A certain number of writers are of opinion that the air of itself emits a report as it traverses the cold and moist regions. Iron, they point out, when heated cannot be dipped in moisture without noise. A mass of heated metal when plunged in water causes a loud sputtering as it is cooled; so, according to Anaximenes, air meeting cloud produces peals of thunder; then as it rushes struggling through the obstructions that bar its way it kindles the flame of lightning merely by its escape. XVIII Anaximander refers all the phenomena of thunder to air. Peals of thunder are, he says, the sounds of blows on a cloud. He explains the inequality of the peals by the inequality of the blows. To the question, why it thunders in a clear sky also, he answers that even in absence of cloud the atmosphere is shaken and rent by the bursting forth of air. But why is there thunder sometimes and yet no lightning? The rarity and feebleness of the air render it incapable of producing flame, while yet sufficient to produce sound. Lightning, according to him, then, is really a disturbance where the atmosphere is merely parted and rushes hither and thither, displaying a faint fire that will not issue from its place. As for the thunderbolt, it is the career of the more active and denser air. XIX Anaxagoras says all the phenomena correspond to the descent of some 1 force from the ether to the lower regions. So when the fire encounters cold clouds it emits a sound; when it cleaves them there is a flash; less violence in the fires produces lightning, greater, thunderbolts. 2 XX Diogenes of Apollonia asserts that thunder arises in some cases from 1 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 2 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. XXI