M. de Cassaud looked at the window.

“Yes, that is ingenious,” he said.

He drained off the water, folded back his sleeve, and plunged his arm into the tank.  Then he uttered a little cry.  He drew up into the light an oblong metal can, like a sandwich-case, with the edges soldered together to make it water-tight.  He slipped it into his pocket and turned again to the window.  He looked at it again curiously.

“Yes, that is ingenious,” he said softly, like a man speaking to himself.  Then he led the way back to his office, looking in at the guard-room on the platform to give an order on the way.

The soldered edges of the case were quickly split asunder and a small package of letters written on very thin paper revealed.

“You will let me take these on with me,” pleaded Martin.  “You shall have them again.  But some of them may want a special treatment of which we have the secret.”

M. de Cassaud was doubtful about the propriety of such a procedure.

“After all I found them,” Martin urged.

“It would be unusual,” said M. de Cassaud.  “The regulations, you know——­”

Martin Hillyard smiled.

“The regulations, for you and me, my friend, are those we make ourselves.”

M. de Cassaud would admit nothing so outrageous to his trained and rather formal mind.  But he made a list of these letters and of their addresses as though he was undecided.  He had not finished when a sergeant entered and saluted.  The attendant of the sleeping-car had been taken to the depot.  He had been searched and a pistol had been found upon him.  The sergeant laid a very small automatic Colt upon the table and retired.  M. de Cassaud took up the little weapon and examined it.

“Do you know these toys, Monsieur Hillyard?” he asked.

“Yes.  They are chiefly used against the mosquitoes.”

“Oh, they will kill at twenty-five paces,” continued the Commissaire; and he looked quickly at Hillyard.  “I will tell you something.  You ran some risk last night when you explored that water-tank.  Yes, indeed!  It would have been so easy.  The attendant had but to thrust the muzzle of this through the opening of the window, shoot you dead, raise an alarm that he had caught you hiding something, and there was he a hero and you a traitor.  Yes, that is why I said to you the little opening in the window was ingenious!  Ah, if he had caught you!  Yes, if he had caught you!”

Martin was quick to take advantage.

“Then let me have those letters!  I will keep my French colleagues informed of everything.”

“Very well,” said M. de Cassaud, and he suddenly swept the letters across to Hillyard, who gathered them up hastily and buttoned them away in his pocket before de Cassaud could change his mind.

“It is all very incorrect,” said the Commissaire reproachfully.

“Yes, but it is the war,” replied Hillyard.  “I have the authority of the attendant of the sleeping-car for saying so.”