The comparatively warm floods which, in consequence of this great law of circulation, come pouring into the Arctic seas, naturally require some time before they are sufficiently chilled to be converted into ice; and as sea-water has its maximum of density, or, in other words, is heaviest a few degrees above the freezing-point of water, and then necessarily sinks, the whole depth of the sea must of course be cooled down to that temperature before freezing can take place. Ice being a bad conductor of heat, likewise limits the process of congelation; for after attaining a thickness of ten or fifteen feet, its growth is very slow, and probably even ceases altogether; for when floating fields, or floes, are found of a greater thickness, this increase is due to the snow that falls upon their surface, or to the accumulation of hummocks caused by their collision. Thus, by the combined influence of these various physical agencies, bounds have been set to the congelation of the Polar waters. Were it otherwise, the Arctic lands would have been mere uninhabitable wastes; for the existence of the seals, the walrus, and the whale depends upon their finding some open water at every season of the year; and deprived of this resource, all the Esquimaux, whose various tribes fringe the coasts in the highest latitudes hitherto discovered, would perish in a single winter. If the Arctic glaciers did not discharge their bergs into the sea, or if no currents conveyed the ice-floes of the north into lower latitudes, ice would be constantly accumulating in the Polar world, and, destroying the balance of nature, would ultimately endanger the existence of man over the whole surface of the globe. [Illustration: 33. GLACIER DISCHARGING.] CHAPTER IV. ARCTIC MARINE ANIMALS Populousness of the Arctic Seas.--The Greenland Whale.--The Fin Whales.--The Narwhal.--The Beluga, or White Dolphin.--The Black Dolphin.--His wholesale Massacre on the Faeroe Islands.--The Orc, or Grampus.--The Seals.--The Walrus.--Its acute Smell.--History of a young Walrus.--Parental Affection.--The Polar Bear.--His Sagacity.--Hibernation of the She-bear.--Sea-birds. The vast multitudes of animated beings which people the Polar Seas form a remarkable contrast to the nakedness of their bleak and desolate shores. The colder surface-waters almost perpetually exposed to a chilly air, and frequently covered, even in summer, with floating ice, are indeed unfavorable to the development of organic life; but this adverse influence is modified by the higher temperature which constantly prevails at a greater depth; for, contrary to what takes place in the equatorial seas, we find in the Polar Ocean an increase of temperature from the surface downward, in consequence of the warmer under-currents, flowing from the south northward, and passing beneath the cold waters of the superficial Arctic current.