TIDAL DATUM PLANES 87 
very small range. Hence in tabulating tides of the daily type, secondary tides with 
ranges less than a tenth of a foot are disregarded for the purposes of high and low water 
tabulation. 
In tides of the daily type, therefore, it is preferable to completely disregard the 
occasional secondary tides in determining mean high water, and use but one high water 
a day, the higher high water. Mean high water for the daily tide is thus the same as 
mean higher high water, and the simple definition of mean high water becomes appli- 
cable to all types of tide. 
Primary Determination 
A primary determination of mean high water is based directly on the average of 
the high waters over a 19-year period. And if there were no change in sea level from 
one 19 year period to another, we would expect two different 19 year determinations 
of mean high water at any place to agree, unless some change in tidal regime had taken 
place. 
At Baltimore, continuous tide observations are available for the 46 year period 
1903-1948. If we take the first 19 year series, 1903-1921, mean high water on the 
fixed staff reads 4.648 feet, while for the last 19 year series, 1930-1948, it reads 4.916 
feet, a difference of 0.268 foot. Sea level for the first 19 year series reads 4.107 feet 
and for the last 19 years, 4.368 feet. With reference to the sea level for the respective 
19 year series, therefore, mean high water at Baltimore for the period 1903-1921 was 
0.541 foot above sea level and for the period 1930-1948, 0.548 foot above sea level. 
The difference of 0.007-foot between the two latter values is scarcely significant in view 
of the disturbing effects of wind and weather. 
At Seattle, tide observations covering the 50 year period 1899-1948 are available. 
Three slightly overlapping 19 year series may be formed from these observations; 
1899-1917; 1915-1933 and 1930-1948. The mean high water for each of these series 
on the staff is 7.881 feet, 7.897 feet and 8.011 feet, respectively. The difference between 
the first and second primary determinations is 0.016 foot and between the first and 
third is 0.130 foot. Referred to the respective 19 year sea levels, mean high water 
above sea level is, respectively, 3.837 feet, 3.842 feet and 3.849 feet, the difference 
between the first and the third being 0.012 foot. This again is so small as to be scarcely 
significant. 
It appears therefore that primary determinations of mean high water above sea 
level are in practical agreement. But since sea level at many places appears to be 
subject to a slow change, for precise purposes the datum of mean high water must be 
specified with regard to the 19 year series used. 
Secondary Determination 
Primary determinations of mean high water are practicable at relatively few 
places. At other places this datum can be derived with sufficient precision for most 
purposes from observations covering much shorter periods than 19 years. Two 
methods are available: (1) comparison of simultaneous observations; (2) correction 
by tabular values. 
Hither method requires two separate corrections. Since high water varies peri- 
odically with respect to sea level, the first correction is to derive the value of mean 
