26 



Mr. J. A. Brouu on Simultaneous 



[May 11, 



near the pole formed a kind of cap {calotte) to the globe, that this pass- 

 ing towards the equator in all longitudes, or advancing in sectors, would 

 produce the atmospheric waves. " Les ondes, dans cette hvpothese, 

 devraient se propager en meme temps que les courants polaires des poles 

 vers I'equateur, et dans notre hemisphere du nord vers le sud"*. Tet 

 after a study of the directions of the wind, he concludes, " Les directions 

 des vents n'ont pas des rapports apparents avec les directions des ondes 

 barom etriques" t. 



It is not necessary to criticise these hypotheses fiu'ther. That heated air 

 rises, and that currents of air are associated v^ith great dimmutions of 

 atmospheric pressure, are facts which do not suffice to explain the great 

 semidiurnal atmospheric tide, nor the sudden appearance of atmospheric 

 waves with crests reaching from London to Pekint and breadths of 1000 

 miles §. 



It does not appear impossible, however, that other causes of varying 

 atmospheric pressure may exist than change of the mass of air; in 

 other words, that the attraction of gravitation may not be the only force 

 concerned in the barometric oscillations : the following results, it ap- 

 pears to me, will require some additional cause for their explanation. 



Having found that a marked variation of the horizontal force of the 

 earth's magnetism is produced by the sun's rotation on his axis, and that 

 the period of this rotation shown by the magnetic observations was 

 nearly 26 days, I sought whether some effect might not be produced by 

 the same cause on the atmospheric pressure. A discussion of the ampli- 

 tudes of the diurnal oscillations of the barometer within the tropics for 

 a period of 26 days gave no sufficiently marked result ; if any such 

 period exists the variation due to it appeared small : a similar discussion 

 of the irregular diurnal oscillations in high latitudes gave a large variation. 

 The latter movements are evidently connected with the daily changes 

 which have been studied by Herschel and others. My attention was then 

 directed to the changes of daily mean barometric pressure within the 

 tropics ; and for this investigation, Singapore, a station near the equator, 

 was chosen, w^here the irregularities due to local causes might be sup- 

 posed least. The variations of daily mean pressure there, when projected 

 in curves, were found to resemble those previously obtained by me for 

 the earth's magnetic force : large oscillations (for that region) occupied 

 from 20 to 30 da}^ for some months, then disappeared to reappear later ||. 



* Sur le Climat de la Belgique, 4^ partie, p. 80. 



t Ibid. p. 91. X Ibid, plate 4. § Brit. Assoc. Eeport, 1843, p. 70. 



II The similar disappearance in the case of the magnetic variations has been found 

 due to the opposing actions of the sim and moon. It has still to be determined whether 

 this is not the case also for the barometric oscillations. See paper "On the Variations 

 of the Daily Mean Horizontal Force of the Earth's Magnetism, &c.,'' Proc. Eoy. 

 Soc. vol. xxiv. p. 231. 



