PAPER BY LORD RAYLEIGH. 295 



is evidently one tiling to make this supposition for sonorous vibrations, 

 and another for vibrations of about 21 hours period. If the dissipation 

 were neither very rapid nor very slow in comparison with diurnal 

 changes (and the latter alternative at least seems improbable), the vibra- 

 tions would be subject to the damping action discussed by Stokes.* 



In any case the near approach of Ti to 24 hours, and of Tz to 12 hours, 

 may well be very important. Beforehand the diurnal variation of the 

 barometer would have been expected to have been much more con- 

 spicuous than the semidiurnal. The relative magnitude of the latter, 

 as observed at most parts of the earth's surface, is still a mystery, all 

 the attempted explanations being illusory. It is difficult to see how 

 the operative forces can be mainly semidiurnal in character; and if the 

 effect is so, the readiest explanation would be in a near coincidence 

 between the natural period and 12 hours. According to this view the 

 semidiurnal barometric movement should be the same at the sea level 

 all round the earth, varying (at the equinoxes) merely as the square of 

 the cosine of the latitude, except in consequence of local disturbances 

 due to want of uniformity in the condition of the earth's surface. 



Terling Place, Witham, Dec, 3 889. 



*Pliil. Mag., 1^51 (4), vol. I, p. 305. Also, Rayleigh ; "Theory of Sound," <j 247. 



