MR. STAPLETON was standing in the middle of the library, when Duvall entered. He turned to him excitedly.

"Mr. Duvall," he said, "I have just heard news that I hope will restore my boy to me within the next twenty-four hours!"

"From the woman who just left the house?"

"Yes."

"Who is she?"

"An agent of the police."

"Ah! Are you certain of that?"

"I know only what she says."

Duvall looked at him curiously. "What is the news she has brought you?"

"A message from the scoundrels who have stolen the child. They want a hundred thousand dollars, to return him."

"And she brought you that message?"

"Yes." The banker regarded his questioner uneasily."Does it not seem rather singular, Mr. Stapleton, that a member of the Paris police should come to you with a message from the kidnappers?"

Mr. Stapleton frowned. "I had not considered that aspect of the case, Mr. Duvall. I was—and am—too anxious to get my boy back, to care by whom these fellows deliver their terms."

"What was the message, Mr. Stapleton?"

"I am to drive along the road to Versailles tomorrow evening, leaving here at eight o'clock, and moving at the rate of twelve miles an hour. Somewhere on that road, an automobile in passing will signal me with a blue light. I am then to slow up and toss into the other machine a package containing one hundred thousand dollars. If I do this, and make no attempt to follow or capture the rascals, they agree to deliver the child here—at my house—by the time I return home."

Duvall listened to Mr. Stapleton's words with growing interest. "They are a shrewd lot," he exclaimed. "They will get away in their machine, and have ample opportunity to examine the package to see that it contains the amount they demand. By signaling to confederates at any point along the road, or in another automobile, they can advise them whether or not to return the child."

"But how will they be able to do this, without running the risk of being caught?"

"That is easy. They take the boy to Paris, employ a passerby—a man of their own class, no doubt—for a few francs, to deliver him at your door. To trace them, through that means, will be impossible. If you give them the money, the chances are that they will never be caught."

"Nevertheless, I shall give it to them."

"I expected that, Mr. Stapleton. I can understand your feelings. It is not right, of course, to submit to this blackmail; but no doubt, were I situated as you are, I would do the same thing. Still, it is a great pity."

"Why?"

"Because we have an excellent chance to capture these fellows."

"And lose the boy!"

"Yes, that might be true. Such men are apt to retaliate very promptly, and very severely. They have no pity. I wish I might handle the case to suit myself."

"What would you do?""I would arrange to follow you, in a fast car, keeping say five hundred feet in the rear. I should have several men, well armed, in the car. By watching carefully, with field glasses if necessary, I would observe the car which signaled you with the blue light. When this car passed me, I would follow, but make no move which would alarm the kidnappers until they had given the signal—whatever it is—that would ensure your boy being returned to you. Then I would close in on them, and arrest them."

"Your plan, Mr. Duvall, is open to serious objections. Suppose these men, undoubtedly on the watch, observe that they are being followed. They will give no signal—and I will lose not only my child, but the one hundred thousand dollars as well. No, no, I want no interference in the matter whatever."

Duvall remained a moment in silence. "Very well, Mr. Stapleton, I am under your orders, of course. But I dislike very much to see these fellows get away."

"So do I; but there's no help for it."

"If I can work out a plan for their capture, which will not involve the loss of the boy, you are willing, I take it, to let me go ahead?""Yes; but I insist that you first submit the plan to me."

"Very well. And now, another matter. This woman who brought the message to you is, you say, an agent of the police. Did she attempt to explain how she came by the message?"

"Yes. She was forcibly abducted, last night, carried a long distance out into the country, and the instructions given her. She was brought back to Paris, blindfolded, tonight."

"Mr. Stapleton, what would you say were I to tell you that less than an hour ago I saw this woman in the rooms of Alphonse Valentin, a man whom I suspect to be very deeply concerned in the kidnapping of your son?"

Stapleton started. "Is it possible?" he said. "Have you any idea what she was doing there?"

"No. They seemed on excellent terms, however. Of course, it is not impossible that an agent of the police might pose as a friend of one of the criminals, and thus obtain information. But it looks decidedly queer."

"It does, indeed. Still, as I said before, if I get my boy back, I shall be satisfied." He took a turn about the room, chewing nervously upon his long black cigar. "Now, Mr. Duvall, what is your plan to capture these fellows?"

Duvall sat in deep thought for sometime. "It is not an easy matter, Mr. Stapleton, but there is one way which promises success, and that, too, without interfering with your arrangements to recover your boy."

"What is it?"

"This. It is necessary for us, in some way, to identify the car which gives you the signal of the blue light. It will pass close to you, at a moderate speed. I want you to mark that car, so that it may be recognized at once."

"How can I do that?"

