81 



The tidal datum plane at Boston is mean low water, and the times 

 of high and low water are on the standard time of the 75th meridian. 

 At Sheerness the tidal-datum plane is mean low water of spring 

 tides, and the time is Greenwich time. 



The harmonic constants of the tliree principal semidiurnal com- 

 ponents at these stations are taken as follows from table V, paragraph 

 134. The last two columns show the phase and parallax ages, com- 

 puted from the epochs as indicated in equations (106) and (107). 



The Greenwich times of the moon's phases, apogee and perigee, in 

 January 1937 were, from the Nautical Almanac: 



The times of spring, neap, apogean and perigean tides at Sheerness 

 are immediately determined from these astonomical data by adding 

 the tidal ages. For Boston, they are similarly determined after 

 correcting the times for the difference in longitude by subtracting 5 

 hours. The times of these tides then are: 



These times are indicated in figures 32 and 33. 



148. Exceptionally high and low waters are to be anticipated when 

 the perigean and spring tides nearly coincide. Since the next succeed- 

 ing apogean tide occurs one-half of an anomalistic month, or a little 

 less than 14 days later, and the next succeeding spring tide one-half 

 a synodic month, or a little more than 14 days later, it follows that 

 when the tides after say the new moon are especially large, those after 

 the next (or preceding) full moon are not. 



