Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/56

30 whether on the coast or in the open sea, they being greatest at the Pull and Change of the moon. On examining the cotidal map of the seas round Great Britain and Ireland, it will be seen that the tidal wave coming in from the south-west is divided when it reaches Ireland, one part (A) going round the northern, and the other part (B) going round the southern end of Ireland. The northern branch (A) is again divided, a part (A′) going northward, then round the northern extremity of Scotland, and afterwards southward along the eastern coast of Scotland and England, while the other portion (A″) goes southward into the Irish Sea. The southern portion (B) of the wave coming in from the south-west, when it has passed the S.W. of Ireland, splits—a portion (B′) running northward up the Irish Sea, while the other part (B″) goes east into the English Channel. The first of these (B′) meets the offshoot (A″) from the northern part of the wave in the Irish Sea in a line joining St. John's Point, co. Down, Ireland, with the Calf of Man, and Maughold Head with the centre of Morecambe Bay, England, the range of tide on the Irish coast being 15 feet and in Morecambe Bay 31 feet, which are the greatest ranges in the Irish Sea, on the Irish and English coasts respectively. The second branch (B″) meets the offshoot (A′) from the northern wave of the preceding tide after it has gone north-about round Scotland; thus when they meet, A′ is twelve hours older than B″,—the place of meeting being a line which extends from the North Foreland to the east of Calais, the water rising 24 feet at Beachy Head, England, and 34 feet at Cayeux, on the Prench coast. These meetings of the north and south tidal waves are known as the end or head of the tides.

It will of course be carefully borne in mind that the motion of a tidal wave is quite distinct from that of the water composing the wave. Nowhere in the Irish Sea does the current caused by the tide move faster than five knots per hour; yet the high water nowhere appears to move more slowly than twenty knots per hour; and in the open waters of the Indian Ocean and South Atlantic this is still more striking, as in those seas the high water seems to travel as fast as the moon, or nearly 1000 knots an hour.

The "flow" and "ebb" are regulated by the "rise" and "fall" of the tide—the "flow" commencing after the slack of low water, and ending with the slack of high water, and the "ebb" commencing after the slack of high water, and ending with the slack of low water. At the "head of the tide" there is the greatest rise, while the lowest rise of the tide is at the "nodal" or "