ground.

"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--"

"Oh, that's all right, Flynn," said Captain Badger. "It isn't your
fault, of course. I suppose there is no rule against my going in
there?" and he smiled.

"Certainly not, sir. Any officer may go in," and the guard stepped to
one side.

"Let me have that pass, Tom, and wait here for me," said the Captain.
"I'll see what I can do for you," and the young officer, whose
acquaintance Tom had made at the tests when the government was
purchasing some aeroplanes for the army, hurried off.

He came back presently, and by his face the lads knew he had been
successful.

"It's all right," he said with a smile. "General Waller countersigned
the pass without even looking at it. He's so excited over the coming
test of his gun that he hardly knows what he is doing. Come on in,
boys. I'll go with you."

"Then they haven't tested his gun yet?" Asked Tom, eagerly, anxious to
know whether he had missed anything.

"No, they're going to do so in about half an hour. You'll have time to
look around a bit. Come on," and showing the sentinel the
counter-signed pass, Captain Badger led the two youths into the proving
grounds.

Tom and N