10 U. S. COAST AND GEODETIC SURVEY 



mark 2 between the years 1900 and 1916 and then settled 0.01 foot 

 relative to bench mark 2 or 0.04 foot relative to bench mark 3 between 

 the years 1916 and 1927, but these differences are too small to be 

 conclusive. 



Because of the uncertainty in regard to the exact amount of the 

 fluctuation in the elevation of bench mark 1 due to the causes 

 described and also for want of positive information concerning the 

 methods alwa3^s used for connecting the several tide staffs with the 

 bench mark, no attempt has been made to apply corrections for 

 these fluctuations, but the heights of the tide planes as directly 

 obtained are compiled in the tables which follow. The general 

 uniformity of the results indicate no considerable change in the 

 datum. 



Reduction of records. — Detailed results from each year of observa- 

 tions obtained from the reduction of the tide records for the navy 

 yard are given in Tables 1 to 8, and summaries of the tidal data are 

 included in Tables 25 to 28. The items contained in these tables are 

 explained in the following paragraphs: 



Lunitidal intervals. — The mean high-water lunitidal interval is the 

 average time that elapses between the transit of the moon over the 

 meridian of a place and the followmg high water. Similarly, the 

 mean low-water lunitidal interval is the tune between the transit of 

 the moon and the followmg low water. For brevity these intervals 

 are frequently designated sunply as the high and low water intervals 

 and are represented by the symbols HWI and LWI, respectively. 

 In general the tides are referred to either the upper or lower meridian 

 passage of the moon. The intervals usually vary from day to day 

 with changes in the phase, parallax, and declination of the moon and 

 sun, but the average over a considerable period of time is fairly constant 

 for any particular locality. 



The mean high-water interval is also sometimes called the "cor- 

 rected establishment of the port," and is approximately the same as 

 the "high water, full and change," which applies to the tide at the 

 times of new and full moon. At such times, however, the intervals 

 are usually 10 to 15 minutes greater than the mean. 



The mean high-water interval at the navy yard, as derived from 

 30 years of observations from 1847 to 1876, is 11.44 hours, or 11 hours 

 26 minutes, and the mean low-water interval for the same period is 

 5.23 hours, or 5 hours 14 minutes. The intervals as obtained for the 

 individual years will be found in Table 1 . The averages for the three 

 10-year groups are also given. 



Mean rise interval. — The mean rise interval, abbreviated MRI, is 

 the average time between the transit of the moon and the middle of 

 the period of the rise of the tide. It may be computed by adding the 

 half period of rise to the mean low-water interval, rejecting multiples 

 of the semidiurnal tidal period (12.42 hours) when desired. The 

 mean rise interval is convenient for comparing the time of the tides 

 at different stations when it is unnecessary to distinguish between 

 the high and low water differences. The mean rise interval for the 

 navy yard is 8.34 hours. 



Tropic lunitidal intervals. — Near the times of the maximum decli- 

 nation of the moon there is often considerable inequality both in the 

 intervals and in the heights of the two high waters and also of the 

 two low waters of each day. The term "Tropic tides" is applied to 



