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level of the Northsea during the period 1700—1860. A short extract 
from this communication may follow. 
From 1700—1860 the level of the Y at Amsterdam was daily 
and hourly noted down at the town’s tidal station. As appears from 
these records the difference in height of high water and low water 
remained unchanged all through this period, from which follows that 
the communication between the Y and the Northsea has not sensibly 
changed, and so the results of the changes in the mean level of the 
Y apply also to the mean level of the Northsea. 
From this record I have further derived the most probable level 
of the water of the Y in the first and the last year of observation, 
1700 and 1860, with reference to the zero of Amsterdam (A.P.) at 
the tidal station, and I have found for it —165 mm. in 1700 and 
— 80 mm. in 1860, both with mean errors of +12 mm. 
In the years 1860 and 1875 the difference between the height of 
the zero of Amsterdam at the tidal station and that of the A.P. 
derived from the grooves in 5 stones, which were placed in 5 sluices 
in 1682, was determined by accurate levellings. This difference 
amounted to 82 rom., so that in 1860 the mean level of the Y with 
relation to the A.P. of the 5 stones was —80—82 or —162mm. 
Now what was in 1700 the position of the A.P. at the tidal 
station with respect to the A.P. of the 5 stones? 
As far as is known the grooves in the stones placed a few years 
before 1700 were the only marks by means of which the A.P. was 
accurately determined, and so it is almost certain that the A.P. at 
the tidal station has been derived from the height of these grooves 
by means of simultaneous observations of the height of the water 
at stagnant water. We may further assume that in that determination 
a not much greater error has been made than in the determination 
of the relative heights of the grooves in relation 'to each other, which 
probably has taken place in the same way. How accurate those 
relative heights were then determined, can still be ascertained. In 1875 
it appeared namely from an accurate levelling, that the heights of 
the grooves differed only some mm., at most 8 mm. from each other. 
It is therefore probable that in 1700 the difference of the A.P. of 
the sluice stones and the A.P. at the tidal station will have amounted 
to no more than a few millimeters. The mean height, —165 mm- 
of the level of the Y in 1700 with regard to the A.P. at the tidal 
station may, therefore, also be considered as representing about the 
mean height with respect to the A.P. of the sluice stones, and this 
height being —162 mm. in 1860, we come to the conclusion that 
from 1700 to I860 no perceptible change came in the height of 
