BOOK II
In this division of his work the author discusses various aspects of the atmosphere and offers an explanation of the phenomena which he describes. He distinguishes between the very bright ether on high, and the moist, denser atmosphere which underlies it, but thinks that they must pass insensibly into each other (66). The atmosphere he regards as a continuous non-composite body, capable of great range in tension, and forming the vehicle through which the exhalations from the earth pass outwards to the sky. It does not everywhere possess the same qualities. In its lower parts next the earth it is dense and misty, owing to the terrestrial exhalations, and is there warmed by the earth’s breath, by the reflection of the sun’s rays from the ground, and from the fires, artificial and subterranean, as well as from the warmth communicated by living animals and plants, for life cannot exist without heat. The highest portions of the atmosphere are exceedingly dry, hot, and attenuated, owing to their nearness to the eternal fires and the heat of the heavenly bodies. The middle parts, on the contrary, are intermediate in character, but colder than what lies above and below them (60, 61). It is the lower portions that are subject to the greatest changes, for they receive the earthly elements which involve such constant turmoil. The instability of the air arises also in part from the motions of the earth and from those of the sun, moon, and stars, to which cold, rain, and other atmospheric disturbances are due (56, 61).
Seneca, in passing on to discuss the nature and origin of thunder and lightning, divides the phenomena into three kinds--lightning-flashes, thunderbolts, and thunderings (62). After citing and commenting on the opinions of various philosophers he proceeds to give his own views regarding these appearances. The lightning flash (_fulguratio_) he looks upon as fire widely spread out, the thunderbolt (_fulmen_) as fire condensed and hurled with violence (66). The difference between the two is in force rather than character; a flash is a bolt without strength enough to reach the earth, while a thunderbolt is lightning in its most intense form (69). With regard to the origin of the fire he points out that fire may be artificially produced in two ways: either by percussion, as when stones are struck; or by friction, as when two bits of wood are rubbed against each other. He thinks that probably in both of these ways clouds may emit fire, and that in the violence of storms a source of energy is supplied whereby the warm or smoky exhalations from the earth may be kindled and fall with a fierce glow to the earth (70, 101). These exhalations contain dry and moist bodies, to which heavier elements may be added. A combination of such materials will form a thicker and more solid cloud than one of pure air, and such a cloud may burst with a loud report (78). There can be no peal of thunder unless the hollow clouds are broken up with great violence (76). The characteristic path of the thunderbolt is determined by the oblique current of air in which, while the natural tendency of the fire is upward, the violence of its discharge presses it downwards and compels it to take up a zig-zag course. The peculiar ozone odour noticed during thunderstorms, and long popularly known as the smell of sulphur, is alluded to by Seneca (69, 97) and by Lucretius.[111]
[111] Similar views on thunder and lightning are expressed in the _De Rerum Natura_:
semina quod nubes ipsas permulta necessust ignis habere.--vi. 206.
post ubi conminuit vis eius et impetus acer, tum perterricrepo sonitu dat scissa fragorem.--_Ibid._ 128.
... notaeque gravis halantis sulpuris auras.--_Ibid._ 221.
The discussion of these subjects leads on to a disquisition on the portents that may be drawn from different kinds of thunder and various forms of lightning. Seneca infers from the effects produced by it that lightning possesses an inherent divine power. Among these effects he enumerates some in which he seems to have thoroughly believed, such, for instance, as the smashing of a wine jar already quoted, and the freezing of the wine for the space of three days thereafter. He is thus disposed to attach credit to the opinion that future events are foretold by both lightning and thunder. Yet he cannot change his Stoic faith that fate, that is, the necessity for the happening of all things and all actions, can be set aside by no force, can be altered by no portents, nor averted by any prayer or sacrifice. Though he admits that vows and supplications may be useful to the worshippers, he knows that even these also are included in the decrees of fate.
These reflections lead the philosopher to a characteristic peroration on the moral lessons to be derived from the subjects he has been discussing. From the dangers incident to thunderstorms he passes to the enforcement of the Stoic doctrine that death must be despised, and everything which leads to death will then cease to have any terror.