“Taken separately, none of the three pieces of information leads to much in the way of a conclusion. Taken together, the indications would be that the thing Charles Russell wished to conceal was not in his house, but in the garden. And how could he hope to keep a buried object hidden from a gardener as skilled and conscientious as your father? He was forced to take your father into his confidentiality, but to protect him, he cut all evidentiary ties between himself and the Long family. He paid their salaries in cash, he made no provisions in his will for them, and he and his wife refused a signed document when she lent your parents money to buy the bookstore. So yes, I believe there is something buried in the garden, something your father knew about. Something too sensitive to be locked into a bank's safe-deposit vault, where it would come to light on Charles Russell's death.”

“You may be correct, Mr Holmes, but I assure you, he did not tell me about it.”

“I should be very surprised if he did. However, I should also be surprised if you could not find it.”

“How? What would I be looking for?”

“I have no idea.”

“Then how do you know that it is there?”

“This threatens to become a circular argument,” Holmes said. “I know it's there because it's all that explains the facts. My wife tells me that astronomers posit the existence of an invisible planet by the effects it has on the orbit of other celestial bodies. Thus do I posit the existence of this object.”

“I see. Mr Holmes, I have been in the garden a few times, yes, when I was very young, but I doubt that now I could even find where my father had his vegetables growing—the place is a jungle, I saw that much the other evening.”

Holmes hunched forward over the table, and spoke in a low voice. “Mrs Russell kept a detailed record of the work done in her garden, including a yearly sketch or map of the arrangement of flower-beds and paths, the addition of major plantings, and so on. There is a volume for every year, beginning with the spring of 1903. The years she spent in England, 1907 to 1911, are missing, but there is one made dated March 1906, and one made in the autumn of 1912 after her return.”

“None of them, I would assume, have a spot marked ‘X' with the Stevensonian suggestion to ‘dig here'?” Long asked it with a smile.

“Alas, no. However, I believe your father may have acknowledged the presence of some object of supreme importance in the arrangement of the garden itself, whether he was instrumental in its concealment or simply told of its presence after the deed was done.”

“How do—ah.” Long sighed. “You are thinking of my father's commitment to the principles of feng shui.”