28 TJ. S. COAST AND GEODETIC SURVEY 



slacks and strengths of the current, the minus sign indicating that the 

 current phases occur earlier than at the principal station and the plus 

 sign that they occur later. The strength of flood interval is the average 

 interval between the transit of the moon over the local meridian and 

 the time of the following strength of flood. The direction of the 

 strength of flood is given in degrees as reckoned from the true north by 

 way of the east. The flood and ebb velocities represent the average 

 velocities at the strength of flood and ebb. The observations at the 

 current stations Hsted in the last column furnished the basis for the 

 data given in this table, but small adjustments have been made, and 

 the results are given in rounded numbers. 



Current charts. — Currents in Boston Harbor are schematically 

 represented for each hour of the tide at Commonwealth Pier No. 5 in 

 Figures 14 to 25, which are based primarily upon the observations of 

 1926. The general direction in which the curreqt is flowing is repre- 

 sented by arrows and the average velocities in knots and tenths of 

 knots by small figures near the arrows. At the times of spring and 

 perigean tides the velocities will usually be greater and at the times 

 of neap and apogean tides less than indicated. Winds and other 

 meteorological conditions may also affect both the velocity and direc- 

 tion of the current. 



Near the times of high and low water, the currents slacken and are 

 more or less irregular in direction. The velocities, usually small at 

 these times, are not included in the charts. The maximum velocity is 

 reached three or four hours after high and low waters. The average 

 velocity at strength of current through the entrances to the harbor 

 is approximately \]/2 knots. Inside the harbor this velocity dimin- 

 ishes to about 1 Imot or less, except that through Nantasket Gut east 

 of Peddocks Island the average velocity at strength of current is 23^ 

 knots. 



TEMPERATURE AND DENSITY OBSERVATIONS 



Temperature and density observations of the sea water in Boston 

 Harbor have been made from time to time in connection with tide 

 observations, and the mean results which are available have been 

 compiled in Tables 42 to 44. The results of such observations at the 

 navy yard from February to December in 1863 are given in Table 42. 



Incidental to the tide observations at Commonwealth Pier Np. 5, 

 South Boston, the observer takes a temperature and density obser- 

 vation of the sea water each day the tide station is visited. There 

 are no requirements that he visit the station at any specified hour nor 

 that the hour selected should be the same each day. Neither is he 

 required to make these visits on Sundays or holidays. The times 

 actually selected for the visits have probably covered nearly all 

 hours of daylight but appear to be more frequent between 3 and 5 

 o'clock in the afternoon. It may be assumed that the observations 

 are distributed approximately imiformly over all phases of the tide. 

 The observed densities were corrected and reduced to temperature 

 15° C. Table 43 contains the monthly means and extremes of the 

 temperatures and densities observed. 



Incidental to the special current survey of 1926, temperatures and 

 densities were taken at three different depths at nearly all of the 

 stations occupied. The densities were reduced to temperature 15° 

 C, and the results have been compiled in Table 44. 



