"No, they're going to do so in about half an hour. You'll have
time to look around a bit. Come on," and showing the sentinel the
counter-signed pass, Captain Badger led the two youths into the
proving grounds.

Tom and Ned 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