'Ob 



24. The computation of the amphtude and initial phase follows the 

 general method heretofore described for the diurnal, semidiurnal, 

 and short-period components. For the fortnightly and monthly 

 components, a component month replaces the component day. It is 

 divided into 24 parts, corresponding to the component hours. A pre- 

 pared tabulation designates the daily sums to be taken as the height 

 at each component "hour." These heights are then summed and 

 averaged, and the amplitude and phase of the component computed 

 therefrom. For the annual and semiannual components, the com- 

 ponent year similarly replaces the component day. Since the average 

 of the observed heights during a calendar day contains residuals of 

 the short-period components (other than the S components) the 

 amplitudes and phases of the long-period components are corrected 

 by the process of elimination heretofore referred to. 



MEAN VALUES AND EPOCHS OF COMPONENTS 



102. Mean values. — The amplitude of each component of the actual 

 tide increases and decreases with the changing inclination of the 

 moon's orbit to the plane of the earth's Equator, and the amplitudes 

 computed from a particular set of observations must therefore be 

 corrected before they may be used at another period. The correction 

 is based on the logical assumption that the change in the actual 

 components is proportional to the change in the corresponding equilib- 

 rium components. For convenience, the amplitude of each com- 

 ponent of the actual tide is reduced to its mean value, which is obvi- 

 ously independent of the period from which it was derived. 



103. Epochs. — The computed initial phase of each component is 

 that at the beginning of the particular set of observations from which 

 the component was derived. The phase of a component of the actual 

 tide depends upon the accidental configuration of the sea bed, while 

 that of the corresponding component of the equilibrium tide is depend- 

 ent upon astronomical causes alone. Since both components have 

 the same speed, the difference in their phases, at any tidal station, 

 is constant at all times. This difference is called the epoch of the 

 component and is conventionally represented by the general symbol k 

 (kappa). It may readily be found by taking the difference between 

 the initial phase of the equilibrium component, at the zero hour of the 

 observations (as determined from astronomical data) and the initial 

 phase of the actual component, as determined from the observations. 

 The phase of the actual component at any other origin of time can 

 then be found by applying its epoch to the phase of the equilibrium 

 tide at that time. The epoch of a particular component is designated 

 by the symbol for its amplitude with a degree mark. Thus the epoch 

 of the M2 component is designated as M2°. 



