National Geographic : 1898 Nov
474 WHAT IS THE TIDE OF THE OPEN ATLANTIC? inhalfalunar . . . day." Just as the hand that supports a pendulum may maintain its motion by a gentle lateral move ment, so the moon's attraction may apply a periodic impulse to a body of water deep and wide enough to oscillate in half a lunar day, and thus make its oscillations perpetual. Admiral Fitzroy,* in 1863, republished some suggestions of his own of earlier date, that the North Atlantic tides (among others) seemed better accounted for as an "'oscillation,as of water in a basin ; or a libration, as a mass of jelly," than as a progression of a southern tide wave. His argument points to irregularities in any system of cotidals, the absence of signifi cant tide in the Plata estuary, opening fairly to the supposed ocean tide, and the relation between times of high water on op posite shores. In the North Atlantic he found high water on the American shore fairly synchronous with low water in Eu rope. In 1879 Mr Henry Mitchellt pointed out that high tide is fairly synchronous from Newfoundland to Hatteras, omitting the Gulf of Maine. Moreover, along this outer coast flood tide current sets to southwest and ebb to northeast. These two facts and the phenomena of the Gulf of Maine are more intelligible on the hypothesis of an oscillating North Atlantic than on any other. The current would result from the northeast-southwest trend of the coast, confining an ocean oscillating east and west, a portion of the westward motion being resolved parallel to the coast. THE STATIONARY WAVE IN THE NORTH ATLANTIC It has been noted above that Dr Whewell's data of 1836 showed him that the American cotidals were imperfect. Though he did not redraw the line, he stated that the xii-hour cotidal should be nearer the coast, and Dr Bache drew it closely contouring from Nantucket to Hatteras and south. It is well established now that, omitting the Gulf of Maine and other enclosed areas, the tides are fairly synchronous from New Foundland to Florida. The great Atlantic oscillation belongs to the deep basin. Across the continental shelf, both east and west, the disturbance is trans mitted as a progressive wave. and of course delayed in trans mission. As a rough outline of the Atlantic basin, I have dotted in figure 1 the portions less than 2,000 fathoms deep, not that * Weather Book, Appendix on Tides. tAnn. Report U. S. Coast and Geodetic Survey, p. 175. 1Ann. Report U. S . Coast and Geodetic Survey, 1857.