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."