given over to a proving ground, where many of the big guns are
taken to be tested.

Tom and Ned reached New York City without incident of moment,
and, after a night spent at a hotel, they went to the Battery,
whence the small government steamer leaves every day for Sandy
Hook. It is a trip of twenty-one miles, and as the bay was rather
rough that day, Tom and Ned had a taste of a real sea voyage. But
they were too experienced travelers to mind that, though some
other visitors were made quite ill.

A landing was made on the bay side of the Hook, it being too
rough to permit of a dock being constructed on the ocean side.

"Now we'll see what luck we have," spoke Tom, as he and Ned,
inquiring the way to the proving grounds from a soldier on duty,
started for them. On the way they passed some of the
fortifications.

"Look at that gun!" exclaimed Ned, pointing to a big cannon
which seemed to be crouched down in a sort of concrete pit. "How
can they 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 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