and then, as though operating on a schedule which must not, under any
circumstances, be changed, the firing ceased, and the crowds once more
filled the streets, for it was yet early in the night.

The next morning the boys went to report, as they did each day,
expecting that they might be called back to duty. They also found, after
being told that their leave was still in effect, that some of the
aviators who had gone up the night before, to try to locate the German
gun, were on hand.

"Now we can ask them what they saw," suggested Jack.

"That's what we will," assented Tom.

But the airmen had nothing to report. They had ascended high in search
of a hostile craft carrying a big gun, but had seen none.

They had journeyed far over the German lines, hoping to discover the
emplacement of the gun, if a long range cannon was being used. But they
saw nothing.

"Not even flashes of fire?" asked Tom.

"Oh, yes, we saw those," an aviator said. "But there were so many of
them, and in so many and such widely scattered places, that we could
not tell which one to bomb. We did manage to hit some, though with what
effect we could not tell."

"Then the German gun is still a mystery," observed Tom.

"It is. But we shall discover it soon. We will never rest until we do!"

So more and new and different theories continued to be put forth
regarding the big cannon, if such it was. Ordnance experts wrote
articles, alike in London, Paris, and New York, explaining that it was
possible for a cannon to be within the German lines and still send a
shell into the French capital. But few believed that it was feasible.
The general opinion was that the gun was of comparative short range, and
was hidden much nearer Paris than the sixty or seventy-odd miles away,
beyond which stretched the German line of trenches.

Meanwhile Tom, though making careful inquiries, had learned nothing of
his father. He did not feel it would be wise to cable back home, and ask
what the news was there.

"It might spoil dad's plans if I did that," said Tom to his chum, "and
it would worry the folks in Bridgeton to know that I haven't yet seen
him in France. No, I'll just have to wait."

And wait Tom did, though there is no harder task in all the world.

It was one morning, after a night bombardment on the part of the
Germans, that Jack, who had been out for a morning paper, came rushing
into the room where Tom was just awakening.

"Great news, old man! Gre