THE EEASON WHY. 135 



" His lightnings enlightened the world : the earth saw and trembled." 



PSAiM XCVII. 



582. Why do these phenomena produce electricity? 



Because they disturb the equilibrium of the electric force, and 

 produce positive and negative states of electricity. 



583. When does lightning occur ? 



When clouds, charged with the opposite electricities approach, 

 the forces rush to each other, and combine in a state of equilibrium. 



584. Why does lightning attend this movement of the 

 forces of electricity ? 



Because the atmosphere, being- unable to convey the great charges 

 of electricity as they rush towards each other, acts as an insulator, 

 and lightning is caused by the violence of the electricity inferring 

 its passage. 



585. Does lightning ever occur when the conducting power 

 is equal to the force of the electricity? 



No ; electricity passes invisibly, noiselessly, and harmlessly, 

 whenever it finds a sufficient source of conduction. 



CHAPTER XXVIII. 



586. Why does lightning sometimes travel through a " zig- 

 zag" course? 



Because the electricity, being resisted in its progress by the air, 

 flies from side to side, to find the readiest passage. 



587. Why does lightning sometimes appear forked ? 



Because, being resisted in its progress by the air, the electricity 

 divides into two or more points, and seeks a passage in different 

 directions. 



588. Why is lightnmg sometimes like a lurid sheet ? 



Because the flash is distant, and therefore we see only the 

 reflection. 



