TIDAL DATUM PLANES 13 
Fic. 3.—Combinations of semidaily and daily constituent tides. 
also in the resultant high waters to some extent, the greater the difference in the times the 
greater the inequality in the high waters. When the time differences are such that 
the two constituents are at sea level at the same time, the inequality in the tide will 
appear in equal degree in both high and low waters, as pictured in the lower diagram. 
As the times of the constituent tides continue to differ still more, the equality in the 
high waters will increase and the inequality in the low waters will decrease until finally 
as represented in the middle diagram when the high waters of the two constituents 
occur at the same time, the mequality will be exhibited wholly in the high waters. 
With regard to the inequality in height of tide, therefore, tides may be grouped 
into three classes: (1) those in which the inequality is featured principally in the high 
waters; (2) those in which the inequality is featured principally in the low waters; 
(3) those exhibiting imequality in approximately equal degree in both the high and 
the low waters. 
Thus far we have discussed only the combination of daily and semidaily constitu- 
ents of equal ranges. In the tides as they actually occur, however, the ranges of the 
two constituents differ at different places. This gives rise to new features in the in- 
equality, which a consideration of the lower diagram of Figure 3 will make clear. If 
the range of semidaily constituent remains as pictured in that diagram but the range 
of the daily constituent is greater, it is obvious that the lower high water will become 
lower and the higher low water will become higher. When the range of the daily con- 
stituent is taken twice that of the semidaily, it will be found that the lower high water 
and the higher low water have the same height, giving rise to what is known as the 
