National Geographic : 1898 Nov
WHAT IS THE TIDE OF THE OPEN ATLANTIC? 473 cotidal line is to be regarded as the crest-line of a great wave, sweeping from shore to shore, as it might be seen by an eye far above the earth. The characteristic feature of such a wave is that every point of the ocean is regarded as first rising, then falling. Such was probably Whewell's conception, and it is wide spread today; yet with the abundant data of today it is not pos sible to comprehend how a progressing wave should adapt itself so completely to the shores as is found to be the case. An ad vancing wave would doubtless tend to adjust itself to the shores of an estuary, but the adjustment observed is more than a tendency. Opposed to this conception is that of a stationary wave, con ceived to have a medial point without vertical motion, called a node. Contemporaneous with a rise of water on one side of this node occurs a fall on the other. For the ocean there is no pro gression of high water; the whole water body'swashes alternately east and west. For an ocean to oscillate about a node in adjust ment to the moon's apparent motion is only possible with a given relation between depth and width. By counting the os cillations in 5 or 10 seconds with various depths of water in a bowl or tumbler, the reader may satisfy himself that for each combination of width and depth there is a constant period of oscillation. If the North Atlantic has such an oscillation in a period of a lunar half day, it must have the width and depth that correspond. GROWTH OF THE LATER VIEW The first suggestion of such an oscillation was by Young:* " We may therefore consider the Atlantic as a detached sea about 3,500 miles long and 3 miles deep." The depth he assumes from theoretical considerations. He considers that the wave from the southern ocean might meet the local oscillation about Gibraltar, when it would doubtless superpose itself upon it. The moon's relation to the motion of the detached ocean is thus suggested by Dr Young : t " The oscillations of the sea, . . . consti tuting the tides, are subject to laws exactly similar to those of pendulums capable of performing similar vibrations in the same time and suspended from points which are subjected to . . regular vibrations, of which the . . . periods are completed * Natural Philosophy, vol. 1, p. 581. t Nicholson's Journal, 1813, August, p. 217.