I puzzled over the letter all that evening, and most of the next day—and then suddenly the solution came to me. It was so simple, too. The figure 4 was the clue. Read every fourth word in the letter, and an entirely different message appeared. "Essential should see you quarry seventeen eleven four."
The solution of the figures was easy. Seventeen stood for the seventeenth of October—which was to-morrow, eleven was the time, and four was the signature—either referring to the mysterious Number Four himself—or else it was the "trade-mark" so to speak, of the Big Four. The quarry was also intelligible. There was a big disused quarry on the estate about half a mile from the house—a lonely spot, ideal for a secret meeting.
For a moment or two I was tempted to run the show myself. It would be s
