Meteorology. — ''The semi-dmmnl horizontal oscillation o f the f ree 

 atmosphere up to 10 km. above sea level deduced from pilot 

 balloon observations at Batavia." By Dr. W. van Bemmelkn 

 and Dr. J. Boekema. (Communicated by Dr. J. P. v. d. Stok). 



(Communicated in the meeting of May 26, 1917). 



The great regularity of the semi-diurnal variation of the air 

 pressure in the whole equatorial zone, as well as the constancy of 

 its amplitude and phase all over the earth prove that the atmos- 

 sphere as a whole also performs a regular semi-diurnal oscillation. 

 Above all it was Jui.. Hann who brought to light the simple laws 

 commanding this phenomenon, while Margules proved this pheno- 

 menon probably to be a phenomenon of resonance by making it evident 

 that an infinitely thin shell of the atmosphere has a period of oscil- 

 lation of its own of neai'ly 12 hours and consequentlj' will resound 

 to a diurnal disturbance as caused by the sun's radiation. 



From the wind observations on mountain tops in Europe and 

 North America and also on those in southern British India Hann ') 

 deduced that this variation of the air pressure is accompanied by a 

 horizontal wind oscillation possessing an amplitude of some deci- 

 meters pro sec. 



This horizontal atmospheric oscillation may be called an important 

 geophysical phenomenon. Thus Arth. Schuster founded his theory 

 of the diurnal variation of terrestrial magnetism on the presence of 

 the above oscillation also in the very upper layers of the atmosphere, 

 and it might, therefore, be desirable to try and obtain more inform- 

 ation by observations in the free atmosphere, where disturbances 

 caused by convection will be of less influence than they must be 

 on high mountain tops. 



Though this has already been done to some extent by means of 

 cloud observations, no exhaustive nor distinct results could be obtained 

 in this way. 



The only suitable method of observation consists in a series of 

 pilot balloon observations, which, however, are so complicated that 



') Sitzungsber. d. Ak. d. W. in Wien 1908. 



