246 Principal Features of Tidal Phenomena 



The average duration of two tides is, therefore, 24 h and 50 min, and 

 the duration of one tide corresponds on the average exactly to half an 

 average lunar day. This fact led to relate each meridian passage of the moon 

 to the subsequent high and low water respectively. The difference in time 

 between the time of high water and the transit remains constant, and this 

 is a characteristic value for each locality. It is called the mean high water 

 lunitidal interval and it measures, in hours, the mean time difference between 

 the transit of the moon and the occurrence of the next high water. Similarly, 

 the low water lunitidal interval gives the average number of hours and 

 minutes between the transit and the time of low water. The establishment 

 is the (apparent) local time of high water occurring at new or full moon, 

 or the high water lunitidal interval when the transit (just preceding the tide) 

 occurs at noon or midnight. At full moon or at new moon (syzygies) the 

 moon and the sun pass through the meridian simultaneously and, as the 

 civil time is counted from the passage of the sun through the meridian, the 

 lunitidal interval indicates at what time, at full or new moon, high water 

 occurs at a given locality. 



An exact comparison of the intervals between the transits and high or 

 low water shows that, in general, these intervals deviate from the mean 

 high-water interval by amounts of ±1 h. When the sun's tidal effects shorten 

 the lunitidal interval, causing the tides to occur earlier than usual, there is 

 said to be a priming of the tide; when from the same cause, the interval is 

 larger than usual, there is said to be a lagging. However, these deviations 

 are not arbitrarily distributed; they return at regular intervals of about half 

 a month. Therefore, this is called the semi-monthly inequality in the time 

 of high water, to which corresponds an identical inequality in the time of 

 low water. It is easy to compare these inequalities with each single transit 

 of the moon and to establish that both values are correlated. The semi- 

 monthly inequality is a function of the phase of the moon. The mean semi- 

 monthly inequality is the average value of the inequalities during one year. 

 This mean semi-monthly inequality depends on the phase of the moon, and 

 its period is one-half of a synodic month (29-53 days) = 14-77 days. The 

 upper part of Fig. 104 shows, for an idealized locality with a mean high 

 water lunitidal interval (establishment) of 6 h, this fortnightly inequality. 

 The numbers at the abscissa mean days after new moon (black circle). We 

 can summarize as follows: 



High water and low water follow the upper and lower transit of the moon 

 in definite average intervals, which can differ according to localities. However, 

 each interval for a place, has different semi-monthly inequalities and their 

 main period is a half of a synodic month, which shows that these inequalities 

 are related to the phase of the moon. 



Neither is the range of the tide a constant quantity, and it varies with 

 the same period of a half of a synodic month. At the times of new and full 



