But night finally approached and then the great airship went silently aloft, ready to hover over the prison ground. Fortunately there was little wind; and she could be used as a balloon, thus avoiding the noise of the motor. "The next thing I do, when I get home," remarked Tom, as they drifted along. "Will be to make a silent airship. I think they would be very useful." With Mr. Borious in the pilot house, to point out the way, Tom steered through the fast-gathering darkness. The Russian had soon become used to the airship, and was not at all afraid. "Can you go just where you want to, as a balloon?" asked the new guide. "No, but almost," replied Tom. "At the last moment I've got to take a chance and start the motor to send us just where we want to go. That's why I think a silent airship would be a great thing. You could get up on the enemy before he knew it." "There are the prison barracks," said the guide a little later, his talk being translated by Mr. Petrofsky. Below and a little ahead of them could been seen a cluster of lights. "Yes, that looks like a line of prisoners," remarked Ned, who was peering through a pair of night glasses. "Where?" asked Tom eagerly, and they were pointed out to him. He took an observation, and exclaimed: "There they are, sure enough. Now if your brother is only among them, Mr. Petrofsky, we'll soon have him on board." "Heaven grant that he may be there!" said the exile in a low voice. A moment later, the Falcon, meanwhile having been allowed to drift as close as possible to the dimly-seen line of prisoners, Tom set in motion the great motor, the propeller blades heating the air fiercely. At the sound there was a shout on the ground below, but before the excitement had time to spread, or before any of the guards could form a notion of what was about to take place, Tom had sent his craft to earth on a sharp slant, closer to the line of prisoners than he had dared to hope. Mr. Petrofsky sprang out on deck, and in a loud voice called in Russian: "Peter! Peter! If you are there, come here! Come quickly! It is I, your brother Ivan who speaks. I have come to save you--save you in the wonderful airship of Tom Swift! Come quickly and we will take you away! Peter Petrofsky!" For a moment there was silence, and then the sound of some one running rapidly was borne to the ears of the waiting ones. It was followed, a moment later, by angry shouts from the guards. "Quick! Quick, Peter!" cried the brother, "over this way!" For an instant only the exile showed a single electric flash light, that his brother might see in which direction to run. The echo of the approaching footsteps came nearer, the shouts of the guards redoubled, and then came the sound of many men running in pursuit. "Hurry, Peter, hurry!" cried Mr. Petrofsky, and, as he spoke in Russian the guards, of course, understood.