26 



by the varying push that would come from a heaving of the beds of 

 the oceans of the magnitude and sequence of the equihbrium tides. 

 Since the equihbrium tides move across the oceans at the same speed 

 as the moon and the sun, or upward of 660 miles per hour in the 

 range of latitudes of the United States, the actual tides generally lag 

 behind the equilibrium tides by a substantial interval of time. But 

 while the range and the times of high and low water vary widely 

 from those of the equilibrium tide, the jjeriods of the fluctuation of the 

 components of the actual tide must clearly be exactly the same as 

 the periods of the forces which cause them, and consequently conform 

 exactly to the periods of the equilibrium tides. Since however the 

 oceans may be expected to respond differently to the diurnal fluctua- 

 tions of the tide producing forces than to the semidiurnal fluctuations, 

 the diurnal and semidurnal elements of the actual tides are generally 

 displaced with respect to the respective timing of these elements of the 

 equilibrium tides. Similar displacements may be expected in the 

 various components into which the diurnal and semidurnal tides may 

 be resolved. Last of all the ranges of the various components of the 

 actual tides bear a definite relationship to the range of the like com- 

 ponents equilibrium tides, in that small components of the latter will 

 produce small fluctuations in the actual tide; and the variations of the 

 various components at any tidal station, due to the longitude of the 

 moon's node (par. 36) correspond to the variations in the equilibrium 

 tide. 



47. Meteorological tides. — Besides the systematic fluctuations due to 

 the tide-producing forces, irregular fluctuations of the oceans are 

 caused by winds and the varying barometric pressure over their sur- 

 face. These accidental fluctuations are called meteorological tides. 



48. Examples of tides. — The recorded tides at a few representative 

 places are shown in figure 15. It is apparent that these tides vary 

 widely as to type. The effect of the diurnal tidal variations in tlie 

 tide at San Francisco and Galveston may be especially noted. 



