102 THE OCEAN. 



continuations of the primitive swelling. Arrested by the American 

 continent, which bars its passage, the tidal wave would rebound 

 towards the north, and follow the contours of the oceanic valley, like 

 a torrent enclosed in a mountain gorge. Striking the coasts of 

 America and those of the Old World, under the same latitude, at the 

 same time, and at an equally oblique angle, it reaches almost simul- 

 taneously, on either side of the Atlantic, the Bay of Fundy and the 

 Irish Channel, where its highest known elevation is observed. The 

 tidal wave accomplishes this passage of about 6000 miles, from the 

 Cape of Good Hope to the British Isles, in about fifteen hours. But 

 its entire voyage, from the centre of the Antarctic Ocean, must have 

 lasted more than a day, and in consequence of the gradual slacken- 

 ing of speed of the waters on the shores of Great Britain, it is only 

 after two days and a half that the tidal wave reaches the mouth of 

 the Thames. Thus the moon would have had time to raise five suc- 

 cessive tides in the Pacific Ocean before the motion of the liquid 

 mass would have been propagated to the entrance of the North Sea. 

 Such is the theory which the labours of \Vliewell have caused to 

 be long considered as the verj^ expression of truth. Nevertheless, 

 It is not certain that things occur in this way. In fact, it is ascer- 

 tamed that m each oceanic basin the tide seems to start from the 

 centre, and to be propagated in aU directions parallel to the general 

 direction of the coasts. We may naturaUy conclude from this, that 

 each great division of the ocean, considered as an isolated sea, is 

 really the cradle of the tides which break upon the surrounding 

 shores. What confirms this idea, too, which appears so probable at 

 first, IS that the various oceans are separated from one another by 

 spaces where the regular tide is hardly perceptible. Thus between the 

 feouth and North Atlantic, whose, precise boundary may be defined 

 by the promontory of St. Roque and Cape Yerde, there exists a wide 

 zone where the tide hardly changes the maritime level more than about 

 23 to 27 inches, as at the islands of Ascension and St. Helena. Be- 

 sides, according to the theory of AYhewell, the tidal wave on the 

 coasts of the Argentine Republic and Brazil, ought to propagate it- 

 self from south to north ; whilst, on the contrary, the movement 

 proceeds from north to south, from Pernambuco to the mouth of the 

 La Plata.* When we see a tidal wave rise ofi" the bank of New- 

 foundland, in the deepest part of the Northern Atlantic, it is 

 not therefore necessary to consider this as the same wave which 

 twelve hours before was raised near the bank of LaguUas at the en- 

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



