Geophysics. — "On the determination of tidal constants from obser¬ 
vations performed with horizontal pendulums” By Dr. J. P. 
VAN DER STOK. 
(Communicated in the meeting of April 28, 1909). 
1. When applying the theory worked out with astronomical 
accuracy of the analysis of tidal observations we are checked in 
practice by disturbing influences of meteorological nature which may 
be great with respect to the quantities to be calculated, making thus 
the accuracy of the theory as well as that of the results illusory. 
These disturbing influences are greatest when we calculate the tides 
of long duration, where meteorological factors of irregular nature 
play such a preponderant part, that the definition of the constants 
is only then possible if it is based upon series of such long duration 
that those disturbances can be regarded as eliminated. Each investi- * 
gation ought then to be preceded by a study of those disturbances 
and a taxation based upon the latter of the demand to be put as a 
minimum for the length of the series. 
For most tides of short duration these objections do not hold, or 
at all events in a much less degree, they do however for those tides 
whose period differs but little from that of mean solar time and which 
are connected with it in a systematic way, namely the sidereal 
tides proper K x and AT, and the sidereal tide improper P. 
As the disturbances of meteorological nature appearing here have 
not only an irregular but also a regular character, the former can 
be eliminated by making the calculation over a great many years, but 
not the latter and only by means of more or less uncertain hypotheses 
shall we be able to come closer to our aim. 
Thus. e. g. the tides AT, and P can only then be calculated out of 
the observations if we assume that the diurnal meteorological tide 
S x remains constant during the whole year, a supposition which is 
certainly inaccurate because the mean amplitudes of the motion <S\, 
finding its origin in the land- and seawind, must be considerably 
less in winter than in summer. 
As lucky circumstance can be regarded that *$>, is generally small 
a. o. on European coasts, where however on the other hand K x and 
P too, are abnormally small, abnormally, namely, with respect to the 
value evaluated out of the theory of equilibrium. In tropical regions 
& is in many places not small, and only where, as in the Java sea, 
K x and P are great, an approximative determination of K x is possible, 
