GENERAL EXPLANATION 11 
Figure 13. Let ABCD be the current curve of a simple rotary tidal current with the 
origin of the coordinates at O.. Then OA, OB, OC, ete. will represent the velocity and 
direction of the tidal current at different intervals of time. Let the nontidal current 
be represented in velocity and direction by the line O0’O. Then lines drawn from O/ 
to A, B, C, ete., will indicate by their lengths and directions the resultant currents cor- 
responding respectively to the tidal currents OA, OB, OC, etc. It will be noted that 
the shape of the current ellipse is not changed by the introduction of the nontidal 
current, but the origin O is shifted to a position O’ in a direction opposite to that of the 
nontidal current and by an amount representing its velocity. If the velocity of the 
nontidal current exceeds that of the tidal current, the origin O will be shifted to a posi- 
tion O’’ outside of the ellipse. In this case the resultant current will not veer entirely 
around the compass, but its direction will vary within an arc limited by tangents drawn 
from O’’ to the ellipse. 
Ficure 13.—Effect of nontidal current on rotary tidal current. 
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