30 G. H. KIlUHAN ON THE CHESIL BEACH, DORSETSHIKE, 



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 f ) 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 Eoreland to 

 the east of Calais, the water rising 24 feet at Bcachy Head, Eng- 

 land, 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 no- 

 where 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 " com- 

 mencing 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 "hinge- 



* We now refer to the tides in sea-channels open at each end. In some 

 of the confined bays and estuaries, the rise is higher, due to counter and 

 converging tides. Thus, in the Bristol Channel there are three "flow" tides 

 entering it at the same time — one going N.E. from Land's-End, another eastward 

 from St. David's Head, and a central one due to the " offing tide." These, meet- 

 ing near the head of the bay, raise the water on an average 47 feet at spring tides, 

 but, when aided by a south- westerly wind, raise it at Chepstow between 

 50 and 60 feet. 



