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DIFFERENCE IN TIDES. 125
a series of observations on the surface of the w-aters pursued during several years that the probable existence of 'a total variation of little more
than three inches between high and low water can be ascertained. Near
Stralsund the difference is only one and a half inches, and near Memel it
hardly exceeds an inch. The much more considerable variations which
occur in the- level of the sea arise from the winds, the currents, or the
pressure of the atmosphere. Rapid oscillations of nearly three feet have
been sometimes seen to occur; but these are the seiches, similar to those
of the Lake of Genoa.* The force of the winds alone is sometimes sufficient to lower by little more than three feet the level of the sea in certain straits, a* well as in the gulfs of Esthonia and Finland.f
The laws of the phenomena of the mouths of rivers differ entirely in the
seas with 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 clay to a great height, it parses 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 Rio Magdalena 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 ;J 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 can not 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 Rhone 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 Egyptian 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 Cartha-
gena, 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 numerous; still we may cite Dantzig on
the Vistula, Stettin on the Oder, and Galatz on the Danube.§
In seas with high tides the principal ports are found, on the contrary,
* See the chapter entitled Lakes.
t Von Sass, Bulletin de VAcademie de St. Petersbourg, t. viii., 6.
X See the chapter entitled Rivers.
§ Ernest Desjardins, De Vembouchure du Rhone.
| Title | The ocean, atmosphere, and life |
| Creator | Reclus, Elisée |
| Publisher | Harper |
| Place of Publication | New York |
| Date | 1873 |
| Language | eng |
| Type | Books/Pamphlets |
| Title | 00000140 |
| Type | Books/Pamphlets |
| Transcript | DIFFERENCE IN TIDES. 125 a series of observations on the surface of the w-aters pursued during several years that the probable existence of 'a total variation of little more than three inches between high and low water can be ascertained. Near Stralsund the difference is only one and a half inches, and near Memel it hardly exceeds an inch. The much more considerable variations which occur in the- level of the sea arise from the winds, the currents, or the pressure of the atmosphere. Rapid oscillations of nearly three feet have been sometimes seen to occur; but these are the seiches, similar to those of the Lake of Genoa.* The force of the winds alone is sometimes sufficient to lower by little more than three feet the level of the sea in certain straits, a* well as in the gulfs of Esthonia and Finland.f The laws of the phenomena of the mouths of rivers differ entirely in the seas with 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 clay to a great height, it parses 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 Rio Magdalena 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 ;J 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 can not 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 Rhone 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 Egyptian 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 Cartha- gena, 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 numerous; still we may cite Dantzig on the Vistula, Stettin on the Oder, and Galatz on the Danube.§ In seas with high tides the principal ports are found, on the contrary, * See the chapter entitled Lakes. t Von Sass, Bulletin de VAcademie de St. Petersbourg, t. viii., 6. X See the chapter entitled Rivers. § Ernest Desjardins, De Vembouchure du Rhone. |
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