106 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Theoretically, the number of components is 

 very large, but most of them are of such small 

 magnitude that for general purposes they may be 

 disregarded. Even in the prediction of tides by 

 the use of tide-predicting machines it is rarely 

 necessary to take into account more than 30 com- 

 ponents. For an understanding of the principal 

 features of the tide at any place the five principal 

 components described above are sufficient. 



The component tides are usually referred to 

 as harmonic components, and for convenience 

 the five principal harmonic components are listed 

 in table 1. 



Table 1. — Principal harmonic components of the tide 



By subjecting the tide observations at any 

 place to the harmonic analysis the amplitudes and 

 epochs of the harmonic components at that place 

 are determined. Collectively, the amphtudes and 

 epochs of the component tides at any place are 

 called the harmonic constants. 



Of the five principal harmonic components listed 

 in table 1 three have periods of about 12 hours, 

 while the remaining two have periods of about 

 24 hom-s. All components with periods of about 

 12 hours are called semidiurnal or semidaily com- 

 ponents, while those with periods of about 24 

 hours are called diurnal or daily components. 



Any component tide is represented by the 

 cosine curve and is completely specified by two 

 characteristics, namely, the amplitude and the 

 epoch. Figure 27 represents a component tide. 

 The amplitude is the maximum ordinate rep- 

 resented by BE or FC; hence, the amplitude of 

 a component tide is half the range of that tide. 

 The epoch is the time, in angular measure, elapsing 

 between the meridian passage of a hypothetical 

 tide-producing body and the high water of its 

 tide. In figure 27, the curve ABCD represents the 

 rise and fall of a component tide for a complete 

 period, A being the instant of meridian passage of 

 the particular hypothetical tide-producing body 

 considered. The height of this component tide 

 at any instant is measured vertically up or down 



Figure 27. — Component tide. 



from the line GH which represents the undis- 

 turbed sea level, while the time is measured along 

 this line from G to H. Hence, the distance GE 

 expressed as an angle, which is (GE-hGH) 360°, 

 is the epoch of the component. 



In this connection it should be noted that the 

 tide-producing forces depend on the relative posi- 

 tions of earth, moon, and' sun. Hence, at any 

 given instant the tide-producing forces are dis- 

 tributed over the earth in a regular manner vary- 

 ing with longitude and latitude. But the response 

 of the different seas to the same set of ..ide-pro- 

 ducing forces is so profoundly modified by the 

 hydrographic features of the different oceanic 

 basins that the tides at different places differ pro- 

 foundly. To understand the use of harmonic 

 constants in determining types of tide, it will be 

 of advantage to consider briefly the combination 

 of component tides. 



Suppose that at a given place conditions are 

 such as to bring about a daily and a semidaily 

 component of the same range. What is the nature 

 of the resulting tide? 



Obviously the two constituents may have var- 

 ious time or phase relations. In figure 28, three 

 cases are considered. The semidaily constituent 

 is represented by a dotted curve; the daily con- 

 stituent by a dashed curve. The height of the 

 resultant tide at any moment is clearly the sum of 

 the heights of the constituent tides at that 

 moment. In the figure the resultant tide is 

 indicated by the full line curve. 



In the upper diagram of figure 28, the two con- 

 stituents have such time relations that their low 

 waters occur at the same time, and the resultant 

 tide is one in which the inequality in morning and 

 afternoon tides is featured in the low waters and 

 is exemplified by the tide at Cedar Keys on August 



