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.