TIDAL DATUM PLANES 83 
Obviously some of the variations must be due to the corresponding changes in sea 
level. On comparing the diagram for Atlantic City in Figure 40 with the diagram of 
monthly heights of sea level for Atlantic City in Figure 24 it is seen that there is a 
close parallelism between the two diagrams. It follows therefore that there must be 
an annual variation in high water similar to that in sea level. 
Figure 41 gives the average heights of monthly high waters at the three stations 
as determined from the 19-year period 1930-1948. The three curves thus represent 
the annual variation in the height of high water at the respective stations. A com- 
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 
Atlantic City 
Los Angeles 
Pensacola 
Fic. 41.—Annuai variation in high water, Atlantic City, Los Angeles, and Pensacola. 
parison of each of the diagrams of Figure 41 with the corresponding diagrams in Figures 
26, 27, and 28 shows very close parallelism. The horizontal line in each diagram 
represents the height of mean high water at that station. 
Monthly high water at any point is thus subject to variations both periodic and 
nonperiodic in character; and in both of these it follows closely the like variations in 
sea level at that point. 
Yearly High Water 
Turning now to the variations in the height of yearly high water, it is found, 
as was to be expected, that these are much smaller than the variations from month to 
month. In Figure 42 are shown the yearly heights of high water for the 25-year period 
1924-1948 at Boston, Los Angeles, and Pensacola. Generally, consecutive heights of 
yearly high water at any one of these places are seen to differ by not more than a few 
hundredths of a foot, but occasionally this difference may be as much as 0.2 foot. 
