1910.] TERRESTRIAL MAGNETIC DISTURBANCES. 141 



Between the two main whirls, which we shall designate as 

 the " polar whirls," there is another — '' the equatorial whirl " — the 

 motion of which is anti-clockwise looking again from the south 

 pole to the north pole the air currents (trade winds) blowing west- 

 wards. And, as known, on each border of the two sets of whirls, 

 there exists a belt of high barometric pressure. The direction of 

 motion of the air in the whirls is conditioned by the deflecting effect 

 of the earth's rotation on the air-currents, which are primarily set 

 in motion because of the temperature differences between the 

 polar and the equatorial regions. 



Considering first the polar whirls, since the air in them has a 

 motion relative to that of the earth, having a greater velocity than 

 that of the earth's rotation, the currents of air are made to cut 

 across the lines of magnetic force, and that too in the regions of 

 the earth where the most effective induction component — the verti- 

 cal component of the magnetic force — is strongest. The electric 

 currents thereby induced would, in general, follow a direction at 

 right angles to the motion of the conducting air currents. Consult- 

 ing a chart of the winds for various seasons, it is seen that the 

 precise distribution of the induced electric currents will be rather 

 complicated. In fact the determination of the exact course of 

 Foucault currents is, in any case, not a simple matter. The dis- 

 placement of the magnetic axis of the induced electric current 

 system with reference to that of the earth, as dependent on the 

 relative magnetic permeability of the two media involved, introduces 

 another factor to be taken into account. 



Since the equatorial atmospheric whirl is opposite to the polar 

 ones, the Foucault negative currents, if brought about, would, 

 generally speaking, be reversed and go around the earth in an 

 opposite direction to the polar negative currents. But the equatorial 

 currents cut the vertical component of the earth's magnetic force 

 in regions of small vertical force, which in fact reduces to zero over 

 the magnetic equator, hence they are considerably weaker than the 

 polar ones. Their strength is further diminished by the fact that 

 in the upper equatorial regions the whirl may be reversed (the anti- 

 trades) and so there would be superposed on the surface equatorial 

 electric currents a set of opposite electric currents. 



