Ou Eapid Periodic Variations of Terrestrial Magnetism. 73 



of gases. 111 tins respect, many interesting physical problems 

 may be suggested which seem worth special consideration. 



On the other hand, if a more or less limited portion of the 

 atmosphere be subjected to the oscillation of vertical type, the 

 period will surely depend on the extent of the area disturl)ed. 

 According to a personal communication by Prof. Sano, such a 

 mode is actually possible and the period will decrease with the 

 area disturbed. At least, the periods shorter than five minutes 

 may partly be explained in this way. The period uf vertical 

 vibration may, however, also depend on the modes of laminar 

 structure of the atmosphere in the distribution uf the temperatures 

 and the wind velocities of the different layers. The inquiry in 

 these directions involves many intricated problems for mathe- 

 matical physicists, and may better be left for the specialists in 

 these lines. 



If the vertical vibration of the atmosphere be the actual 

 cause of the magnetic pulsation as appears most "probable, the 

 investigations of the latter phenomena will in any case afford 

 very valuable material for studying the actual physical conditions 

 of the upper atmos})here, and may offer probably an unexpectedly 

 wide field for new researches in different directions. 



According to Prof. Sano, mechanical disturbances of any 

 kind must gradually suljside into a regular vertical natural 

 vibration, which is comparatively slowly dissipated. This tlieoreti- 

 cal result is in harmony with the observed phenomena, viz., 

 the subsidence of an abrupt change of the terrestrial field into 

 a train of regular damped waves, and also the peculiar persistent 

 character of some trains. These points will be touched on once 

 more in ^ 28. 



28. Finally, let us consider the last crucial test of the 

 different h3^pothese^. the peculiar ])e!iaviour of the rotational 

 oscillation of the horizontal components. If we assume the 

 observed sense of rotation for different hours, as generally 

 applicable for the entire northern hemisphere, the distribution 

 of the different senses of rotation in different hours will be roughly 

 represented l)y the annexed figure (Fig. 23). where the semicircles 



