58 On the late Exceptional Season. 



currents be caused in a stationary conductor in presence of a 

 magnetic core of variable power, but also in a conductor 

 moving across the lines of force of a constant magnet. The 

 question arises, have we on the earth such moving 

 conductors ? In answer to this, let us reflect what takes 

 place at the equator. When once the anti-trades have 

 reached the upper regions of the atmosphere, they will 

 become conductors from their tenuity ; and as they pass 

 rapidly over the lines of the earth's magnetic force, we may 

 expect them to be the vehicles of an electric current, and 

 possibly to be lit up as attenuated gases are when they 

 conduct electricity. May not these form the Zodiacal light ? 



" Such moving currents will, of course, re-act on the 

 magnetism of the earth. We may therefore suppose that 

 somewhat sudden and violent changes are likely to take 

 place in the earth's magnetism at those seasons at which 

 the earth's great wind-currents change most rapidly. May 

 not this account for the excess of disturbances at the 

 equinoxes ? 



" Besides the anti-trades, there are also, no doubt, " con- 

 vection-currents," caused by the daily progress of the sun, 

 taking place in the upper regions of the earth's atmosphere. 

 May not these also be the vehicle of currents as they cross 

 the lines of the earth's force, and account, to some extent at 

 least, for the daily variations of terrestrial magnetism ? and 

 may not this be the reason of the likeness observed by 

 Mr. Baxendall, between the curves denoting the daily 

 progress of the wind, and those denoting the variation of 

 the declination magnet 1 Such currents (in as far as they 

 are electric conductors), taking place in the upper regions of 

 the atmosphere, would not be felt by the earth-current wires 

 at Greenwich, and I think Mr. Airy has noticed that this is 

 the case. But the tidal wave represents a motion of a 

 conductor on the earth's surface, with two peviods in one 

 lunar day. This motion cannot produce a very great 

 secondary current ; but may it not be sufiicient to account 

 for the lunar-dinrnal magnetic variation, which is also very 

 small ? 



'' Such a current taking place in a conductor electrically 

 connected with the earth's upper surface ought 'to be felt by 

 the Greenwich wires ; and, if I am not mistaken, Mr. Airy 

 has detected a current of this nature. 



" May we not also imagine that there are two varieties of 

 aurora — one corresponding to stationary conductors under 



