Lindsey Davis - Alexandria

Clearly our kind of man. And as we all made our way to my uncle's house on a hired cart – Heron crippled, Helena pregnant, me completely whacked – he even made jokes about us being borne home like a bunch of walking wounded after life's battles.

Aulus and Albia had arrived back. Vast numbers of scrolls from the Library had been recovered in Rhakotis and transferred back to where they came from, under military guard.

Fulvius and Pa, looking tense, were going out. Cassius confessed to Helena that my conniving relatives were desperate to snatch back money they had paid over to Diogenes. They wanted to find where he had stashed the cash. Knowing traders, retrieving their deposit might prove impossible. His banking would be done in cunning hideaways; the money might even already be tied up undetectably in a knotted skein of investments.

Cassius said there would be plenty of food and drink tor us to entertain our famous visitor. There was indeed, and so we had a memorable evening. It was nowhere near as formal as the night we dined with the Librarian, but all the more enjoyable for that. Helena and I, Aulus and Albia were delighted by Heron, who was so secure in his enlightened cleverness he could freely share his enjoyment of ideas with anybody who would listen.

This was the mental conjuror who invented the self-trimming oil lamp, the inexhaustible goblet and slot machines to dispense holy water. Not for nothing was he known as the Machine Man. We already knew of him from his work with automata, famous devices he made for theatres and temples: noises like thunder, automatic opening doors using fire and water, moving statues. He had produced a magic theatre, which could roll itself out before an audience, self-powered, then create a miniature three-dimensional performance, before trundling away to resounding applause. As we sat enthralled, he told us how he once made another that staged a Dionysian mystery rite; it had leaping flames, thunder and automatic Bacchantes who whirled in a mad dance around the wine god on a pulley-driven turntable.

Not all of his work was frivolous. He had written on light, reflection and the use of mirrors; useful stuff on dynamics, with reference to heavy lifting machines; on the determination of lengths using surveying instruments and devices such as the odometer that I had myself seen used in transport; on the area and volume of triangles, pyramids, cylinders, spheres and so forth. He covered mathematics, physics, mechanics and pneumatics; he was the first to write down what was called the Babylonian method of calculating square roots of numbers. He collected information about military war machines, particularly catapults.

The most fascinating gadget he told us about was his aeolipile, which he modestly translated as a 'wind ball'. His design for it used a sealed cauldron of water, which was placed over a heat source. As the water boiled, steam rose into pipes and into the hollow sphere. As I understood it, this resulted in rotation of the ball.

'So what could it be used for?' asked Helena intently. 'Some kind of propulsion? Might it move vehicles?'

Heron laughed. 'I do not consider this invention to be useful, merely intriguing. It is a novelty, a remarkable toy. The difficulty of creating sufficiently strong metal chambers makes it unsuitable for everyday applications – but who would need it?'

Eventually, it became impolite to demand yet more stories. Heron was willing to talk, a man eager to spread his knowledge and deservedly keen to report his own ingenuity. Still, he must find himself asked the same questions over and over again; that must become tedious. He could probably dine out every day of the week with devotees, though I noticed he ate wisely and drank only water. We all liked him. He flattered us by seeming to like us. Helena was particularly impressed that he encouraged us to let the children run around. 'What is the point of knowledge, but to improve the lot of future generations?'

Since they were allowed to be with us, the novelty of being amongst the adults soon palled; Julia and Favonia quickly took it as natural and were for once well behaved. I wished Uncle Fulvius had seen it. Mind you, they might have sensed his attitude; things could have been very different.

Time for business.

'Heron, before we break up this delightful party, you wanted a word with me, you said – and I would like to pick your brains about a puzzle too.'

He smiled. 'Falco, we may have been beguiled by the same problem.'

Aulus jumped in: 'Marcus, are you going to ask how the Librarian came to be found dead in a locked room?'

I nodded. We all fell silent as the great inventor settled down once more to fascinate us. He certainly liked to be the centre of attention, yet had a winning attitude that made his holding court endurable.

'I knew Theon. I heard about how he was found. A locked room – its lock worked from outside – and its key missing.'

'We have now found the key,' Aulus quickly informed him. 'The ancient scholar Nibytas had it.'

'Ah – Nibytas! I knew Nibytas too…' Heron let his quiet smile suffice as comment. 'I have considered deeply how this mystery can be explained.' He paused. He was wickedly keeping us in suspense. 'Could it be ropes and pulleys? Could Theon have worked some pneumatic device from within his private sanctum? Could some incredibly impractical criminal have set up a crack-brained mechanical killing machine? Impossible, of course – you would have found the machine afterwards… Besides, this is outside my sphere,' he said tactfully, 'but most murderers tend to act on impulse, don't they, Falco?'

'More often than not. Even premeditating killers are often quite stupid.'

Heron acknowledged this and continued: 'When I was told that the eminent Nicanor had been first on the scene, my mind took flight extravagantly, I must admit. I know Nicanor also -' He favoured us with his sweetest, most mischievous smile of all. 'I have often thought I would like to harness Nicanor's bluster. That energetic material would surely work some miraculous device!'

Heron paused again so we could all laugh at his joke.

'So do you have a theory?' Helena prompted gently.

'I have a suggestion. I will call it no more. I cannot prove my idea with mathematical rules, nor to the high legal standard you would require, Falco. Sometimes, however, we should not seek answers that are intricate or outrageous. Human nature and material behaviour may suffice. I took myself to the Librarian's room, to inspect the scene of this mystery of yours.'

'I wish I had been there with you, sir.'

'Well, you may visit again and test my ideas at your leisure. I propose nothing complicated. First,' said Heron, making it all sound so logical I felt ashamed not to have seen this myself, 'over the centuries, the great Library has suffered many times from the earthquakes we experience here in Egypt. 'Young Albia squeaked and bounced about; Aulus nudged her quiet. 'The building has withstood the shocks -' He chortled. 'So far! One day, who knows? Our whole city lies on low land, scored and silted by the Nile Delta. Perhaps it may yet slip into the sea…' He fell silent, as if troubled by his own speculations.

It was Aulus who worked out where the original comment had been heading. 'The doors to the room stick – one very badly.'

Heron revived. 'Ah, excellent young man! You catch my drift. The door sticks so much, I myself could not open it. Earthquake damage has shifted the floor and doorframe; routine maintenance has failed to address this problem. Were that my room, I should devote myself to arranging some system of artificial exodus, in case I found myself trapped one day…'

'So you think Theon found himself stuck?' suggested Albia.

'My dear, I think he never knew the doors had been locked. I suspect his death was entirely coincidental to what happened with the key.'