Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/298

Rh 284 the Gulf-stream, and therefore, indirectly, of return cold currents from the polar regions. It seems hardly less certain that, to some extent at least, differences of tempera- ture, and therefore of density, must occasion movements in the mass of the oceanic waters. The discussion of this subject, however, belongs to another part of this work.‘ The main facts for the geological reader to grasp are—that a system of circulation exists in the ocean ; that warm currents move round the equatorial regions, and are turned now to the one side now to the other by the form of the continents along and round which they sweep; that cold currents set in from poles to equator; and that, apart from actual currents, there appears to be an extremely slow “creep” of the polar water under the warmer upper layers towards the equator. 3. Waves and Ground-Swell.—A gentle breeze curls into ripples the surface of Water over which it blows. A strong gale or furious storm raises the surface into waves. The agitation of the water in a storm is prolonged to a great distance beyond the area of the original disturbance, and then takes the form of the long heaving undulations termed ground-swell. Waves which break upon the land are called breakers, and the same name is applied to the ground-swell as it bursts into foam and spray upon the rocks. The concussion of earthquakes sometimes gives rise to very disastrous earthquake-waves, as already explained. The height and force of waves depend upon the breadth and depth of sea over which the wind has driven them, and the form and direction of the coast-line. The longer the “fetch,” and the deeper the water, the higher the waves. A coast directly facing the prevalent wind will have larger waves than a neighbouring shore which presents itself at an angle to this wind or bends round so as to form a lee-shore. The highest waves in tlie narrow British seas probably never exceed 15 or 20 feet, and usually fall short of that amount. The greatest height observed by Dr Scoresby among the Atlantic waves was 43 fcet.2 Gronnd—swell propagated across a broad and deep ocean produces by far the most imposing breakers. So long as the water remains deep and no wind blows, the only trace of the passing ground-swell on the open sea is the huge broad heaving of the surface. But when the water shallows, the superﬁcial part of the swell travelling faster than the bottom begins to curl and crest as a huge billow or wall of water, which ﬁnally bursts with enormous force against the shore. Such billows, even when no wind is blowing, often cover the cliffs of the north of Scotland with sheets of water and foam up to heights of 100 or even nearly 200 feet. At Dunnet Head during 11orth-westerly gales the windows of the lighthouse, at a height of upwards of 300 feet above high-water mark, are said to be sometimes broken by stones swept up the cliffs by the sheets of sea water which then deluge the building. A single roller of the ground-swell 20 feet high falls, according to Mr Scott Russell, with a pressure of about a ton on every square foot. M r Thomas Stevenson conducted some years ago a series of experiments on the force of the breakers on the Atlantic and North Sea coasts of Britain. The average force in summer was found in the Atlantic to be 611 lb per square foot, while in winter it was 2086 lb, or more than three times as great. But on several occa- sions, both in the Atlantic and North Sea, the winter breakers were found to exert a pressure of three tons per 1 The reader may consult Maury’s Physical Geography Qf the Sm, but more particularly Dr Carpenter's papers in the Proceeilings of the Royal. uciely for 1869-73, and Journal of R. G'eograplu'cal Society for 1871-77, on the side of temperature; and Hersehel’s I'll]/sical (r'co_r/raphy, and Dr Croll's Climate and Time, on the side of the winds. 9 Brit. Assoc. Rep., 1850, p. 26. The reader will find a table of the observed heights of waves round Great Britain in Mr T. Stevenson's treatise on Ilarbours, p. 20. G E O L O (1 Y [IIL l)Y_'-IXl(‘.- L. square foot, aml at Dunbar as much as three tons and a half.3 lesides the waves produced by ordinary wind action, others of an extraordinary size and destructive power are occasionally caused by a violent cyclone-storm. The mere diminution of atmospheric pressure in a cyclone must teml to raise the level of the ocean within the cyclone limits. But the further furious spiral in-rushing of the air towards the centre of the low pressure area drives the sea onward, and gives rise to a wave or succession of waves having great destructive power. Tlms, on 5th October 1864, during a great cyclone which passed over Calcutta, the sea rose in some places 24 feet, and swept everything before it with irresistible force, drowning upwards of -18,000 people. 4. Ice on the Sea.—In this place may be most con- veniently noticed the origin and movements of the ice which in eireumpolar latitudes covers the sea. This ice is derived from two sourees——(1) the freezing of the sea itself, and (2) the seaward prolongation of land-ice. 1. Three chief types of sea-ice have been observed. ((1.) In the Arctic sounds and bays the littoral waters freeze along the shores and form a cake of ice which, upborne by the tide and adhering to the land, is thickened by successive additions below, as well as by snow above, until it forms a shelf of ice 120 to 130 feet broad a11d 20 or 30 feet high. This shelf, known as the ice-foot-, serves as a platform on which the abundant debris loosened by the severe frosts of an arctic winter gathers at the foot of the cliffs. It is more or less completely broken up in summer, but forms again with the early frosts of the ensuing autumn. The sur- face of the open sea likewise freezes over into a continuous solid sheet which in summer breaks up into separate masses sometimes of large extent. This is what navigators term ﬂue-ice, and the separate ﬂoating cakes are known as ﬂm-s. Ships fixed among these ﬂoes have been drifted with the ice for hundreds of miles until at last liberated by its disrup- tion. In the Baltic Sea, off the coast of Labrador and elsewhere, ice has been observed to form on the sea—bottom. It is known as ground-ice or anchor ice. In the Labrador ﬁshing-grounds it forms even at considerable depths. Seals caught in the lines at those depths are brought up sometimes solidly frozen. 2. I11 the Arctic regions the vast glaciers which drain the snow-ﬁelds and descend to the sea extend for some distance from the land until large fragments break off and float away seawards. These detached masses are icebergs. Their shape and size greatly vary, but lofty peaked forms are common, and they sometimes rise from 200 to 300 feet above the level of the sea. As only about a ninth part of the ice appears above water, these larger bergs must some- times be from 2000 to 3000 feet thick from base to top. They consequently require water of some depth to float them, but they are often seen aground. In the Antarctic regions, where one vast sheet of ice envelops the land and extends into the sea as a high rampart of ice, the detached icebergs often reach a great size, and are charac- terized by the frequency of a ﬂat tabular form. II. ‘GEOLOGICAL Wonk or THE SEA. I. INFLUENCE ON CL1M.i'r1-:.—'ere there no agencies in nature for distributing temperature, there would be a regu- lar and uniform diminution in the mean animal tempera- ture from equator to poles, and the {sol/wrmul lines, or lines of equal heat, would coincide with lines of latitude. But no such general correspondence actually exists. If we look at a chart of the globe with the isothermal lines drawn across it, we shall find that their divergences from the parallels are striking, and most so where they approach and cross the ocean. Currents from warm regions raise the 3 Trans. Roy. Sue. Edz'n., xvi. 25 ; treatise on IIa7'bo.'u‘s, p. 42.