National Geographic : 1898 Nov
WHAT IS THE T1DE OF THE OPEN ATLANTIC? 475 the continental shelf attains anything like that depth, but the descent from the shelf on east and west falls rapidly to that fig ure. The ocean basin is thus slightly larger than the parts lelt white on this sketch. The Atlantic basin is seen to approach much nearer the Span ish and African coasts than the American or the English and Scandinavian. Sable island, east of Nova Scotia, lies close to the margin of the continental shelf and has its high water 6h. 28m. after high water on the west coast of Spain and about two hours before the actual American coast farther west, just as the Spanish coast has its tides earlier than the British isles and northern Europe generally, where a true progressive wave exists and travels across the shallow waters. This oceanic basin is so shaped and proportioned as to possess an oscillation period of half a lunar day, and twice a day the moon's attraction inclines its surface now east, now west. The figures for Sable island and Spain show that low water on the east coincides with high water on the west. As the ocean basin is not bounded by straight lines, every tongue of deep water that advances among shallows toward the land transmits the tidal impulse synchronously with the swaying of the Atlantic. In the shallows progressive waves carry the impulse further. Whole bays respond to the oceanic movement, and only in exceptional areas can cotidals be truly drawn. The Irish channel in Whewell's second chart and the Gulf of St Lawrence well illustrate the limitations of the cotidal. The great coastwise ebb and flow of the Atlantic currents govern the long lines of bars and sand islands of the eastern United States. It is noteworthy that the so-called Atlantic ridge, really a broad, gentle swell, must occupy about the same position as the node of the ocean oscillation. One is tempted to speculation on possible accumulations of finest ocean silts in this stiller axis of the swaying mass through the long ages of geologic time. One may wonder again if the moon's periodic impulse does not for bid a departure of the ocean basin from the form demanded for an oscillation in harmony with lunar time-in other words, whether the moon may not have contributed to the permanence of oceanic basins in governing oceanic tides. The tide must re sist any attempt to change its period.