350 ELECTRICITY. 



so that the wire L K may stand in the situation 

 I M, in which case the conductor H O is not dis- 

 continued ; and you will observe, that the explo- 

 sion will have no effect upon the piece of wood 

 L M, this remaining in the hole unmoved ; which 

 shows the usefulness of the metallic conductor. 



Farther, unscrew the brass ball H from the wire 

 H I, so that this may remain pointed ; and with 

 only this difference in the apparatus, repeat both 

 the above experiments ; and you will find that the 

 piece of wood I M is in neither case moved from 

 its place, nor any explosion heard. 



To know that lightning and the electric matter 

 are the same is a great step in natural philosophy ; 

 but we must still remain ignorant of the causes of 

 many of the appearances which accompany light- 

 ning, so long as our knowledge with the properties 

 of electricity is so very imperfect. We know that 

 the clouds are almost always electrified, sometime 

 positively and sometimes negatively ; when these 

 come near each other, a discharge, or flash of 

 lightning, is generally the consequence. When a 

 cloud is highly charged with electric matter, and 

 there is no other cloud near that can attract it, 

 it then strikes that part of the earth which is 

 nearest. 



Whatever may be the cause that disturbs the 

 equilibrium of the electric matter in the atmo- 

 sphere, it may easily be conceived, that when such 

 disturbance happens in the upper and highly rari- 

 fied regions of the air, the equilibrium will be re- 

 stored by electric corruscations darting through 

 the partial vacuum. This consideration accounts 

 for the aurora borealis, which has commonly a 

 darting or undulating motion between two oppo- 

 site parts of the heavens. The aurora borealis, ox 



