74 



Art. 9. -ï. Teratla 



represent tlie hemispheres for day and night respectively. The 

 distril^ution, expressed in other words, is such that when tlie 

 periodic magnetic field is directed toward due south, the WP]- 

 component, just passing zero, is increasing toward the direction 



Fi.a. '2d. 



fiiçWiSiè€ 



^a^^5<.êi 



6>.:n 



M,N 



Qi'am. 



fieen. 



Qym. 



of the meridian at 6^ a.m., or 6^ p.m., lor all points of a.m., 

 or p.m. region respectively. A similar variation might have 

 been produced, if there existed two sets of horizontal atmospheric 

 oscillations corresponding to the zonal harmonics of the second 

 order, the one having its axis at the earth's axis and the other 

 in a direction passing through the equator at the meridians 

 corresponding to 6^' p.m. and G^ p.m.; the two component 

 vibrations having a proper phase difference. Purely mechanical 

 vibration of such a type must, however, have a more or less 

 definite, and certainly much longer period, and is scarcely apt to 

 explain the observed phenomena in the case of the most frequent 

 short waves. 



Another suggestion is that along the atmospheric current 

 of more or less linear cliaracter, running near the place of 

 observation, a sinuous motion is propagated, in which case 

 the periodic disturbance may become rotatory and the sense 

 of rotation will be opposite on both sides of the current. It 

 seems to suffice, therefore, to assume a belt of current making 

 no considerable angle with the meridian at (f' or 12Î' on which 

 a sinuous motion is propagating with a proper velocity- The 

 assumption is so far not utterly contradictory to the assumption 

 of the circular current mentioned earlier, since the latter has 



