"Yankee, eh?" repeated Foyle. "So is Grell. There won't be any one in the finger-print department at this time of night. We'll go along and have a search by ourselves, I think. If we've not got him there, Pinkerton of the U. S. National Detective Agency is staying at the Cecil. We'll get him to have a look over our man and say whether he recognises him." "Very good, sir. There's one other thing. When I searched this man I found this. I don't know if you can make anything out of it. I can't." He handed across an envelope already torn open, addressed to "The Advertisement Dept., The _Daily Wire_." Within were two plain sheets of notepaper and a postal order. On one was written: "Dear Sir, please insert the enclosed advts. in the personal column of your next issue.--John Jones." On the other were two advertisements-- "R.F. You are closely watched. Don't forget 2315. Don't forget 2315. G. "E. £27.14.5. To-morrow. B." "Very curious," commented Foyle. "Copy them out carefully and have 'em sent to the paper. They can't do any harm. Now let's get along." The fog hung heavy over a muffled world as they walked down Victoria Street. Green, whose wits were a trifle less supple than those of his chief when imagination was required, put a question. Foyle answered absently. The mysterious advertisements were not altogether mysterious to him. He recalled the cipher that had been found at Grave Street, and decided that there was at least room for hope in that direction. Besides, there was at least one man now in custody who knew something of the mystery, and, even if he kept his lips locked indefinitely, there was a probable chance of a new line of inquiry opening when his identity was discovered. And even if finger-prints and Pinkerton failed to resolve that, there was still the resource of the newspapers. With a photograph scattered far and wide, the odds were in favour of some one recognising its subject. As Foyle switched on the lights in the finger-print department, Green sat down at a table and with the aid of a magnifying-glass carefully scrutinised the prints which he carried on a sheet of paper. Ranged on one side of the room were high filing cabinets divided into pigeon-holes, numbered from 1 to 1024. In them were contained hundreds of thousands of finger-prints of those known to be criminals. It was for the detectives to find if among them were any identical with those of their prisoner. The whole science of finger-prints for police purposes resolves itself into the problem of classification. It would be an impossible task to compare myriads of records each time. The system employed was absurdly simple to put into execution. In five minutes Green had the finger-prints of the two hands classified into "loops" and "whorls" and had made a rough note. "W.L.W.L.L. "L.W.W.L.W."