96 



tide does not often fall. It might be better named the mean lower 

 low water of spring tides. 



183. In the United States, mean low water of spring tides is taken 

 by the Coast and Geodetic Survey as the average of the two low waters 

 nearest the successive times of spring tides, these times being deter- 

 mined by adding the phase age (par. 144) to the hour of new or full 

 moon. The variations in the diurnal inequality at the times of spring 

 tide are thereby eliminated. With this procedure four tidal heights 

 per month enter the computation both of mean low water and of mean 

 high water of spring tides; but observations must extend over a long 

 period to afford a mean in which the other systematic and the acci- 

 dental variations in the tide are satisfactorily eliminated. 



184. Approximate values. — It is shown in appendix II that the 

 height of mean high water above half -tide level, and of mean low 

 water below half-tide level is equal to the amplitude of the M2 com- 

 ponent increased by a relatively small correction due to the displace- 

 ment by the other components of the time of high and low water. 

 The S2 component does not have, therefore, any large effect on the 

 elevation of mean high or low water or the mean tidal range. At the 

 time of spring tides, however, the S2 component is in conjunction with 

 the M2 component, and the height of high water is increased, and of 

 low water decreased, by substantially its amplitude. It follows there- 

 fore that mean high water of spring tides, as the term is used in the 

 United States, is closely approximated by adding the value of S2, as 

 computed by harmonic analysis, to the corrected elevation of mean 

 high water, as determined by observ action ; and mean low water of 

 spring tides by subtracting this value from the established mean low 

 water. In other words: 



LWOST-MLW-S2 (109) 



HW0ST=MHW+S2 (110) 



Sg=Mn+2S2 ■ (111) 



For example, the elevation of mean low water below half-tide level 

 at Fort Hamilton, New York Harbor, is determined from observations 

 extending over a long period, to be 2.37 feet, and the value of S2 at this 

 station is 0.44. The elevation of low water of spring tides, from equa- 

 tion (109) is then 2.81 below half-tide level. Its value computed 

 directly from observations extending over several years is 2.79. Sim- 

 ilarly at the Presidio of San Francisco, the elevation of low water of 

 spring tides from equation (109) is 2.37 feet below half -tide level, 

 whUe its elevation from direct observation, is 2.36 feet. The corre- 

 spondence is, therefore, very close at these stations. 



