43 
equipotential projections be removed, the angle between these two 
may be more easily seen and designated as a, in the figure. It follows 
that: 
AF=AC tana=Zwsin yV tan CaN es we en) 
It is seen from this that the force due to virtual friction varies 
directly as the tangent of a and the velocity of the current. There is, 
however, very little data bearing on the value of this angle a for 
various currents at different places and under different conditions. 
In order to determine various values of a (contemplating of course 
that there are several assumptions), simultaneous observations should 
be madg regarding the direction and the rate of flow of the current 
with the aid of current meters, and at the same time observations for 
temperature and salinity should be taken in a section at right angles 
Fie. 22.—An illustration similar to Fig. 21, but with all but one of the dynamic isobaths erased 
and all but one of the parallel lines of flow erased 
across the current. By such a means the virtual friction, as repre- 
sented by the line AF, can be tabulated directly. The value of this 
effect, however, as it enters the present discussion, may be safely 
stated, rarely exceeds a magnitude of 0.05 to 0.07 knots per hour, 
either to accelerate or to retard the flow of a steady current, and such 
ciphers being relatively insignificant can be disregarded in the prac- 
tical determination of currents. 
EFFECT OF BOTTOM CONFIGURATION ON CURRENTS 
Frictional retardation attains considerable proportions, however, 
between flowing water particles in contact with the fixed configura- 
tions of an ocean basin. Ekman has suggested as a result of mathe- 
matical investigations that if an ocean current proceeding along in a 
steady manner moves in over a gradually shallowing shelf, it will 
tend to be deflected more and more to the right in the Northern 
Hemisphere. On the other hand, if by a continuance the bottom 
begins to recede deeper and deeper from the surface, then the cur- 
rent will be proportionately deflected to the left. This phenomenon 
