far from the
hill, and on this plain had been thrown up earthworks behind which the
Union forces would take their stand in an effort to reduce the
Confederate stronghold.

"They're going to fire!" cried Estelle as they came within sight of the
gun, and saw, by the activities of the men about it, that a shot was
about to be delivered.

Alice covered her ears with her hands, and Russ and Paul stood on their
tiptoes and opened their mouths wide.

"What in the world are they doing that for?" asked Estelle.

"I can't hear a word you say!" called Alice, making her voice loud, to
overcome her own hearing handicap.

"There she goes!" cried Russ.

The earth trembled as flames and smoke belched from the muzzle of the
cannon, and the girls screamed.

Something black was seen for an instant in the air amid the swirl of
smoke, and then another portion of the hill was seen to lift itself up
into the air and dirt and stones were scattered about.

"A good shot!" observed Russ, letting himself down off his tiptoes.
"That would make a dandy scene for the film."

"That's right," agreed Paul, also letting himself down and closing his
opened mouth.

"Why did you do that?" asked Estelle, when the echoes of the firing had
died away. "Why did you stand on your toes, and open your mouths?"

"To lessen the shock to our ear drums," answered Paul. "It is the
concussion, that is, the rushing back of air into the vacuum caused by
the shot, that does the damage. By opening your mouth you equalize the
air pressure on the inside and the outside of your ear drums, just as
you do when you go through a river tunnel. When there is a partial
vacuum outside your ear, the air inside you presses the drum outward,
and by opening your mouth--or by swallowing you make the pressure
equal. Sometimes the pressure outside is greater than the pressure
inside, and you must also equalize that before you can be comfortable."

"But that wasn't why you stood on your toes," Alice said.

"No; we did that to have less surface of our bodies on the ground so the
vibration would be less. If one could leap up off the earth at the exact
moment a shot was fired it would be much better, but it is hard to jump
at the right instant, and standing on one's toes is nearly as good. Then
you present only a comparatively small point which the vibrations of the
earth, caused by the explosion of the gun, can act upon."

"That's a good thing to remember," Estelle said. "Are t