XII
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There are, I have just said, three phenomena—lightnings, thunderbolts, thunderings: the last is simultaneous in occurrence with the others, but its sound reaches us subsequently. Lightning (i.e. sheet) merely reveals fire, the thunderbolt (forked lightning) actually despatches it on its mission. The former is, so to speak, a threatening and feint without a blow, the latter a stroke and a blow. There are some of the facts connected with the phenomena of thunder and lightning on which there is general agreement, others on which there is much diversity of opinion. For example, there is agreement that they occur in the clouds and issue from the clouds; further, it is agreed that lightning of both kinds is either composed of fire or at any rate presents the appearance of fire. But to pass on to the points which are disputed—some authorities believe that the fire is actually resident in the clouds, some that it is merely produced for 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 as due to light. Again, a certain author says that the sun’s rays accumulate through recurrent intersection, 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.
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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, 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 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 unseasoned 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 likewise 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 according to the variety of impact in the clouds; the larger cavity in some clouds, the smaller in others account for the variety. That air violently driven out is fire, which is called sheet lightning when it forms a fitful flame of no great violence. We see the flash before we can hear the sound: eyesight is swifter than hearing, and far outstrips it.