48 



days in the period. Stencils may be prepared in wiiich the summations, 

 are made for 1 and only 1 component hour in each component day. 

 In such stencils 1 of the observed heights is omitted at the shifts of the 

 component hour when it is longer than the mean solar hour, and 2 com- 

 ponent hours are assigned to the same observed height at the shifts 

 when the component hour is shorter than the mean solar. The latter 

 system is not generally favored. 



89. Secondary stencils. — The speed of the Ki component is so nearly 

 that of the solar day that its component hours shift with respect to 

 the solar hours but once in about 8 days. To facilitate the computa- 

 tion of this component from a year's observations, it is permissible to 

 assemble on standard sheets the 7-day sums of the observed hourly 

 heights, and to use these sums, instead of the daily observations, in 

 computing the hourly heights of this component. A number of com- 

 ponents similarly have speeds so close to those of others that they may 

 be computed from the 7-day hourly sums of their primary component. 

 The stencils prepared for such computations are called secondary 

 stencils. 



90. Number of days of observations required. — To eliminate a com- 

 ponent by harmonic analysis, the tidal observations should extend 

 over a period, or multiple thereof, in which the successive values of 

 the component to be eliminated, at the component hours of the com- 

 ponent sought, run through their entire range of values, both positive 

 and negative (par. 78). If a is the speed of the component A, which is 

 to be segregated, and b the speed of a component B which is to be 

 eliminated, component B gains b — a degrees on component A each 

 solar hour. Its successive daily values at any component hour of A 

 will then run through their whole range of values in 360°/24 (b — a) = 

 15/(6 — a) days, or 360/(6 — a) hours, the synodic period of the two 

 components (par. 54). A consideration of the relative speeds of the 

 tidal components establishes a minimum period of 14 days for diurnal 

 and 15 days for semidiurnal components. The periods adopted by the 

 United States Coast and Geodetic Survey are 14-15, 29, 58, 87, 105, 

 134, 163, 192, 221, 250, 279, 297, 326, 355, and 369 days. The stand- 

 ard period for a complete analysis for tide predicting purposes is 369 

 days. A period of 29 days affords, however, fair determinations if 

 corrections are applied to eliminate the residual effect of interfering 

 components. 



91. Computation of amplitude and initial phase. — The determination 

 of the heights of the various components at their component hours has 

 been described in the preceding paragraphs. The amplitude and initial 



