ON THE TIDES. 449 



distinctly observable, 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 communi- 

 cate 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 coin- 

 cide, and the effects will be doubled ; and the same will happen 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 tide coincides with the low water of another. 

 The tides at the port of Batsha 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 unequal 

 tides which happen in succession every day, by combining with two equal 

 tides another tide, independent of them, and happening only once a day ; 

 then, if a point be so situated in the canal which we have been considering, 

 that the effects of the two equal semidiurnal tides may be destroyed, those 

 of the daily tides only will remain to be combined with each other ; and 

 their joint result will be a tide as much greater than either, as the diagonal 

 of a square is greater than its side ; the times of high and low water being- 

 intermediate between those which belong to the diurnal tides considered 

 separately. Thus, in the port of Batsha, the greater tide probably arrives 

 at the third lunar hour directly from the Pacific ocean, and at the ninth 

 from the gulf of Siam, having passed between Sumatra and Borneo ; so that 

 the actual time of high water is at the sixth lunar hour. The magnitude of 

 this compound tide is by no means inconsiderable ; it sometimes amounts 

 to as much as 13 feet. (Plate XXXVIII. Fig. 523, 524.) 



Besides the variations in the height of the sea, which constitute the tides, 

 a current is observed in its most exposed parts, of which the general direc- 

 tion is from east to west. This current comes from the Pacific and Indian 

 oceans, round the Cape of Good Hope, along the coast of Africa, then 

 crosses to America, and is there divided and reflected southwards towards 

 the Brazils, and northwards into the Gulf stream which travels round the 

 gulf of Mexico, and proceeds north eastwards into the neighbourhood of 

 Newfoundland, and then probably eastwards and south eastwards once 

 more across the Atlantic. It is perhaps on account of these currents that 

 the Red Sea is found to be about 25 feet higher than the Mediterranean : 

 their direction may possibly have been somewhat changed in the course of 



2G 



