1881.] on Magnetic Dlnlurhance, Aurora', and Earth Currents. 071 



have a layer of air wliicli for air is a comparatively good conductor of 

 electricity ; here, then, we have not a lagging of tlie magnet behind 

 the conductor (as described in the early part of my lecture), but a 

 lagging of the conductor beliind the magnet, and hence we may expect 

 a current or a gradual heaping up of electricity in the air in the 

 opj^osite direction to the current in the earth's crust. Thus whilst 

 the tidal wave in the earth's crust would cause a current in a telegraph 

 wire from the equator towards the poles, the regular tidal waves in 

 the atmosphere would cause the gradual transfer of positive electricity 

 from the 2)oles towards the equator. This transfer may be of the 

 nature of a current of electricity or of a mass of air carrying a static 

 charge of electricity with it, for as Professor Eowlaud has shown that 

 the motion of a static cliarge will produce magnetism, so we may 

 expect from the principles of conservation of electricity that a change 

 in the position of a magnet will, under such circumstances, produce 

 motion of the static charge of electricity. When the air becomes 

 charged up to discharging point, then we may get the sudden dis- 

 charges such as the aurora in the air and the earth currents in the 

 earth, and since the conducting layer of air approaches nearer to the 

 earth in the colder polar regions, possibly within less than 20 miles 

 of the earth's surface, it may be found that the discharge of the aurora 

 may even take place from earth to air by gradual slow discharge, 

 aided as it may be by the state of moisture of the air and by change of 

 temperature and other causes. 



[W. G. A.j 



