Juve motioned to his companion to be silent and listen. "Fandor, you hear what that man's singing; the one drinking at the bar?" "Yes, 'The Blue Danube.'" "Well, that gives me the answer. We shall soon be on Loupart's tracks. By the way, are you armed?" "If you won't run me in for carrying concealed weapons I'll confess that Baby Browning is in my pocket." "Good. Now, then, listen to my directions. Loupart was seen at the markets this morning by two of my watchers, and you may be sure he hasn't been lost sight of since. Reports I have received indicate that he will presumably go to the Chateaudun cross-roads and from there to the Place Pigalle, in the direction of Doctor Chaleck's house. We shall nab him at the cross-roads. Needless to say we are not going to keep together. As soon as our man comes in sight you will pass on ahead, walking at his pace on the same pavement and without turning round." "And if Loupart doesn't appear?" "Why then--" began Juve. "The deuce! There's another customer whistling 'The Blue Danube.' It's time to be off." "Are those your agents whistling?" asked Fandor, as they left the shop. "No." "What! Isn't it a signal?" "It is, and you'll be able to find your trail by the passers-by who whistle that air." While talking, the journalist and the detective arrived at the Chateaudun cross-roads. Juve cast an eye over the ground. "It's six o'clock. Be off and prowl around Notre Dame de Lorette. Loupart will probably come out of that wine-shop you see to the right. You can easily recognise him by his height and a scar on his left cheek." "Look here, Juve, why should these people whistle 'The Blue Danube' if they are not detectives?" Juve smiled. "It's quite simple. If you whistle a popular tune in a crowd, some one is bound to take it up. Well, the two men I put to watching Loupart this morning were whistling this same tune, and now we are meeting persons who caught the air." Fandor crossed the road and proceeded toward Notre Dame de Lorette to the post the detective had allotted to him. The man hunt was about to begin. III BEHIND THE CURTAIN The Cité Frochot is shut in by low stone walls, topped by grating round which creepers intertwine. The entry to its main thoroughfare, shaded by trees and lined with small private houses, is not supposed to be public, and a porter's lodge to the right of the entrance is intended to enforce its private character. It was about seven in the evening. As the fine spring day drew to a close, Fandor reached the square of the Cité. For an hour past the journalist had been wholly engaged in keeping track of the famous Loupart, who, after leaving the saloon, had sauntered up the Rue des Martyrs, his hands in his pockets and a cigarette in his mouth. Fandor allowed him to pass at the corner of the Rue Claude, and from there on kept him in view. Juve had completely disappeared.