National Geographic : 1898 Nov
WHA T 18 THE TIDE OF THE OPEN A TLANTIC? 467 vertical motion. It is a stationary wave with a central node. As with a pendulum, successive oscillations are in the same period, but the period may be changed by changing the depth of water. If the nodal axis lies north and south, as when the east end of the vessel has been lifted, the motion of the water particles is simultaneously to the west, then simultaneously to the east. A fall on the east corresponds to a rise on the west, the amount of rise and fall depending on distance from the node and (much more) on local configuration. Stationary waves may be studied in a tumbler of water, and the experiment should be tried. THE EARLIER VIEW It is usual in tidal discussions to assume a general case of con venient conditions and come later to the real problem-the tides in the case of nature. The general case supposed was a sphere uniformly covered with water. The moon was considered to have the power of heaping up the waters at the points of the earth nearest to itself and farthest away. The deepening of the waters at these two points would be accompanied by a shallow ing around a circle equatorial to these points as poles. Thus the ocean would assume the shape of a prolate spheroid with longer axis always pointed at the moon. T he earth would always have its two high waters at its opposite points, with low waters between. In the mean 6h. 13m.-a half lunar day-would in tervene between high and low and between low and high. This slheroidal shell would seem to revolve about the earth with the moon, alternately elevating and depressing the water surface of any place. The first assumption to reject for the actual world is the earth's uniform envelope of ocean. The Atlantic is barred east and west by continents. The apices of a tidal spheroid can not come to this water body in a daily swing about the earth. When the moon is over the eastern border of the ocean it might heap the waters there in a tide that would accompany it in its apparent westward path across the ocean; but at the American continent this action must for the moment cease. Each ocean would see the birth and death of a tidal wave at its eastern and western bounds. Below the southern continents, in latitude 600, is a ring of continuous ocean, with tides probably simultaneous, 1800 apart.* This belt alone, then, conforms to ideal conditions. It is hard to * South Georgia and Aukland island, near this circle, are distant 9h. 15m. of longi tude; their tides differ in time 9h. 47m.