"I will place in the bottom of your machine a small device, consisting of a rubber bulb, equipped with a small nozzle, projecting through a hole in the body of the car. The bulb will be filled with indelible red stain. When you stand up, to toss the package of money to the kidnappers, you must press this bulb with your foot. The two cars will then be side by side. The pressure on the bulb will discharge a blast of the red stain against the body and wheels of the car opposite you. It will then be a simple matter to identify it.""Yes—yes. I see that. But what then?"

"The car, in passing you, will be headed for Paris. Undoubtedly it is the intention of these fellows to enter the city. I shall station myself at the Porte de Versailles, and I will arrange to have other men, members of the detective bureau, stationed at the neighboring gates in the fortifications. All cars entering the city will be momentarily halted. The one which bears upon its body or wheels the red stain will be seized, its occupants arrested."

"But suppose they have not yet notified their confederates to return the boy to me?"

"In that event, I feel certain that the child will be found in the automobile with them. Look at the thing as you would, were you in their place. They are forced to act with great quickness. Were they to signal, by lights or otherwise, to persons along the road, they could hardly hope to get the boy to your house before you yourself return there. They know you will return home immediately at your best speed as soon as you have delivered the money to them. What more likely, then, that they will have the boy with them in the car, will drive to some prearranged point in Paris, and deliver him to the person who will bring him to your house? That would seem, to my mind, their most probable plan."

"And if not—if the child is not with them?"

"Then there are but two courses open to them. The first is to signal, by lights or otherwise, to their confederates, before they enter Paris. If they do this, the boy will be returned to you, and we will capture the men as well. The only other alternative, of course, is for them to notify their confederates after they enter Paris."

"But, if you arrest him at the barrier, they cannot do that, and my boy will not be sent back."

"That is true; but I do not think they will wait to notify their confederates until after they enter Paris."

"Why not, Mr. Duvall?"

"First, because of the danger of being observed, in the crowded streets of the city. Secondly, because I do not think the child is in Paris at all. The woman who brought you the message from the kidnappers, I understand, saw the child at a point some distance in the country. It seems unlikely that these men would run the risk of conveying the child into the city, in broad daylight. By having the boy with them in the car, they avoid all danger of signaling anybody. They merely inspect the package of money, run into Paris, fully believing themselves for the time being safe, drop the child at a convenient point, divide the plunder, and scatter to their respective hiding places. Criminals of this sort know perfectly well that they are far safer, hiding in a big city, than fleeing through the country in an automobile. I feel scarcely any doubt that they have the child with them."

"But if he is still in the country, and they wait until after they are in Paris before notifying their confederates?"

"Then the latter are obliged to journey a long distance out into the country, get the child, and bring him back to your house. That would require a considerable period. They could not possibly do it before you return home."

Mr. Stapleton considered the matter for a long time in silence. "Your arguments seem sound, Mr. Duvall," he presently observed. "Like yourself, I am anxious to capture these fellows. It makes my blood boil, to think of their getting away. Of course, your deductions may be wrong.""Then at least we will get the perpetrators of the crime, and it is most likely that one of them, at least, may be persuaded to turn state's evidence, and disclose the whereabouts of your son."

Mr. Stapleton pondered the matter with great care. Evidently he feared any course of action which did not insure the return of the child.

"It seems to me, Mr. Stapleton," the detective went on, "that you owe it to the public to let me make this effort to capture these fellows. It is a grave danger to the community, to have such rogues at large. Let me try my plan. Even if it fails, you are no worse off than you are now. The attempt cannot in any way be traced to you."

"Very well," said the banker, nervously. "It is a chance—that's all. However, since it seems to involve no breach of faith on my part, I am willing to take it."

"Good! I will bring the device I spoke of to your house tomorrow, and attach it to your car. Your man FranÇois will drive you, I presume."

"Yes."

"You trust him?""I have no reasons for not doing so. And besides he will know nothing of the affair. His part will be merely to drive the car, as I direct him."

Duvall thought for a moment. "You will not, of course, give him his instructions until the last moment—just before you start."

"No. That will be best, I think."

"Undoubtedly. And to avoid any possible interference, I think I had better not attach the identifying device of which I have spoken to your car until late tomorrow afternoon, immediately before you set out. Then, if by any chance your chauffeur is in this plot, he will have no opportunity to give a warning."

"Very well. I think, however, that your precautions are needless. There has been nothing whatever brought out to connect FranÇois with this matter."

"I know; but it is well to be careful. You will leave here tomorrow evening, at eight o'clock?"

"Yes. Promptly at eight."

"You might do well to have someone with you, some member of the police, perhaps."

"The instructions expressly forbid it.""Ah—I see. These fellows are shrewd." He took up his hat. "Until tomorrow then. Good night."

"Good night."