the time of spring tides, than at other times in the month. Charts 

 of foreign waters sometimes give the intervals at such times, instead of 

 the mean intervals, designating them as HWI, F. & C, and LWI, 

 F. & C, respectively. 



9. The charts of the United States Coast and Geodetic Survey and 

 other publications show the average luni tidal intervals at representa- 

 tive tidal stations. By computing from the Nautical Almanac the 

 times of upper and lower meridian transits of the moon at the place 

 on any day, the times of high water on that day can be approximately 

 determined. Although rarely of practical importance, the method of 

 computation is of interest. 



The Nautical Almanac gives the Greenwich mean solar time of the 

 moon's upper and lower transits across the meridian of Greenwich 

 for each day in the year. This time obviously is the interval, or hour 

 angle, between passage of the (mean) sun and the passage of the 

 moon over the Greenwich meridian. This interval increases at the 

 average rate of 25.2 minutes every 12 hours, or 2.1 minutes per hour. 

 If then the longitude of a given place is L° west of Greenwich, the 

 transit of the (mean) sun over its meridian will be L°/15 hours later 

 than the transit over the Greenwich meridian, and the interval between 

 the transits of the (mean) sun and of the moon over the meridian 

 of the place, or the local mean solar time of the moon's transit, will be 

 the Greenwich time of transit increased by 2.1 L°/15 minutes. For 

 example, the high-water interval at Sandy Hook, long. 74° W., at the 

 entrance to New York Harbor, is 7*^.35'". For April 12, 1936, the 

 Almanac gives: 



Upper Lower 



Moon's transit, Greenwich 3''55™ 16''21°^ 



Correction to Sandy Hook (74/15) 2.1 10™ 10°> 



Local time moon's transit, Sandy Hook 4'^05™ 16''31™ 



Correction to standard time 75° meridian —04™ —04™ 



Standard time moon's transit, Sandy Hook 4''01™ 16^27™ 



Adding the high-water interval to the times of the moon's transits, 

 the approximate times of high water at Sandy Hook are found to be 

 ll'^36°' and 24'^02"'; or 11:36 a. m. April 12 and 12:02 a. m. on April 

 13. The times given in the tide tables are 11 a. m. and 11:24 p. m. 

 on April 12. The time of high and low water found from luni tidal 

 intervals may be in error by half an hour or more. 



10. The difference between the lunitidal intervals at two tidal sta- 

 tions, corrected if necessary for the difference in the longitudes of the 

 stations, gives the average difference between the times of high, or 

 low, water at these stations. The formula for this correction is at 

 once derived from the process of finding the time of high (or low) 

 water from the Greenwich meridian transit of the moon and lunitidal 

 interval, as set forth in paragraph 9. Let G be the time, in hours, of 



