"The 'subs' aren't like the big ships," Captain Nicholson explained. "With such a small crew I know the men personally, and I know I can count on each and every one of them, particularly Old Jansen, and Brown, the gunner's mate. I need not caution the engine-room crew for special watchfulness. Every oiler aft knows a warm bearing would condemn him forever in the eyes of his shipmates." A few moments more and the submarine was racing along toward the mouth of the Euphrates, where the enemy was known to be. Just as dusk was setting in, Brown, the gunner's mate, reported an aeroplane to leeward. Captain Nicholson, Jack and Frank, who stood on the bridge, could just make it out with binoculars. "I hate to use any electricity out of my batteries now," said Captain Nicholson, "for it is likely to be very precious later. However, I don't want to run chances of being discovered. We'll dive." The three made their way below. The entrance was hermetically closed, and soon the tanks were being filled. A moment later the Y-3 began to submerge. At a depth of 60 feet Captain Nicholson trimmed down, and f or an hour the vessel ran along at eight knots, the commander wishing to make sure of complete darkness before coming up. "How do you manage to keep your course under water, captain?" asked Frank. "Simple," was the reply. "Gyroscopic compass." At that moment the man on watch at the bell receiver reported the sound of a ship's propellers above. Captain Nicholson turned his place at the periscope over to his first officer and listened himself. The steady rhythmic beat was well off the port bow. At Captain Nicholson's command, the main ballast tank was emptied until the conning tower was well awash. Then the commander, Frank and Jack went up to have a look around, for the airship, as well as for the vessel. "Those sky pilots," said the commander, "maintain that they can see us and get us with bombs at any depth. However, I see nothing of our friend. Looks like he had lost his bet this time." They returned below, and Frank put his eye to the periscope. Almost instantly he made out the outline of large vessel of some kind. He reported this to Captain Nicholson, who brushed him quickly aside and peered into the periscope himself. "Merchant vessel of some kind," he said aloud. "We haven't time to fool with him now. May be able to get him as we come back." Once more now the three made their way to the bridge. The clouds had gradually thickened and it was very dark. "I wish it would rain," mused Captain Nicholson, "or that we would at least have a dull sunrise, for it will be better suited for our work. Brown says he's sure we'll be favored with suitable weather because of the righteousness of our cause; but I am pinning my faith to the barometer, which has already fallen two points." "Well, I hope everything goes all right," said Jack.