TIDES AND CURRENTS IN BOSTON HARBOR 



27 



however, that the observations at station D46 were made many years 

 ago, and it is not impossible that changes such as the recent improve- 

 ment in the North Channel may have materially affected the current 

 in this locality. 



Of Moon Head. — From an examination of Table 40 and by refer- 

 ence to Figure 10 it will be noted that in the passage between Moon 

 Head and Long Island, and also between Thompson and Spectacle 

 Islands, the results from the recent observations in 1926 differ materi- 

 ally from those of the earlier series in regard to the time of the turning 

 of the current. The relation of the current from observations of 1848 

 at station D47 with the current in 1926 at stations Wll and W20, and 

 also with the current at the principal current station W16, is illus- 

 trated in Figure 13. In the earlier observations the phase of the cur- 

 rent corresponded approximately with those for the harbor generally 



Fig. 13.— TYPICAL CURRENT CURVES FOR STATIONS D47, Wll, W20, AND W16 



but in the later series the times of the phases have been advanced two 

 or three hours. 



As these passages connect the main current stream through Presi- 

 dent Roads with the current stream entering Quincy Bay through 

 Nantasket Roads, the currents through these passages are virtually 

 cross currents which would depend to a considerable extent upon the 

 relative volume of water passing through the main channels. Such 

 currents might be affected to a considerable extent by changes in the 

 configuration of the harbor. 



Condensed current data. — For convenience of reference there have 

 been compiled in Table 41 condensed current data for selected locali- 

 ties. The time differences refer to the current at the principal station 

 W16, off Deer Island Light at the entrance to President Roads. These 

 differences, which are given to the nearest five minutes, represent the 

 average differences for all phases of the current, including both the 



