112 THE OCEAN. 



estuary, are due almost wholly to the regular breezes and the tem- 

 pests, which depress the waves on one side and raise them on the other. 

 Then, too, as the land winds generally predominate during the morn- 

 ing, and are replaced in the evening by the sea-breezes, the ebb and 

 flow, obedient to the alternating impulses of the atmosphere, suc- 

 ceed each other every twelve hours ; the tide rises in the afternoon and 

 falls the next morning.* This apparent anomaly is easily explained 

 by the meeting of high and low water at the entrance of the estuary. 

 The tidal waves which flow to the south on the Brazilian side, and to 

 the north on the side of Patagonia, do not strike the coasts at the same 

 instant daily. They follow each other at an interval of several 

 hours, and the lateral currents which diverge from them succeed one 

 another at the mouth of the estuary of La Plata, so as to maintain the 

 liquid mass at nearly the same level. At the moment when the ebb 

 of the northern tide is about to occur, the southern flow takes 

 place, the pressure of which, exercised in the contrary direction, 

 prevents the waters from falling ; then when a new tide from the 

 coasts of Brazil presents itself, the surface of the sea is already 

 lowered in the southern latitudes. The swellings would intersect 

 each other, and on the line of interference the water would be subject 

 to no oscillations. 



It is probable that to causes of a similar kind we must at- 

 tribute the formation of those diurnal, and always very slight, tides 

 which occur at the mouth of the Mississippi, on the coasts of New 

 Ireland, at Port Dalrymiile in Tasmania, to the south of Australia, 

 near King George's Gulf, in the Gulf of Tonquin, in the Bay of 

 Bahr-el-Benat, in the Persian Gulf, in the White Sea, and in many 

 other parts of the ocean. These slow changes of level, the ebb and 

 flow of which each lasts twelve hours, present, like ordinary tides, the 

 greatest diversity in their phenomena, according to the direction of 

 the winds and the currents, the respective positions of the sun and 

 moon, and the parts of the sea where this equiKbrium of the waters 

 is established. On the moving surface of the ocean, all the undula- 

 tions, whatever may be their cause, are mixed and confounded, and in 

 this ceaseless changing and mingling of the waves it is impossible 

 to discern, without long and patient research, the part taken by 

 each agent in disturbing the perfect repose of the sea-level. The 

 problem can be solved in a general manner only, without taking 

 account of details that have been as yet imperfectly observed. Thus, 

 it is known, that in the port of Vera Cruz, and on the neighbouring 

 * Martin de Moussy, Confederation Argentine^ t. i. p. 78. 



