274 THE TIDES, 



the coast it is widely different ; hence its velocity along the Eastern coast 

 of England varies from 35 miles to 160 miles per hour. 



In the open ocean, where nothing intervenes to obstruct the course of 

 the tidal wave, it travels probably with regularity ; and it may be presumed 

 that its height is also inconsiderable. But when this wave, from an open 

 ocean, approaches a narrow channel — such as the Bristol or English 

 Channel — from being hemmed in, as it were, it forms a tide current. 

 Now, along the centre of such a channel the tidal wave would travel with 

 much greater speed than on the sides. Hence the distances at which the 

 hour-marks represent high water will be wide apart in the centre, and 

 transverse to its general direction ; while, on the shores, the direction of 

 the wave would be altered, and it will approach parallel to the shore ; 

 hence the hour-marks will be close together, and parallel to the general 

 direction of the main tide-current. 



The tide-wave, advancing through the contracting channel, towards the 

 end becomes of great height, and, as at Bristol, and in the Bay of Fundy, 

 sometimes rises to the enormous height of 60 or 70 feet; just in the same 

 manner that the surf runs up a shelving beach. 



The variation in the height of the Tide (as is found to be the case in 

 some parts of the coast of France) between places near each other, and 

 having high water at the same time, is to be accounted for by the con- 

 vex form of the ti-dal wave. 



In some parts of the world, as in Australia, Kamschatka, &c., the Tides 

 offer very singular anomalies. At Adelaide, in South Australia, it is high 

 water only once in the twenty-four hours, and that during the night. This 

 arises from what are called interferences, whereby two distinct sets of tidal 

 waves, in their combination, produce apparent rest.* 



(226.) One of the mos:t important circumstances of this subject is, that, 

 in an open channel, the flood current (the current which runs till high 

 water) will continue running for three hours afterwards, or till half -ebb ; 

 and the ebh current, which then begins, will run after low water till half- 

 flood. The time of slack water is intermediate between the times of high 

 water and low water. In proportion as the channel is obstructed at the 

 farther end, the flood current runs for a shorter time after flood; and in a 

 closed creek, the flood current ends at high water.f 



Another error to correct is this : — " That the time of the change of 

 current, or the time of slack water, as it may be termed, never coincides 

 with the time of high water, except close inshore, and within its influence ; 

 the interval is generally considerable. Great confusion has arisen from 

 these two times not being properly distinguished." — Phil. Trans., 1833, 

 ^age 162. 



(227.) The Establishmeiit of the Fort. — The vulgar establishment of the 

 port is the interval of time by which the time of high water follows the 

 moon's transit on the day of the new and full moon. This is, corrected, the 

 mean value of the interval, freed from the semi-menstrual inequality. Its 

 value at the London Docks is l*" 26™ minutes, by the mean of all the obser- 

 vations. — Phil. Trans., 1834, page 19. 



• Phil. Trans., 183^1, page 154. r Phil. Trans., 1833, page 215. 



