do much
to-day. Had to come back on account of mist, and we didn't see enough to
pay for the petrol used. Want to come along?"

"Oh, I might, yes."

Tom and Jack went up, as did several more. But the two remained up
longer than did the others, and Jack was somewhat surprised to see his
chum suddenly head for the German lines, but at an angle that would take
him over them well to the south of where the observation work had been
carried on.

"I wonder what he's up to," mused Jack; "Guess I'd better follow and
see."

There was not much chance of an aerial battle at that hour, for dusk was
coming on. There had been no bombing squadron sent out, which would
have accounted for Tom going to meet them, and Jack wondered greatly at
his chum's action.

Still there was no way of asking questions just then, and Jack followed
his friend. They sailed over the German lines at a good height, and Jack
could keep Tom in view by noting the lights on his plane.

These were also seen by the Germans below, and the anti-aircraft guns
began their concert, but without noticeable effect. None of the Hun
airmen seemed disposed to accept a challenge to fight, so Tom and Jack
had the upper air to themselves.

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