explosion came to their ears from behind the big pile of earth
and concrete that formed the bomb-proof around the testing
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."