160 Intelligence and Miscellaneous Articles. 



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, react 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, convection-cur- 

 rents, 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. JBaxendell between the curves denoting the daily progress of 

 the wind and those denoting the variation of the declination magnet ? 

 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 periods in one lunar day. 

 This motion cannot produce a very great secondary current ; but may 

 it not be sufficient to account for the lunar-diurnal magnetic varia- 

 tion, 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 a very rapidly 

 changing core, and the other to rapidly moving conductors under a 

 constant core ? And might not an aurora of the latter kind indicate 

 the approach of a change of weather ? 



These remarks are thrown out in order to invite comment and 

 criticism, and they will have served their purpose if they direct 

 attention to the part that may be played by moving conductors in 

 the phenomena of terrestrial magnetism. It will be noticed that 

 these remarks do not touch upon the mysterious and interesting 

 connexion believed to exist between magnetic disturbances and the 

 frequency of solar spots. 



P.S. — Since writing the above, Sir W, Thomson has called my 

 attention to a paper by him in the Philosophical Magazine for De- 

 cember 1851, in which it is suggested that moving conductors may 

 play a part in the phenomena of terrestrial magnetism, — Monthly 

 Notices of the Royal Astronomical Society, December 10, 1869. 



8ft. 



