ches down again."

"Oh, you mean a disappearing gun."

"That's it, Ned. See, it works by compressed air," and Tom showed his
chum how, when the gun was loaded, the projectile in place, and the
breech-block screwed fast, the officer in charge of the firing squad
would, on getting the range from the soldier detailed to calculate it,
make the necessary adjustments, and pull the lever.

The compressed air would fill the cylinders, forcing the gun to rise on
toggle-jointed arms, so that the muzzle was above the bomb-proof wall.
Then it would be fired, and sink back again, out of sight of the enemy.

The boys looked at several different types of big rifled cannon, and
then passed on. They could hear firing in the distance, some of the
explosions shaking the ground.

"They're making some tests now," said Tom, hurrying forward.

Ned followed until, passing a sort of machine shop, the lads came to
where a sentry paced up and down a concrete walk.

"Are these the proving grounds?" asked Tom. "This is the entrance to
them," replied the soldier, bringing his rifle to "port," according to
the regulations. "What do you want?"

"To go in and watch the gun tests," replied Tom. "I have a permit," and
he held it out so the soldier could see it.

"That permit is no good here;" the sentry exclaimed.

"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