Initially I saw nothing, but I leaned in closer … and could just perceive the outline of another large hatch on the opposite side of the sphere from the front opening. It was well-hidden among the rivets and piping.

‘Go on,’ said Holmes.

‘Then Dario, he gets in from the front and the girl locks it very good, many locks in front here.’ Fricano spun the cauldron, so the front now faced us again. That opening remained ajar and I got another strong whiff of that awful smell. I sneezed. Fricano closed the front hatch.

‘Hmm,’ said Holmes. ‘I suppose Borelli normally slips out the back hatch and makes his exit through the split in the curtain behind this contraption?’ Holmes spun the sphere again, so the back faced him. He opened the secret hatch an inch or so, then closed it. He ran his hand along the edges.

‘Yes. Normally he is out in five seconds through the back.’

Fricano attempted to spin the thing again facing front, but Holmes stopped it from moving.

‘But not this time?’ he said.

‘No.’

‘Why did he not come out?’ asked Holmes.

‘I do not know. Maybe he fainted.’

‘There are fumes, Holmes,’ I offered. ‘Shall I take a closer look now?’

‘It’s bad, doctor,’ said Hamilton, who had shadowed me and now stood behind Fricano.

‘In a moment, Watson. Stay with me. Mr Fricano, do continue. Were you not waiting to help him out?’

‘No, at this part, I am under the stage, tending to the fire. Another man, Paolo, is in back. The girl pretends to light the fire onstage, which I help from below. When Borelli did not come out, Paolo ran to get me.’

‘What then?’

‘Paolo yells at me that Dario is still inside the cauldron. Since I cannot stop the fire, I ran up from backstage and put my hands though the curtain, trying to open. But the hatch will not open.’

Holmes unlatched the secret back hatch easily, opened it and closed it again.

‘Hmm. It seems to work fine now. Ah, but what is this small bolt?’

‘What bolt?’ Fricano’s eyes widened.

Holmes leaned in, pulled out his magnifying glass and said, ‘This bolt here that – yes, if I slide it like so – would secure the hatch shut. Seems to have been attached recently. Look, the soldering or brazing is pewter-coloured. The rest here is brass or copper.’ Holmes handed me his glass. ‘With this shut, the person inside could not open this door.’

‘Oh God, we tried to pull it open but did not see,’ said Fricano.

With magnification, the bolt indeed looked different from the nearby pipes and ornamentation. I would never have noticed, either. It was a subtle piece of work. I handed the glass back, nodding.

‘Yes,’ said Holmes. ‘Nicely hidden. What happened next? In the act, that is. The audience sees Borelli get inside. The front hatch is apparently secured. What does the girl do?’