164 FIRST YEAR SCIENCE 



with electricity. As the electrical charge increases, dis- 

 charges take place which cause lightning flashes. These 

 discharges occur along the lines of least resistance and are 

 often very irregular and forked. As tall objects are 

 likely to offer good paths for the discharge, it is safest to 

 keep away from trees and walls during a thunder-storm. 



The air becomes greatly agitated by the lightning dis- 

 charges and makes us aware of this by the noise of the 

 thunder, just as the agitation of the air caused by the 



THUNDER-STORM CLOUDS. 



discharge of a gun is made apparent to us by what we call 

 the noise of the report. Since sound travels at about the 

 rate of a mile in five seconds and the lightning discharge 

 is practically instantaneous, the noise from different parts 

 of the discharge will reach us at different times and to 

 this and the echoing from clouds or hills is due the roll of 

 the thunder. The distance of the flash can be told ap- 

 proximately by dividing the number of seconds between 

 seeing the flash and hearing the thunder by five. 



Frequently in the evening flashes called heat lightning 

 are seen near the horizon. These are due to the reflection 

 on clouds of flashes of lightning in a storm which is below 

 the horizon. Thunder-storms occur sometimes in winter. 

 They are very prevalent in the tropics. 



