Pliny (Hist. Nat. ii. 43) writes: “Si in nube luctetur flatus aut vapor, tonitrua edi: si erumpat ardens, fulmina; si longiore tractu nitatur, fulgetra. His findi nubem, illis perrumpi. Etesse tonitrua impactorum ignium plagas.” cf. Sen., Quæst. Nat. ii. 12.