(191) 
Further let the difference between the mean sea-level and the 
height of half-tide be 4, the mean sea-level Z, high water V, low 
water £. Half-tide is }(V-+ #) and the range of the tide(V — £), 
The causes, which have an influence on the value of A, are four 
in number: 
Ist. the range of the tide (V — EZ); 
2nd, the mean sea-level (Z); 
3rd, the time of the year; 
4th, the presence of ice. 
In the last mentioned case, I am not alluding to the fact of the ice 
preventing the working of the tide-gauge, for I consider this to be an 
interruption, but to the fact, that the presence of ice at a certain 
distance from the tide-gauges, deforms the tide-curve. This defor- 
mation is, i my opinion, one of the most interesting researches on 
tides. 
I propose to solve the following question. What corrections are 
wanted for Delfzyl in the value A, deduced from a. certain number 
of years, in order to find that quantity for separate months ? 
The data, which I had at my disposal were the values of V and 
E for 18 years (July 1881—July 1899) the values of 2 for 7 years 
(1884—1890) viz. the height at 2, 5, 8 and 11 o’clock, and in 
addition the height at 2 and 8 o’clock for 8 years (1891—1898). 
The mean range of the tide at Delfzyl is, according to these 
data 2750 m.m., the mean sea-level Z is according to the calcula- 
tions of the above-mentioned commission 128 m.m., reckoned from 
the zero of the tide-gauge during the years 1884—1890. The mean 
value A during these 7 years is 193 m.m., so we find that the 
mean of half-tide is 128—193 mm. = — 65 mm. Tide-curves 
of spring and neap-tides accompany this paper. 
It is difficult to determine, how much each of the four causes, 
influences A at Delfzyl, as they often modify A in the same direction. 
So, during the year, the correction for each of the three first-named 
causes, is generally a sinusoide of about the same amplitude and 
the same phase. | 
It is therefore necessary, to adopt a certain definite value for one 
of these causes. I assume that the correction, due to the first cause, 
is proportional to the difference of the mean range of the tide and 
the observed value of that range V—ZE and that their proportion 
is equal to the ratio of the mean values A and V—E, or ceteris 
paribus, A is always proportional to V—Z£. In substance this will be 
14 
Proceedings Royal Acad. Amsterdam. Vol. II. 
