Some of your friends the philosophers, a critic may say to me, in 1 giving an explanation of shooting stars have told us that some parts of the atmosphere contract fire which is drawn from these same higher regions, and that the fires are kindled by the glow of the ether. Yes, but I reply that it makes all the difference whether the fire is alleged to fall from the ether, which is incompatible with its nature; or whether it is asserted that from its fierce glow the heat leaps the boundary between it and the lower regions, firing them by its power. For on the latter assumption, the fire does not fall from the upper region, which is impossible, but is kindled in the lower. Surely, too, when a widely spread conflagration occurs in one of our cities, we 2 see detached blocks which have for long been heated by the fire from a distance at last catch fire of themselves. So in the upper atmosphere, which is endowed with the power of drawing fire, in all probability there are cases of ignition from the heat of the superposed ether. In nature there is never a sudden transition from one element to a totally different one. Hence there must be some congruity between lowest ether and highest atmosphere; conversely highest atmosphere cannot be wholly dissimilar to lowest ether. On the confines the two elements pass so imperceptibly into one another that at a particular point there might well be doubt whether one is in atmosphere or in ether. XV Some of the Stoics believe that air, being interchangeable with other elements such as fire and water, does not derive from without a fresh cause of fire; it kindles itself by its internal motion. Then in dissipating masses of thick, compact clouds it necessarily emits a loud noise from the bursting of such large bodies. Besides, the very conflict of the resisting clouds contributes to the energy of the fire. In the same way the hand contributes to the cutting power of an instrument, but the actual cutting is done by the steel. XVI Let me now explain the difference between the flash and the bolt of lightning which you naturally wish to know. The flash is the fire widely spread out, the bolt is the condensed fire hurled with violence. Let me use a homely illustration. We sometimes join our two hands in order to take up water in them; then we squeeze our palms together and squirt out the water like a syringe. Imagine something like this to take place in the clouds. When they are compressed the restricted space drives out the air between them, setting it on fire at the same time, and hurling it forth like a cannon ball. The missiles from our balistae[1] and scorpions[44] give forth a loud noise as they are hurled. [44] The ancient counterparts of cannon. XVII