16 U. 8. COAST AND GEODETIC SURVEY 
stated that the range during the month varied from 50 percent below its average value 
to 50 percent above. 
The range of the tide at any place may be very greatly affected by wind and weather. 
But quite apart from the effects of disturbed meteorological conditions, the range varies 
in accordance with the astronomical positions of sun and moon relative to the earth, 
the principal variations being due to the phase, parallax, and declination of the moon, 
the tide at different places responding in different degree to each of these three causes. 
In the semidaily type of tide, the variations in range during a month are largely 
those due to the phase and parallax of the moon, the variation due to the declination being 
but of minor importance. In response to the parallax variation, the tides rise higher 
and fall lower than the average when the moon is near its perigee, and rise and fall 
least near the times of apogee. In response to the phase variation, the tides rise higher 
and fall lower near the times of new and full moon, and rise and fall least near the times 
of the moon’s quadratures. As a rough figure, 50 percent variation in range from the 
monthly average may be taken as characteristic of the semidaily type of tide. 
As noted before, diurnal inequality is manifested both in the heights and in the 
times of the tide. But so far as tidal datum planes are concerned, only the height 
inequality is of importance, and hence diurnal inequality in times may here be left 
out of consideration. 
The Daily Type of Tide 
The daily type of tide may be defined as one in which one high and one low water 
occur in a tidal day. Figure 5, which pictures the rise and fall of the tide at Pensacola, 
Florida for the month of June 1934, may be taken as representative of this type of tide. 
Figure 5 shows that the tide at Pensacola went through two periods of variation 
in range, with the minima occurring on the 5th and 20th and the maxima on the 12th 
and 28th. (The fluctuation from the latter part of the 19th to the early hours of the 
20th, which was obviously due to disturbed weather conditions and not to tidal causes, 
is left out of consideration in this connection.) In June 1934, it will be recalled, the 
moon was over the Equator on the 5th and 19th, and in the tropics or at its semimonthly 
maximum declination on the 12th and the 26th. In the daily type of tide, therefore, the 
variation in range is principally in connection with the declination of the moon, while | 
in the semidaily type of tide it is principally in connection with the phase and parallax 
of the moon. 
For the month of June 1934, the range of tide at Pensacola averaged 1.44 feet. 
The least range occurred on the 20th when it was 0.5 foot, and the greatest range oc- 
curred on the 28th when it was 2.5 feet. On a percentage basis, the least range was 
65 percent less than the average monthly value, while the greatest range was 74 percent 
above that value. In the daily type of tide, therefore, the variation in range of tide 
during the month is considerably greater than in the semidaily type. 
It should be noted that places where the tide is at all times of the daily type are 
rather uncommon. At Pensacola, for example, there frequently occur two high and 
two low waters during the day at the times when the moon is over the Equator. In 
Figure 5 it will be noted that on the 5th of the month, towards the end of the day, there 
was a marked change in the slope of the tide curve which reveals the effects of the semi- 
daily constituents of the tide. If the tide for the greater part of the month at any place 
is of the daily type, the tide at that place is designated as belonging to the daily type. 
