FRICTIONAL OR STATIC ELECTRICITY 187 



222. Danger from Lightning. If a cloud filled with 

 one kind of electricity comes near the earth when the 

 latter is filled with the opposite kind, the cloud may dis- 

 charge its electricity to the earth. If any tall object, 

 such as a tree, a steeple, or a house, happens to be near 

 where the cloud discharges, the electricity will often pass 

 down it to the earth. In this way houses are sometimes 

 injured and set on fire and great trees are split up into 

 small pieces. Sometimes, too, human beings and animals 

 are struck and killed. It is not safe, therefore, to stand 

 under a tree or close to a high house during a thunder 

 storm. 



223. Forms of Lightning. We see lightning in several 

 different forms; sometimes its flash is straight, sometimes 

 it looks forked or zigzag, sometimes it is round like a ball, 

 and sometimes it spreads over the clouds like a sheet of fire. 

 When a thunder cloud is near the earth, the flash comes 

 straight down, because there is but little air for it to pass 

 through. When, on the other hand, the cloud is at a con- 

 siderable distance from the earth, the air in the path of the 

 lightning is made denser or thicker by being pushed together, 

 and as lightning can pass more quickly through thin than 

 through thick air, it flies from side to side so as to pass 

 where the air is thinnest. This makes its path zigzag or 

 forked. When there is a great charge of electricity in a cloud 

 it sometimes forces its way through the air in the shape of a 

 ball. What is called sheet lightning is either the reflection 

 or shine on clouds of a stroke of zigzag lightning which is 

 too far off to be seen, or light discharges of electricity from 

 clouds which have not enough in them to make zigzag 

 lightning. 



