TIDAL CUREENTS. 117 



CHAPTER XIY. 



TIDAL CURRENTS. — RACES AND WHIRLPOOLS. — TIDAL EDDIES. — RIVER TIDES. 



The popular belief is that the oscillations of the tides are always ac- 

 companied by currents changing regularly with the ebb and flow, 

 and tending alternately in one direction or the other. This is, it 

 is true, a pretty frequent phenomenon, especially at the mouths of 

 rivers. Usually when the water rises, a tidal current rushes at the 

 same time towards the shore and into the estuaries of rivers ; then 

 when the level of the liquid mass falls, a return or low- water current, 

 swelled by the fresh water from inland, flows again towards the open 

 sea. Nevertheless, this coincidence of the horizontal currents with the 

 vertical oscillations of the ocean is far from being reproduced with 

 regularity in all parts. The tide, being merely a swelling of the sea, 

 can rise without the least movement occurring in one direction or the 

 other. A remarkable example of this is seen in the Irish Sea, so rich 

 in maritime phenomena. In the middle of the channel which separ- 

 ates the Isle of Man from Ireland, the sheet of water keeps perfectly 

 tranquil between the contrary currents, though the water at this 

 place rises more than 18 feet during the spring tides. On the other 

 hand, as one can see at Oourtown, on the coast of Arklow, the cur- 

 rent determined by the meeting of opposing tides can have a great 

 speed where the surface of the sea neither rises nor falls.* Finally, 

 the same wave can follow a constant direction across two contiguous 

 regions of the sea, one of which is at ebb and the other at flow. 



The currents which occur in straits in consequence of difierences of 

 level, are sometimes extremely violent ; and by their abrupt changes, 

 their eddies and whirlpools may be classed among the most dangerous 

 phenomena of the ocean. Thus the entrance to the Gulf of Normandy 

 and the Channel Islands is rightly dreaded by navigators because 

 of the terrible speed which the tidal currents attain there. The 

 Blanchard Race, a strait which separates the Cape of La Hogue from the 

 island of Alderney, is the first of these terrible marine defiles where 



* See above, p. 113. 



