[Illustration: THE _BERENGARIA_ _A former German ship now belonging to the Cunard Line._] Thousands of men work on these huge steel structures, and a “skin” of steel is riveted on the outside of these frames. In the bottom and extending a little way up the sides a second “skin” is placed on the inside of the ribs. These two coverings make up the “double bottom.” Girders for decks are put in place, great rooms are left for boilers, engines, and other equipment, the shafts are installed, the engines and boilers are bolted in place, and finally, when the ship is getting fairly well along toward completion, she is launched. That is, the great timbers that have been holding her in place are sawed in two, and the great vessel slides down the ways into the water. After she is launched the infinite number of tasks still untouched are attended to, and finally she is completed—a marvellously complicated and wonderfully perfect fabrication, into which almost every industry in a nation has put something. These hulls are huge and are tremendously strong, yet so great are the dimensions of the ship, so great her weight, that should her giant hull touch a rock the heavy steel plates would curl up like paper, the frames would bend like tin, and driven head on against a cliff or an iceberg the great structure would crumple its bow, twist its great frames, and might become a total wreck. Modern ships that are propelled by machinery use two principal methods of propulsion, paddle-wheels and screw propellers. Paddle-wheels bear a very close resemblance to mill-wheels. They are merely great circular structures with paddles attached at intervals around the circumference which, when the wheel is partly submerged and set to turning, strike the water one after the other and so propel the hull to which the wheel is attached. These wheels are sometimes arranged amidships, one on each side, and sometimes but one wheel is used (in this case it is much broader) at the stern, or rear end of the vessel. This equipment is not satisfactory for ocean-going ships, for heavy seas sometimes crush the paddle-wheels. River steamers, however, and particularly shallow-draft river steamers, find this means of propulsion satisfactory. The other method of propelling ships—that is, by screw propellers—is more important, and for use at sea is practically universal.