Part II 

 TIDES AND TIDAL CURRENTS 



Chapter VII 



Principal Features of Tidal 

 Phenomena 



1. Review of the Phenomena 



One of the most regular and important phenomena is the tides. They manifest 

 themselves by the rhythmic rise and fall of the water, mostly twice a day. 

 It is evident that these vertical displacements of the surface are accompanied 

 by horizontal displacements of the water masses, called tidal currents. The 

 vertical and horizontal displacements are different manifestations of the same 

 phenomenon. The rhythmic rise and fall of the sea is only occasionally dis- 

 turbed by atmospheric processes. Sometimes these disturbances are quite 

 strong (storm surges), but always does the regularity of the phenomenon 

 return after the meteorological influence has ceased. The perpetuity and 

 regularity of the tides makes an attentive observer realize that a force con- 

 stantly present, is the cause of the tidal phenomena and governs their course. 

 The fact that on all shores of the oceans and adjacent seas these periodical 

 movements are of the same nature, although different as to their extent, 

 shows that tides are a worldwide phenomenon and must be governed by 

 a system of forces which acts everywhere in the same manner and to which 

 all the water-masses of the earth are similarly subjected. Such a system of 

 forces can only be of a cosmic nature, i.e. it must be related to the position 

 occupied by the earth in space. 



The basic phenomenon is the periodical rise and fall of the water surface, 

 which occurs twice in somewhat more than a day. The highest water of a tide 

 is called high water, the lowest water low water, the rise of the water is 

 designated as flood, the fall as ebb. The difference in height between low 

 water and high water is called the range of the tide. As the successive high 

 waters are as different as the successive low waters, one can distinguish a rise 

 of the water during the flood and a fall of the water during the ebb. The 

 arithmetical mean of both is the mean range of the tide. 



The average interval between two successive high waters or between two 

 successive low waters is 12 h and 25 min. Consequently, the high and low 

 water is retarded from one day to the next by 50 min. This points to re- 

 lationship with the motion of the moon, because the moon also is retarded 

 daily by 50 min on its passage through the meridian. 



