6 MANUAL OF CURRENT OBSERVATIONS 
as the line of zero velocity. If we now assume a nontidal current of velocity n in an 
ebb or negative direction, its effect will be to diminish all the ordinates of the curve 
by the constant n. In the figure this will be equivalent to raising the line of zero veloc- 
ity to a new position A’B’ at a distance n above the original datum. With the new 
datum it is evident that all ebb velocities have been increased and flood velocities have 
been diminished or changed into ebb velocities. The times of the strength of flood 
and strength of ebb have not been affected, but the slack before flood has been retarded 
and the slack before ebb advanced, thus diminishing the duration of the flood period 
and increasing the duration of the ebb period. If the assumed nontidal current has 
a velocity in the ebb direction that exceeds the velocity of the tidal current, the result 
will be represented in the graph by raising the line of zero velocity to a position A’’B’’, 
and the curve then shows a continuous flow in the ebb direction with minimum velocity 
occurring at the time of the strength of the tidal flood current and maximum velocity 
at time of strength of tidal ebb current. 
12. Relation of time of current to time of tide——The relation of current to tide 
depends upon whether the tidal movement is primarily of progressive or stationary 
wave type. In a progressive wave the times of flood strength and ebb strength occur 
FicureE 9.—Effect of nontidal current on reversing tidal current. 
theoretically at the times of high and low water, respectively, with slack water mid- 
way between high and low water. In a stationary wave slacks come at the times of 
high and low water and the strength of current midway between high and low water. 
Tidal movements are, however, somewhat complicated and usually include both pro- 
eressive and stationary waves, and the actual relation between the current phases and 
the tidal phases will vary in different Iccalities. For any one locality, the relation usu- 
ally approximates to a constant which may be determined from a comparison of 
observational data. 
13. Relation of velocity of current to range of tide.—In the progressive type of 
tide wave the velocity of the current is expected to be the strongest where the range 
of tide is greatest, but in an area where the stationary wave predominates the strongest 
currents usually occur in the vicinity of the axis of oscillation where the range of tide 
is the least while the weakest currents are found in the vicinity of the loops of the 
wave where the rise and fall of the tide is greatest. In a combination of progressive 
and stationary wave movement the relation between current and tide becomes some- 
what complicated. 
14. For any one place, if it is assumed that the velocity of the tidal current is 
proportional to the speed at which the tide rises and falls, it may be shown that the 
