180 TERRESTRIAL PHENOMENA. 



light. The Aurora (W) is not, therefore, to be itself re- 

 garded as a cause of the perturbation, but as the result of 

 a state of telluric activity excited to the production of a 

 luminous phenomenon ; an activity which manifests itself, 

 on the one hand, by the fluctuations of the needle, and, on 

 the other, by the appearance of the brilliant auroral light. 

 The magnificent phenomenon of coloured polar light is the 

 act of discharge, the termination of a magnetic storm, as 

 in the electric storm, an evolution of light (lightning) indi- 

 cates the restoration of the equilibrium in the distribution 

 of the electricity. The electrical storm is usually confined 

 to a small space, beyond which the state of electricity in the 

 atmosphere remains unchanged. The magnetic storm, on 

 the contrary, manifests its influence on the march of the 

 needle, over large portions of continents, and far from the 

 place where the evolution of light is visible, as was first re- 

 discovered in our own age by Arago. It is not improbable 

 that, as clouds of threatening appearance and heavily 

 charged with electricity, do not always proceed to the point 

 of discharge by lightning, owing to frequent transitions in 

 the electrical state of the atmosphere, so magnetic storms 

 may produce great disturbances in the ordinary diurnal 

 march of the magnetic needle over a wide range, without 

 its necessarily following that the equilibrium of distribution 

 must be restored by explosion, or by luminous effusions from 

 the pole to the equator, or from pole to pole. 



If we desire to collect into one view all the features of the 

 phenomenon, we may describe the commencement and 

 successive phases of a complete appearance of the Aurora as 

 follows : Low down on the horizon, about the part where 

 it is intersected by the magnetic meridian, the sky, which 



