4
TIDAL AND CURRENT DATA
 turbance, tidal travel becomes a matter of observation rather than of calculation. If such observations are sufficiently long continued, an empirical basis for foretelling the periods of high and low water at a given port may be arrived at.
   The interval at each port between the passage of the moon and the actual time of high water F. and C. is known as “the establishment” of that port. From this record a time-table of tides may be calculated, the connection between the periods of F. and C. following a defined sequence. The Admiralty charts supply this information at every port. Taking as an illustration the chart of the Rivers Orwell and Stour, the following phrase will be found:—
   “H. W. F. and C. at Harwich XI hours 56 minutes. Springs rise 12J feet, neaps io| feet, neaps range 7J feet.”
   This phrase indicates the hour of high water at Harwich at the periods of new and full moon and the amounts of mean tidal lift.
   The National Physical Laboratory has designed the India Office Tide Predicting Machine, which is a modification of Lord Kelvin’s apparatus now in South Kensington Museum. It can be used for predicting the tides, and preparing tide-tables, for any year, for any port for which the constants requisite for setting the machine have been determined by observation.
    The principle of the method may be briefly stated as follows: The tidal motion at any port can be analysed into a number of simple tides, i.e. simple harmonic oscillations of sea-level. The periods of these oscillations for the different component tides are determined from astronomical data, and are the same for all ports. The amplitudes and the phases of the several component tides are different for different ports. Twenty-four such components are included in the machine at the National Physical Laboratory. The amplitudes and phases of these twenty-four components are the constants above referred to, which it is necessary to know for any particular port in order to set the machine for predicting the tides.
   These amplitudes and phases are determined from actual