17. While sea-currents move the ice in close and continuous lines, winds produce great disturbances in their movement, and open long “leads” in the direction of their course, which often alternate with strips of the thickest pack-ice. This movement of the ice varies with each accumulation of floes, as its rate of motion depends on the height of the ice-field, which then acts as a sail. It is ascertained by experience that calms, on the other hand, have the remarkable property of breaking up the ice. The knowledge and application of these circumstances are essential to the Arctic navigator. If the course of a ship lies across or against a current, it is constantly deflected. The deflection on the coast of East Greenland, for example, amounted to five, even ten miles, within twenty-four hours; hence the importance of choosing routes with and not against the course of currents. 18. Lastly, it is of the greatest moment to choose betimes an appropriate winter harbour, and it is therefore necessary to keep near the coast towards the close of the season for navigation. To find one suitable for shelter during the winter in an unknown Arctic region is a matter of great difficulty, for it very often happens, that the ice drifts out from these “docks”[9] in the storms which constantly occur, or perhaps the “dock” is so sheltered, that the ice, if it breaks up at all, breaks up only in the following summer. Shallow bays which freeze almost to the bottom, lying under the lee of a current or within a fiord, are the most appropriate spots in which to winter. CHAPTER III. THE PENETRATION OF THE REGIONS WITHIN THE POLAR CIRCLE; THE PERIOD OF THE NORTH-WEST AND NORTH-EAST PASSAGES. 1. Around the lonely apex of the Pole stand cairns of stone which serve to mark the points to which the restless spirit of human enterprise and discovery has penetrated. In its zenith wheels the sea-gull in its flight, and the harpoon-persecuted seal finds on its ice-floes an unapproachable asylum; but the Pole itself remains the goal which no human effort has yet reached.