"No good?" faltered Tom.
"No, it has to be countersigned by General Waller. And, as he's on the proving grounds now, you can't see him. He's getting ready for the test of his new cannon."
"But that's just what we want to see!" cried Tom. "We want to get in there purposely for that. Can't you send word to General Waller?"
"I can't leave my post," replied the sentry, shortly. "You'll have to come another time, when the General isn't busy. You can't get in unless he countersigns that permit."
"Then it may be too late to witness the test," objected the young inventor. "Isn't there some way I can get word to him?"
"I don't think so," replied the sentry. "And I'll have to ask you to leave this vicinity. No strangers are allowed on the proving grounds without a proper pass."
CHAPTER VI
TESTING THE WALLER GUN
Tom looked at Ned in dismay. After all their work and planning, to be thus thwarted, and by a mere technicality! As they stood there, hardly knowing what to do, the sound of a tremendous explosion came to their ears from behind the big pile of earth and concrete that formed the bomb-proof around the testing ground.