( 192 ) 
he case. I adopted as this proportion 1/,,, the correction is therefore 
Is (V —BL—2750) mm. 
As concerns the second correction we see that when the sea-level 
is higher, A is smaller than the mean value, and vice-versa. 
We do not know exactly the law governing these small changes, 
because various unforeseen circumstances e.g. storms influence them. 
The only thing that can be done, in my opinion, is to take for this 
correction a quantity proportional to the deviation of the mean sea-level. 
As it was my intention to deduce the value of 7 from that of half-tide, 
which is known, I adopted for the value of that correction a quantity 
proportional to the deviation from the mean height at half-tide 
(— 65 mm.). 
After comparing the same months of different years, I found 
that this correction amounts to about 7/o) or 7/39 of the value of 
that deviation i.e. Jao or 3/59 of (V+ E+ 130). I have adopted 
Ios (V + E + 130). 
That the height of Z has an influence on the form of the tide- 
curve, is probably due to the mud-banks in the Dollard. The surface 
that must be covered, constantly changes with the level of the sea, 
and so for equal tide-ranges, the quantity of water flowing in and 
out of the Eems at Delfzyl, is much greater for high sea-levels 
than for low. | 
Both corrections being applied another annual correction is still 
wanted. For this correction I adopted an annual sinusoide, the 
amplitude and the phase of which can be easily determined. The 
amplitude is in round numbers 10 mm. and the greatest positive 
value occurs about the 1st of July. From the observations in the 
seven-yearly period, there is no evidence of the existence of a half- 
yearly sinusoide. Considering also the heights at 2 and 8 o'clock 
for the period 1891—1898, there appears to be a semi-annual sinus- 
oide, but the amplitude is very small, and it is questionable whether 
the sinusoide derived from those observations is not different from 
the meau sinusoide. It is better to entirely neglect this correction. 
After appiying these corrections, the values of the sea-level for 
some months, still show great negative divergencies. It is obvious 
that these are exactly the months in which we have a very low 
temperature, and in which there must have been ice. But as the 
mean temperature of a month is not an exact proof of the presence 
of ice, I adopted as a datum the thickness of the ice according to 
the observations at Den Helder (see the Proc. Kon. Inst. van Inge- 
nieurs) as quoted in the following table. 
