LV CAUSE OF THUNDER 99 



adds that if the clouds merely collide with each 

 other, the kind of blow needed to produce an 

 explosion is given, but not completely ; clouds do 

 not meet through their whole extent, but only part 

 with part. And again, soft substances do not 

 resound unless knocked against hard ones ; a wave 

 is not heard unless when it beats on the hard shore. 

 But fire, which is soft, says an opponent, when let 

 into water, also a soft substance, produces sound in 

 being extinguished. Well, suppose it is so, it makes 2 

 for the opposite view which I urge. For it is not 

 really the fire that makes the sound, but the air 

 escaping through the water that is quenching it. 

 Granted that fire is both produced and extinguished 

 in the cloud, it arises from air and friction. Well 

 then, it is urged, may not some of the shooting 

 stars plunge into a cloud and be extinguished ? 

 Even supposing that such a thing can and some- 

 times does occur, it does not remove the difficulty. 

 It is not the occasional chance cause but the natural 

 normal one that we are in search of. Suppose I 

 admit the truth of your contention that occasionally 

 after thunder fires gleam in the heavens much like 

 shooting and falling stars. Yet this does not prove 3 

 that the thunder was caused by them ; it merely 

 shows that the thunder occurred simultaneously 

 with this other phenomenon. Clidemus asserts that 

 a lightning flash is an empty reflection, and not real 

 fire; for in the same way after nightfall a gleam 

 appears from the motion of oars in water. His 

 illustration is not on all fours with the phenomenon. 

 In the latter case the gleam is seen actually within 

 the water ; in the former, in the atmosphere, it 

 bursts and leaps out of its element. 



