“I've had a series of peculiar dreams. Two of them served to remind me about the earthquake and the period afterwards, events I had forgotten entirely, but the third is still puzzling. It involves a secret compartment in a house—nothing particular happens, I just pass by and know that it's there. I don't know what the imagery means. Probably nothing, but it would have been satisfying to have discovered a hidden vault under the house or something.”

Long nodded impassively and the conversation turned to the collection of furniture the cellar contained, some of which was going to have to come out through the coal-cellar doors. They ate the food and drank wine and pale tea, and when they were replete, Long patted his lips with his table napkin and spoke hesitantly.

“I wonder, about your hidden room. Do you know of the writings of Father Matteo Ricci?”

Russell shook her head, but Holmes got a faraway look on his face.

“Ricci was a Jesuit in the sixteenth century who went to China, as a missionary of course, although as was the habit of the Jesuits, he learnt as much as he taught. Many of his writings are in Chinese, which somewhat limits his fame in the West. But one of the things he tried to teach the Mandarins concerned the mnemonic arts. I believe Western philosophers have something of a tradition of memory training.”

“Ignatius of Loyola,” Holmes supplied, his own memory having performed its retrieval, “founder of the Jesuit order. And Pliny has a section on memory experts, I believe, as do several Mediaeval works on oration.”

“What does this have to do with locked rooms?” Russell asked.

“Ricci's technique involves the construction of memory palaces,” Long told them. “One visualises a large building—real or imagined, palace or basilica—and furnishes it with items that stimulate specific memories.”

“The problem being,” Holmes commented, “that the formulation and retention of the myriad rooms and furnishings alone requires a prodigious memory.”

“And,” Long added, with an air of finally being permitted to reach his central thesis, “there is nothing to guarantee that a room once furnished will not be closed off and forgotten. To have its lock turned, as it were.”

“I see,” Russell said. Her chin had come up and one light brown eyebrow had arched delicately above the frame of her spectacles: scepticism, and a trace of indignation that this stranger would presume to know her mind. Before she could voice her objections aloud, Holmes firmly turned the conversation to books and Chinese philosophy, and in a while they were lighting their after-dinner cigarettes and arguing amiably over the bill.

She was still silent when they stood to leave, rousing herself only to say the necessary words of farewell to the bookseller. Outside, the fog had thickened into a clean, grey version of a London particular, and Holmes relaxed into its protection, hooking her hand through his left arm as they set off for downtown.