OF TIDES, - 321 



water, with respect to the primitive tides, are equally distant from 

 the middle of the lake. 



The tides may be direct in a lake one hundred fathoms deep and 

 less than eight degrees wide ; but if it be much wider, they must 

 be inverted. Supposing the depth a mile, they will be direct when, 

 the breadth is less than 25 ; but if a sea, like the Atlantic, were 

 lifty or sixty degrees wide, it must be at least four miles deep, in 

 order that the ft 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 resistance; but where the tides are direct, they must be retard- 

 ed by the effect of a resistance of any kind ; and where they are 

 inverted, they must be accelerated ; 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 eleva- 

 tion or depression of the ocean in its neighbourhood ; and such de- 

 rivative 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 effect of the main ocean, in about a day and a 

 half; but this opinion does not appear to be justified by observa- 

 tion ; for the interval between the times of the high water belong- 

 ing to the same tide, in any two places between Brest and the Cape 

 of Good Hope, has not been observed to exceed aboTkt twelve 

 hours at most; nor can we trace a greater difference by consparing 

 the stale of the tides at the more exposed situation of St. Helena, 

 the Cape Verd Islands, the Canaries, the Madeiras, and the Azores, 

 which constitute such a succession as might be expected to have 

 indicated the progress of the principal tide, if it had been such as 

 Mr. 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 a south, and the 

 original tide, which happens in this widely extended ocean, where 

 Us depth is sufficiently uniform, must take place, according to the 

 theory which lias been advanced, at ?ome time before the sixth 

 VOL. III. Y 



