“‘How stupid of me!’ he exclaimed.  He turned quickly and pointed to a narrow slit cut in the brass plate of the front door.  ’The house has a private letter-box,’ he said, ’and I had not thought to look in it!  If we had gone out as we came in, by the window, I would never have seen it.  The moment I entered the house I should have thought of securing the letters which came this morning.  I have been grossly careless.’  He stepped back into the hall and pulled at the lid of the letter-box, which hung on the inside of the door, but it was tightly locked.  At the same moment the postman came up the steps holding a letter.  Without a word, Lyle took it from his hand and began to examine it.  It was addressed to the Princess Zichy, and on the back of the envelope was the name of a West End dressmaker.

“‘That is of no use to me,’ Lyle said.  He took out his card and showed it to the postman.  ‘I am Inspector Lyle from Scotland Yard,’ he said.  ’The people in this house are under arrest.  Everything it contains is now in my keeping.  Did you deliver any other letters here this morning?’

“The man looked frightened, but answered, promptly, that he was now upon his third round.  He had made one postal delivery at seven that morning and another at eleven.

“‘How many letters did you leave here?’ Lyle asked.

“‘About six altogether,’ the man answered.

“‘Did you put them through the door into the letter-box?’

“The postman said, ’Yes, I always slip them into the box, and ring and go away.  The servants collect them from the inside.’

“’Have you noticed if any of the letters you leave here bear a Russian postage-stamp?’ Lyle asked.

“’The man answered, ‘Oh, yes, sir, a great many.’

“‘From the same person, would you say?’

“‘The writing seems to be the same,’ the man answered.  ’They come regularly about once a week—­one of those I delivered this morning had a Russian postmark.’

“‘That will do,’ said Lyle, eagerly.  ’Thank you, thank you very much.’

“He ran back into the hall, and, pulling out his penknife, began to pick at the lock of the letter-box.

“‘I have been supremely careless,’ he said, in great excitement.  ’Twice before when people I wanted had flown from a house I have been able to follow them by putting a guard over their mailbox.  These letters, which arrive regularly every week from Russia in the same handwriting, they can come but from one person.  At least, we shall now know the name of the master of this house.  Undoubtedly, it is one of his letters that the man placed here this morning.  We may make a most important discovery.’

“As he was talking he was picking at the lock with his knife, but he was so impatient to reach the letters that he pressed too heavily on the blade and it broke in his hand.  I took a step backward and drove my heel into the lock, and burst it open.  The lid flew back, and we pressed forward, and each ran his hand down into the letter-box.  For a moment we were both too startled to move.  The box was empty.