TERRESTRIAL MAGNETISM, ETC. 275 



brilliant displays of aurora are to be seen in 

 northern and southern polar regions, often as far 

 from either pole as our own latitudes, and some- 

 times perhaps as far as the equator, and over both 

 northern and southern hemispheres simultaneously. 

 Though it is not quite certain that there is not 

 always a display of aurora borealis or australis, or 

 both, at the time of a magnetic storm, it is quite 

 certain that no display of aurora, even of the 

 faintest to be visible, is ever seen without marked 

 disturbances of a delicately poised magnetic needle 

 in any part of the world. 



The electric telegraph has made known to us 

 another allied disturbance the underground 

 electric storm which is found always to accompany 

 the magnetic storm and auroral display. A fourth 

 agency, atmospheric electricity, has its storms too ; 

 and these produce great disturbances of the ordi- 

 nary daily electric " earth current " discovered in 

 every telegraph wire whether aerial or submarine. 



But though the thunderstorm produces dis- 

 turbances of earth currents, and though disturbances 

 of earth currents are also produced by some cause 

 which produces also auroral displays and magnetic 



T 2 



