120 



thej can only be applied in those regions where the atmospheric 

 conditions are quiet enough to retain the series of observations 

 within practicable limits. 



Experience gained by means of pilot balloon observations at 

 Batavia justified the supposition that here favourable results might 

 be acquired by such investigation, and therefore during the last 

 few years we have continued the ascension of pilot-balloons started in 

 1909. As early as 1912 ascensions at 2 p.m. and 7 p.m. were 

 added by the first mentioned of us to the ascensions which before 

 tiiat date usually took place at 8 a.m., in order to gather further 

 knowledge of the phenomenon of land and sea breezes. The results 

 obtained induced him to start in 1913 an extensive series of observ- 

 ations at different hours between 6 a.m. and 6 p.m., more in 

 particuhir witfi a view to study the diurnal and semi-diurnal variation 

 of the wind. However, at the time he was not able to have nocturnal 

 ascensions made of balloons carrying lights, like those that first 

 took place in 1912, but in 1914 we again proceeded to these 

 nocturnal observations, when the latter of us joined the investigation. 



A continuation of these nocturnal observations was checked, 

 however, by the outbreak of the war in August 1914, so that only 

 a series of day balloons could be sent up and not before the latter 

 part of 1915 was it possible to have an extensive series of nocturnal 

 ascensions made. 



From the above it will appear that on account of various circum- 

 stances, partly not under our control, the ascensions have by no 

 means been conducted in such a manner as would iiave been most 

 suitable to the investigation, i.e. equally distributed over the day 

 and during the same season. 



These drawbacks have partly been neutralized by the following 

 circumstances. "" 



For the deduction of the semi-diuriial variation, which is the most 

 important and which it is our aim to investigate, it is sufficient to 

 divide the observations over half a whole-day, as was usually 

 done before. 



The observations were made in periods as little disturbed by 

 showers as possible, this being even more feasible for half days, or 

 shorter parts of a day, than for whole-days. 



The .ascensions took place within the semester May /November, 

 i.e. principally with northern declination of the sun and eastern winds. 



Of the greater part of the ascensions the altitude reached by the 

 balloons has been calculated trigonometrically from double-observation. 



The direction and length of the bases were the following : 



