74 U. S. COAST AND GEODETIC SURVEY 



it should not be checked. If, however, the tide has become diurnal 

 and only one high and one low water occur during the tidal day, these 

 should both be checked. The checked heights are to be summed 

 separately for the high and low waters, the sums being entered in the 

 spaces provided, with the number of observations written above in 

 small figures. The mean of the higher high waters {HEW) and the 

 mean of the lower low waters {LUW) are then obtained, each result 

 being carried to two decimal places. 



236. The diurnal high water inequality {DHQ) is obtained by sub- 

 tracting the mean of all high waters from the mean of the higher 

 high waters, and the diurnal low water inequality {DLQ) is obtained 

 by subtracting the mean of the lower low waters from the mean of 

 all low waters. 



237. Correction for longitude of moon's node. — There is a long- 

 period variation in the range of tide due to changes in the inclination 

 of the moon's orbit to the Equator. When the longitude of the 

 moon's node is 0° the inclination of the orbit to the Equator is at a 

 maximum and the average range of the four tides of the day is less 

 than usual; and when the longitude of the moon's node is 180° the 

 inclination is at a minimum and this range of tide is greater than 

 usual. The time required for the longitude of the moon's node to 

 pass through the cycle of 360° is approximately 19 years. In addition 

 to the variations caused by changes in the longitude of the moon's 

 node, the diurnal inequalities are subject to variations from month to 

 month caused by changes in the declination of the sun. Because of 

 these variations certain factors are necessary in order to reduce to 

 mean values the ranges and inequalities obtained from short series 

 of observations. At the primary tide stations, the correction need 

 not be applied to the results for each individual month, it being 

 smflScient to correct the annual means as needed. If the series 

 of observations extends over a period of 19 years, the factors are 

 unnecessary. 



238. Tables containing the factor F{Mn) for reducing the observed 

 range of tide to its mean value and factor F^ for reducing the diurnal 

 inequalities DHQ and DLQ to their mean values are given on page 

 75. These tables cover the years 1941 to 1950, inclusive, and are 

 based upon tables 6, 14, and 32 of Harris' Manual of Tides. Similar 

 tables covering the years 1891 to 1950, inclusive, will be found in 

 Special Publication No. 135, Tidal Datum Planes. The factor F{Mn ) 

 was computed for the middle of each calendar year, but as the factor 

 changes very slowly, the tabular value may be used for any month 

 in the year without material error. The factor F^ was computed for 

 the middle of each calendar month. The table includes also the mean 

 of the monthly factors for each year which may be taken as the factor 

 for correcting the yearly inequalities to their mean values. 



239. The factor F(Mn) depends not only upon the year of observa- 

 tion but also upon the relation of the diurnal to the semidiurnal wave 

 in the locality, this relation being expressed approximately by the 



formula — ^^ %7 ^^, or if harmonic constants are available by 



Mn 



the ratio :^lT_^, For stations along the Atlantic coast of the United 



Mo. 

 States from Maine to Florida this relation is generally small, and, if 



