230 
BULLETIN OF THE BUREAU OF FISHERIES 
in the cross sectional areas due to the slope of the bar the irregularity in the hori- 
zontal movements of water flowing over the bar is more pronounced than it is in the 
channel. 
Current-meter readings were made on calm days when there was only slight 
interference caused by the wind. The results of the observations are presented in 
Figures 18, 19, 20. It is obvious from an examination of Figures 18 and 19 that the 
periods of slack water in the channel near Shell Point occur about the times of the 
tide, and that the strength of the tide comes between the times of high and low 
water. In this respect tidal conditions at Onset Harbor, like those of Long Island 
Sound, present the characteristics of stationary wave motion. An interesting 
feature of the tidal currents at Shell Point is the appearance of two velocity peaks 
during the flood tide, which are probably due to a temporary piling up of water in a 
Figure 19. — Surface'ebb currents in the channel at Shell Point, August 30 and 31, 1927 
narrow entrance at Shell Point (see fig. 15) and blocking the horizontal movement of 
water on the bar. The maximum velocity (20 centimeters per second) was observed 
during flood tide. As can be noticed from Figures 18 and 19, flood-tide currents in 
Onset Harbor are stronger than ebb currents, and the changes in the velocities dur- 
ing the receding tide are more gradual than they are during the flood tide. 
From a biological point of view, it is of interest to determine the current velocities 
on the bar which is exposed at low water and where, as had been determined by previ- 
ous observations, the setting of oysters is usually good. Because of the gravelly 
bottom over the bar, the receding tides leave no pools of water; hence the oyster 
larvae that set on the bar are undoubtedly brought in with the incoming or outgoing 
tide. Observations made on July 30 (fig. 20) show that flood-tide currents on the 
bar are rather irregular and reach higher velocities (73 centimeters per second) than 
in the channel, and that the difference between the velocities at flood and ebb cur- 
rents is even more pronounced than it is in the channel. 
