Principal Features of Tidal Phenomena 



247 



moon the relative positions of sun and moon are such that the high water 

 produced by one of those bodies occurs at the same time as that produced 

 by the other; and so also with the low waters; these tides are called spring 

 tides and have a greater range than any others of the lunar month, and at 



| 5 





Fig. 104. Semi-monthly inequality in time and in range of high water. 



such time we have the highest high tides as well as the lowest low tides, and 

 the tidal range is at its maximum. The time of occurrence of spring tides 

 is called spring time. The interval between new or full moon and spring tides 

 is called the retard and can be regarded constant for any given place. The 

 minimum range (neap tides) occurs with a similar retard after the first and 

 last quarter of the moon (quadratures) at neap time. The time difference 

 between the meridian passage of full or new moon and the occurrence of 

 the spring tide is called the age of the tide and is given in days. In the lower 

 part of Fig. 104 the semi-monthly inequality in the range of the tide is 

 illustrated for an ideal locality, where the age of the tide is 1 day, the ampli- 

 tude (= one-half of the range of the tide) of a spring tide 3 m, of a neap 

 tide 1 m. The chart datum coincides for most European countries with the 

 mean low water of the spring tide. The height of the high water above chart 

 datum will, therefore, be at spring tide 6 m, neap tide 3£ m, that of low 

 water at spring tide (according to the definition) m, at neap tide 2\ m. 

 The mean sea level is in this case 3 m. 



The semi-monthly inequality, in height and time is introduced by the sun. 

 Apart from these two most important inequalities, there are still others which 

 can be obtained from the residual terms remaining after elimination of the 

 semi-monthly inequalities or from the harmonic analysis (see p. 299). The 

 more important inequalities of this kind are: 



(a) Monthly inequality. The range of the tide becomes a maximum soon 

 after the moon is in perigee and a minimum soon after she is in apogee. 



