A Riddle in Bronze

to see a distinctive pattern that was unlike any of the others.
I heard a metallic squeal, and turned to see Roberta spinning the cylinder in the lathe, which was rattling and rumbling once more. Curls of swarf flew from the newly-sealed cylinder, and I saw the misshapen end cap become gleaming and smooth under the cutting tool's sharpened bit. I realised Roberta had almost finished, and so I pressed the remaining discs until I had the round dozen she'd asked for.
She completed her work at the same time, and, after stalling the lathe and dousing the cylinder in a bucket of water, she headed for the stairs. "Bring those," she called over her shoulder, indicating the discs.

Chapter 7
We found the professor asleep still, with Mrs Fairacre in attendance. The housekeeper spied the cylinder in Roberta's hands, and shook her head, her lips thinning. "You're meddling with things you don't understand, Miss Twickham. One day you'll both go too far." Then she glanced at me. "And now there's another innocent roped into your schemes. Have you not learned your lesson?"
"Hush, Mrs Fairacre. I promised Septimus a full disclosure after I've seen to my father."
"If he lasts that long," said the housekeeper darkly. "You do what you must, but I will not be made to watch. It's unnatural."
As she left us, I recalled the frantic scrawl in the ledger. Had my predecessor been asked to help as well? Was that why he'd been driven out of his mind? As I stood there clutching the metal discs, I wondered whether to walk out of that house and never look back — contract be damned. But then I caught Roberta looking at me, her face flushed from the fire, and I realised I could not abandon her. My contribution, no matter how minor, might help to save the professor, and I could not flee from such a responsibility.
"Hold this for me," murmured Roberta, and she added the newly-fashioned cylinder to my load. It was about five inches long, half as big again as the smaller versions I'd already seen, and heavier than it looked. I still had no idea as to its purpose, but I guessed I would shortly find out.
Roberta left, returning moments later with the two haversacks. She took out the tripods and placed them one at each end of the room, then began unfolding the fine metal netting I'd seen earlier. "Take this end for me," she said, holding it out. "Careful. It's fragile."
I took the net as requested, and watched as she extended the tripod and fastened her end to a pair of hooks. I copied her, and we moved the tripods apart until the net stretched across the room. The professor was seated on the far side, between the net and the bay window, and as a shaft of sunlight gleamed on the fine wiring I wondered at its purpose. It reached neither floor nor ceiling, so could not be intended to catch anything, and it wasn't strong enough to stop the professor, so it was not meant to trap him either.
Roberta now took the metal discs and began placing them about the room, forming a circle around the professor. I noticed she omitted one disc from the pattern, leaving an opening next to the bay window. Then she took the cage from the haversack and removed the broken cylinder, dropping the latter into the bag. She took the new, larger cylinder from me, and after some fiddling and adjustments she got it mounted in the centre of the lantern-like cage, where it stood like a metal candle. Then she placed the lantern device on a side-table near the professor.
I should add that every step of this process was carried out with practiced ease, as though Roberta had performed the routine numerous times. For my part, I tried to absorb every detail, adding to the abundant questions I was saving for later.
Now Roberta returned to the haversacks, opening the one I had not managed to inspect earlier. She fished around inside, the contents clanking as she did so, then withdrew a tool much like a spanner or wrench. But this device had a round bulge in the middle of the handle, and each end had four prongs rather than the usual two. Hanging the tool from her overalls, Roberta took out a skullcap fashioned from the same fine metal as the net, with a broad metal band to hold it in shape. Finally, she withdrew two glass bottles, one containing amber fluid and the other, clear. These had cut-glass stoppers, and looked like they belonged in a drinks cabinet rather than a tool bag. She set them on a side table, then beckoned me over.
"What do you want me to do?" I asked her.
"Hold the bottles for me, and I beg you not to drop either of them."
Once I had them in hand, Roberta ducked under the net and approached her father. Without ceremony she placed the metal cap on his head, adjusting it to her liking before turning to me. "Bring them here, if you please."
The bottles were cold to the touch, unnaturally so, and I felt my fingers turning numb as I obeyed. Roberta took the bottle containing the amber fluid and, holding the stopper tightly in place, tipped it over and back again several times. Then she pulled the stopper, and I stepped back in alarm as a greenish mist rose from the neck of the bottle. "Wh-what is that?" I asked.
"It's a highly volatile liquid," she explained. "The bottle must be stoppered at all times, for the contents would soon disperse if not."
That didn't explain why the amber fluid gave off a green gas, but Roberta was busy and I didn't want to interrupt again. She took the stopper, which contained several drops of the liquid, and dabbed them on the metal cap. Then she replaced the stopper and handed me the bottle, taking the other in its place. This one