XX
1
2
Diogenes of Apollonia asserts that thunder arises in some cases from fire, in some from air. Fire precedes those it produces, to herald them. Those that are attended with rattling noise, but without flash, are produced by air. Either sound or flash, I grant, can and sometimes does occur without the other. Still, their powers are not distinct, each may be produced by each. For will any one say that air borne with great violence, when it can produce sound, will not also produce fire? Will not every one grant, too, that fire as well as air may sometimes burst the clouds without darting from them, for example, if it has burst through a few of the clouds, but is buried beneath an accumulation of them? So fire will pass into air, and lose its shining appearance in cutting through some cloudy obstacles and kindling what is within. Add now another inevitable result—the rush of the thunderbolt sends out blasts of air and drives them before it, and raises a wind behind it through the great extent of its impact on the atmosphere. Thus, through the vibration caused by the wind which the fire drives in front of it, all objects quiver before they are actually struck by the bolt of lightning.