Physical Science in the Time of Nero: Naturales Quaestiones (1910) - Page 16

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XIV.	The explanation previously given (Book I, Chapter 15) regarding meteors is consistent with this. There may be an analogy to what happens in fires, where isolated groups of houses catch fire through the gradual accumulation of heat.	65
XV.	Some Stoics believe that spontaneous combustion occurs within the air.	66
XVI.	The difference between a flash of lightning and a bolt.	66
XVII.	Some explain the noise of thunder as the result of hot meeting cold, similar to plunging hot iron into water.	67
XVIII.	Anaximander attributes these effects to air and explains all such phenomena by referring to it.	67
XIX.	Anaxagoras argues that it is the ether—the upper air—that acts upon the lower atmosphere to produce them by sending down fire.	68
XX.	Diogenes of Apollonia believes that fire and air interact, producing one another, as can be observed in various phenomena.	68
XXI.	Discarding previous authorities, I offer an independent explanation. A flash and a bolt are both forms of fire; they differ only in intensity.	69
XXII.	The analogy of fire on earth must apply to the heavens. Lightning is caused either by impact or friction. Hurricanes are a sufficient cause for the former.	70
XXIII.	Friction between clouds and the air may also be a cause. The fires produced in this way are insubstantial and vanish quickly.	71
XXIV.	Fire naturally rises because of its lightness, just as water naturally sinks. However, in the case of a lightning bolt, it is forced downward against its nature, much like a "weeping" tree.	71
XXV.	But it is argued that wet clouds produce fire. How can this be?	72
XXVI.	There is no contradiction in a single cloud containing both potential fire and water. A log may burn at one end while exuding moisture at the other. On two separate occasions, an island was thrown up by fire in the Aegean Sea, with fire overcoming water. Furthermore, clouds are actually required for lightning; any apparent exceptions are merely illusions.	72
XXVII.	Different kinds of thunder. The growling and the crashing sounds, and their causes.	75
XXVIII.	For thunder to occur, clouds of a specific shape must meet in a specific way. A bladder does not burst with a loud noise if it is merely cut; a broad, simultaneous impact across the entire cloud is necessary for an explosion.	76
XXIX.	The correct shape and the rupture of the cloud are necessary conditions. Compare this to the sound of drums and similar instruments.	77
XXX.	Some argue that clouds are not necessary for thunder, citing the eruptions of Mount Etna and the destruction of the army of Cambyses, where particles of sand acted as the medium for thunder and lightning. However, in these cases as well, a cloud was likely formed—perhaps one denser and more solid than one made of mere air—before the sound was emitted.	77
XXXI.	The strange effects of lightning.	78
XXXII.	Portents and events: their undeniable and widespread connection.	79
XXXIII.	Thunderbolts. The threefold division of the art of interpreting them.	81