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the mean sea-level and half-tide (the mean of high and low water), 
during several years or months. 
This has been done by the above-mentioned Commission, in their 
calculation of the mean sea-level for several years for Den Helder. 
Dr. H. G. VAN DE SANDE BAKHUYZEN also in his communication 
“On the variation of latitude,” to the meeting of the Royal Aca- 
demy (24 of Febr. 1896), assumed that the mean value of that 
difference during a month was a constant quantity at Den Helder. 
In both cases this was perfectly justified, as this value for the 
annual means is very nearly constant at Den Helder, and in the last 
case differences that may exist, are eliminated by the method of 
determination. However, the supposition that the difference is con- 
stant is not true for the annual means at all stations, and is 
certainly not so, for the monthly means at some stations. 
I intend to trace those causes, which produce a difference in this 
value, and to find its range for one tide-gauge. I took Delfzyl for 
the observation-station, as at Delfzyi the difference between half-tide 
and the mean of the sea-level, is greater and more variable than at 
any other station in our country. From another point of view, 
Delfzyl would not be so advantageous, as there a comparison with 
tide-gauges in the neighbourhood is not possible. 
Before proceeding further, a few words, to point out the impor- 
tance of the law of the variation of that difference, are necessary. 
The knowledge of the mean sea-level is not only important for the 
annual means, but also for the monthly means, as we can deduce 
from them the annual variation, and also because an exact knowledge 
of the monthly means, assists in the detection of the unavoidable 
changes of the zero’s in the automatic tide-gauges, and the deter- 
mination of their values. As the high and low water marks are 
always determined in the first place, their mean is naturally known; 
therefore it saves much trouble, if it is possible to deduce from that 
mean value, the true mean sea-level, as the hourly observations can 
be then neglected. Besides, in the event of interruptions, which 
happen frequently in using the automatic tide-gauges, it 1s much 
easier to guess, the positions of high and low water, than the hourly 
heights, as high and low water are independent of the exact time. 
Moreover meteorological circumstances have, by the retardation or the 
acceleration of the tide, a greater influence on the hourly heights 
than on high and low water. 
The mean sea-level can therefore be deduced more exactly from 
the height of half-tide, the difference of both being known, than 
from the hourly observations when some of these must be guessed. 
