124 The Sky 
When the discharge has ceased to flow, the atoms of air re- 
turn to their normal states and no more light is generated. 
Thunder is caused by the violent expansion of air near the 
stroke and the resultant compressional wave which spreads 
out from the disturbance. A small electric spark produces a 
simple snapping sound; the sounds from the different parts of 
a long lightning stroke arrive at our ears at different times, 
causing a prolonged rumbling sound. Despite the violence 
of lightning itself, thunder is seldom heard more than fifteen 
miles away. Large cannon have been heard much farther. 
The lapse of time between a lightning flash and the ensu- 
ing thunder is a good index of the distance of the lightning 
stroke. The number of seconds elapsed between lightning 
and thunder divided by five gives the distance roughly in 
miles. 
KINDS OF LIGHTNING 
There are several distinct kinds of lightning observed: 
streak lightning, bead lightning, rocket lightning, ball light- 
ning, and sheet lightning. 
Streak lightning is the ordinary kind of lightning that we 
have already discussed. It is a discharge along an erratic, 
broken, zigzag path, often branching off in different direc-_ 
tions. The cause of its erratic paths is the lack of homogeneity 
of the atmosphere. Certain portions of the air are more re- 
sistant to the current’s passage than others and force the dis- 
charge to follow a crooked path. 
Occasionally a streak of lightning is observed to break up 
into a string of bright “beads’—hence it is known as bead 
lightning. 
Sometimes a progressive growth in the length of a light- 
te 
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