THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPPIICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MA Y 1859. 



XLIX. On the Semidiurnal OsciVation of the Barometer. 

 By W. S. JevonSj Assaijer, Sydney Branch of the Royal Mini*. 

 1. '1^0 explain the remarkable double oscillation of the baro- 



L meter which occurs during each day in most parts of 

 the world, a theory has been proposed by the eminent meteoro- 

 logists Dove and Sabine, and has, I think, been favourably 

 recr'ived by the scientific world, which I yet venture to call m 

 question. It is concisely stated in Sabine's Notes to ' Cosmos f/ 

 and is somewhat as follows : — In all countries which are not far 

 removed from large surfaces of water, the barometer on an ave- 

 rage stands highest about 9 a.m. and 9 p.m., and it stands low- 

 est" at the nearly intermediate hours of 2 or 3 p.m., and i or 5 

 A.M. (sunrise). This phajnomenon, being of daily occurrence, 

 can only depend ultimately on the sun's heat and the rotation 

 of the earth ; the intermediate causes yet admit of debate. Now 

 as the temperature of the air only undergoes a single oscillation 

 daring the day, it could only, and in some parts of the world 

 does only occasion a single minimum of the barometer during 

 the day, and a single maximum at sunrise; it is supposed, in 

 fact, that this is always the effect produced by the agency of 

 temperature. But the aqueous vapour contained in the air also 

 undergoes a daily variation ; its elastic force is greatest in the 

 daytime and least at night, and this varying force must equally 

 affect the barometrical column. Now the two undulations thus 

 continually produced in an independent manner are superini- 

 posed 81) as to disguise each other, and the result is the semi- 

 diurnal oscillation of pressure under consideration. It is the 

 great deficiency of aqueous vapour at sunrise which cau.ses a 



* Communicated by the Author, 

 t Vol. i. p. xcix. 

 Phil. May. S. 1. Vol. 17. No. 115. May 1859. Y 



