as to be not only beyond ordinary observation, but to be unreachable by the type of planes available at Paris.
"If we could only find a piece of the shell we could come nearer to guessing what sort of gun fired it," remarked Tom, as the two Air Service boys rested a moment from their hard, terrible labors.
"Do you mean if it was dropped from an airship it wouldn't have any rifling grooves on it?" asked Jack.
"That's it. A bomb, dropped from an aeroplane, would, very likely, be only a sort of round affair, set to explode on contact or by a time fuse. But if it was a shell fired from a long-range gun, there might be enough of it left, after the explosion, to observe the rifling."
"There isn't a gun with a range long enough to reach Paris from the nearest German lines, unless they have broken through," said Jack.
"Well, the last may have happened; though I should think we'd have got some word of it in that case. There'd be fierce fighting if the Germans tried that, and we'd rush reinforcements out in taxicabs as the Paris soldiers went out once before."
"Do you think then," asked Jack, as they went back, after their brief respite, to their appalling labors, "that they have a gun long
