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at Helder in the years 1855 -1912, and as the results of the years 
1893—1912 were not at my disposal, Mr. Gockinea, Chief engineer 
Director of the “Waterstaat”, was so good as to let me have the 
monthly averages of these years. 
2. before I give an account of how the monthly averages were 
used by me, it is desirable to explain the exact significance of the 
observation material. The tide curve of Helder, with its double 
maximum, has an asymmetrical form, which differs considerably from 
a sinecurve, so that to deduce the exact mean sea level during a 
day from the observations, one must either determine the area of 
the surface enclosed by the tide curve with a planimeter, or, as 
will also be sufficiently accurate, determine the average value of the 
24 hourly heights. From the daily means one can then deduce the 
monthly means. 
It will be clear that the work which is necessary to calculate 
all the observations in this way for the more than 21,000 days from 
1855 to 1912 is very great; fortunately for our purpose we can use 
an easier way, as we do not need to know the actual mean heights, 
but only their mutual differences. If the tidal curve were symme- 
trical with respect to the mean sea level, the half of the sum of high 
and low water would correspond to the mean sea level of that day ; 
but the form is not symmetrical, and even changes periodically, so 
that there is not only a difference between the half of the sum 
of high and low water, and the mean sea level, but this difference 
changes from day to day. If, however, we determine the average 
form of the tide curve during the period of a month, then we get 
a fairly constant shape, and for such a period one may assume, that 
the difference between the half sum of all the high and low waters 
and the mean sea level is almost constant. This assumption will 
differ even less from the truth, if we take the average of a great 
number of monthly means from different years, which is the case 
with my calculations. 
On these grounds I have taken as the monthly means of the 
sea level the half of the sum of the high and low waters during 
these months, deduced from the registered tidal curves in the years 
1855—1912. 
These monthly means show rather marked deviations from the 
annual mean, due partly to the yearly and half-yearly sun tide, 
and partly to the regularly changing meteorological conditions. From 
58 years, I found for Helder the following mean values for yearly 
means—monthly means in millimetres. 
