586 ^ LECTURK XLVII. 



The particular forms of the channels, through which the tides arrive at dif- 

 ferent places, produce in them a great variety of local modifications; of 

 which the most usual is, that from the convergence of the shores of the chan- 

 nels, the tides rise to a much greater height than in the open sea. Thus at 

 Brest the height of the tides is about 20 feet, at Bristol 30, at Chepstow 40, 

 at St. Maloes ^0; and at Annapolis Royal, in the Bay of Fundy, as much 

 sometimes as 100 feet ; although perhaps in some of these cases a partial 

 oscillation of a limited portion of the sea may be an immediate effect of the 

 attraction of the luminary. In the Mediterranean the tides are generally 

 inconsiderable, but they are still perceptible; at Naples they sometimes 

 amount to a foot, at Venice to more than two feet, and in the Euripus, for 

 a certain number of days in each lunation, they are very distinctly observ- 

 able, from the currents which they occasion. In the West Indies, also, and in the 

 gulf of Mexico, the tides are less marked than in the neighbouring seas, perhaps 

 on account of some combinations derived from the variations of the depth 

 of the ocean, and from the different channels by which they are propagated. 



In order to understand the more readily the effects of such combinations, 



we may imagine a canal, as large as the river of Amazons, to communicate 



at both its extremities with the ocean, so as to receive at each an equal series 



of tides, passing towards the opposite extremity. If we suppose the tides to 



enter at the same instant at both ends, they will meet in the middle, and 



continue their progress without interruption : precisely in the middle the 



times of high and low water belonging to each series will always coincide, 



and the effects will be doubled; and the same will liappen at the points, 



where a tide arrives from one extremity at the same instant that an earlier 



or a later tide comes from the other ; but at the intermediate points the 



effects will be diminished, and at some of them completely destroyed, where 



the high water of one ti^e coincides with the low water of another. The 



tides at the port of Batslia in Tonkin have been explained by Newton from 



considerations of this nature. In this port there is only one tide in a day ; 



it is high water at the sixth lunar hour,or at the moon's setting, when the moon 



has north declination, and at her rising, when she has south declination; 



and when the moon has no declination there is no tide. In order to explain 



this circumstance, we may represent the two xmequal tides which happen in 



succession every day, by combining with two equal tides another tide, in- 



