"What's that?" cried Ned, as the earth shook.
"Just trying some of the big guns," explained the sentry, who was not a bad-natured chap. He had to do his duty. "You'd better move on," he suggested. "If anything happens the government isn't responsible, you know."
"I wish there was some way of getting in there," murmured Tom.
"You can see General Waller after the test, and he will probably countersign the permit," explained the sentry.
"And we won't see the test of the gun I'm most interested in," objected Tom. "If I could only—"
He stopped as he noticed the sentry salute someone coming up from the rear. Tom and Ned turned to behold a pleasant-faced officer, who, at the sight of the young inventor, exclaimed:
"Well, well! If it isn't my old friend Tom Swift! So you got here on my permit after all?"
"Yes, Captain Badger," replied the lad, and then with a rueful face he added: "But it doesn't seem to be doing me much good. I can't get into the proving grounds."
"You can't? Why not?" and he looked sharply at the sentry.
"Very sorry, sir," spoke the man on guard, "but General Waller has left orders, Captain Badger, that no outsiders can enter the proving grounds when his new gun is being tested unless he countersigns the permits. And he's engaged just now. I'm sorry, but—"