Ocean and Oceanography

In addition to these seas notice must be taken of the sub ordinate marginal features, such as gulfs and straits. Gulfs may be classified according to their origin as due to fractures of the crust or overflowing of depressed lands. The former are either the ex tensions of oceanic depressions, e.g., the Arabian sea, or such caldron-depressions as the gulf of Genoa or rift-depressions like the Gulf of Aden. Compound gulfs are formed seawards by fracture and landwards by the overflowing of depressed land, e.g., the Bay of Biscay, Gulf of Alaska and Gulf of the Lion. Gulfs formed by the overflowing of depressed lands lie upon the con tinental shelf, e.g., the Gulf of Maine, Bay of Fundy.

Straits have been formed (I) by fracture across isthmuses, longitudinally as in the Strait of Bab-el-Mandeb, or transversely as in the Strait of Gibraltar; (2) by erosion, e.g., the Strait of Dover; (3) by overflowing subsided land, as in the straits of Bering, Torres and Formosa.

Surface of the Ocean.

If the whole globe were covered with a uniformly deep ocean and if it were of uniform density, the surface would form a perfect ellipsoid of revolution. At any point a sounding line would hang in the line of the radius of curvature of the water surface. But the water surface is broken by land, and the mean density of the substance of the land is 2.6 times as great as that of sea-water, so that the gravitational at traction of the land must necessarily cause a heaping up of the sea around the coasts, forming what has been called the continental wave, and leaving the sea-level lower in mid-ocean. Hence the geoid or figure of the sea surface is not part of an ellipsoid of rotation but irregular. The differences of level between dif ferent parts of the geoid have been greatly overestimated in the past ; F. G. Helmert has shown that they cannot exceed 200 metres and are probably much less. Recent pendulum observa tions have shown great local differences in density in the earth's crust, a marked deficiency of mass under high mountains and excess under the bed of the ocean. There is thus a condition of isostasy or equilibrium between them as a general rule. The in tensity of gravity at the surface of the sea far from land has been measured on several occasions. The inequalities of the geoid are not over ioo metres. Distortion of the ocean surface may also arise from meteorological causes and be periodic or unperiodic in its occurrence, but it does not amount to more than 1 or 2 metres. Atmospheric precipitation poured into the sea by the

great rivers must necessarily create a permanent rise of the sea level at their mouths, and from this cause the level round the coasts of rainy lands must be higher than in mid-ocean. The level of the Skagerak should be about o.6 metre higher than that of the open Norwegian sea between Jan Mayen and the Lofoten islands, the level of the Gulf of Finland at Kronstadt and of the Gulf of Bothnia at Haparanda should similarly be 0.4 metre higher than that of the Skagerak. The level of the Mediterranean has also been determined by exact measurements to be from 0.4 to 0.6 metre lower than that of the Atlantic on account of evaporation. The atmosphere affects sea-level also by its varying pressure, the difference in level of the sea surface from this cause between two given points being 13 times as great as the difference between the corresponding readings of the mercurial barometer. In the north tropical belt of high pressure south of the Azores the atmospheric pressure in January is 2 2MM. higher than in the Irminger sea; hence the sea-level near the Azores is almost 0.3 metre lower than in the northern sea. The Baltic is fuller in the time of the summer rains than in winter, when the rivers and lakes are frozen and most of the precipitation on the land is snow. A similar range occurs on the Dutch coast in the North sea, where the maximum level is reached in October, the month of highest rainfall, and there is a range of 200MM. to the minimum level at the time of least rainfall in early spring.

In the monsoon regions the half-yearly change from on-shore to off-shore winds produces noticeable differences in level, the total range being 237mm. The influence of wind on water-level is most remarkable in heavy storms on the flat coasts of the North sea and Baltic, when the rise may amount to very many feet. In the region of tropical hurricanes the converging wind system of a circular storm causes a heaping up of water capable of devastating the low coral islands of the Pacific. The old speculations as to a great difference of level between the Mediterranean and the Red sea, and on the two sides of the Isthmus of Panama, have been proved by precision levelling to be totally erroneous.