Pompeii

Popidius? Holconius? Brittius? Cuspius?' He waited. It was all a charade. The aquarius would be dead by now, whatever they said: Corax had been itching to slit his throat. 'I'll take that as agreement. Shall we drink to it?'
    He reached for the decanter but stopped, his hand poised in mid-air. The heavy crystal glass was not merely shaking now: it was moving sideways along the polished wooden surface. He frowned at it stupidly. That could not be right. Even so, it reached the end of the sideboard and crashed to the floor. He glanced at the tiles. There was a vibration beneath his feet. It gradually built in strength and then a gust of hot air passed through the house, powerful enough to bang the shutters. An instant later, far away – but very distinctly, unlike anything he, or anyone else, had ever heard – came the sound of a double boom.

Hora sexta

    [12:57 hours]

'The surface of the volcano ruptured shortly after noon allowing explosive decompression of the main magma body... The exit velocity of the magma was approximately 1,440 km. per hour (Mach 1). Convection carried incandescant gas and pumice clasts to a height of 28 km.
Overall, the thermal energy liberated during the course of the entire eruption may be calculated using the following formula:
Eth = V · d · T · K
where Eth is in joules, V is the volume in cubic km., d is specific gravity (1.0), T is the temperature of the ejecta (500 degrees centigrade), and K a constant including the specific heat of the magma and the mechanical equivalent of heat (8.37 × 10 14 ).
Thus the thermal energy released during the A.D. 79 eruption would have been roughly 2 × 10 18 joules – or about 100,000 times that of the Hiroshima atomic bomb.'
Dynamics of Volcanism

    Afterwards, whenever they compared their stories, the survivors would always wonder at how differently the moment had sounded to each of them. A hundred and twenty miles away in Rome it was heard as a thud, as if a heavy statue or a tree had toppled. Those who escaped from Pompeii, which was five miles downwind, always swore they had heard two sharp bangs, whereas in Capua, some twenty miles distant, the noise from the start was a continuous, tearing crack of thunder. But in Misenum, which was closer than Capua, there was no sound at all, only the sudden appearance of a narrow column of brown debris fountaining silently into the cloudless sky.
    For Attilius, it was like a great, dry wave which came crashing over his head. He was roughly two miles clear of the summit, following an old hunting trail through the forest, descending fast on horseback along the mountain's western flank. The effects of the poisoning had shrunk to a small fist of pain hammering behind his eyes and in place of the drowsiness everything seemed oddly sharpened and heightened. He had no doubt of what was coming. His plan was to pick up the coastal road at Herculaneum and ride directly to Misenum to warn the admiral. He reckoned he would be there by mid-afternoon. The bay sparkled in the sunlight between the trees, close enough for him to be able to make out individual lines of surf. He was noticing the glistening pattern of the spiders' webs hanging loosely in the foliage and a particular cloud of midges, swirling beneath a branch ahead of him, when suddenly they disappeared.
    The shock of the blast struck him from behind and knocked him forward. Hot air, like the opening of a furnace door. Then something seemed to pop in his ears and the world became a soundless place of bending trees and whirling leaves. His horse stumbled and almost fell and he clung to its neck as they plunged down the path, both of them riding the crest of the scalding wave, and then abruptly it was gone. The trees sprang upright, the debris settled, the air became breathable again. He tried to talk to the horse but he had no voice and when he looked back towards the top of the mountain he saw that it had vanished and in its place a boiling stem of rock and earth was shooting upward.

    From Pompeii it looked as if a sturdy brown arm had punched through the peak and was aiming to smash a hole in the roof of the sky – bang, bang: that double crack – and then a hard-edged rumble, unlike any other sound in Nature, that came rolling across the plain. Ampliatus ran outside with the magistrates. From the bakery next door and all the way up the street people were emerging to stare at Vesuvius, shielding their eyes, their faces turned towards this new dark sun