Page:Popular Science Monthly Volume 74.djvu/533

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A body of water resembling the artificial one just described is the portion of the Atlantic Ocean (shown by one form of shading) extending from South America to southern Greenland and resting against the western coast of Africa and Europe. A section of this basin or

 This diagram shows the binodal oscillation going on in the North Atlantic Ocean. When it is high water at either end, it is low water off Morocco and vice versâ. The depth of the southern portion being greater than the depth of the northern, a half wave-length in the former exceeds a half wave-length in the latter—the period in each case being very nearly 12 lunar hours.

system is shown by means of a diagram (Fig. 2). One nodal line passes near the Cape Verde Islands and another lies westerly from Ireland. When it is high water on the coasts of Guiana and Brazil, it is also high water around southern Greenland, and it is then low water along the coast of Morocco, Spain and Portugal. The Roman numerals upon the small map of the world show that high water occurs at eight o'clock, Greenwich lunar time, for the South American and Greenland ends of this basin and at two o'clock for the central or Morocco portion. On account of the extensive openings to the eastward and northward of this basin or system, progressive waves are formed which contribute to the tides around the British Isles and Arctic Archipelago, and are chiefly responsible for the tides of the Arctic Ocean. Since progressive waves can not arise suddenly, their effects are felt over a large portion of the system now under consideration, and they tend to obscure the theoretical nodal lines which cross it.

The tides along the Atlantic Coast of the United States are produced in the body of water which extends from this coast to the Antarctic Continent by way of Cape of Good Hope. This is shown upon the smaller chart of the world (Fig. 4) by one of the types of shading. The northern portion of this region is not greatly influenced by progressive waves because the openings through its northwestern, or United-States, boundary are not large. Consequently, the theoretical