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 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!" "No more murder than the use of torpedoes in naval warfare, as far as I can see," replied Arnold coolly. "Remember, too," he continued in a sterner tone, "that fortress belongs to the power that flogged Radna and has captured Natasha. Come, let's see what execution you can do." He crossed the deck and set the other gun by its scale, saying as he did so-- "Put your finger on the button and press when I tell you." Colston did as he was bid, and as his finger touched the little knob his hand was as firm as though he had been making a shot at billiards. "Now!"