Then he signalled to the engine-room, the fan-wheels rose from the deck, as if by their own volition, and, as soon as they reached their places, began to spin round faster and faster, until Colston could again hear the high-pitched singing sound that he had heard as the Ariel rose from Drumcraig Island.

At the same time the speed of the vessel rapidly decreased; the side propellers ceased working, and the stern-screw revolved more and more slowly, until the speed came down to about thirty miles an hour.

By this time the great fortress of Kronstadt could be distinctly seen lying upon its island, like some huge watch-dog crouched at the entrance to his master's house, guarding the way to St. Petersburg.

"Now," said Arnold, "we can go outside without any fear of being blown off into space." They went out and walked forward to the bow. Arrived there they found two of the men, each with a curious-looking shell in his arms. The projectiles were about two feet long and six inches in diameter, and were, as Arnold told Colston, constructed of papier-maché. There were 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..."

"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 as though it had been swept off the face of the earth. I can hardly see even the ruins of it. Surely that's murder rather than war!"