REDUCTION OF CURRENT RECORDS 79 
90° 77 Ke~. 
/ \ 
180° 
o 
2710 
FIGURE 33.—Graphic solution of formulas (12) and (13). 
head will result partly from any difference in the range of tide and partly from any 
difference in the times of the high and low waters. Theoretically, the current velocity 
will vary as the square root of the difference in head, will be a maximum when this 
difference is greatest, and will be zero or slack water when the difference is zero. 
Actually, because of inertia and friction, there will usually be a lag of some minutes in 
the response of the currents to this difference in head. 
191. Predictions of hydraulic currents in a strait, based upon the difference in the 
tidal head at the two entrances, may be made by means of harmonic constants derived 
from the tidal constants for the entrances, allowance being made for lag and the fact 
that the current velocity varies as the square root of the difference in head. Let the 
two ends of the strait be designated by A and B, with the flow from A toward B con- 
sidered as flood or positive and the flow in the opposite direction as ebb or negative. 
With the waterway receiving the tide from two sources, the application of the terms 
“flood” and “ebb” must be somewhat arbitrary, and care should be taken to clearly 
indicate the direction assumed for the flood movement. 
192. In the following discussion tidal constants pertaining to entrances A and B 
will be distinguished by subscripts a and 6, respectively, and those pertaining to the 
