The big gun which Mr. Pertell had secured to make more realistic the war play he was preparing for the films, was an old fashioned siege rifle, made toward the close of the Civil conflict. It had not been used more than a few times, and then it had been stored away in some arsenal. The director, hearing of it, had secured it to fire at a certain hill on Oak Farm.

This hill would, in the motion pictures, form a stronghold of the Southern forces and it would be demolished by shells from the large cannon, and then would follow a charge on the part of the Union soldiers.

Real shells, with large explosive charges in them, would be used, but it is needless to say that when the shots were fired at the hill the players taking the parts of the Southerners would be at a safe distance.

"They're just trying it out now," observed Russ, who with Paul, was walking over the fields with Alice and Estelle. "Mr. Pertell wants to get the range, and decide on the best places from which to make the pictures. I think we'll film some to-morrow if it's a good day."

"What's the matter with your eyes, Estelle?" asked Paul, as he looked at her. "Were you working in the studio to-day? I know those lights always affect my sight."

"Why, no, I wasn't in the studio," and then Estelle realized why her eyes were so inflamed—it was from crying. She gave Alice a meaning glance, as though to enjoin silence, but she need have had no fears. Alice would not betray the secret.

The big gun had been mounted on a level piece of land, not 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 they going to fire again?"

"It looks so," observed Russ. "But if they knock away too much of the hill there won't be any left for the pictures to-morrow."

"I believe they want to make the top of the hill flat," said Paul. "They are going to have some sort of hand-to-hand fight on it after the Unionists capture it," he went on. "I heard Mr. Pertell speaking of it."

"There goes another!" cried Alice, as she saw the same preparations as before and one man standing near the gun to pull the lanyard, which, by means of a friction tube, exploded the charge.

Once more the projectile shot out and, burying itself in the soft dirt of the hill, threw it up in a shower.

"That'll save me a lot of work!" exclaimed a voice behind the young people, and, turning, they saw Sandy Apgar smiling at them. "That's a new way of plowing," he went on. "It sure does stir up the soil."

"Won't it spoil your hill?" asked Alice.

"Not so's you could notice it. That hill isn't wuth much as it stands. It's too steep to plow, and only a goat could find a foothold on it to graze. So if you moving picture folks level it for me I may be able to raise some crops on it. Shoot as much as you like. You can't hurt that hill!"

The men at the gun signaled that they were going to fire no more that day, and then, as it was safe, the young folks made a trip to see the extent of damage caused by the shells.

Great furrows were torn in the earth and the stones, and the top of the hill, that had been rounding, was now quite flat, though far from being smooth.

The next day had been set for filming the scenes with the big gun in them. Contrary to expectations, no pictures could be taken, as the throwing up of the earthworks had not been finished. But a number of men from both "armies" were set to work, and as it afforded good practice for the militia they were called on to dig trenches, throw up ridges of earth, and go through other needful military tactics.

The girls had no part in the scenes with the big gun, except that, later on, they were to act as nurses in the hospital tent.

On top of the hill a force of Confederates would be stationed, and they were to reply to the fire of the big gun. Of course, when the projectiles struck the hill the soldiers would be a safe distance away, but by means of trick photography scenes would be shown just as if they were sustaining a severe bombardment.

"Is everything ready?" asked Mr. Pertell, a few days after the setting up of the big gun, during which interval a sort of fort had been constructed on the hill and a redoubt thrown up.

"I think so," answered Russ. "We couldn't have a better day, as far as sunshine is concerned. I'm ready to film whenever you are."

"I'll give the word in a minute. Paul, you're in charge of a detachment of Union soldiers that storms the hill as soon as the big gun has silenced the battery there."

"Very well, sir."

The big gun rattled out its booming challenge and was replied to by volleys from the rifles of the Confederates on the hill and by their field artillery, which they hurriedly brought up.

Shot after shot was fired, and one after another the Confederate cannon were disabled. They were blown up by small charges of powder put under them, set off by fuses lighted by the Confederates themselves, but this did not show in the picture, and it looked as though the Southern battery was blown up by shots from the big gun.

"All ready now, Paul! Lead your men!" yelled the director, who was standing near Russ and his camera. "Rush right up the hill. Stop firing here!" he called to those in charge of the big gun.

But something went wrong, or some one misunderstood. As Paul was charging up the hill at the head of his little band, Russ, turning his head for an instant, saw a man about to pull the lanyard of the big gun.

"Don't shoot! Don't shoot!" he yelled. "It's aimed right at Paul and his fellows!"

But Russ was too late. The man pulled the cord. There was a deafening roar, a cloud of smoke, a sheet of fire, and a black projectile was sent hurtling on its way against the hill, up the side of which Paul was climbing with his soldiers.