All at once there appeared in the sky directly ahead of the _Monitor_ a line of red lights. A German birdman, circling above on patrol duty, had observed the wake of the periscopes and had touched off a night flare. Like a giant pyrotechnic display the red globules floated in the air. No sooner had the light appeared than there came the boom of a big gun on shore and a huge shell screeched seaward over the _Monitor_. Instantly the coast defense guns of the Germans crashed into action. And now, from the rear of the _Monitor_ and her consorts, came the answering crash of the great guns on the mighty ships of the Allies. The furious battle was on! German aviators, having sighted the American submarines, were bending all their energies on wiping out the "invisible mosquitoes." Guided only by the telltale wake of the periscopes, they were dropping huge depth bombs at random. One of them splashed into the sea directly astern of the _Monitor_. "We will have to act quickly now," urged McClure as he rang for full speed. So far, so good; not a mine had been encountered. The _Monitor_ was worming her way unmolested into the heart of the enemy stronghold! Convinced now that he had passed the guardian line of nets and mines, McClure decided to dive and run for the inner recesses of the enemy harbor. Now down, out of sight, out of hearing of the big guns, the _Monitor_ relentlessly pursued her course, flouting the danger that lurked on every hand. At last came the moment when the commander of the submarine flotilla decided to climb to the surface and make an observation. With every man standing to his post of duty and every nerve strained for the next move, the signal to pump out ballast was given and the _Monitor_ turned her diving rudders to ascend. Soon her periscopes thrust their eyes out of the depths. "Look! there, off our port bow!" exclaimed Jack to his chief. McClure saw it, too, at the same moment---a giant German battle cruiser lying broadside to the harbor entrance and every gun belching forth fire at the allied fleet outside! Responding to the control of her commander, the _Monitor_ swung off sharply to port and drove straight for the cruiser. McClure's right hand groped for the torpedo button as he sighted on the German battle cruiser. He released a torpedo! CHAPTER XXV CHLORINE GAS