TIDES AND CURRENTS IN BOSTON HARBOR 23 



The compass dial is free to move so that the needles may assume a 

 north-and-soiith direction. The velocity dial, through a system of 

 reducing: gears and parallel magnets, rotates with an anemometer 

 wheel which is actuated by the current. At intervals of 30 minutes 

 the electric lamp is automatically flashed and a picture is taken show- 

 ing certain numerals on the velocity and compass dials. The numeral 

 on the velocity dial indicates the accumulated motion of that dial 

 due to the movement of the current. The difference between two 

 successive readings, as interpreted by a rating table, gives the average 

 velocity of the current for the half-hour period. The numeral on the 

 compass dial indicates the direction of the current at the moment the 

 picture was taken. 



Stations occupied. — During this survey 56 current stations were 

 occupied in Boston Harbor, the locations being indicated in Figures 

 7 to 11. At stations Wl, W16, and W37 the observations covered 

 periods from 6 to 14 days. At the other stations the period of obser- 

 vations ranged from one-half to two days. 



In general observations were made at each station with both the 

 current pole and the Price meter, the latter being set at depths 

 approximately equal to two-tenths, five-tenths, and eight-tenths of 

 the depth at each station. These observations were usually made in 

 groups at half-hourly intervals, each group consisting of a pole obser- 

 vation and two meter observations at each of the depths, the direc- 

 tion of the current at each depth being obtained by the bifilar current 

 indicator. Supplementarv observations with the Pettersson meter 

 were obtained at stations' W2, W5, W9, WIO, W33, W49, and W54, 

 the results practically agreeing with those obtained by the Price 

 meter. 



Reduction of records. — Following the method generally employed 

 m the reduction of current observations, the velocities as observed 

 at approximate half-hourly intervals were plotted on cross-section 

 paper with the flood velocities above and the ebb velocities below the 

 zero line representing slack water. A smooth curve was then drawn 

 following the general line of the plotted velocities. From these curves 

 the times of slack water and the times and velocities of the current 

 at strength of flood and ebb were tabulated. 



The times of slack water and strength of current were then com- 

 pared with the times of high and low water at the primary tide 

 station at Commonwealth Pier No. 5, and the observed velocities 

 were reduced to mean velocity by comparison with the range of tide 

 at the primary tide station. The directions of the current as 

 observed on the pelorus were corrected for ship's head and the 

 variation and deviation of the ship's compass and are therefore 

 represented in true azimuth as reckoned from the north toward the 

 east. 



The reductions were made separatelj^ for each depth at which 

 observations were taken, and the results are given in Table 34. The 

 station numbers are as originally assigned in the survey, with the 

 letter " W" prefixed to indicate that the stations were those occupied 

 by Lieutenant Woodworth in 1926. 



Harmonic analysis.— The hourly velocities at stations Wl, W16, 

 and W37 were reduced by harmonic analysis. Station Wl is located 

 at the entrance to Nantasket Roads, Station W16 is at the entrance 

 to President Roads, and Station W37 is in the channel between 



