394 



GENERAL SCIENCE 



Lightning. Lightning is caused much in the same way. A great 

 deal of electricity is carried up from the earth while water is evap- 

 orating. Also a great deal of electricity is generated by the friction 

 of clouds and wind. When clouds are very high above the earth, 

 flashes of electricity will pass between them, but if a cloud is near 

 the earth, the flash of electricity will pass between the cloud and 

 some object on the earth. We commonly say the lightning strikes 



FIG. 274. Volta entertaining his friends by generating static electricity. 



the object. Lightning rods have been placed on buildings in order 

 to protect them from this great spark of electricity. The rod 

 extends slightly above the highest point of the building, and runs 

 down the side of the building into the moist earth. Buildings that 

 are well protected by lightning rods are very seldom struck by light- 

 ning, since the current of electricity passes down the rods into the 

 ground. Very often, too, no spark is seen at all. Such rods are very 

 dangerous if they are not well insulated from the buildings. 



