LVII RAREFACTION AND FRICTION 101 



way a leaden bullet is liquefied when discharged 

 from a sling, and falls in drops by reason of 

 atmospheric friction just as it would do through 

 fire. Bolts of lightning are more numerous in 2 

 summer, for the reason that there is most heat at 

 that season. Fire naturally starts more readily 

 when the friction is in warmer air. A flash of 

 lightning which merely gleams and a bolt which is 

 discharged are produced in exactly the same way. 

 But there is less force in the former case and less 

 fuel. To put my opinion on the point shortly : a 

 bolt is just lightning in its most intense form. So 3 

 then, when a body of the nature of heat or smoke 

 is exhaled from the earth and, meeting with clouds, 

 is for a long time rolled about in their hollows, at 

 last it bursts out. Since it possesses no strength, it 

 is merely a flash. But when lightnings have more 

 material and burn with fiercer glow, they not merely 

 become visible, but also fall to the earth. 



LVIII 



SOME writers are firmly convinced that the lightning 

 bolt always returns to the clouds. Others hold that 

 the bolt settles in the ground, at least when its fuel 

 is heavy, and when it has comparatively little force 

 in its stroke as it glides down. But why, it may be 

 asked, does the bolt make its appearance suddenly, 

 and is there not a continuous trail of fire ? It is on 

 account of the extreme rapidity of its motion ; it 

 fires the air at the same moment as it bursts 

 through the cloud. By and by when the motion 

 ceases, the flame subsides. For the course of the 

 air that forms the bolt is intermittent, which pre- 



