TIDES AND CURRENTS IN BOSTON HARBOR 61 



Table 20. — Mean tide level on tide staff, Coynmonwealth Pier No. 5 



The above heights are referred to the tide staff installed in September, 1922, the zero of which is 21.45 feet 

 below bench mark 7. 



Table 21. — Mean tide level referred to sea-level datum, Commonwealth Pier No. 5 



Above table is derived by referring the values in Table 20 to the sea-level datum defined on p. 41. 

 Table 22. — Mean sea level on tide staff. Commonwealth Pier No. 5 



The above heights are referred to the tide stafl installed in September, 1922, the zero of which is 21.45 feet 

 below bench mark 7. 



81772—28 5 



