580 LKCTURE XLVII. 



deep, in Older that the time of high water might coincide with that of the 

 moon's southing. 



Hitherto we have considered the motion of the water as free from all resist- 

 ance ; but M'here the tides are direct, they must be retarded by the effect of 

 a resistance of any kind ; and where they are inverted, they must be accele- 

 rated ; a small resistance producing, in both cases, a considerable difference in 

 the time of high water. 



Where a considerable tide is observed in the middle of a limited portion of 

 the sea, it must be derived from the effect of the elevation or depression of 

 the ocean in its neighbourhood ; and such derivative tides are probably combined 

 in almost all cases with the oscillations belonging to each particular branch of 

 the sea. Mr. Laplace supposes that the tides, which are observed in the most 

 exposed European harbours,are produced almost entirely by the transmission of 

 the cfl^'ect of the main ocean, in about a day and a half; but this opinion does 

 not appear to be justified by observation; for the interval between the times of 

 the high water belonging to the same tide, in any two places between Brest and 

 the Cape of Good Hope, has not been observed to exceed about twelve hours 

 at most; nor can we trace a greater difference by comparing the state of the 

 tides at the more exposed situations of St. Helena, the Cape Verd Islands, 

 the Canaries, the ISladeiras, and the Azores, which constitute such a suc- 

 cession as might be expected to have indicated the progress of the principal 

 tide, if it had been such as My. Laplace supposes. The only part of the 

 ocean, which we can consider as completely open, lies to the south of the two 

 great continents, chiefly between the latitudes 30® and 70° south, and the ori- 

 ginal tide, which happens in this widely extended ocean, where its depth is suffi- 

 ciently uniform, must take place, according to the theory which has been ad- 

 vanced, at some time before the sixth lunar hour. It sends a wave into the At- 

 lantic, which is perhaps 12 or 13 hours in its passage to the coast of France, but 

 certainly not more. This tide, which would happen at the sixth lunar hour 

 after the moon's transit, if there were no resistance, is probably so checked by 

 the resistance, that tjie water begins to subside about the fourth, and in some 

 seas even somewhat earlier, although in others it may follow more nearly 

 its natural course. There is scarcely a single instance which favours the 



