"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?"