' TIDES AND CURRENTS IN BOSTON HARBOR 11 



the tides at this time. From an harmonic analysis of the observations 

 for the year 1869, the following tropic intervals were obtained for the 

 navy yard: Tropic higher high-water interval — that is, the interval 

 referring to the higher of the two high waters of the day at the time 

 of the tropic tides — 11.36a hours; tropic lower high-water interval, 

 referrmg to the lower of the two high waters of the day, 11.58b hours; 

 tropic higher low-water interval, referring to the higher of the two 

 low waters of the day, 5.15a hours; and tropic lower low-water inter- 

 val, referring to the lower of the two low waters of the day, 5.46b 

 hours. The intervals marked "a" refer exclusively to the moon's 

 upper meridian passage at the time of its north declination and to 

 the lower meridian passage at the south declination; the intervals 

 marked "b" refer exclusively to the moon's lower meridian passage 

 at its north decimation and to the upper meridian passage at the 

 south declination. 



From an inspection of these tropic intervals, the sequence of the 

 tides is higher high water to lower low water followed by the lower 

 high and higher low water. 



Greenwich intervals. — The lunitidal intervals which are usually 

 computed and published refer to the transits of the moon over the 

 local meridian of the place where the tides have been observed. Such 

 intervals do not afford a direct means for comparing the time of the 

 tide at stations located in different longitudes but require certain 

 corrections for the difference in longitude. If the lunitidal intervals 

 for all stations were referred to the transits of the moon over some 

 standard meridian, such as the meridian of Greenwich, they would 

 afford a direct means of obtaining the actual difference in the time of 

 tide without further correction. In west longitude the Greenwich 

 intervals exceed the local intervals by an amount equal to the average 

 time elapsed between the transit of the moon over the meridian of 

 Greenwich and the transit over the local meridian. For the longitude 

 of the navy yard, and also for Commonwealth Pier No. 5 in Boston, 

 this is 4.90 hours. Applying this difference to the local lunitidal 

 intervals and rejecting multiples of the semidiurnal period when 

 desired, the corresponding Greenwich intervals are obtained. The 

 Greenwich intervals for mean high water, mean low water, and mean 

 rise of tide are included in Table 25. 



Duration oj rise oj tide. — The mean duration of rise of tide, abbre- 

 viated DR, is the average time required for the water to rise from its 

 minimum height at low water to its maximum height at high water. 

 Similarly the mean duration of fall is the average time required for 

 the tide to fall from high to low water. Since the average period 

 between two consecutive high waters or between two consecutive low 

 waters is the semilunar day, or 12.42 hours, the sum of the mean 

 duration of rise and the mean duration of fall must equal this amount. 

 The mean duration of rise for the navy yard from 30 years of obser- 

 vations is 6.21 hours, and the mean duration of fall is the same. 

 Therefore, the average time required for the tide to rise from low to 

 high water at the navy yard is the same as the average time required 

 for it to fall from high to low water. The duration of rise for indi- 

 vidual years will be found in Table 1. The average for three 10-year 

 groups are also given. 



High and low water heights. — The mean high and low water heights 

 at the navy yard for each of the 30 years of observations by the Coast 



