188 APPLIED SCIENCE 



224. Cause of Thunder. When lightning passes through 

 air it leaves a vacuum, and the air rushing in to fill it makes 

 the noise which we call thunder. We do not usually hear 

 this until some time after the flash of lightning because light 

 travels more than a million times faster than sound. When 

 the thunder cloud is at a distance, the sound comes to us 

 little by little and we then call it rolling thunder; but when 

 the cloud is near the earth the sound comes in one great 

 crash. You can generally tell how far off a thunder cloud is 

 by noting how long the time is between the flash of lightning 

 and the sound of the thunder. If you can count five as 

 slowly as the tick of a clock between the two, you may be 

 sure that the cloud is more than a mile away. 



225. Use of Lightning Rod. Lightning on its way to the 

 earth always follows the best conductor and consequently 

 will leap from side to side to find a building or a tree. It is 

 attracted to pointed things rather than to round or blunt 

 things, and for this reason lightning rods are made with sharp 

 points. Buildings properly fitted with lightning rods are 

 safe from being struck by lightning, because the rods lead 

 the electricity into the earth. When a cloud filled with 

 electricity comes over the rods, the electricity will flow 

 down them until the cloud is discharged. We see no flash 

 and hear no thunder; and we may feel sure that the building 

 will not be struck. The tops of lightning rods are usually 

 silvered or gilded, so that they will not rust and become worth- 

 less. The lower end of the rod must be carried down into 

 damp earth; if the earth is dry it is better to carry the end 

 into a well, because dry earth is not so good a conductor as 

 moist earth and the lightning might leap from the rod at the 

 lower end and go into the cellar of the building. High chim- 



