"Here! Come back!" shouted the warship's captain as he saw Mr. Henderson's head disappearing from view. "Come back I say!" But with a quick movement the inventor pulled down the manhole cover and clamped it. Then he sprang to the conning tower, and, with a jerk, opened the levers that admitted water to the tanks. The _Porpoise_ began to sink slowly, and then more suddenly, so that, in less than a minute, she was out of sight beneath the waves, and the angry, gold-laced captain was staring in wonderment at the place where the submarine had been. The spot was marked only by a few bubbles and some foam. "I guess he'll wait some time for an explanation," spoke Mr. Henderson, as he started the big screw and sent the _Porpoise_ ahead at a swift pace. "That was rather a narrow escape," observed Jack, standing at the foot of the conning tower stairs and talking to Andy and Mr. Henderson, who was steering. "It certainly was," agreed the professor. "I have not yet become used to seeing things very far ahead in the dimness caused by being under water. But we'll soon get used to it. Luckily, the _Porpoise_ was not damaged by the shock." For several hours the _Porpoise_ was kept on her course. She behaved handsomely, and nothing excepting slight and easily remedied defects were found. The professor steered well out to sea, increasing both the forward speed and the depth to which the vessel sank. Presently the craft came to a stop with a little jolt. "What's the trouble?" asked Mark, somewhat alarmed. "Nothing at all," replied the professor with a smile, as he stepped out of the conning tower and entered the engine-room. "I thought it was time for dinner so I stopped the ship. We are now resting on the ocean bed, about half a mile below the surface. Look!" As he spoke he slid back the slides covering the plate glass windows. The boys saw that the ship rested in the midst of an immense forest of sea weed. Some of the stalks were as large around as trees. In and out among the snake-like, waving branches swam big fishes. It was a weird, but beautiful sight. "Come, Washington, serve dinner," ordered Mr. Henderson, and the colored man soon had a good meal prepared. Few repasts have been eaten under such strange circumstances. Desiring to be back at his secluded dock by nightfall, Captain Henderson soon started the _Porpoise_ up again. Without any accidents the return trip was made and by nine o'clock the _Porpoise_ rode safely at the dock where she had been launched. The night was spent in the cabin on shore. Early the next morning Mr. Henderson paid a visit to the ship, to make a thorough examination by daylight, and see if the craft had suffered any damage. "I think you and Mark will have to make a trip to town," he said to Jack at the breakfast table. "I need a new monkey wrench and some other tools and some small pieces of machinery. I'll give you a list of them, and you can bring them back in a valise, for they will be quite numerous."