Page:Popular Science Monthly Volume 74.djvu/536

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The diminution of the mean range of tide in going westward from Portugal indicates the existence of a no-tide point westward from the Azores. Other no-tide points occur in European and American arms of the Atlantic Ocean. The no-tide point between Holland and England was first pointed out by Dr. Whewell and the one in the Adriatic Sea by Dr. Sterneck.


Indian-Ocean Tides

The Indian Ocean contains two strips of water well suited to the production of tides. The first extends from the northwestern coast of Australia to the coast of Somali Land and Arabia, the second, from the southern coast of Australia to the southern coast of Africa and Madagascar—the southern edge of this strip resting upon the Antarctic Continent. The tides of the Bay of Bengal depend upon those generated in the first strip, while the horizontal shading in the area between Madagascar and Hindustan, taken in connection with the tidal hours, indicates that an oscillation exists in this area which depends chiefly upon the rise and fall of the tide at the southern end of Mozambique Channel.

The stationary character of the tide in the Bay of Bengal is shown not only upon the small chart of the world, but also upon the chart of cotidal lines which covers the Indian Ocean (Fig. 6). A vertical section is shown by means of a diagram (Fig. 3).

The distance southward from the southern coast of Australia to the Antarctic Continent is less than a half wave-length of the lunar tide. It is more nearly equal to a half wave-length of the solar tide. Consequently the tides due to this oscillation are chiefly solar, and the tidal hours are enclosed in parentheses for the sake of distinction.

Fig. 6 shows that a great diversity of ranges of tide occurs in the Indian Ocean. Between Ceylon and Sumatra the mean range of tide is about one foot, while at the head of the Gulf of Martaban the range is nearly 14 feet. In the mouths of the Ganges the mean range is nearly 10 feet. At the southern extremity of Hindustan Peninsula the mean range of tide is about 1.5 feet while at the head of the Gulf of Cambay it is 23 feet. On the western coast of Madagascar the range of tide is 8 or 10 feet, while at Tamatave, upon the eastern coast, it is 1.5 feet. At Freemantle, near the southwestern corner of Australia, the range of the semidaily tide is 0.4 foot, while at Collier Bay and King Sound on the northwestern coast the mean range of tide is about 24 feet.

Between Madagascar and Hindustan is a no-tide point where the rise and fall of the semidaily tide vanishes.


Pacific-Ocean Tides

The tides in the Pacific Ocean are produced by two systems of oscillations which are distinguished from each other by the two kinds of