"Here, help me hold him!" Mr. Mack called to some of his friends who had come in with him. "We'll see what this is all about. Who can explain?" he asked, looking at Bert, Harry and Frank, in turn. "He can," said Bert, pointing to the former circus boy. At this the stranger took a good look at Frank, and he seemed much worried. "I see you know me," said Frank with a smile. The man muttered something to himself. In a few words Frank told how he had been cheated by the old twenty dollar Confederate bill the man had passed on him some time ago, in the lumber office. "And when I saw that man, to-day, for the first time since, hiding around your store," went on Frank to Mr. Mack, "I thought perhaps he was up to some of his old tricks. He went in as soon as you went out, and I saw him give your clerk the same kind of a bad bill he gave me. Only I gave him eighteen good dollars in change." "But I didn't," said George Smith with a grateful look at Frank. "I was warned in time." "I tell you it is all a mistake," said the man. "You had better let me go." "The only place you will go to is prison," cried Mr. Mack. "Take him away, Constable Sprigg," he said to one of the men who had come into the store with him. "Take him away!" So the man who had cheated Frank, and who had nearly cheated Mr. Mack, was locked up in jail. It was found that he had many Confederate bills with him. That money was once good in the Southern States, during war-times, but now it is of no value, and will not buy even a stick of candy. Of course grown persons could not be fooled by the Confederate bills, but boys, who had never seen any of that money, might be easily deceived. And it was on boys that the man played his tricks, giving them bad twenty dollar bills for some small purchase, and getting good money in change. "He just waited until Mr. Mack went out of his store," explained Frank, "and he knew only a boy was left in charge. That's how he tricked me, waiting until Mr. Mason was out of the office." "Well, you did me a good service," said Mr. Mack, "and if ever you are in need of work, I'll give you a place in my store to help George when I am out." "I guess Frank is going back in the lumber business," said Bert. The next day Mr. Mason came in answer to the letter he had received about Frank. He brought with him the bad twenty dollar bill the man had cheated Frank with, and a little later the dishonest man was taken away by a policeman, and put in a place where he would have to work hard as a punishment for cheating honest persons. The Bobbseys never saw him again. Everyone said Frank was very smart to catch the cheat as he had done. Mr. Mason received back his twenty dollars, for the man had some good money in his pockets when arrested. "And now are you ready to come back with me, Frank?" asked Mr. Mason, when everything had come out right. "I--I guess so," was the rather slow answer.