Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/297

Rh ocean Movi-:.rE."rs.] I. Movmrnxrs or THE OCEAN. These maybe grouped as——(l) tides, currents, and (3) waves. 1. ’1’i¢les.—'l‘liese are oscillations of the mass of the oceanic waters caused by the attraction of the sun and moon. 'c have at present to deal with them merely in so far as their geological bearings are concerned. I11 a wide deep ocean the tidal elevation probably produces no perceptible geological change. It passes at a great speed; in the Atlantic its rate is 500 geographical miles an hour. But as this is merely the passing of an oscillation whereby the particles of water are gently raised up and let down again, there can hardly be any appreciable effect upon the deep ocean bottom. When, however, the tidal wave enters a narrow and shallow sea, it has to acconnnodate.itself to a smaller ch mnel, and encounters more a11d more the friction of the bottom. Hence, while its rate of motion is dimin- ished, its height and force are increased. It is in shallow water and along the shores of the land that the tides acquire their main geological importance. They there show them- selves in an alternate advance upon and retreat from the coast. Their upper limit has received the name of lily]:- 1r1te2- marl-, their lower that of low-water mark, the space between being termed the beach. If the coast is precipitous, a beach can only occur in the shelving bays and creeks, since elsewhere the tides will rise and fall against a face of rock, as they do on the piers and bulwarks of a port. On s-ueh rocky coasts the li11e of high water is sometimes ad- mirably deﬁned by the grey crust of barnacles adhering to the rocks. Where the beach is ﬂat, and the rise and fall of the tide great, an area of several hundred square miles of sand or mud may be laid bare i11 one bay at low—water. The height of the tide varies from zero up to 60 or 70 feet. It is greatest where, from the form of the land, the tidal wave i- c_)_)1)c(]. up within a narrow inlet or estuary. Under such circumstances the advancing tide sometimes gathers itself into one or n1ore large waves, and rushes furiously up be- tween the converging shores. This is the origin of the “bore” of the Severn, which rises to a height of 9 feet, while the rise and f all of the tide there amounts to 40 feet. In like manner the tides which enter the Bay of Fundy, between Nova Scotia and New Brunswick, get more and more cooped up as they ascend that strait, till they reach a height of 70 feet. Vhile the tidal swelling is increased in height by the shallowness and convergence of the shores, it gains at the same time force and rapidity. No longer a mere oscillation or pulsation of the great ocean, the tide acquires a true movement of translation, and gives rise to currents which rush past headlands and through narrows in powerful currents and eddies. The rocky and intricate navigation of the west of Scotland and Scandinavia furnishes many admirable illustrations of the rapidity of these tidal currents. The famous whirlpool of Corryvreckan, the lurking eddies in the Kyles of Skye, the breakers at the Bore of Duncans- bay, and the tumultuous tideway, grimly named by the northern ﬁshermen the Merry Men of Mey, in the Pentland Firth, bear witness to the strength of these sea rivers. At the last-mentioned strait the current at its strongest runs at the rate of 10 miles an hour, which is fully three times the speed of most of our larger rivers. 2. C'm°rents.——Recent researches in ocean temperature have disclosed the remarkable fact that beneath the surface layer of water affected by the temperature of the latitude there lies a vast mass of cold water, the bottom tempera- ture of every ocean in free communication with the poles being little above and sometimes actually below the freezing point of fresh water. In the North Atlantic a temperature of 40° Fahr. is reached at an average depth of about 800 fathoms, all beneath that depth being progressively colder. GEOLOGY 283 In the equatorial parts of the same ocean the same tempera- ture comes to within 300 fathoms of the surface. In the South Atlantic, off Cape of Good Hope, the mass of cold water (below 40°) comes likewise to about 300 fathoms from the surface. This distribution of temperature proves that there must be a transference of cold polar water towards the equator, for in the ﬁrst place the temperature of the great n1ass of the ocean is much lower than that which is normal to each latitude, and in the second place it is lower than that of the superﬁcial parts of the earth’s crust underneath. On the other hand, the movement of water from the poles to the equator requires a return movement of compensation from the equator to the poles, and this must take place in the superﬁcial strata of the ocean. Apart therefore from those rapid river-like streams which traverse the ocean, and to which the name of currents is given, there must be a general drift of warm surface water towards the poles. This is doubtless most markedly the case in the North Atlantic, where besides the current of the Gulf—stream there is a prevalent set of the surface waters towards the north- east. As the distribution of life over the globe is everywhere so dependent upon temperature, it becomes of the highest interest to know that a truly arctic submarine climate exists _ everywhere in the deeper parts of the sea. With such uniformity of temperature we may anticipate that the abyssal fauna will be found to possess a corresponding sameness of character, and that arctic types may be met with even 011 the ocean-bed at the equator. But besides this general drift or set, a leading part in oceanic circulation is taken by the more deﬁned streams termed currents. The tidal wave only becomes one of translation as it passes into shallow water, and is thus of but local consequence. But a vast body of water, known as the Equatorial Current, moves in a general westerly direction round the globe. Owing to the way in which the continents cross its path, this current is subject to consider- able deflexions. Thus that portion which crosses the Atlantic from the African side strikes against the mass of South America and divides, one portion turning towards the south and skirting the shores of Brazil, the other bend- ing north-westward into the Gulf of Mexico, and issuing thence as the well-known Gulf—stream. This equatorial water is comparatively warm and light. At the same time the heavier and colder polar water moves towards the equator, sometimes i11 surface currents like those which skirt the eastern and western shores of Greenland, but more generally as a cold 11nder—current which creeps over the ﬂoor of the ocean even as far as the equator. Much discussion l1as arisen in recent years as to the cause of oceanic circulation. Two rival theories have been given. According to one of these the circulation entirely arises from that of the air. The trade-winds blowing from either side of the equator drive the water before them until the north-east and soutl1-east currents unite in equatorial lati- tudes into one broad westerly-ﬂowing current. Owing to the form of the land portions of this main current are deflected into temperate latitudes, and, as a consequence, portions of the polar water require to move towards the equator to restore the equilibrium. According to the other view the currents arise from diﬁ"erences of temperature (and according to some, of salinity also); the warm and light equatorial water is believed to stand at a higher level than the colder and heavier polar water; the former, therefore, flows down as it were polewards, while the latter moves as a bottom inﬂow towards the equator; the cold bottom water under the tropics is constantly ascending to the sur- face, whence, after being heated, it drifts away towards the pole, and on being cooled down there, descends and begins another journey to the equator. There can be no doubt that the winds are directly the cause of such c11rrents as