It happens that one of Mr.Hornby's customers in South Africa is interested in a diamond mine, and, although transactions in precious stones form no part of the business of the house, he has, from time to time, sent parcels of rough diamonds addressed to Mr.Hornby, to be either deposited in the bank or handed on to the diamond brokers.
"A fortnight ago Mr.Hornby was advised that a parcel of stones had been despatched by the _Elmina Castle_, and it appeared that the parcel was an unusually large one and contained stones of exceptional size and value.

Under these circumstances Mr.Reuben was sent down to the docks at an early hour in the hope the ship might arrive in time for the stones to be lodged in the bank at once.

Unfortunately, however, this was not the case, and the diamonds had to be taken to the works and locked up in the safe." "Who placed them in the safe ?" asked Thorndyke.
"Mr.Hornby himself, to whom Mr.Reuben delivered up the package on his return from the docks." "Yes," said Thorndyke, "and what happened next ?" "Well, on the following morning, when the safe was opened, the diamonds had disappeared." "Had the place been broken into ?" asked Thorndyke.
"No.

The place was all locked up as usual, and the caretaker, who had made his accustomed rounds, had heard nothing, and the safe was, outwardly, quite undisturbed.

It had evidently been opened with keys and locked again after the stones were removed." "And in whose custody were the keys of the safe ?" inquired Thorndyke.
"Mr.Hornby usually kept the keys himself, but, on occasions, when he was absent from the office, he handed them over to one of his nephews--whichever happened to be in charge at the time.