REDUCTION OF CURRENT RECORDS 81 
90° 
180 
FiGuRE 34.—Graphic solution of formulas (18) and (19). 
If we now let V equal the average velocity of the current at time of strength as deter: 
mined from actual observations and assume that the corresponding difference in water 
level is 1.02 times the difference resulting from the principal constituent M2, we may 
obtain an approximate value for the constant (C) by the formula 
CVE RODNID en a oe ee ee a uiean (21) 
in which M, is the amplitude of the constituent M, in the harmonic expression for the 
difference in head. The application of the factor (C) to all theconstituent amplitudes 
in this expression has the effect of changing the height units into units representing 
the square of the velocity of the resulting current. 
196. The lag in the current is usually determined by a comparison of the times of 
strengths and slacks from actual observations with preliminary predictions of the 
corresponding phases based upon the harmonic constants derived by the method just 
described. This lag expressed in hours is multiplied successively by the speed of each 
constituent and the result applied to the preliminary epoch for that constituent. 
