THE VARIETY IN TIDES ^ 



By H. A. Marme:r 

 United States Coast and Geodetic Survey 



The usual explanation of the tide in textbooks of physical geog- 

 raphy or astronomy makes of it a simple phenomenon. It is shown 

 that the gravitational attraction of sun and moon on the earth gives 

 rise to forces which move the waters of the sea relative to the solid 

 earth. It is further shown that these forces have different periods, 

 but that the predominant ones have a period of about half a day; 

 therefore there are two high waters and two low waters in a day. 

 And finally it is shown that the moon plays the leading role in 

 bringing about the tide, for the tide-producing power of a heavenly 

 body varies directly as its mass and inversely as the cube of its 

 distance from the earth. 



On the bases of these general considerations, numerous featui-es 

 of the tide can be explained. Thus, at the times of new and full 

 moon, when the tide-producing forces of sun and moon are in the 

 same phase, the tides are greater than usual, while at the times of 

 the moon's quadratures the tides are less than usual. In the same 

 way, when the moon is in perigee, or nearest the earth, greater tides 

 result than when the moon is in apogee or farthest from the earth. 

 To be sure, it is known that the tides at different places vary in 

 time, in range, and in other features. But these differences are 

 ascribed to modifications brought about by local hydrographic fea- 

 tures. And thus the whole phenomenon is seemingly reduced to 

 simple terms. 



To the navigator who is familiar with the Seven Seas, however, 

 this very much oversimplifies the subject. For he finds that the tides 

 at different places vary not only in time and in range, but also in 

 character of rise and fall. Quite apart from differences in time 

 and in range, which may be regarded as merely differences in degree, 

 it is found that tides present striking differences in kind. There is, 

 in fact, an almost bewildering variety in tides. 



Take for example the actual records of the rise and fall of the 

 tide at three such well-known places as Norfolk, Va., Pensacola, 



1 Reprinted by permission from the United States Naval Institute Proceedings, vol. 60, 

 no. 3, whole no. «73, March 1934. Copyrighted: United States Naval Institute. 

 Annapolis, Md. 



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