TIDAL DATUM PLANES 15 
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Fic. 4.—Tide curve, New York, June 1934. 
tively large. Likewise, from the 18th to the 20th the inequality is small (except for the 
disturbing effect of a strong wind on the 19th), while from the 25th to the 27th it is 
again relatively large. If we examine a lunar table for June 1934, we find that on the 
4th of the month and again on the 19th the moon was over the Equator, while on the 
12th and 26th it was in the tropics. 
The semidaily type of tide, therefore, exhibits some inequality, especially when the 
moon is at its maximum semimonthly north or south declination. The question there- 
fore arises, how great must the inequality in the tide become before that tide ceases to 
to be classed as a semidaily tide? This question it will be more convenient to consider 
in connection with the mixed type of tide. For the present it will be sufficient to note 
that while the semidaily type of tide exhibits diurnal inequality, the magnitude of this 
inequality in relation to the range of the tide is small. 
For the month shown in Figure 4, the tide in New York Harbor averaged 4.4 feet. 
The smallest range during the month occurred on the 19th, when the height difference 
between the morning high and low waters was 2.5 feet. The greatest range occurred on 
the 26th and again on the 28th when the difference between afternoon high water and 
the following low water was 6.6 feet. On a percentage basis, the smallest range was 
43 percent less than the average range for the month, while the greatest range was 50 
percent greater than the monthly average. In round numbers, therefore, it may be 
