When these icebergs are separated by open spaces, through which vessels can be navigated, the pack ice is said to be open. But it often happens that mountains of ice occur partly submerged, where one edge is retained under the principal mass, while the other is above the water. Scoresby once passed over a _calf_, as English mariners call these icy mountains, but he trembled while he did so, dreading lest it should throw his vessel, himself, and crew into the air before he could pass it. The aspect of the ice-fields varies in a thousand ways. Here it is an incoherent chaos resembling some volcanic rocks, with crevices in all directions, bristling with unshapely blocks piled up at random; there it is a strongly-marked plain, an immense mosaic formed of vast blocks of ice of every age and thickness, the divisions of which are marked by long ridges of the most irregular forms; sometimes resembling walls composed of great rectangular blocks, sometimes resembling chains of hills, with great rounded summits. In the spring, when a thaw sets in, and the fields begin to break up, the pieces of light ice which unite the great blocks into unique masses are the first to melt; the several blocks then separate, and the motion of the water soon disperses them, and the imprisoned ships find a free passage. But a day of calm is still sufficient to unite the dispersed masses, which oscillate and grind against each other with a strange noise, which sailors compare to the yelping of young dogs. When a ship is shut up in one of these floating ice-fields, inexplicable changes sometimes occur in the vast incoherent aggregations. Vessels, which think themselves immovable, are found in a few hours to have completely reversed their positions. Two ships shut in at a short distance from each other were driven many leagues without being able to perceive any change in the surrounding ice. At other times ships are drawn with the floating ice-fields, like the white bears, who make long voyages at sea upon these monster vehicles. In 1777 the Dutch vessel, the _Wilhelmina_, was driven with some other whaling ships from eighty degrees north back to sixty-two degrees, in sight of the Iceland coast. During this terrible journey the ships were broken up one after the other. More than two hundred persons perished, and the remainder reached land with difficulty. Lieutenant De Haven, navigating in search of Sir John Franklin, was caught in the ice in the middle of the channel in Wellington Strait. During the nine months which he remained in captivity, he drifted nearly thirteen hundred miles towards the south; and the ship _Resolute_, abandoned by Captain Kellet in an ice-field of immense extent, was drifted towards the south with this vast mass to a much greater distance.