68 



U. S. COAST AND GEODETIC SURVEY 



the first of each calendar month, when the series commences with 

 January 1 : 



221. The tabulated hourly heights are to be expressed in feet and 

 tenths and will be referred to the same datum as adopted for the 

 high and low waters, the heights being scaled from the record in the 

 same manner as described for the high and low water tabulations. 

 The hourly heights are to be taken on the exact hour and allowance 

 must be made for any time error in the record. This is especially 

 necessary if the heights are to be used in an harmonic analysis of the 

 observations. 



222. Tide observers who tabulate their own records should retain 

 at the end of each month the last incomplete sheet of the hourly 

 heights in order that it may be completed when the record for the 

 followine: month is available. 



INTERPOLATIONS 



223. Before beginning the reductions, if any portion of the record 

 is lost, it is desirable that the missing tides be supplied by interpola- 

 tion. Interpolated tides should be enclosed in brackets to distinguish 

 them from observed tides. 



224. If the heights of alternate high waters are plotted on cross- 

 section paper it will be found that fairly smooth curves may be drawn 

 through the plotted points. Such a graph affords a means of inter- 

 polating for missing high waters where only a few tides have been 

 lost. When only a few tides are missing the alternate heights for 

 several days before and several days after the break may be plotted 

 and spaces left for the missing values. The smooth curve connecting 

 the observed values will generally determine the missing values with, 

 sufficient accuracy. Similar curves may be drawn for alternate low 

 waters. The times as well as the heights will be found to plot in 



