P TIDELESS AEEAS IN THE SEA. 



113 



coast, the winds have a marked preponderance, for they sometimes 

 maintain the surface of the sea at the same level during whole days. 

 At the mouths of the Mississippi, where the daily tide has a rise 

 of little more than 14 inches, it is not less regular in its pro- 

 gress, and its total height each day represents exactly the difference 

 of level between the two composing waves, which have crossed each 

 other. Finally, the tide at Tahiti, nearly 12 inches high, is the re- 

 sult of many more oscillations ; for four tides, coming from the four 

 cardinal points, meet each other there, all differing in their speed 

 and their hour of high water. It is not surprising that in the 

 middle of this general intersection of the tides of the Pacific Ocean, 

 that of Tahiti is almost completely neutralized.* 



The Irish Channel, so well studied by Beechey, presents a very 

 curious example of a perfect equilibrium of waters, and that almost 

 opposite the Bristol Channel, where the sea rises and falls alter- 

 nately above 48 feet. That part of the Channel whose surface 

 remains at rest borders on the Irish coast not far from the little 

 town of Courtown, to the south of Arklow. There, neither rise 

 nor fall in the waters has ever been observed, though the cur- 

 rents of the ebb and flow run along the coast alternately, with a 

 speed of nearlj^ 4 J miles per hour. The point where the waters are 

 always in equilibrium may be considered as a kind of " hinge " on 

 which the tides turn. Their amplitude is greater and greater in 

 proportion as they are distant from this tranquil region, to the north- 

 east towards Holyhead and Liverpool, to the south-east towards Mil- 

 ford Haven and Bristol. In the North Sea, the meeting of high and 

 low water, not far from the Straits of Dover, is marked by another 

 centre of equilibrium, which seems to oscillate between the coasts of 

 Holland and those of England, according to the atmospheric and 

 marine currents, and the movements of the heavenly bodies. In this 

 place, Hewitt has ascertained that the tide rises two feet only ; and 

 it is in this region where the waters keep almost always at the same 

 level, that the largest and most numerous sand-banks are deposited. 



It appears that the two tidal currents which meet near the Straits 

 of Dover, the one coming directly from the Atlantic, the other 

 from the North Sea, do not follow the centre of the Channel, and 

 consequently do not encounter each other directly. The rotation of 

 the earth which in the northern hemisphere displaces all moving 

 bodies towards the right, causes each of the tidal waves to diverge 

 in this direction. In the Channel the tidal wave, which is directly 

 * Fitzroy, Adventure and Beagle, Appendix to Vol. II. p. 290. 



