With a feeling of absolute security against further pressures and movements, the crew returned on board, and once more the cabin echoed to the light-hearted laughter which had been interrupted by the "nip." The hardy Norsemen who formed the party were as happy as they were brave, and throughout the years they were together there was nothing but good-humour and merriment among them. After the preliminary experience of how the _Fram_ conducted herself during a "nip," little attention was paid to the ceaseless noise and roaring set up by the moving ice. Often she was forced up out of the line of movement, but the men in her cabin sat quiet; she was able to "sail herself" without any help on that ice-locked sea. The existence of this constant movement of the ice formed a very important discovery in Arctic knowledge. A brief explanation of the causes and the effects may make this clear, and, at the same time, show how it is that such huge mountains of ice are formed in the depth of winter when the Polar Sea was currently supposed to be frozen into one great silent moveless ice-field. As winter sets in within the Arctic Circle, the sea which flows between the northern coasts of Europe, Asia, and America becomes covered with ice to the shores, thus forming an enormous field of ice some two thousand miles across. This, lying on the surface of the water, often having a thickness of from thirty to fifty feet, checks, but cannot control the tides. The ebb, on one hand, leaves vast tracks of ice, previously afloat, straining on the ground, cracking so as to form enormous fissures and weakening the surface resistance. On the other hand, the flood tide is welling and pressing against the overlying barrier of ice and lifting it up until it cracks and opens, the pressure underneath lifting the separated masses on to their neighbours, which in turn resist with all their weight and grind back upon the masses beyond, until with the turn of the tide the forced-up masses gravitate down again, tumbling, crashing, bounding and rebounding one upon the other. Meanwhile the ice lowered by the ebb tide has formed a restricted crust against which the flood tide, backed up by the weight of the disturbed masses, uses its energy as a man uses his shoulder to lift a load. It is a battle between the resistance and the energy of nature, and usually energy wins along the line of the least resistance. Here, when once a point gives way, the accumulated energy concentrates. The "point" may be an area of ice a hundred miles square and fifty feet thick, and this tremendous mass, moved by the immeasurable force of the water pressure beneath it, grinds upon its surroundings and upon itself. Huge masses are pushed up on to the surface of the pack, crushing, grinding, and splintering as they go, their weight causing the under ice to bend and crack, and so add to the confusion of the struggle. Mass meets mass in a test of strength, and, failing to climb over one another, crush