Archive image from page 612 of Discovery Discovery discovery0304londuoft Year: DISCOVERY 263 only a small fraction of the total surface of separation, and breaking constantly occurs along old pressure- ridges. The heaviest pressure-ridges are formed b}' a some- what different process. Suppose a crack has divided a heavy floe into two pieces, each perhaps several miles square, and that they then move together again. They will rarely come together so that they fit exactly, for there will nearly always be some lateral displacement. Most of the pressure between the two will be taken on a few pr
Archive image from page 612 of Discovery Discovery discovery0304londuoft Year: DISCOVERY 263 only a small fraction of the total surface of separation, and breaking constantly occurs along old pressure- ridges. The heaviest pressure-ridges are formed b}' a some- what different process. Suppose a crack has divided a heavy floe into two pieces, each perhaps several miles square, and that they then move together again. They will rarely come together so that they fit exactly, for there will nearly always be some lateral displacement. Most of the pressure between the two will be taken on a few projecting corners. In addition there may the Endurance, became more common towards the end of the winter. It did not as a rule occur during gales, but generally in periods of calm, often shortly before a gale. We may perhaps get an idea of its causes from the following analogy. Wind Effects on Drifting Pack-ice Suppose a jig-saw puzzle, emptied out on a smooth table, is pushed along by means of a ruler. This ma}' be taken as representing pack-ice drifting under the influence of a wind. The analogy is not quite accurate. Fig. 2.—.SH.\CKI,ET0X AO STANDING BETWEEN HUMMOCKS OF ICE. By cctrteiy of WiUiam Heinemaiin, also be a twisting or screwing motion of the ice. If the pressure goes on, a buckling of the floes at the regions of contact wiU occur. Blocks from one floe wUI be driven up over the surface of the other, and in this way ridges are formed of blocks perhaps 5 or 6 ft. thick. This process is known as ' rafting.' Pressure-ridges seldom reach a great height. Thirty feet was very in the Weddell Sea, and from 10 to 15 ft. above the water-line may perhaps be taken as an average height. The formation of pres- sure-ridges by screwing and rafting is the greatest danger to which a ship drifting in the pack-ice is exposed. This type of pressure, during the drift of since the wind, when conditions are steady, will act all over the surface of the ice, and not jus
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