Below them the boys could see flashes of fire as the various guns were discharged; and at one point in the lines there was quite an artillery duel, the French batteries sending over a shower of high explosive shells in answer to the challenge from the Boches.
It was not until Jack had followed his chum back to Camp Lincoln, and they had made a landing, that a conversation ensued which was destined to have momentous effect.
"Jack, did you notice the peculiar colored lights away to the north of where we were flying?" asked Tom, as they divested themselves of their fur garments.
"You mean the orange colored flare, that turned to green and then to purple?" asked Jack.
"That's it. I thought you'd see it. I wonder what it means?"
"Oh, perhaps some signal for a barrage or an attack. Or they may have been signaling another battery to try to pot us."
"No, I hardly think so. They didn't look like signal fires. I must ask Major de Trouville about that."
"What?" inquired the major himself, who was passing and who heard what Tom said.
"Why, we noticed some peculiar lights as we were flying over the German lines in the dark. There was an orange flare, followed by a green light that changed to purple," answered Tom.
"There was!" cried the major, seemingly much excited. "You don't mean it! That's just what we've been hoping to see! Come, you must tell Laigney about this."