rEBEUAET 21, 1919] 



SCIENCE 



183 



4 6 8 10 XII ; 4 6 8 10 M ? 4 6 8 10 XII ; 4 6 8 10 M 2 4 6 8 10 mi ;« 6 e 10 M 2 4 6 8 10 XII Z 4 6 a 10 M 2 4 6 B 10 Xll ? 



Fig. 5. Balogram, Grand Turk Island, West Indies. 



meters to perhaps 500 meters thick, the pas- 

 sage of the air billows, like the passage of 

 waves in shallow water, necessarily produces 

 greater or less corresponding changes in the 

 pressure on tlie bottom — changes that, as 

 shown in Fig. 4, appear as a series of ripples 

 in the record of a sensitive barograph. 



FlO. 6. Average daily barometric curves, Key 

 West, Florida. 



During the summer, when air billows rarely 

 form near the surface, though frequently at 

 greater altitudes, esiiecially that of the cirrus 

 cloud, barometric ripples do not appear. 

 This, doubtless, is because wave disturbances 

 in air, as in water, do not penetrate far be- 

 neath the wave level. 



Semidiurnal Pressure Changes. — It has been 

 known now for more than two and a half 

 centuries that there are approximately regular- 

 daily variations in the height of the barometer 

 that culminate in two maxima and two min- 

 ima in the course of twenty-four hours. Dur- 

 ing particularly calm weather these fluctua- 

 tions are conspicuous on the current barogram, 

 as shown in Fig. 5, obtained at Grand Turk 

 Island, West Indies, and always revealed by 

 averages, no matter how masked by storm 

 conditions, as shown by Fig. 6, based on' 

 records, covering fourteen years, at Key West,. 

 Fla. 



Although, as stated, this phenomenon of the 

 semidiurnal variation of the atmospheric pres- 

 sure has been familiar almost since the in- 

 vention of the mercurial barometer, and al- 

 though repeatedly studied, its cause or causes 

 remained until quite recently wholly unknown. 



Some of the observed facts in regard to this 

 twe^-e-hour cyclic change of pressure are : 



(a) The maxima occur at, roughly, 10 o'clock 

 A M. and 10 o'clock p.m. 



(6) The minima occur at, roughly, 4 o'clock 

 A.M. and 4 o'clock p.m. 



(c) The amplitude, when other things are 

 substantially equal, varies with place approxi- 

 mately as the square of the cosine of the 

 latitude. 



