Now, of a sudden, there came a faint "boom" across the water. "Can she have sighted us, sir?" asked Frank anxiously. "Had we not better dive at once?" "No, I don't think she has sighted us," replied Lord Hastings. "Then what is she shooting at?" "You've got me. But that is what we shall have to find out." Lord Hastings went below, where he ordered the course of the D-16 altered slightly, so as to bring her closer to the far side of the distant cruiser. Then he returned on deck. Time after time the boom of the big guns could be heard, and those aboard the D-16 were at a loss to make out what the cruiser was firing at. Not another speck was visible on the broad expanse of the Baltic Sea. "There must be something wrong," said Lord Hastings. "But I can't imagine what----Hello, she's hoisting her flag." "Can you make it out, sir?" asked Frank. Lord Hastings looked long and carefully through his glass. "Russian," he said at last. "Good," exclaimed Frank. "We are among friends at any rate." "I wouldn't be too sure," said his commander. "It may be a German ship that has hoisted the Russian flag for a purpose." "Well, we'll soon see now," said Frank. Now the submarine had approached so close that Lord Hastings deemed it advisable to submerge, so that the vessel might not be seen. This was done, and standing at the bottom of the periscope below, the commander of the D-16 took in the scene about him. Occasional clouds of smoke issued from the cruiser, indicating that her guns were still in action, but for the life of him Lord Hastings could not make out the object of her shots. Also, the cruiser was maneuvering in a strange way, and Lord Hastings could not account for this either. He stepped aside and Jack took a look through the periscope. Peering long and carefully, he stepped back suddenly with a cry. "What's the matter?" asked Lord Hastings. "I've made out the cause of the trouble, sir," said Jack. "What is it?" demanded his commander. "Why, sir," replied the lad, "the cruiser is being followed by a submarine--I can just make out her periscope in the distance. That's what the cruiser is firing at. And her strange maneuvering is caused by the fact that she is trying to escape torpedoes. So far she must have been successful." "In that event," said Lord Hastings, "it is up to us to lend the cruiser a hand. Signal full speed, Mr. Templeton." Jack obeyed, and the swift British submarine dashed madly through the water. Suddenly Jack, who was at the periscope, perceived a puff of smoke issue from the forward turret of the cruiser, and a moment later a shell plowed up the water near the D-16. "We'll have to submerge at once, sir," he said. "The cruiser has sighted us and takes us for an enemy. One of those shells may hit us." "All right," said Lord Hastings. "First let me have a look."