TIDES AND CURRENTS IN BOSTON HARBOR 19 



they represent the currents near the surface. Directions of the cur- 

 rent were obtained by angles between the float and objects on shore. 



The results from these observations are shown in Table 31. The 

 station numbers refer to those assigned in the original reductions but 

 are prefixed by the letter "D " to signify that they are stations occu- 

 pied by the party of Lieutenant Davis. Numbers omitted refer to 

 stations for which insufficient data were obtained or to stations which 

 were reoccupied after a preceding number had already been assigned. 

 Following the location and dates on which the observations were 

 made, the length of observations is given to the nearest half or whole 

 day. 



The times of the slacks and strengths are referred to the tides as 

 observed at the Boston Navy Yard, the slack before flood and the 

 flood strength being referred to the low waters and the slack before 

 ebb and ebb strength to the high waters. In general the current 

 phases follow the tidal phases to which they are referred, but if the 

 current occurs earlier than the tide, it is indicated in the table by a 

 minus ( — ) sign. The time difl'erences are given in hours and decimals. 



The direction for the flood and ebb strength is reckoned in degrees 

 from the true north as 0°, through east as 90°, south 180°, and west 

 270°. The velocities of the flood and ebb strength have been cor- 

 rected by factors depending upon the range of tide at the navy yard. 



The flood duration from slack before flood to slack before ebb and 

 the ebb duration from slack before ebb to slack before flood are given 

 in hours and decimals. 



Mean current hour. — The mean current hour as given in this table 

 is expressed in solar time and is the mean interval between the transit 

 of the moon over the meridian of Greenwich and the time of the 

 strength of the flood current modified by the times of slack water and 

 strength of ebb. In computing the mean current hour an average is 

 obtained of the intervals for the following phases: Flood strength, 

 slack before flood increased by one-fourth semilunar day (3.10 hours), 

 slack after flood decreased by one-fourth semilunar day, and ebb 

 strength increased or decreased by one-half semilunar day (6.21 

 hours). Before taking the average the four phases are made compar- 

 able by the addition or rejection of such multiples of the semilunar 

 day (12.42 hours) as may be necessary. 



CURRENT OBSERVATIONS, 1860, 1861, AND 1863 



A survey of Boston Harbor was undertaken in 1860, and current 

 observations were made between July 30 and October 10, 1860, under 

 the direction of Assistant Henry Mitchell. Additional observations 

 were made between August 4 and September 19, 1861, and between 

 August 8 and August 29, 1863, under the direction of Assistant 

 Mitchell. 



Surface velocities were obtained by a log line. Observations for 

 different depths were made by using two cylinders or two globes con- 

 nected by wire, the upper cylinder or globe supporting the lower one, 

 which was set at various depths. By this method the observed cur- 

 rent was the resultant of the current at the surface and at the depth 



