Page:Tom Swift and His Giant Cannon.djvu/61

Rh "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.