128 THE OCEAN. 



the level of the sea in certain straits, as well as in the gulfs of 

 Esthonia and Finland.* 



The laws of the phenomena of the mouths of rivers differ entirely 

 in the seas w^ith strong tides, as the northern Atlantic, and in those 

 with insensible oscillations, like the Baltic and the Mediterranean. 

 In the estuaries where the sea rises regularly twice a day to a great 

 height, it passes over every obstacle, bars, or sand-banks accumulated 

 at the entrance to the mouths of rivers ; while in those places where 

 the level of the sea remains always the same, the dikes of mud or 

 sand deposited parallel to the coasts between the fresh and salt 

 waters, always close the entrance to the river. Thus the Kio Mag- 

 dalena, and the Arato, in the Antilles ; the Rhone, the Po, and the 

 Nile in the Mediterranean, spread their liquid mass over bars which 

 are often hardly a yard at the lowest part ; f while the river of 

 the Amazons, the St. Lawrence, the Gironde, and the Thames, allow 

 free passage to ships at all hours. 



This diversity of fluvial laws, according to the height of the 

 oscillations of the tide, has the most important consequences for 

 the commerce of regions watered by great rivers. In general 

 the ports of the rivers without tide cannot be established at the 

 mouth itself, because of the want of water, and merchants are obliged . 

 to choose a locality situated on the sea- coast at a certain distance 

 from the sandy mouths of the river for their emporiums. Thus 

 Marseilles, where almost all the commerce of the great basin of the 

 Hhone is transacted, is constructed on the shores of a deep bay of 

 the Mediterranean, far from the peninsulas of mud between which 

 the river discharges itself. Alexandria, the great port of the Egyp- 

 tian delta, lies to the west of the alluvial delta of the Nile ; Venice 

 is far from the mouths of the Po ; Leghorn protects its port from 

 the approach of the Arno ; Barcelona is not at the entrance to the 

 Ebro ; and Carthagena in the West Indies and Santa Maria are only 

 in communication with the great Magdalena by means of hardly 

 navigable canals. The exceptions to this rule are not very numer- 

 ous, still we may cite Dantzig on the Vistula, Stettin on the Oder, and 

 Galatz on the Danube. J 



In seas with high tides the principal ports are found, on the con- 

 trary, not on the maritime coast-line, but on the rivers, and even 

 at a certain distance from the mouth, not far from the place where 



* Yon Sass, Bulletin deV Academic de St. Petershourg, t. viii. 6. 



t See in Vol. I. the chapter entitled, Hirers. 



I Ernest Dcsjardins, de rembouchure du Rhone. 



