( 708 ) 



1 have confined myself to the mean of the 5 years 1700 — 1704 and 

 1856 — 1860. To these means we must tuhl the variations dui'ing a 

 period of tv\a) years, wliieh are probably smaller than 3 mm. and 

 5 mm. the values which would follow from the periods of 18 

 years; instead of these I adopt 1 mm. and 4 mm. and consequently: 



adopted A. P. above mean sea level in 1700 = 164-1-1=165 mm. 

 A.P. „ „ „ „ „ 1860=76-^4= 80 „ 



For the mean error of these values 1 have derived ±12 mm. 



As yet it remains undecided whether the variation from 165 mm. 

 to 80 mm. is due to a slow variation in the mean level of the North 

 sea on our coast, or to a variation of the adopted A P. in the tidal 

 station either caused by the sinking of tlie whole station or of the wooden 

 floor, or by accidental or perhaps intentional changes in the height 

 of the A.P. on the gauging rod which during the period from 1700 to 

 1860 has certainly been renewed several times. 



Some data towards the solution of this dilemma may be borrowed 

 from the elevations of the bench marks in the 5 sluices : Oude 

 Haarlemmersluis, Nieuwebrugsluis, Kraansluis, Westindischesluis and 

 Kolksluis; these bench marks have been established in 1682, and 

 consist of grooves cut in stones indicating the elevation of the A.P. 

 The good mutual agreement between the heights of the grooves in the 

 year 1875 whicli appeared from the levelling made by our meml)er 

 Dr. Lely (the largest ditference between them amounted to only 

 8 mm.) proves that those grooves have been placed with the greatest 

 care, and makes us confident that in 1700, when the first observations 

 in the tidal station were made, the zero on the gauging rod agreed 

 well with that on the stones placed in the sluices some years earlier. 



We are therefore entitled to assume with a high degree of probability 

 that in 1700 the A.P. on the 5 sluices was 165 mm. above the mean 

 level of the Y. 



In 1860 Stamkakt l)y a levelling has com[)ared the height of the 

 A.P. in the tidal station at that time with the heights of two bench 

 marks in the tower of the St. Anthoniewaag. He found : 



lower bench mark 3208,4 mm. above A.P. in the tidal station 

 higher ,, ,, 3705,4 mm. ,, ,, ,, ,, ,, „ 



In the same year Dr. Stamkakt and Mr. \. d. Stekk have also 

 determined by means of levelling the ditference in height between the 

 higher bench mark in the St. Anthoniewaag and the grooves iji the 

 5 sluices (Versl. en meded. XVII p. 277—284). From these observu- 



