fire that, Tom? The muzzle points directly at the stone wall. Does the wall open when they want to fire?"
"No, the gun raises up, peeps over the wall, so to speak, shoots out its projectile, and then crouches 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.
