Page:EB1911 - Volume 21.djvu/1005

 Antarctic Ice-Conditions.—It is difficult to decide whether the ice of the polar regions should be dealt with as a geological formation or a meteorological phenomenon; but in the Antarctic the ice is so characteristic a feature that it may well be considered by itself. So far as can be judged, the total annual precipitation in the Antarctic region is very slight, probably not more than the equivalent of 10 in. of rain, and perhaps less. It was formerly supposed that the immense accumulation of snow near the South Pole produced an ice-cap several miles in thickness which, creeping outward all round, terminated in the sea in vast ice-cliffs, such as those of Ross’s Great Barrier, whence the huge flat-topped ice-islands broke off and floated away. Evidence, both in the Graham Land and in the Victoria Land areas, points to a former much greater extent of the ice-cap. Thus Shackleton found that the summit of Mt Hope, in 83° 50′ S., which stands 2000 feet above the ice of the surrounding glaciers, was strewn with erratics which must have been transported by ice from the higher mountains to the south and west. In McMurdo Sound, as in Graham Land, evidence was found that the surface of the ice-sheet was once at least a thousand feet above its present level. These facts appear to indicate a period of greater snowfall in the geologically recent past—i.e. a period of more genial climate allowing the air to carry more water vapour to the southern mountains. Whatever may have been the case in the past the Antarctic glaciers are now greatly shrunken and many of them no longer reach the sea. Others project into the sea a tongue of hard ice, which in the case of the Drygalski glacier tongue is 30 m. long, and afloat probably for a considerable distance. Some of these glacier tongues of smaller size appear now to be cut off at their shoreward end from the parent glacier. At one time the Victoria Land glacier tongues may have been confluent, forming a great ice barrier along the coast similar to the small ice-barriers which clothe the lower slopes of some of the islands in Gerlache Strait. The Great Ice Barrier is in many ways different from these. Captain Scott showed that it was afloat for at least 400 m. of its extent from west to east. Sir Ernest Shackleton followed it for 400 m. from north to south, finding its surface in part thrown into long gentle undulations, but with no evidence of the surface being otherwise than level on the average. The all-but forgotten experiments and cogitations of Biscoe convinced that shrewd observer that all Antarctic icebergs were sea-ice thickened with snow “accumulated with time.” The recent expeditions seem to confirm this view to a great extent in the case of the Barrier, which, so far as the scientific men on the “Nimrod” could see, was formed everywhere of compressed nevé, not of true glacier ice. Instances have been seen of tabular bergs floating with half their bulk above water, showing that they are of very much less density than solid ice. The thrust of the glaciers which descend from the western mountains upon the Barrier throws it into sharp crevasses folds near the point of contact, the disturbance extending 20 m. from the tip of the Beardmore glacier, and the seaward creep of the whole surface of the Barrier is possibly due to this impulse, the rate of movement at the eastern side of the Barrier was found to be at the rate of 500 yds. per annum for the seven years between Scott’s and Shackleton’s expeditions.

Pack ice composed of broken-up sea-ice and fragments of icebergs appears to form a floating breakwater round the Antarctic area. It is penetrated by powerful steamers with ease or with difficulty according to the action of the wind which loosens the pack when it drives it towards the open sea, and closes it up when it drives it against a coast or a barrier of fast ice. At every point but one around the circumpolar area the pack, be it light or dense, appears to extend up to the southern permanent ice or land, though, as in the Weddell Sea, the pack seems at times to be driven bodily away. The exceptional region is the opening of the Ross Sea east of Cape Adare, where a comparatively narrow band of pack ice has always been penetrated by the resolute advance even of sailing ships and led to an extensive open sea to the south. No doubt the set of the ocean currents accounts for this, but how they act is still obscure. The great flat-topped ice-islands which in some years drift out from the Antarctic area in great numbers are usually met with in all parts of the Southern Ocean south of 50° S., and worn-down icebergs have been sighted in the Atlantic even as far north as 26° 30′ S. The greater frequency of icebergs in the Southern Ocean in some years is attributed to earthquakes in the Antarctic breaking off masses of the floating edge of the Barrier.

Antarctic Climate.—Although a vast mass of observations has recently been accumulated, it is not yet possible to treat of the climate of the South Polar region in the same broad way as in the case of the North Polar region. The following table shows the mean temperatures of each month and of the year at all the stations at which the Antarctic winter has been passed. The result is to show that while the winter is on the whole less severe at high latitudes than at equal latitudes in the north, the summer is very much colder, and has little relation to latitude. Even in the South Orkneys, in latitude 60°, in the three warmest months the air scarcely rises above the freezing point as an average, while in Shetland (60° N.) the temperature of the three summer months averages 54° F. But on the other hand, the warmest month of the year even in 77° S. has had a mean temperature as high as 30°. A study of the figures quoted in the accompanying table shows that until longer records