Two things were at once apparent to Tom and Jack as they hurried out of their pension. One was that the people of Paris were not seeking shelter after the warnings as quickly as they had done at first, and the other was that there was evident curiosity on all sides to see just what damage would be done, and from which direction it would come. With an almost reckless disregard for their safety, if not for their lives, the Parisians fairly flocked out of doors to see the results of the Huns' bombardment. It was in vain that the police and military urged them to seek safety in cellars or the places provided.

This time only one shell fell near enough to Tom and Jack to make the explosion heard, and that was so faint as to indicate that it was some distance off. What damage had been done could only be guessed at.

"But we'll find out where it is, and go take a look," said Jack.

"Maybe it'll hit right around here if we stay," suggested his chum.

"Well, I'm not taking that chance," Jack went on. "Let's find out where it landed this time."

This they could do through their acquaintance with the military authority of the district where they were then staying. A telephonic report was at once received, giving the quarter where the shell had landed. It had fallen in one of the public squares, and though a big hole had been torn in the ground and pavement, and several persons killed and wounded, no material damage had been done. As for any military effect of the shell, it was nil.

The firing was done in the early evening hours, and Tom and Jack learned that, almost to the second, the shots were fifteen minutes apart.

There was one theory that an underground passage had been made in some manner to within a comparatively few miles of Paris, and from that point an immense mortar sent up the shells in a long trajectory.

Another theory was that traitors had let the Germans through the French lines at a certain place, so they could get near enough to Paris to bombard it.

And of course the gigantic airship theory had its adherents.

But, for a time at least, no one would admit the possibility of a gun with range sufficient to shoot into Paris from the nearest German lines. The range, sixty-odd miles, seemed too great for practical belief, however nicely it might work out in theory.

"And you must remember that the gun, if gun it is, couldn't be in the very first German line," said Tom, who had studied ordnance. "It must be at least ten miles back, to allow for sufficient protection from the French guns. That would make it shoot about seventy-two miles, and I don't believe any gun on earth could do it!"

"Neither do I," added Jack. "We've got to dope out something else. But this isn't finding your father, Tom."

"I know it, and I don't mind admitting I'm clean discouraged about him, Jack. If he's alive why doesn't he send me some word? He must know where I am, and, even if he doesn't know I'm in Paris, they would forward any message he might send to our aeroplane headquarters."

"That's right. But what are you going to do about it?"

"I hardly know. He may still be in Paris, but it's such a big city that it's hard to find him. Then, too, I'm thinking of something else."

"What's that, Tom?"

"Well, dad may not want us to know where he is."

"Why in the world would he want such a thing as that?"

"Well, he might be followed, or bothered by spies. Perhaps he has come over to do some special work for the French or English army people. Maybe a spy was after him just before the big German gun wrecked his Rue Lafayette house. He may have considered this a good chance to play dead, and that's why he doesn't send some word to me."

"That's a good theory. But it isn't very comforting."

"No, but there isn't much comfort in war times. We've got to make the best of it."

"I guess you're right, Tom. Now do you want to go look at the latest work of the Hun?"

"Might as well. The bombardment seems over for the night."

"I wonder why it is they don't fire after dark."

"Probably afraid of giving the location of their cannon away by the flashes. They'd be seen at night; but during the day, if they used smokeless powder, or a smoke screen in case they can't get smokeless powder for such a big gun, it would be hard to locate the place where the shots come from. So we're comparatively safe after dark, it seems."

Later this was not to prove to be the case, but it was when Tom spoke.

The boys went to the section of the city in which the last shells had fallen. While comparatively little damage had been done, a number of persons had been killed and injured, children among them. Some fragments of the shells were picked up, but not enough to make certain any particular theory in regard to the gun.

"But if it's a gun, where could it be placed?" queried Tom of an officer. "The Germans haven't broken through, have they?"

The French officer shook his head.

"No. And please God they will never get through," he said. "But there is a gun somewhere, I am sure of that."

"Do you mean to say within ten or fifteen miles of Paris?" Jack wanted to know.

"I can not be sure. It is true there may have been traitors. We have them to contend with as well as spies. But our line is intact, and at no point along it, near enough to it to fire into Paris from an ordinary gun, can the Germans be found."

"Then it must be an extraordinary gun," suggested Jack.

"It may well be—perhaps it is. Yet, as I said, there may have been traitors. There may be a gun concealed somewhere closer to Paris than we dream. But we shall find it, messieurs! Who knows? Perhaps you may be the very ones yourselves to locate it, for we are depending on you soldiers of the air."

And it was not long before this talk came back to Tom and Jack with impressive recollection.

And meanwhile the bombardment of Paris went on, usually during the late afternoon or early morning hours—never at night, as yet.

Yet with all the frightfulness of which the unscrupulous Huns were capable, it was impossible to dampen for long the spirits of the French. Soon they grew almost to disregard the falling shells from the hidden German gun. Of course there were buildings destroyed, and lives were lost, while many were frightfully maimed. But if Germany depended on this, as she seemed to, to strike terror to the hearts of the brave Frenchmen the while a great offensive was going on along the western front, it failed. For the people of Paris did not allow themselves to be disheartened, any more than the people of London did when the Zeppelins raided them.

Indeed one Paris paper even managed to extract some humor out of the grim situation. For one day, following the bombardment, a journal appeared with "scare" headlines, telling about eleven "lives" being lost. But when one read the account it was discovered that the lives were those of chickens.

And this actually happened. A shell fell on the outlying section and blew up a henhouse, killing nearly a dozen fowls and blowing a big hole in the ground.

There were other occasions, too, when the seemingly superhuman bombardment was not worth the proverbial candle. For the shells fell in sections where no damage was done, and where no lives paid the toll. Once a shell went through a house, passing close to an aged woman, but not hurting her, to explode harmlessly in a field near by.

And it was with such accounts as these that the Paris papers kept up the spirits of the inhabitants. Meanwhile the Germans kept firing away at quarter-hour intervals, when the gun was in action.

"I wonder if there is any chance of us getting in at the game?" questioned Jack of Tom one night.

"I shouldn't be surprised. As that officer said, they'll have to depend on the aircraft to locate the gun, I'm thinking."

"And you think we have a chance?"

"I don't see why not," replied Tom. "We've been off duty long enough. I'd like to get back behind the propeller again, and with a drum or two of bullets to use in case we sight a Hun plane. Let's go and send word to our captain that we've had enough of leave, and want to go out again."

"All right. But what about your father?"

"Well, I don't know what to say," answered Tom. "I'm about convinced that he wasn't killed, or even hurt, in any of the bombardments of Paris. But where he is I don't know. I guess, as a matter of duty to France, I'll have to let my private affairs go and—"

At that instant there sounded an explosion the character of which the two boys well knew by this time.

"The big gun again!" cried Jack.

"Yes, and they're firing after dark!" added Tom. "This may be just the chance the airmen have been waiting for—to locate the piece by the flashes. Come on out and see what's doing!"

Together they rushed from their room.