“And now for a few words as to how business is conducted in Mr. Hornby’s establishment.  The samples of gold are handed over at the docks to some accredited representative of the firm—­generally either Mr. Reuben or Mr. Walter—­who has been despatched to meet the ship, and conveyed either to the bank or to the works according to circumstances.  Of course every effort is made to have as little gold as possible on the premises, and the bars are always removed to the bank at the earliest opportunity; but it happens unavoidably that samples of considerable value have often to remain on the premises all night, and so the works are furnished with a large and powerful safe or strong room for their reception.  This safe is situated in the private office under the eye of the principal, and, as an additional precaution, the caretaker, who acts as night-watchman, occupies a room directly over the office, and patrols the building periodically through the night.

“Now a very strange thing has occurred with regard to this safe.  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.  But on this occasion the keys did not go out of his custody from the time when he locked up the safe, after depositing the diamonds in it, to the time when it was opened by him on the following morning.”