280 SCIENCE OF COMMON THINGS. 



Lightning. Three forms of lightning. 



Water, in passing into steam by artificial heat, or in evaporating by the 

 action of the sun or wind, generates large quantities of electricity. The 

 crystallization of solids from liquids, all changes of temperature, the 

 growth and decay of vegetables, are also instrumental in producing elec- 

 trical phenomena. 



1S1Q What is lightning f 



Lightning is accumulated electricity, generally dis- 

 charged from the clouds to the earth, but sometimes 

 from the earth to the clouds. 



1S3O What causes the discharge of an electric cloud? 



When a cloud overcharged with electric fluid ap- 

 proaches another which is undercharged, the fluid 

 rushes from the former into the latter, till both contain 

 the same quantity. 



18J31 Is there any other cause of lightning besides the one just men- 

 tioned ; 



Yes ; sometimes mountains, trees, and steeples will 

 discharge the lightning from a cloud floating near, and 

 sometimes the electricity passes from the earth, into 

 the clouds. 



1833 How high are the lightning clouds from the earth? 



Sometimes they are elevated four 01* five miles high, 

 and sometimes actually touch the earth with one of 

 their edges ; but they are rarely discharged in a thun- 

 der storm when they are more than seven hundred 

 yards above the surface of the earth. 



1833 What is a thunder storm ? 



The disturbance caused" in the air when successive 

 discharges of accumulated electricity take place. 



1S34 Into how many kinds has lightning been divided ? 



Three. 



1835 What are they ? 



The zig-zag lightning, sheet lightning, and hall light- 

 ning. 



1836 Why is lightning sometimes forked ? 



Because the lightning cloud is at a great distance ; 

 and the resistance of the air is so great that the electri- 

 cal current is diverted into a zig-zag course. 



