Currents 61 
The Sargasso Sea mentioned above is a large area of water 
bounded by the Gulf Stream on the North and West, the 
North Equatorial Current on the South. It derives its name 
from the fact that large quantities of sargassum weed are 
usually found floating there. The region was once regarded 
by seamen with superstitious dread. 
TIDAL CURRENTS 
On a much smaller scale than their gigantic brothers are 
the tidal currents. Their motion is derived from the inequali- 
ties of the heights of tides at different places. At the inlet to 
a bay there are likely to be four reversals of current each 
day: flood, ebb, flood, ebb. A flood current follows low water; 
an ebb current, high water. The periods between alternate 
floods and ebbs are called slack water. 
The tidal currents for many harbors can be obtained from 
Current Tables published by the U.S. Coast and Geodetic 
Survey. 
Offshore tidal currents are likely to be of a more complex 
nature. The current may take on a kind of rotary motion, 
completing the circle each half day. Such a current is com- 
pletely defined by two quantities: set (direction of flow), 
and drift (velocity of current in knots). It is important to re- 
member that the direction of currents is the direction toward 
which the current flows. This is different from wind direc- 
tions, which are measured by the direction from which the 
wind blows. A northwest wind might easily whip up a south- 
east current! 
BORES 
In a river with a broad mouth the current of the river tends 
to prevent the tide from rising. After a struggle the tide over- 
