. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . thenshow the progress of the wave for each configuration of the tide-generators. But as yet the only cotidal maps made are those foethe mean eemi-diurnal tide, and only for the configuration of newand full moon. The knowledge of the tides is not very accuratethroughout the world, and therefore in the maps which we give itis assumed that the same interval elapses at all places between newand full moon and spring tide. At spring tide, as we have seen in (87) and (88),hj = (lI-)-M,)cos2(\(-^i),since A- fi,
. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . thenshow the progress of the wave for each configuration of the tide-generators. But as yet the only cotidal maps made are those foethe mean eemi-diurnal tide, and only for the configuration of newand full moon. The knowledge of the tides is not very accuratethroughout the world, and therefore in the maps which we give itis assumed that the same interval elapses at all places between newand full moon and spring tide. At spring tide, as we have seen in (87) and (88),hj = (lI-)-M,)cos2(\(-^i),since A- fi, becomes then ctjual to As a rough approximationspring tide occurs when the moons transit is at one oclock at nightor in the day. We only assume, however, that it occurs simultan-eously everywhere. Now let t bo the Crcenwich mean time of highwater, and I the E. long, in hours of the place of observation, then,the local time of high water being the time of the moons transitplus the interval, and local time being Greenwich time plus £.long., we hare T = /i/(7-<r) -/ = ,j/i-J +l,. 0. 6.—Cotidal lines of the world. where ;< is in acgrees. Therefore, if we draw over the ocean asuccession of lines defined by equidistant integral values of theGreenwich time of high wafer, and if we neglect the separation ofthe moon from the sun in longitude in twelve hours, the successivelines will give the motion of the semi-diurnal tide-wave in one hour. §41. Cotidal Zines of the World. No recent revisal of cotidal lines has been made with the aid ofthe great mass of tidal data which is now being accumulated, andwe therefore reproduce (fig 6) the chart of the world prepared bySir George Aiiy for his article on Tides and Waves. The partsof the world for which data are wanting are omitted. The Romannumerals upon the cotidal lines denote the hour in Greenwich timecf high water on the day of new or full moon. Airy remarks(§§ 575-584) that the cotidal lines of the North Atlantic are ac-c
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