8. A _hummock_ is a protuberance raised upon any plane of ice above the common level. It is frequently produced by pressure, where one piece is squeezed upon another, often set upon its edge, and in that position cemented by the frost. Hummocks are likewise formed by pieces of ice mutually crushing each other, the wreck being heaped upon one or both of them. To hummocks, principally, the ice is indebted for its variety of fanciful shapes, and its picturesque appearance. They occur in great numbers in heavy packs, on the edges, and occasionally in the middle of fields and floes, where they often attain the height of thirty feet or upwards. 9. A _calf_ is a portion of ice which has been depressed by the same means as a hummock is elevated. It is kept down by some larger mass, from beneath which it shows itself on one side. 10. A _tongue_ is a point of ice projecting nearly horizontally from a part that is under water. Ships have sometimes run aground upon tongues of ice. 11. A _pack_ is a body of drift-ice, of such magnitude that its extent is not discernible. A pack is _open_ when the pieces of ice, though very near each other, do not generally touch, or _close_ when the pieces are in complete contact. 12. A _patch_ is a collection of drift or bay-ice, of a circular or polygonal form. In point of magnitude, a pack corresponds with a field, and a patch with a floe. 13. A _stream_ is an oblong collection of drift or bay-ice, the pieces of which are continuous. It is called a _sea-stream_ when it is exposed on one side to the ocean, and affords shelter from the sea to whatever is within it. 14. _Open-ice_, or _sailing-ice_, is where the pieces are so separate as to admit of a ship sailing conveniently among them. 15. _Heavy_ and _light_ are terms attached to ice, distinguishable of its thickness. 16. _Land-ice_ consists of drift-ice attached to the shore; or drift-ice which, by being covered with mud or gravel, appears to have recently been in contact with the shore; or the flat ice resting on the land, not having the appearance or elevation of icebergs. 17. A _bight_ is a bay in the outline of the ice. 18. A _lane_ or _vein_ is a narrow channel of water in packs or other large collections of ice. When the sea freezes, the greatest part of the salt it contains is deposited, and the frozen mass, however spongy, probably contains no salt but what is natural to the sea-water filling its pores. Hence the generality of ice, when dissolved, affords fresh water. As, however, the ice frozen altogether from sea-water does not appear so solid and transparent as that procured from snow or rain water, the whale-fishers distinguish it into two kinds, accordingly as it affords water that is potable, or the contrary, as it appears to have been the product of fresh or salt water.