no A TEXTBOOK OF OCEANOGRAPHY 



passage by a certain interval of time. This interval varies 

 greatly for different ports, but is fairly constant for any given 

 port, and the time of high-water at any port on the day of new 

 or full moon is known as the vulgar establishment of the port. 



The establishment of a port is the interval between the time 

 of high-water and the preceding meridional passage of the 

 moon at new or full moon. Exact observation proves that 

 the interval between the moon's meridional passage and the 

 following high-water is not always the same, but alters with 

 the age of the moon. On the days of the four principal phases 

 of the moon it is the same, but from new moon to the first 

 quarter, and from full moon to the last quarter, high-water 

 occurs earlier, and in the other two quarters of the moon later, 

 than would be the case if the time-interval were invariable. 



There are other minor variations in the rise of the tide. In 

 many places the tides are during half the year higher in the 

 forenoon than in the afternoon, the reverse being true for the 

 other half-year. 



The character of individual tides is subject to great 

 irregularities. In bays, gulfs, estuaries, and other enclosed 

 sea areas, the time occupied by the flood-tide is much smaller 

 than the ebb, and this is more noticeable at springs than at 

 neaps. The peculiarities of the tides depend to a great extent 

 on the locality where the observations are made. On small 

 oceanic islands the rise is very small, seldom exceeding a 

 metre. In other localities apparently only one tide is notice- 

 able. In enclosed seas such as the Mediterranean and Baltic 

 the tides are so slight as to be only perceptible to careful 

 investigation. In narrow straits, bays, and estuaries, the 

 highest tides are attained. In certain extreme cases tidal 

 waves are formed. In long, narrow channels, such as estu- 

 aries, the flood-tide runs for less than an hour and a half, 

 while the ebb-tide runs over eleven hours. Such a sudden 

 rush of water is accompanied by a phenomenon known as a 

 "bore," which is occasionally a tidal wave of considerable 

 height and velocity. In narrow channels of this description 



