National Geographic : 1906 Jun
VOL. XVII, No. 6 WASHINGTON JUNE, 1906 LIE J HK1P IPBL COTIDAL LINES FOR THE WORLD' By R. A. HARRIS U. S. COAST AND GEODETIC SURVEY BEFORE calling attention to the accompanying charts, which are supposed to represent the lines of simultaneous high water at each hour and half hour of Greenwich lunar time, it seems well to remark that many sys tems of lines could be laid down which would satisfy all reliable data; for, with very few exceptions, observations away from land have never been attempted. In fact the lines could generally be drawn across the land instead of the water, the only requirement being that they be so numbered as to agree with the known times of tide along the coasts. On account of the difficulties connected with the mechanism of the tides, and the want of sufficient data, it has heretofore been possible to incorporate but little rational theory into the charts of cotidal lines covering oceanic depths. The object of this paper is to give a general idea of a system of cotidal lines for semi-daily tides so constructed as to agree closely with observational data and tolerably well with rational the oretical considerations. Numerous cotidal charts covering various parts of the world, including those here shown, together with a more detailed account of the tides represented upon them, consti- tute an appendix to the report of the Coast and Geodetic Survey for the year 1904. Upon referring to the chart of the world (Supplement) it will be seen that even for oceans where the depth is fairly uniform the cotidal lines are in some places crowded together and in other places spread apart. The range of tide also undergoes great changes in value, as will be noted later on. One nearly simultaneous region whose tidal hour is XII extends easterly from the Atlantic coast of the United States, the range of tide decreasing from about 4 or 5 feet along this coast to one foot on the northeastern coast of Porto Rico. In going southeasterly from this island the time of tide changes rapidly, and the range of the semi-daily tide is less than one foot throughout the greater portion of the Lesser Antilles. If the tide of this region forms part of a stationary wave of which the United States is an end boundary or loop, the Greater Antilles a lateral boundary, and whose nodal line lies easterly from Porto Rico, we ask at once, Where are the other loops, lateral boundaries and nodal lines? Is there a VI hour region which can be associated with the XII hour region? When it is *An address to the Eighth International Geographic Congress, recently held in the United States.