We walked down the entry to the side door at which I had been admitted by Mrs. Schallibaum on the occasion of my previous visits. Thorndyke inserted the latchkey, and, as soon as we were inside, shut the door and walked quickly through into the hall, whither I followed him. He made straight for the front door, where, having slipped up the catch of the lock, he began very attentively to examine the letter-box. It was a somewhat massive wooden box, fitted with a lock of good quality and furnished with a wire grille through which one could inspect the interior.

“We are in luck, Jervis,” Thorndyke remarked. “Our visit has been most happily timed. There is a letter in the box.”

“Well,” I said, “we can’t get it out; and if we could, it would be hardly justifiable.”