Mr. Ackerman paused a moment. "By 1871 there was an American line between Philadelphia and Liverpool. In the meantime, ever since 1861, there had been a slow but steady advance in ocean shipbuilding. Although iron ships had gradually replaced wooden ones the side-wheeler was still in vogue, no better method of locomotion having been discovered. When the change from this primitive device to the screw propeller came it was a veritable leap in naval architecture. Now revolutions in any direction seldom receive a welcome and just as the conservatives had at first hooted down the idea of iron ships, asserting they would never float, so they now decried the use of the screw propeller. Indeed there was no denying that this innovation presented to shipbuilders a multitude of new and balking problems. While the clipper ships had greatly improved the designs of vessels the stern was still their weakest point and now, in addition to this already existing difficulty, came the new conundrums presented by the pitch, or full turn of the thread, in the screw propeller; also the churning of the current produced by the rapidly whirling wheel, which was found to retard the speed of the ship very materially. Valiantly engineers wrestled with one after another of these enigmas until they conquered them and put shipbuilding on the upward path where it has been ever since. In time steel ships replaced the cruder vessels of iron; finer types of engines were worked out; the wireless and the many electrical devices which herald approaching foes and announce the presence of icebergs have been invented; until now the ocean liner is practically safe from all perils except fogs, icebergs and submarines." He stopped a moment with eyes fixed on the glowing logs that crackled on the hearth.