three blades projecting from the outside, and running spirally
from the point to the butt. These fitted into grooves in the inside
of the cannon, which were really huge air-guns twenty feet long,
including the air-chamber at the breech.

The projectiles were placed in position, the breeches of the guns
closed, and a minute later the air-chambers were filled with air at a
pressure of two hundred atmospheres, pumped from the forward engines
through pipes leading up to the guns for the purpose.

"Now," said Arnold, "we're ready! Meanwhile you two can go and load
the two after guns."

The men saluted and retired, and Arnold continued--

"Just take a look down with your glasses and see if they see us. I
expect they do by this time."

Colston put his field-glass to his eyes, and looked down at the
fortress, which was now only six or seven miles ahead.

"Yes," he said, "at any rate I can see a lot of little figures
running about on the roof of one of the ramparts, which I suppose are
soldiers. What's the range of your gun? I should say the fortress is
about six miles off now."

"We can hit it from here, if you like," replied Arnold, "and if we
were a thousand feet higher I could send a shell into Petersburg.
See! there is the City of Palaces. Away yonder in the distance you
can just see the sun shining on the houses. We could see it quite
plainly if it wasn't for the haze that seems to be lying over the
Neva."

While he was speaking, Arnold trained the gun according to a scale on
a curved steel rod which passed through a screw socket in the breech
of the piece.

"Now," he said. "Watch!"

He pressed a button on the top of the breech. There was a sharp but
not very loud sound as the compressed air was released; something
rushed out of the muzzle of the gun, and a few seconds later, Colston
could see the missile boring its way through the air, and pursuing a
slanting but perfectly direct path for the centre of the fortress.

A second later it struck. He could see a bright greenish flash as it
smote the steel roof of the central fort. Then the fort seemed to
crumble up and dissolve into fragments, and a few moments later a
dull report floated up into the sky mingled, as he thought, with
screams of human agony.

For a moment he stared in silence through the glasses, then he turned
to Arnold and said in a voice that trembled with violent emotion--

"Good God, that is awful! The whole of the centre citadel is gone a