On Eapid Periodic Variations of Terrestrial Mag-netism. (59 



question is, win' and in what manner these \ariations are 

 produced. Schuster attributed the origin of the current to the 

 induction caused by the daily oscillation of the upper atmosphere, 

 which may be regarded as a conducting layer enveloping the 

 entire earth. The first suggestion strongly appealing to us is 

 that this diurnal wave may occasionally^ be accompanied by 

 numerous trains of secondary waves with decidedly shorter 

 periods compared witli tlie daily or semidiurnal periods. An 

 analogy is afforded in the case of the secondar}' undulations of 

 oceanic tides. '^ Whatever may l)e the cause, any mechanical 

 wave motion produced in the upper atmosphere will produce 

 corresponding waves of the induced current whicli will very 

 much resemble the case discussed in § 23. SujDposing such to be 

 actually the case, let us see how far tlie assumption may Ije 

 justified. 



Taking first the case of the progressive waves, a serious 

 difficulty confronting the assumption is to explain the enormous 

 velocity of propagation as obtained in the preceding calculation 

 wdiich is far greater than the sound velocity, even if the upper 

 atmosphere be entirely of hydrogen at the temperature of the 

 stratosphere. The case Avould become more favorable only, if 

 by any modification in the method of estimation, we could 

 l)riug down the calculated vehjcit}- to a plausible range. 



Another alternative hypothesis of the stationary wave, 

 where the wave length is uf the order of magnitude of the earth's 

 merichan, must then be considered. If the atmospheric oscillation 

 be purely mechanical and he one of the modes of the natural 

 tangential vil)ration of the ivhole atmosphere, the period corre- 

 sponding to such harmonics of low order can scarcely be of the 

 order of a few minutes, as is most frequently observed, in so far 

 as we can judge from the results of investigation^^ on the 

 phenomena of the daily variation of atmospheric pressure. 



These considerations alone seem to lead us rather to 



1) K. Honda, T. Terada, Y. Yoshida and D. Isitani, Secondary undulations of Oceanic 

 Tides, Journal of the Coll. of Sc, Imp. Univ., Tokyo. 24, 1908. 



2) Rayleigh, Phil. Mafj., [5] 29, 1890, p. 173: M. Margules, Sitz. bor. d. kön. Akad. d. Wiss. 

 z. Wien, 99, 1890, p. 204, <-tr.; H. Lamb, Proc. Eoy. Soc. A 84, 1911, p. 551. 



