and the Aurora Borealis. 365 



the horizon. Here, then, it is possible for the electric fluid of 

 the atmosphere to descend into the earth without the terres- 

 trial magnetism opposing to it an obstacle directly, provided 

 it follows the direction of the dipping needle ; but in follow- 

 ing that direction the electricity has a longer path to travel 

 to reach the surface of the earth, and consequently suffers a 

 greater resistance than if it could descend vertically. At the 

 magnetic pole the dipping needle takes a vertical position, in 

 consequence of which the resistance opposed by terrestrial 

 magnetism to the propagation of the electricity in the vertical 

 direction is here equal to nil. From all this it follows that, 

 all other circumstances being equal, the resistance to the flow of 

 the electric fluid from the atmosphere to the surface of the earth 

 is greater at the equator and in the equatorial regions than at a 

 certain distance from that circle, and that the resistance dimi- 

 nishes as the dip of the magnetic needle to the earth increases. 



It is here that, in my opinion, we must seek the chief cause 

 of the fact that in the equatorial regions the electric fluid of 

 the atmosphere descends to the earth by strong disruptive dis- 

 charges, and in high latitudes chiefly by slow and feeble Sow- 

 ings, forming more or less continuous electric currents. If 

 we continue to charge with opposite electricities two insulated 

 bodies at a suitable distance from each other, the electricity at 

 last traverses the intervening space, producing sparks, and 

 the two bodies are discharged. That the discharge thus 

 effected may be powerful and instantaneous, it is necessary 

 that the bodies be good conductors, and the resistance of the 

 intervening space great. If the resistance is slight, the dis- 

 charge commences while the charge is yet feeble. In propor- 

 tion as the resistance grows weaker, the discharge takes on 

 more and more the form of a continuous current. In the 

 equatorial regions, or in general in lower latitudes, the force 

 of terrestrial magnetic induction acts with very great inten- 

 sity in rendering the atmosphere electropositive, and, accord- 

 ing to what has just been said, the resistance to a discharge is 

 also very great. When the aqueous vapour of the air con- 

 denses so as to form cloud, this becomes charged with the 

 electric fluid accumulated in the air in the locality. The cloud, 

 which is a good conductor, therefore takes an electropositive 

 charge. It is unnecessary to say that negative clouds may 

 also in turn be formed under the inductive influence of posi- 

 tive clouds produced in this way. If now the clouds have 

 become sufficiently electric, the electric fluid may escape to 

 the earth by means of an instantaneous discharge. These 

 discharges, or thunder-claps, then, take place when clouds are 

 formed and when the electric resistance between them and the 



