yO Art 9.-T. IVrada: 



since it is an open question whether in tlie liiglier part of our 

 atmosphere there may occur any remarkable vertical convection, 

 as is the case in the troposphere. At any rate, the fact that an 

 abrupt increase of the general fiekl is most frequently accom- 

 panied by a train of remarkable pulsations, is in harmony with 

 the above idea that the increase of the current is associated with 

 the increase of the vertical impulsive motion of the atmosphere 

 which subsides into a vertical natural vibration. The charac- 

 teristic disturl^ance of this type occurs as a rule near midnight. 

 On the other hand, examples are by no means rare, especially 

 in the evening, where an abrupt increase of the horizontal 

 component occurs, but not at all accompanied l)y a train of waves. 

 How is the fact to be explained ? It may at first be suggested 

 that in this case the abrupt increase of tlie current takes place 

 in the direction of the general terrestrial field; but as a matter 

 of fact, the increment of the Y-component in such a case is not 

 generally of a different order of magnitude compared with that 

 of the X- component. Another possible altervative is that in the 

 hours which are near the boundary between the day and night 

 hemispheres, the regular vertical vibration of the atmosphere 

 is not so easy as at midnight. The latter assumption seems 

 in some measure plausible, if Ave consider very probable 

 heterogeneity of the atmosphere in that region forming the 

 transition stage of the illuminated and non-illuminated halves. 

 The minimum of the frequency of pulsations of all periods falls 

 actually in the evening hours (Fig. 8). The morning hours 

 are not necessarily in the same condition as the evening hours. 

 In the former, the atmosphere is being rapidly heated up 

 and ionized, but on the night side of tlie boundary a dormant 

 homogeneity of the night-atmosphere may probably prevail; 

 whereas in the latter, the gases are cooling down and the ions 

 recombining, and gradually passing into the nightly condition. 

 In the morning, the transition may therefore be abrupt and the 

 area of the heterogeneous atmosphere comparatively small, 

 W'hile in the evening the heterogeneity may extend to a consid- 

 erable area of the earth's surface. 



