TIDAL WAVE FORMATION DIRECTION AND VELOCITY If the earth were uniformly covered with water, the tidal-wave would flow regularly every lunar day from east to west—highest directly under the moon, and lowest on either side. But the continuity of the ocean is broken by the intervention of the land masses, the only uninterrupted ex panse being a belt between 30° and 70° S. lat., where the deep waters of the Southern Ocean extend right round the globe ; and it is here that the true tide-waves are originally formed, and as they advance westwards are deflected, first into the Indian, then into the Atlantic Ocean. Minor tides may, probably, be generated in the tropical parts of all the great oceans, especially the Pacific, but the true tidal-waves in every part of the ocean receive their primary impulse in, and are propagated from, the Southern Ocean.
The progress of the tidal-wave may be marked on a chart by a series of lines connecting all places whose "estab lishments " are the same. These lines, called co-tidal lines, show the path of the great tidal-wave from its origin in the Southern Ocean to its final subsidence in the remote Northern Seas. In examining the accompanying map of co-tides, it must be remembered that the tides in the Atlantic and Indian Oceans are simply the continuation of the tidal-waves formed in the Southern Ocean, between Cape Horn and Balleny Islands. Off Van Diemen's Land it is high-water at 12. The tidal-wave, moving more and more obliquely to the coast, passes Cape Leeuwin at 5. Seven hours later it reaches Cey lon, and at 1 it deflects past the Cape of Good Hope into the Atlantic. Pressing up the Atlantic, at first in a south-westerly direction, it crosses the equator, and, inclining more to the north-west, appears off Newfoundland and Cape Verd 13 hours after passing the Cape of Good Hope. Four hours later the wave extends from the south coast of Iceland, by the south west of Ireland, to Cape Ushant in France. It then divides into three branches—the main branch sweeping along the western coasts of Ireland and Scotland, and dividing off the north of Scotland, one turning south into the German Ocean, the other proceeding north along the coast of Norway, reaching North Cape in 10 hours. The second branch presses through
St. George's Channel into the Irish Sea, and meets a minor tide-wave from the north, off Courtown on the north-east coast of Ireland. The third branch proceeds up the English Chan nel, arriving at Dover in 7 hours, and meeting the northern tide-wave in the German Ocean off the mouth of the Thames. It will be readily seen that the tides are considerable only in areas open to the general course of the tidal-wave from the Southern Ocean ; for this reason the Mediterranean and other inland seas are almost tideless.
The velocity of the tidal-wave is greatest where it passes over the deepest and most open water. In the Southern Ocean it attains a velocity of 1,000 miles an hour, but in more limited and shallow areas, such as the North Sea, it scarcely travels 50 miles an hour. The absolute height of the tide is considerable only in channels or inlets opening broadly to its course, and gradually getting narrower. In the open Southern Ocean the wave is scarcely six feet high, in the Indian and Atlantic Oceans about ten feet ; but in such a cul-de-sac as the Bay of Fundy or the Bristol Channel, it rises to 50 or 60 feet, and forms an impetuous bore or head of water, advancing with great rapidity. All rivers whose estuaries are open to the course of the wave are tidal rivers, and frequently the ascending wave attains such a height and velocity as to be dangerous, and often most destructive. Thus the tidal-bore of the Tsientang River, in China, rushes up the river in huge waves 30 feet high, at a rate of 25 miles an hour. In the Bristol Channel the wave is pressed into a narrowing channel, and rises at Chepstow to nearly 40 feet, and thence forma a bore 8 or 9 feet high, which ascends the Severn rapidly to a considerable distance. Such bores also occur in the Hooghly, Garonne, Amazon, and other rivers.