xii THUNDER AND LIGHTNING 63 



some authorities believe that the fire is actually 

 resident in the clouds, some that it is merely pro- 

 duced fcr the occasion, and that it does not exist 

 until it issues out. But yet there is no agreement 

 as to what brings out the fire. One explains it 3 

 as due to light. Again, a certain author says that 

 the sun's rays accumulate through recurrent inter- 

 section, and kindle the fire. Anaxagoras asserts that 

 it is distilled from the ether, that from such heat in 

 the sky many sparks fall which the clouds enclose 

 and retain for a long time. 



Aristotle supposes that the fire does not gather 

 in the clouds any long time previously, but rather 

 that it bursts out at the same instant as it is formed. 

 His opinion runs thus : Two elements of the world, 4 

 land and water, lie in its lower part ; each exhales 

 its peculiar emanation. The vapour of earth is 

 dry, resembling smoke, and produces wind, thunder, 

 and lightning ; the breath of water, on the other 

 hand, is moist, and produces rain and snow. 

 But that dry vapour from the earth, to which 

 [as mentioned] winds owe their origin, on account 

 of its accumulation in large masses, is subject to 

 violent lateral pressure when it is condensed for 

 the formation of clouds. Thereupon it strikes the 5 

 adjacent clouds over a larger surface, and the 

 blow reverberates loudly [in thunder]. The effect 

 is analogous to that produced by the crackling of 

 flame from the moisture contained in green un- 

 seasoned firewood. In this case the air enclosed 

 in the wood has some moisture in it, and when it 

 accumulates it bursts out in the flame. So like- 

 wise the air which, as I said a little ago, is driven 

 out through a collision of two clouds, cannot burst 

 or leap out without noise. The sound varies 6 



