( 710 ) 



this invariability we may deiive fVoin the observations the subsidence 

 of the A. P. in the tidal station with regard to the A. P. derived from 

 the marks in the 5 sluices, amounting to 165 — 80 = 85 mm. between 

 1700 and 1860. 



The method by which the height of the water in the tidal station 

 was obtained and the possible causes of the subsidence of the zero on the 

 rod added to the invariability of the 5 grooves in the sluices and hence of 

 a fairly large part of the ground of Amsterdam with regard to 

 the sea, render the idea very probable that this subsidence has a pure 

 local character and that we are not entitled to derive any results with 

 regard to the subsidence of a larger part of the ground of Amsterdam. 



It has often been asked what the Amsterdam zero represents. 

 Our colleague Dr. van Diesen has devoted to this subject an 

 interesting study in which he has gathered from old documents 

 everything which may help us to tind how this zero has been 

 established. With certainty nothing can be derived from it. But 

 the observations show : 1 that in 1700 the A. P. was 165 mm. ± 

 12 mm. above the mean sea level in the Y, 2 that the height of the 



318 



mean high water was =159 mm. ± 1 mm. above the same mean 



^ 2 



sea level, and we conclude thence that both in J 700 and 1860 the 

 A. P. within the limits of the errors of observation agreed with the 

 mean high water in the Y. 



Astronomy". — "0?i the masses and elements of Jupiter s satellites, 

 and the mass of the system, (continued), by Dr. W. de Sitter. 

 (Communicated by Prof. J. C Kapteyn). 

 III. The great inequalities. 



The values of these, derived from the heliometer-observations of 

 1891, 1901 and 1902, have been collected in Table III, together 

 TABLE III. GREAT INEQUALITIES. 



