SCIENCE OF COMMON THINGS. 287 



What occasions the rolling of thunder ? Aurora- borealis. 



vacuum, and that the air rushing in to fill it produces the sound. Any 

 explanation that has yet been ottered is not altogether satisfactory. 



1374 What occasions the rotting of the thunder % 



It has been ascribed to the effect of echo ; but the 

 true cause probably is, that the sound is developed by 

 the lightning in passing through the air, and conse- 

 quently separate sounds are produced at every point 

 through which the lightning passes. 



187*5 Why is thunder sometimes one vast crash? 



Because the lightning cloud is near the earth ; and 

 as all the vibrations of the air (on which sound depends) 

 reach the ear at the same moment, they seem like one 

 vast sound. 



187*0 Why is the thunder generally heard several moments after the 

 /lash? 



Because it has a long distance to travel. Lightning 

 travels nearly a million times faster than thunder ; if, 

 therefore, the thunder has a great distance to come, it 

 will not reach the earth till a considerable time after 

 the flash. 



187"? Can we not tell the distance of a thunder cloud by observing the 

 interval which elapses betiveen the flash and the peal f 



Yes ; the flash is instantaneous, but the thunder will 

 take a whole second of time to travel three hundred and 

 eighty yards ; hence, if the flash be five seconds before 

 thunder, the cloud is nineteen hundred yards off. 



i. e. 380 x 5=1900 yards. 



187*8 What is the aurora borealis or northern lights f 



JLuminous appearances seen in the sky at night-time. 

 Sometimes streaks of blue, purple, green, red, etc., and 

 sometimes flashes of light, are seen. 



187*9 What is the cause of the aurora borealis or northern lights ? 



Electricity in the higher regions of the atmosphere 

 is undoubtedly an active agent in producing this pheno- 

 menon. 



188O Is the aurora ever seen in other parts of the heavens than towards 

 the north f 



In the northern hemisphere it always appears in the 



