314 



The Harmonic Analysis of Tidal Observations 



(like in the North Atlantic Ocean). Consequently the following rules can 



be applied for the computation of time and height of the high water: 



Time of the high water = true solar time of the culmination of the moon + 



+ lunitidal interval + correction for declination + correction for 



parallax + diurnal inequality — equation of time. 



Height of the high water = height of the high water above chart datum + 



+ correction for declination + correction for parallax -f- diurnal 



inequality. 



The corrections should only be taken for the moon whereas those for 



the sun being small can be neglected. The lunitidal interval is equal to 



the mean lunitidal interval + the semi-monthly inequality; likewise the 



height of the high water above chart datum = the mean height of the high 



water + the semi-monthly inequality. For a certain harbour tables and graphs 



are made up in advance, which facilitate considerably the actual computation. 



As an example, we refer to the curves of the lunitidal intervals for 



Portsmouth and Aden given in Fig. 126. The former harbour is characteristic 



14 16 18 20 22 2 4 6 6 10 12 

 • o ... • 



Hours after passage of moon through menoian, hr 



Fig. 126. Curves for the lunar tide interval and range for Portsmouth and Aden. (For 

 Portsmouth 28 h have been deducted (Darwin).)* 



for its semi-diurnal tides, in the latter the diurnal inequality is strongly notice- 

 able, wherefore a difference has to be made between day and night. Let us 

 try to find time and height of the high water in Aden, for instance, on 

 17 March 1889. At that particular date, the moon passes through the local 



* The curve for Aden, March must bz shifted upwards by 0-3 hours. 



