328 OF TIDES. 



points the effects will be diminished, and at some of I hem com- 

 pletely destroyed, where the high water of one tide coincides with 

 the low water of another. The tides at the port of Batsha in Ton- 

 kin have been explained by Newton from considerations of this na- 

 ture. 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 everyday, 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 Iheir 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 sepa- 

 rately. Thus, in the port of Batsha, the greater tide probably ar- 

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

 the ninth from the gulf of Siam, haviug 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 incon- 

 siderable ; it sometimes amounts to as much as 13 feet. 



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



the tides, the attractions of the sun and moon are also supposed to 



occasion a retardation in its rotatory motion, in consequence of 



which it is left a little behind the solid parts of the earth ; and a 



current is produced, of which the general direction 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 Jn the course of many ages, and \viUi 



