“And please observe,” he concluded, as the gun sank down into its lair again, “how that armored hatch-cover protects the gun-crew from shrapnel or falling bombs.” I followed him to the conning-tower, or, as he always spoke of it, the turret. The little round bandbox of the _Holland_ has developed into a tall, tapering structure, sharply pointed fore and aft to lessen resistance when running submerged. Above the turret was a small navigating-bridge, screened and roofed with canvas, where a red-haired quartermaster stood by the steering-wheel, and saluted as we came up the ladder. The lieutenant put the engine-room telegraph over to “Start,” and a mighty motor throbbed underneath our feet. Then the mooring was cast off, the telegraph put over to “Slow Ahead,” and the _X-4_ put out to sea. “How long a cruise could she make?” I asked. “Four thousand miles is her radius,” answered her commander. “Back in 1915, ten American-designed submarines crossed from Canada to England under their own power.” “Yet it is only a few years since we were told that submarines could only be used for coast defense, unless they were carried inside their mother-ships and launched near the scene of battle,” I remarked. “Or that each battleship should carry a dinky little submarine on deck and lower it over the side like a steam-launch.” “People said the same thing about torpedo-boats,” agreed the lieutenant; “they began as launches--now look at the size of that destroyer smoking along over there. Ericsson thought that any ironclad bigger than a Civil War monitor would be an unwieldy monster. Even John P. Holland fought tooth and nail against increasing the length of his submarines. This boat of mine is five times the length of the old _Holland_, but she’s only a primitive ancestor of the perfect submarine of the future.” “She isn’t a submarine at all,” I replied presently, as the _X-4_ swept on down the coast at a good twenty-two knots, her foredeck buried in foam and the sea-breeze singing through the antennæ of her wireless. “She’s nothing but a big motor-boat.” “And she’s got some big motors,” replied the lieutenant. “Better step below and have a look at them.” I went down through the open hatchway to the interior of the boat and aft to the engine-room. There I found two long, many-cylindered oil-engines of strange design, presided over by a big blond engineer whose grease-spotted dungarees gave no hint as to his rating. “What kind of machines are these?” I shouted above the roar they made. “And why do you need two of them?” “Diesel heavy-oil engines,” he answered. “One for each propeller.” “What is the difference between one of these and the gasoline engine of a motor-car? I know a little about that.” “Do you know what the carburetor is?” asked the engineer. [Illustration: Courtesy of International Marine Engineering. A Submarine Cruiser, or Fleet Submarine (Lake Type).