TIDAL DATUM PLANES 3 
At one place a strong current may accompany a tide having a very moderate rise and 
fall, while at another place a like rise and fall may be accompanied by a very weak 
current. Furthermore, the time relation between current and tide varies widely from 
place to place. At some places the strength of the current coincides with high and 
low water, while at other places the slack of the current coincides with high and low 
water. Hence flood current is not everywhere synonymous with a rising tide nor is 
ebb current synonymous with a falling tide. 
Unfortunately, there is no term in the English language by which to designate 
the whole phenomenon which includes both tides and tidal currents. Frequently ‘‘the 
tide”’ or ‘‘flood and ebb”’ is used in this general sense, and no confusion arises from this 
usage if the context clearly indicates that the term is intended in its general sense. 
For the sake of clearness, however, when the vertical movement of the water is meant, 
tide is to be used, and when the horizontal movement is meant, current is to be used. 
Characteristics of the Tide 
In its rise and fall the tide does not move at a uniform rate. From low water 
the tide begins rising very slowly at first, but at a constantly increasing rate for about 
three hours, when the rate of rise is at a maximum. The rise then continues at a con- 
stantly decreasing rate for the following three hours, when high water is reached and 
the rise ceases. The falling tide behaves in a similar manner, the rate of fall being 
least immediately after high water, but increasing constantly for about three hours, 
when it is at a maximum, and then decreasing for a period of three hours till low water 
is reached. 
The rate of rise and fall and other characteristics of the tide may best be studied 
by representing the rise and fall of the tide graphically. This may be done by reading 
the height of the tide at regular intervals on a fixed vertical staff graduated to feet 
and tenths, plotting these heights to a suitable scale on cross-section paper and drawing 
a smooth curve through these points. A more convenient method is to make use of 
an automatic tide gage by means of which the rise and fall of the tide is recorded on a 
sheet of paper as a continuous curve drawn to a suitable scale. Figure 1 shows a tide 
curve for New York Harbor for the last two days of June 1934. 
In Figure 1 the consecutive numbers from 0 to 24, increasing from left to right, 
represent the hours of the day beginning with midnight. Numbering the hours con-: 
secutively to 24 eliminates all uncertainty as to whether morning or afternoon is 
meant and has the further advantage of great convenience in computation. The 
numbers on the left increasing upward from 0 to 6 represent the height of the tide in 
feet as referred to a fixed vertical tide staff. The tide curve approximates the well 
known form of the sine or cosine curve. 
The rise and fall of the tide at any place is characterized by numerous features 
which differ at different places. Of these features, three may be considered as con- 
stituting the principal features; namely, those relating to the time of tide, to the range 
of the tide, and to the type of the tide. 
The time of tide has reference to the times of occurrence of high and low water 
with respect to the moon’s meridian passage. That is, as a characteristic feature of 
the tide at a given place, the time of tide is specified by the high water and low water 
lunitidal intervals. These intervals are not constant, but vary periodically within 
