VARIETY IN TIDES MARMER 185 



The varieties of tides considered above do not, however, exhaust 

 the variety in tides. In figure 3 are shown the records of the tides 

 at Galveston, Tex., and at Manila, P. I., for the same 4 days of 

 May 1931. At Galveston, for the first 2 days, there are 2 high and 

 2 low waters each day ; but for the last 2 days, there are but 1 high 

 and 1 low water a day. A characteristic feature of the tide during 

 these last 2 days is the long stand of the tide which begins about 4 

 hours after high water, when for a period of about 3 or 4 hours the 

 tide changes but little in height. At Manila, there is a somewhat 

 similar state of affairs; but whereas the stand of the tide here takes 

 place on the rising tide, at Galveston it takes place on the falling 

 tide. Furthermore, the stand of the tide at Galveston occurs above 

 sea level, while at Manila it takes place below sea level. 



On investigation it will be found that most tides can be referred to 

 one or another of the varieties discussed above. Furthermore, it can 

 be shown that these different varieties arise from different combina- 

 tions of two primary constituent tides. In developing the tide- 

 producing forces arising from the attraction of sun and moon, it is 

 found that these forces have different periods, the principal ones 

 being those having a period of a day and of half a day, respectively. 

 The daily tide-producing forces give rise to a tidal constituent hav- 

 ing a period of a day, and the semidaily forces give rise to a tidal 

 constituent having a period of half a day. And it is the varying 

 combinations of these two constituent tides that give rise to the dif- 

 ferent varieties discussed. An example will make this clear. 



Suppose that in a certain sea the tide-producing forces give rise to 

 daily and semidaily tides with different ranges. If the semidaily 

 constituent has much the greater range, it is clear that within that 

 sea the tide will be much like that at Norfolk, morning and afternoon 

 tides differing but little, and the tide Avill be of the semidaily type. 

 If the range of the daily constituent is much the larger the tide will 

 be like that at Pensacola with but one high and one low water in a 

 day, or of the daily type. 



But suppose that the two constituent tides have the same range, 

 what is the character of the resultant tide ? The rise and fall of each 

 of these constituent tides may be represented as in figure 4, the semi- 

 daily constituent by the dotted curve and the daily constituent by the 

 dashed curve. The height of the resultant tide at any moment is 

 then clearly the sum of the heights of the constituent tides at that 

 moment. In figure 4 the resultant tide is indicated by the heavy 

 full-line curve. 



Now the two constituent tides may combine in various ways in 

 regard to time. In figure 4 three cases are considered. In the upper 



