Anstruther strode off down the street, and presently hailed another night cab which was crawling down the road. Rigby sat up and aroused his own driver.

"Here's another five shillings for you," he said. "Keep that cab in front of you in sight, and follow it till it stops. Then you shall have fifteen shillings. Unless I am greatly mistaken, you will not have very far to go."

As a matter of fact, Rigby had summed up the situation quite correctly. The mention of the name of Charlie had given him the clue he required, this same Charlie being none other than the professional cracksman who had been engaged by Anstruther to deliver the letter at the Great Metropolitan Hotel to Ferris. This deduction proved to be absolutely correct, for a little time later the first cab pulled up in front of the tenement house where Seymour had taken up his temporary quarters. Rigby dismissed the cab, and followed cautiously. He was in time to see Anstruther take a key from his pocket, and let himself quietly into the rooms occupied by the individual who was known to his friends and admirers as "Simple Charlie." Then Rigby turned and knocked for admission at the outer door of Seymour's apartments. The latter did not appear in the least surprised to see Rigby.

"I came here quite by chance," the latter explained. "I quite expected to be told that you had not returned home yet. Lady Barmouth's dance might have kept on till daylight."

"I had to come away," Seymour explained. "In fact, I lost sight of Anstruther, and it rather put me out. Can you tell me anything about him? But of course you can, or you would not be here."

Rigby explained at length what had taken place during the past hour. Seymour chuckled as he listened.

"Rather a good joke," he said. "Here is Anstruther looking for his friend 'Simple Charlie,' whilst all the time we have that desirable individual tight by the leg at the Great Metropolitan Hotel. I suppose you can pretty well guess what's going to happen? Anstruther was desperately frightened to-night by my allusion to that set of Cellini plate. He will know no peace of mind until that stuff is removed from Carrington's private safe. There will be another burglary, of a sort, and 'Simple Charlie' has been selected to open the safe. You see, as the safe is not in the vaults, but in Carrington's private office, it would never do to use dynamite there."

"That is all very well," Rigby objected. "But how is Anstruther going to make use of 'Simple Charlie' so long as the latter is in our hands? That seems to be rather an objection."

"Oh, I have thought all that out," Seymour laughed. "From what you told me just now, it is evident that Anstruther means to leave a note for his pal if the latter is away. In the event of 'Simple Charlie' being professionally engaged elsewhere to-morrow night, he will be asked to find a substitute. As we are perfectly well aware of the fact that there is no chance of Anstruther finding his friend at home, it is only logical to assume that he will leave the note behind. In a few moments that note will be in our possession, and we shall be in a position to read it at leisure. Then I will take it the first thing in the morning round to the Great Metropolitan Hotel, and force 'Simple Charlie' to write a suitable reply. Do you follow me?"

"Oh, quite," Rigby said. "You are going to choose your own substitute. Have you fixed upon him yet?"