TIDAL DATUM PLANES 19 
25th when higher high water rose 2.4 feet above the preceding lower high water. The 
least difference occurred on the 19th when the two high waters differed by but 0.1 foot. 
It is of interest to note that on the 5th and again on the 19th, that is, when the 
moon was close to or over the Equator, there was considerably greater inequality in 
the low waters than in the high waters. If observations on those days only were 
available, it would be natural to class the tide here with that form of the mixed type 
in which the inequality is featured principally in the low waters. Because of the 
variations to which tides are subject from day to day, it is necessary to have at least 
a month of observations to determine the character of the tide at any place. 
Turning now to a consideration of the second form of the mixed type of tide, in 
which the inequality is featured principally in the low waters, the tide at Seattle, 
Washington, may be taken for illustration. Figure 7 shows the tide curve at Seattle 
for the month of June 1934. 
For the month shown in Figure 7, the high waters at Seattle averaged 3.88 feet 
above sea level and the low waters averaged 3.73 feet below sea level, giving an average 
range for the month of 7.61 feet. Deriving the heights of the higher and lower tides 
separately, we find higher high water to average 4.90 feet and lower high water 2.93 feet 
above sea level; lower low water 7.25 feet below sea level and higher low water 0.05 
Feet 
5 
June lst 10th 
fe. Hlth | 20th 
30th 
Fic. 7.—Tide curve, Seattle, June 1934. 
