70 U. S. COAST AND GEODETIC SURVEY 



to the time meridian of the place of observation, the correction to be 

 added for west longitude and subtracted for east longitude. In 

 figure 25, the mean unreduced high and low water intervals for Seattle 

 for the month represented are 4.98 hours and 11.12 hours, respectively. 

 As the time meridian for Seattle is 120° or 8 hours west, a correction 

 of +8 hours must be applied to each of these intervals giving 12.98 

 hours and 19.12 hours, respectively, from which the lunar semidiurnal 

 period of 12.42 hours may be rejected, leaving 0.56 hours and 6.70 

 hours, respectively, as the Greenwich high and low water lunitidal 

 intervals from this particular series of observations. 



230. Local lunitidal intervals. — To change from Greenwich to 

 local intervals, it is necessary to apply a correction equal to the time 

 required for the moon to pass from the meridian of Greenwich to 

 the mferidian of the place of observations. A table (p. 73) has been 

 prepared giving the correction for each degree of longitude from 

 1° to 180°, and the value for each minute of arc from 1' to 60' for 

 interpolating between the whole degrees. In changing from Green- 

 wich to local intervals, the tabular value is to be subtracted if the 

 place of observation is in west longitude and added if in east longitude. 

 In order that the local intervals may be positive and less than the 

 half lunar day, the lunar semidiurnal period of 12.42 hours may 

 be added or subtracted as desired. The longitude of Seattle, which 

 is used as an example in figure 25, in 122°20'. In the table on page 73 

 we find the corrections corresponding to 122° and 20' to be 8.418 houts 

 and 0.023 hours, respectively. Combining these we have 8.44 hours 

 as the difference between Greenwich and local intervals at Seattle, 

 and as Seattle is in west longitude this difference is to be subtracted 

 from the Greenwich intervals. To avoid negative results, the Green- 

 wich intervals are first increased by adding the semidiurnal period of 

 12.42 hours, after which the difference 8.44 hours is subtracted leaving 

 4.54 hours and 10.68 hours, respectively, as the local high and low 

 water lunitidal intervals for these observations. 



231. Method of checking intervals. — The mean unreduced inter- 

 vals as obtained above may be conveniently checked by the following 

 method : 



(a) Sum times of moon's transits occurring during month. The 

 number of transits will usually be two less than twice the number of 

 days in the month ; that is, 54 for a 28-day month, 56 for a 29-day 

 month, 58 for a 30-day month, and 60 for a 31-day month. For the 

 sake of uniformity, if the number of transits should be one greater 

 than this, the last transit must be omitted from the sum and the last 

 day be considered as having only the single value. On the other 

 hand, if the number of transits should be one less than the usual num- 

 ber, the deficiency must be supplied by including the last transit of 

 the preceding month. From the sum of the transits subtract the 

 product of 24 hours by the sum of the numerals indicating the days 

 of the month on which only a single transit occurs. For example, if 

 single transits occur on the 3d and 17th of a month, the sum 20 is 

 multiplied by 24, and the product 480 hours is then subtracted from 

 the sum of the transits. Designate the remainder by T, which may 

 be either positive or negative. 



