Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/200

the floor of the deep sea begins, after which the normal value appears again at greater distance from the coast-line. The occurrence of this disturbance of gravity can be represented in somewhat the following way. The observer on the land, who inland has obtained normal values, obtains a maximum at the coast, because he approaches the heavy sima of the ocean-floor, which lies obliquely beneath him. To be sure, this excess of gravity should be counterbalanced by the fact that the uppermost 4 km. are replaced by the less dense sea-water.
Fig. 37.—Disturbance of gravity on the margin of a continent, after Helmert. But these layers lie by the side of and not beneath the observer, and thus instead of reducing gravity again to its normal value, cause a deviation of the plumb-line, in the nature of an attraction of the plumb by the continental platform. The observer on the sea, on approaching the coast, experiences the reverse. The amount of gravitational force is affected by the diminution of mass beneath him; the increase of mass on the land side of him cannot, however, influence the amount, but only the direction of the force of gravity, so that a minimum of gravity results.