MANUAL OF TIDE OBSERVATIONS 69 



fairly regular curves, and times of missing high and low waters may 

 also be determined by this graphic method. 



225. Another method of supplying missing values is by direct com- 

 parison with simultaneous observations at some other station in the 

 vicinity. This method is especially useful in extrapolating values at 

 the beginning and end of a series of observations. 



LUNITIDAL INTERVALS 



226. A Greenwich lunitidal interval is the absolute difference in 

 time between the transit of the moon over the meridian of Greenwich 

 and the time of the occurrence of the following high or low water at 

 any place, separate intervals being obtained for the high and low 

 waters. A local lunitidal interval is the difference in time between 

 the transit of the moon over the local meridian and the time of the 

 following high or low water at a place on the same meridian. The 

 mean local high-water intervals for many places are included in the 

 annual tide tables of this Bureau. The Greenwich intervals have 

 the advantage that they afford a direct means for obtaining the differ- 

 ence in the time of tide at different places. 



227. Form 138 (figs. 24, 25) in which the high and low waters are 

 tabulated provides for the computation of both high and low water 

 lunitidal intervals. The Greenwich transits in hours and tenths, 

 which will be furnished upon request or which may be derived from 

 the American Ephemeris and Nautical Almanac where they are given 

 in hours and minutes, are to be entered in the column of "Moon's 

 transits," the lower transits being distinguished from the upper 

 transits by being enclosed in parentheses. 



228. From the time of each high and low water subtract the time 

 of the first preceding moon's transit and enter the difference in the 

 appropriate column of the form and on the same line as the tide from 

 which it was obtained. In case the time of high or low water is 

 nearly the same as that of the moon's transit, take the transit whiclx 

 precedes the tide by about 12 hours, but in no case must the same 

 transit be used for two consecutive high waters or for two consecutive 

 low waters. The lower transit of the moon applies to both high and 

 low water, just the same as the upper transit does. Wnen the time 

 of the moon's transit is on one day and the following high or low 

 water is on the next day the time of this tide must be increased by 

 adding 24 hours before attempting to subtract the time of the transit. 

 The high-water intervals will usually be approximately 6 hours 

 greater or less than the low-water intervals, but the intervals for each 

 phase of tide will usually agree among themselves within an hour or 

 two. Intervals from the lower transits of the moon are to be indi- 

 cated by parentheses. The high and low water intervals for the 

 calendar month are summed separately and the means obtained to two 

 decimal places. 



229. In these computations the moon's transits are given in Green- 

 wich time while the high and low waters are given in the standard 

 time of the place of observations. To obtain the Greenwich lunitidal 

 intervals it is therefore necessary to apply a correction corresponding 



