National Geographic : 1898 Nov
WHAT IS THE TIDE OF THE OPEN ATLANTIC? the general course of a tidal wave advancing up the Atlantic at least as far as Gibraltar. Dr William Whewell took up the investigation in the thirties. From all the charts, sailing directions, and ocean pilots he could obtain, he computed cotidal hours for points all over the world, being the time of high water on the day of new or full moon. From these data he traced the progression of the tide up the Atlantic to the coasts of Europe and America, deriving it from the belt of ocean to the south. He published his cotidal chart in 1833.* He was fully conscious of the very crude data given him at times by observers who fancied the tides always occurred at the same hour, and he closed his first essay with the warning that the results were only tentative. Figure 1 reproduces the Atlantic portion of this chart. Dr Whewell was moved by this lack of good data to seek the cooperation of the admiralty to have careful observations made simultaneously at least about the British shores. He not only accomplished this, but was enabled in 1835 t to publish obser vations made according to his instructions at 666 stations in America and Europe, with two at the Cape of Good Hope, for every tide between the 8th and 28th of June of that year. The greater part of these were about the British isles, and for this region he published a revision of his chart. For the American coast he contented himself with pointing out some errors in his first chart. The rest of the chart he abandoned until a wide range of good observations should be at hand. DIFFICULTIES OF THE EARLIER VIEW Now defects in the general scheme of cotidals are defects in the theory of a wave progressing up the Atlantic from the south. These defects Whewell found to be based on (1) the extraordi nary manner in which the cotidals contour about the lands, together with the difficulty of including the oceanic islands in the system, and (2) the great difference of epoch of the diurnal wave in Europe and America, together with the identical epoch in Spain and at the Cape of Good Hope, supposed to be sepa rated by a long journey up the Atlantic. * Phil. Trans., 1833, p. 147. This chart is reproduced in numberless excellent works, though abandoned by its author in the first two years of its existence, and it is usually reproduced, even in America, without the correction the author indicated for the xiih. line on our coast. Thus in Young's General Astronomy, for instance, 1889, p. 287. j Phil. Trans., 1836, p. 289.