( 705 ) 

 TABLE I. 



If we assume that in each of the 3 periods of 18 years this 

 difference has been constant, we find for it : 



in the !«* period 1700—1717 319 mm. 

 „ „ 2d „ 1796—1813 318 „ 

 „ „ 3d „ 1843—1860 316 „ 



The mean error of a yearly mean is then ± 14,8 mm. 



If we suppose that from 1700 — 1860 the difference has remained 

 constant, then the difference derived from all the 58 years amounts 

 to 318 mm. and the mean error of a yearly mean to ± 14.4 mm. 

 Therefore we are justified in assuming that the difference has remained 

 constant during the whole period 1700 — 1860 and was 318 mm. ± 

 1,9 mm. 



Moreover we derive from this table that between 1700 and 1860 

 no perceptible change has occurred in the influx and outflow of the 

 water from the Northsea to the Y, no more than in the mean level 

 of the Northsea with regard to the mean level of the Y. 



48 



Proceedings Royal Acad. Amsterdam. Vol. X. 



