TIDES AND CURRENTS IN BOSTON HARBOR 25 



tides at Commonwealth Pier No. 5. The average velocity and 

 direction of the current for each hour after high water are given in 

 Table 36. The average times for the slacks and the strengths for 

 both series are referred to the tides at Commonwealth Pier No. 5, 

 and the velocity and direction at the strength of flood and ebb are 

 also given. The average velocity of the tidal current at strength of 

 flood and ebb is only 0.1 knot. This velocity, being derived from 

 resolved currents, grouped according to the number of hours after 

 high water, is less than would be obtained by selecting the maximum 

 velocities during the periods of flood and ebb regardless of tidal hour 

 or exact direction. The average velocity for both flood and ebb 

 current obtained by such a selection for a 29-day period was 0.3 knot. 



Set of current. — -In Table 37 there is given the set of the nontidal 

 current for each month of observations. This set is very irregular, 

 and the velocity is too small to be of practical importance. The 

 resultant set from the longer series is in an easterly direction with a 

 velocity of 0.02 knot. Because of the irregularity in the direction of 

 the set' from month to month, this velocity derived from the resolved 

 currents for the entire series is less than the average of the set for 

 each month taken regardless of direction. 



Wind reduction. — In order to determine the effect of the wind on 

 the current in this locality, the resolved currents were grouped in 

 accordance with the direction of the wind, and the results are given 

 in Table 38. The current due to the wind had an average velocity 

 from 0.06 to 0.19 knot in a direction usually somewhat to the right 

 of the direction toward which the wind was blowing. 



Harmonic analysis. — The resolved hourly velocities for a 221-day 

 series beginning June 9, 1926, were reduced by harmonic analysis, 

 separate analysis being made for the north and east components. 

 The resulting values for the principal components are given in Table 

 39. As the currents in the locality are chiefly in an easterly and 

 westerly direction, the values of the H's for the east component are 

 somewhat larger than the H's for the north component. The east 

 component of principal element M2 has an amplitude of 0.08 knot, 

 which is consistent with the velocity of the tidal current as given in 

 Table 36. 



SUMMARY OF SURFACE CURRENTS 



For the navigator the currents near the surface are of principal 

 importance, and for convenience the results for these currents from 

 the several surveys are brought together in Table 40 and grouped 

 according to location. For the purpose of this table averages have 

 been taken of the results from the pole observations and from the 

 meter observations to a depth of 14 feet, and the times of slack and 

 strength of current for all stations are referred to the tides at Common- 

 weallSi Pier No. 5, in order that they may be comparable with each 

 other.^^ 



The results from the observations at the various stations listed in 

 this table show a number of minor irregularities. Some of these may 

 be due to location. The current near the shore may differ consider- 

 ably from that in the middle of the channel, and the current at sta- 

 tions near each other may sometimes vary in time, velocity, and direc- 

 tion. Some of the irregularities may be due to changes in the con- 



