TIDAL DATUM PLANES 21 
0 12 0 12 0 12 24 
Feet 
Fic. 9.—Tide curve illustrating the vanishing tide. 
In June 1934, the range of tide at San Pedro averaged 3.71 feet, the difference 
between the higher high waters and lower high waters averaged 2.04 feet and the 
difference between the higher low waters and lower low waters averaged 2.48 feet. 
Corresponding to the moon’s maximum semimonthly declination on the 11th and 26th 
the high-water differences were 2.5 feet on the former date and 3.4 feet on the latter 
date. For the low waters the corresponding differences were 3.3 feet and 3.8 feet, 
respectively. On the 4th and 19th the moon was over the Equator and the correspond- 
ing differences in the high waters and in the low waters were less than a foot. 
It will be noted that on days when the inequality in the high waters and in the 
low waters is large, as for example on the 12th and 25th, the difference between the 
lower high water and higher low water is small. At certain times the difference becomes 
so small that these two tides merge, and thus but one high water and one low water 
occurinaday. This is illustrated by the tide at San Pedro for September 16, 1934, in 
Figure 9. 
In Figure 9 the tides for the three-day period, September 15 to 17, are shown. On 
the 15th there were two high and two low waters, the difference between the morning 
high and low waters being 0.4 foot. On the 16th the morning high and low waters had 
the same height, merging to form what is known as a vanishing tide, resulting in a 
stand of the water for a period of about three hours. On the 17th, two high and two 
low waters again appear, the difference between the morning high and low waters being 
0.3 foot. 
Criteria for the Different Types of Tide 
From the brief discussion of the three types of tide in the previous section, it is 
clear that there are no sharp dividing lines between the different types of tide. For 
general purposes it may be sufficient to define the semidaily type as one in which there 
are two high and two low waters a day with but little inequality, while the mixed type 
is defined as one with two high and two low waters a day which exhibit considerable 
