( 189 ) 
Sy 19° = 0.6 hours, 
My 19502 
N 21955003 
Ky 217 = 0:7. » 
so that the difference amounts to 42 minutes of time, whilst the 
amplitude may be assumed to be twice as large at Sabang — at 
least in the back parts of the bay — as on the road of Oleh-leh. 
The mono- and semi-annual variations are for both places some- 
what different; from the three series of observations at Oleh-leh it 
appears however that in these regions the monthly mean values of 
the sea-level widely differ for different vears, so that a better agree- 
ment might be expected only if the observations extend over a long 
series or at least over simultaneous periods. 
It is of some importance to remark that, whereas the semidiurnal 
tides at Sabang are nearly twice as strong as near Oleh-leh, the 
mono-diurnal tides seem to be amplified in a far less degree. 
This point, concerning the way in which both tides propagate 
and are enlarged or diminished, is of great importance for the 
understanding of the mechanism of tides and requires a thorough 
investigation. 
With a view of elucidating this point tide-gauges ought to be 
established at the entrance and in the back parts of bays and 
estuaries: for these experiments however stations should be chosen 
where the mono-diurnal tides are better marked than at Sabang 
so that an accurate determination of the characteristic constants is 
possible. 
An analysis of tides at different parts of a river in which a tidal 
wave of mixed description propagates would also afford useful data 
for this purpose. 
Hydrography. — “On the relation between the mean sea-level and 
the height of half-tide.’ By H. E. De Bruyn. 
The mean sea-level is the mean of the height of the water ob- 
served at short intervals i.e. every hour. 
Observations have proved, that the mean of 3-hourly observations 
does not practically deviate from this; in this way the mean sea- 
level in the years 1884—1888 has been determined by the Royal 
Geodetical Commission (Annual report of the Commission 1889). 
Generally it is admitted that there is a constant difference, between 
