14 U. S. COAST AND GEODETIC SURVEY 



Mean tide level. — Mean tide level, also known as half tide level, is 

 a plane midway between the mean of all high waters and the mean of 

 all low waters. In the reduction of the long series of observations at 

 the navy yard the mean tide level was obtained for each year of obser- 

 vations, and the results are given in Table 7. The mean tide level for 

 the entire series of 39 years was found to be 20.12 feet above the zero 

 of the tide scale. In Table 8 the mean tide level for each year is 

 referred to the sea-level datum. 



Spring and neap tide level.— Aside from the irregular fluctuations 

 due primarily to meteorological conditions, there appear to be small 

 variations in the mean tide level due to change in the phase of the 

 moon. From the phase reductions for the navy yard covering a period 

 of 30 years, from 1847 to 1876, the spring tide level, or average level at 

 the time of the spring tides, was found to be 0.04 foot below the mean 

 tide level, and the neap tide level, or average level at the time of the 

 neap tides, was 0.04 foot above the mean tide level. 



Mean sea level.— The mean sea level is obtained by taking the aver- 

 age of the hourly heights of the tide over a considerable period of 

 time. For places near the coast mean sea level thus obtained differs 

 very little from the mean tide level derived from the high and low 

 waters. The records for three years — 1868, 1869, and 1871 — at the 

 navy yard were reduced to obtain mean sea level, and the average 

 elevation of mean sea level above mean tide level for these years was 

 0.16 foot. Adding this difference to the mean tide level as obtained 

 from the 39 years of observations, we have 20.28 feet as the elevation 

 of mean sea level above zero of the original tide scale. A discussion 

 of the sea-level datum will be found on page 41 under "Tidal datums." 



Harmonic constants.— Tidsil harmonic constants from an harmonic 

 analysis of the hourly heights of the tide at the navy yard have been 

 determined from a series of 369 days, beginning January 1, 1869. 

 The processes used in the analysis are described in Special Publication 

 No. 98 of the Coast and Geodetic Survey. The harmonic constants 

 from this series of observations are given in Table 28. For many 

 years, until the more recent series of observations was started in 

 South Boston, these constants were used in making the tidal predic- 

 tions for Boston for the annual tide tables of the Coast and Geodetic 

 Survey. 



Ages of the tide. — Inequalities in the tide resulting from changes 

 in astronomical conditions will generally follow the assumed cause by 

 several hours in time. The time "that elapses between the cause and 

 the maximum effect on the tide is called the age. 



The following ages have been derived from the harmonic constants 

 for the navy yard: The phase age, or interval by which the spring 

 and neap tides follow the new and full moon or the quadratures, 38 

 hours; the parallax age, or interval by which the perigean and apogean 

 tides follow the corresponding positions of the moon, 57.7 hours; and 

 the diurnal age, or interval by which the tropic tides follow the maxi- 

 mum north and south declination of the moon, 19.1 hours. 



TIDE OBSERVATIONS, COMMONWEALTH PIER NO. 5 



A primary tide station was established on Commonwealth Pier No. 

 5, South Boston, on May 3, 1921. The float well is a 12-inch iron pipe 

 with a conical casting containing a ^-inch hole at the bottom for the 

 access of water. The pipe is about 23 feet long to the intake, and 



