Vol. XVII, No. 6 



WASHINGTON 



June, 1906 



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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 

 YI 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. 



