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Geophysics. — “On the duration of showers at Batavia.” By 
Dr. J. P. VAN DER STOK. 
Since January 1st 1866 hourly observations of different meteoro- 
logical quantities have been made at the Batavia Observatory and 
also of the rainfall so that, at present, a series of 40 vears is available 
comprising some 25000 rainy hours distributed over about 8200 
showers. 
In less favourable climates it is not practicable to have such 
observations made continuously night and day and, as self-registering 
instruments are subject to frequent interruptions, this series may be 
regarded as a unique material for investigation. 
The purpose of this inquiry is to investigate the distribution of 
showers of variable duration in different seasons, and to apply to 
these frequencies suitable frequency-formulae. 
A rainy hour is defined as every hour during which rain fell, if 
it were only 0.1 m.m.; the duration of a shower is defined as the 
number of subsequent hours in which rain was observed: e.g. by 
a shower of 10 hours’ duration we do not assume incessant rain 
during this period, but that no hour has passed without some rain 
having fallen. 
As during the first decade no observations were made on Sundays, 
the total amount of hours e.g. for January is not equal to: 
40.31.24 — 29760 
but to: (40.31—44) 24 — 28704. 
The results of this inquiry have been summarized in Table I. 
Average values of quantities so divergent and intermittent as 
rainfall are hardly sufficient to convey an adequate idea of the way 
in which this phenomenon affects the climate and it is questionable 
whether excessive quantities or durations are to be inciuded in the 
computation, because even the mean values deduced from a long 
series of observations may be affected to an important degree by 
one shower and thus the meaning which we attach to averages, 
loses its value. 
Consequently the frequencies given in Table II give not only a 
more complete, but also a more accurate idea of this climatological 
factor than the average values of Table I. 
From this summary it appears that in April, almost suddenly, the 
condition of rainfall shows an alteration such, that the probability 
of showers of long duration is considerably reduced; in the next 
months this probability again increases whilst in August the distribution 
