14 U. S. COAST AND GEODETIC SURVEY 
vanishing tide. As the range of the daily constituent is increased still further, the re- 
sultant tide shows but one high and one low water in a day. 
In the tides as they actually occur at different places, not only do the times of the 
semidaily and daily constituents have different relations, but the ranges of the two 
constituents likewise differ. On investigation it is found that if the range of the daily 
constituent is less than twice that of the semidaily constituent, there will be two high 
and two low waters each tidal day; if the range of the daily constituent is between 
two and four times the range of the semidaily, there may be two high and two low 
waters or only one high and one low water a day; but if the range of the daily constit- 
uent is four or more times that of the semidaily, there will be only one high and one 
low water a day. 
It is to be noted that both the daily and the semidaily tide-producing forces vary 
in intensity from day to day, the former being greatest when the moon is at its semi- 
monthly maximum north or south declination, and the latter being greatest when the 
moon is over the equator. The tide at any given place, therefore, exhibits varying 
amounts of inequality within a fortnight. 
With this brief discussion of diurnal inequality, we may now consider the three 
types of tide in greater detail. : 
The Semidaily Type of Tide 
The semidaily type of tide, as implied by the name, is one in which the tidal cycle 
of high water and low water is completed in half a day; and there is the further impli- 
cation that there is but little difference between the corresponding tides of successive 
half-day cycles. In other words, in the semidaily type of tide there are two high and 
two low waters in a day with but little diurnal inequality. Figure 4, which reproduces 
the tide curve for New York Harbor for the month of June 1934, illustrates the appear- 
ance of a tide curve of this type of tide for a period of a month. The horizontal line 
associated with each of the 10-day groups of tide curves represents the plane of mean 
sea level in New York Harbor. The heights of the tide are referred to this plane by 
the scales to the left of the diagram. 
A period of 30 civil days of 24 hours each corresponds almost exactly to 29 tidal 
days of 24 hours and 50 minutes each. And Figure 4 shows that during the 30-day 
period of June 1934 there were 58 high waters and 58 low waters, or two high and two 
low waters in a tidal day. Furthermore, while there is some difference in height 
between the two high or two low waters of a day, this difference is seen to be relatively 
small as compared with the range of the tide. In other words, the diurnal inequality 
in the tide at New York is relatively small and hence the tide here is of the semidaily 
type. 
In passing, it may be noted that Figure 4 shows clearly that the diurnal inequality 
of the tide at New York is greater for the high waters than for the low waters. That is, 
the little inequality that exists in the height of the tide in New York Harbor is exhibited 
principally by the high waters. 
In the section on diurnal inequality it was noted that this feature of the tide varies 
within a period of a fortnight, depending on the declination of the moon. In Figure 4, 
it is seen that from the third to the fifth day of the month, the tide curve exhibits rela- 
tively little diurnal inequality, while from the 11th to the 13th the inequality is rela- 
