104 U. S. COAST AND GEODETIC SURVEY 
The horizontal line associated with each of the diagrams of Figure 49 represents 
for each station the average height of low water below sea level for the 25 years repre- 
sented, this height in feet being given by the figures at the left of the horizontal line. 
The yearly values are seen to range themselves more or less closely along the sme-like 
curves associated with each diagram. 
Referring back to Figure 43 which represents the yearly variation in high water 
relative to sea level, it is seen that the two sets of curves in Figures 43 and 49 are comple- 
mentary. In other words, when the rise of high water from sea level is above the aver- 
age, the fall of low water below sea level will be greater than the average, and vice 
versa. This follows from the periodic variation in range of tide discussed in connection 
with Figure 43. 2 
Low water therefore, like high water is subject to periodic variations from day to 
day, month to month and year to year. And like high water, too, low water is subject 
to the nonperiodic variations found in sea level. 
Definition of Mean Low Water 
In view of the variations to which the height of low water is subject, mean low 
water at any place may be defined simply as the average height of the Jow waters at 
that place over a period of 19 years. 
Yor tides of the semidaily and mixed types of tide, this simple definition is im- 
mediately applicable. But for the daily type of tide the same difficulty comes up as 
in the case of mean high water, namely, whether or not to include the secondary tides 
that occur occasionally. Referring back to this matter (page 86), it is preferable in 
determining mean low water to disregard the secondary tides and use but one low 
water a day, the lower low water. Mean low water for the daily tide is thus the same 
as mean lower low water, and the simple definition of mean low water becomes applicable 
to all types of tide. 
Primary Determination 
A primary determination of mean low water is derived directly as the average of 
the low waters over a 19-year period. And if there were no change in sea level from 
one 19-year period to another, we would expect two different 19 year determinations 
of mean low water at any place to agree, unless some change in tidal regime had taken 
place. 
From the 46 years of continuous tide observations at Baltimore we can get two 
19 year series for the primary determination of mean low water. Taking the first 19 
years, 1903-1921, mean low water averages 3.533 feet on the tide staff, while the 
last 19 years, 1930-1948 averages 3.786 feet, a difference of 0.253 foot. Sea level for 
1903-1921 averaged 4.107 feet and for 1930-1948 it averaged 4.368 feet. Hence with 
reference to sea level for the respective 19-year series, mean low water at Baltimore 
for the period 1903-1921 was 0.574 foot below sea level and for the period 1930-1948 
it was 0.582 foot below sea level. The difference of 0.008 foot between the two latter 
values is so small as to be of little significance, so that mean low water with reference 
to sea level is practically the same for the two 19-year periods. 
