he found it overspread with red snow. We ourselves never saw the phenomenon observed by Parry, and the ice-crystals we met with seldom exceeded the length given above. 14. Field-ice is of a delicate azure-blue colour, and of great density, and there is, in these respects, no difference between that of the Arctic and Antarctic regions. Cook, indeed, calls the South Polar ice colourless, though Sir James Clark Ross speaks expressly of the blueness of its ice-masses. Sea-ice surpasses the ice of the Alps both in the beauty of its colour and in its density. The glorious blue of the fissures is due to the incidence of light, the blue rays of which only are reflected, while the other rays are absorbed. A spectrum observation made in 1869 on a Greenland ice-field gave brownish red, yellow, green and blue. The yellowish spots observed in ice are due to the presence of innumerable microscopic animalculæ. 15. Sea-ice, which, when the cold is intense, is hard and brittle, loses this quality with the increase of temperature till it acquires an incredible toughness, far exceeding that of glaciers; and floes several feet thick bend under mutual pressure before they split. Hence the fruitlessness, especially in summer, of all attempts to loosen the connexion of its parts by blasting with gunpowder.