SCIENCE OF COMMON THINGS. 281 



Sheet and heat lightning. Duration of a flash of lightning. 



1837 How does the resistance of the air make the lightning zig-zag ? 



As the lightning condenses the air in the immediate 

 advance of its path, it flies from side to side, in order 

 to pass where there is the least resistance. 



18SS Why is the flash sometimes quite straight ? 



Because the lightning cloud is near the earth, and as 

 the flash meets with very little resistance, it is not 

 diverted ; in other words, the flash is straight. 



183Q What is sheet lightning? 



Either the reflection of distant flashes not distinctly 

 visible or beneath the horizon, or else several flashes 

 intermingled. 



1830 What other form does lightning occasionally assume ? 



Sometimes the flash is globular, which is the most 

 dangerous form of lightning. 



1831 Does a discharge produce a flash when it passes through good 

 conductors f 



It does not, but passes quietly and invisibly. 



1S3S What is heat lightning f 



Sometimes it is the reflection in the atmosphere of 

 the lightnings of storms very remote, the storms them- 

 selves being so far distant that their thunders cannot 

 be heard. This phenomenon is also occasioned by the 

 play of silent flashes of electricity between the earth 

 and the clouds, the amount of electricity developed not 

 being sufficient to produce any other effects than the 

 mere flash of light. 



1833 Why is lightning more common in summer and in autumn than 

 in spring and winter f 



Because the heat of summer and autumn produces 

 great evaporation, and the conversion of water into 

 vapor always developes electricity. 



1834 How long is the duration of a flash of lightning f 



Arago has demonstrated that it does not exceed the 

 millionth part of a second. 



1835 With ivhat velocity is lightning, or the electric fluid which gives 

 rise to its appearance, supposed to move f 



