"That is Mayes, and no other," said Plummer, when I had made my report and described the man with the eyes. "I've seen him twice, once with his beard and once without. The question now is, whether we hadn't best sail in straight away and collar him. But there's the window at the back, and a ladder, I think you said. Can he reach it?" "I think he might--easily." "And perhaps there's the roof, since he's got the top floor too. Not good enough without some men to surround the house. We must go gingerly over this. One thing to find out is, what is the building behind? Ah, how I wish Mr. Hewitt were here now! If we don't hear from him soon we must send a message. But we mustn't lose sight of No. 8 for a moment." There was a thump at the sitting-room door, and Mr. Moon came puffing in and shouldered himself confidentially against Plummer. "Bloke downstairs wants to see you," he said, in a hoarse grunt that was meant for a low whisper. "Twigged you outside, I think, an' says he's got somethink partickler to tell yer. I believe 'e's a 'nark'; I see him with one o' your chaps the other day." "I'll go," Plummer said to me hurriedly. "Plainly somebody's spotted me in the street, and I may as well hear him." I knew very well, of course, what Moon meant by a 'nark.' A 'nark' is an informer, a spy among criminals who sells the police whatever information he can scrape up. Could it be possible that this man had anything to tell about Mayes? It was scarcely likely, and I made up my mind that Plummer was merely being detained by some tale of a petty local crime. But in a few minutes he returned with news of import. "This fellow is most valuable," he said. "He knows a lot about Mayes, whom, of course, he calls by another name; but the identity's certain. He saw me looking in at No. 8, he says, and guessed I must be after him. He seems to have wondered at Mayes's mysterious movements for a long time, and so kept his eye on him and made inquiries. It seems that Mayes sometimes uses a back way, through the window you saw on the opposite side of the little area, by way of that ladder you mentioned. It's quite plain this fellow knows something, from the particulars about that ladder. He wants half a sovereign to show me the way through a stable passage behind and point out where our man can be trapped to a certainty. It'll be a cheap ten shillingsworth, and we mustn't waste time. If Hewitt comes, tell him not to move till I come back or send a message, which I can easily do by this chap I'm going with. And be sure to keep your eye on the front door of No. 8 while I'm gone."