ed saw so much to interest them that they did not know at
which to look first. In some places officers and firing squads were
testing small-calibre machine guns, which shot off a round with a noise
like a string of firecrackers on the Chinese New Year's. On other
barbettes larger guns were being tested, the noise being almost
deafening.

"Stand on your tiptoes, and open your mouth when you see a big cannon
about to be fired," advised Captain Badger, as he walked alongside the
boys.

"What good does that do?" inquired Ned.

"It makes your contact with the earth as small as possible--standing on
your toes," the officer explained, "and so reduces the tremor. Opening
your mouth, in a measure, equalizes the changed air pressure, caused by
the vacuum made when the powder explodes. In other words, you get the
same sort of pressure down inside your throat, and in the tubes leading
to the ear--the same pressure inside, as outside.

"Often the firing of big guns will burst the ear drums of the officers
near the cannon, and this may often be prevented by opening the mouth.
It's just like going through a deep tunnel, or sometimes when an
elevator descends quickly from a great height. There is too much
outside air pressure on the ear drums. By opening your mouth and
swallowing rapidly, the pressure is nearly equaled, and you feel no
discomfort."

The boys tried this when the next big gun was fired, and they found it
true. They noticed quite a crowd of officers and men about a certain
large barbette, and Captain Badger led them in that direction.

"Is that General Waller's gun?" asked Tom.

"That's where they are going to test it," was the answer.

Eagerly Tom and Ned pressed forward. No one of the many officers and
soldiers grouped about the new cannon seemed to notice them. A tall
man, who seemed very nervous and excited, was hurrying here and there,
giving orders rapidly.

"How is that range now?" he asked. "Let me take a look! Are you sure
the patrol vessels are far enough out? I think this projectile is going
farther than any of you gentlemen have calculated."

"I believe we have correctly estimated the distance," answered someone,
and the two entered into a discussion.

"That excited officer is General Waller," explained Captain Badger, in
a low voice, to Tom and Ned.

"I guessed as much," replied the young inventor. Then he went closer to
get a better look at the big cannon.

I say big cannon, and yet it was