Stevens continued his work of improving the beam-engine for many years. He designed the now universally-used "skeleton-beam," which is one of the characteristic features of the American engine, and placed the first example of this light and elegant, yet strong, construction on the steamer Hoboken in the year 1822. He built the Trenton, which was then considered an extraordinarily powerful, fast, and handsome vessel, two years afterward, and placed the two boilers on the guards--a custom which is still general on the river steamboats of the Eastern States. In this vessel he also adopted the plan of making the paddle-wheel floats in two parts, placing one above the other, and securing the upper half on the forward and the lower half on the after side of the arm, thus obtaining a smoother action of the wheel, and less loss by oblique pressures. In 1827 he built the North America (Fig. 88), one of his largest and most successful steamers, a vessel fitted with a pair of engines each 44-1/2 inches in diameter of cylinder and 8 feet stroke of piston, making 24 revolutions per minute, driving the boat 15 to 16 miles an hour. Anticipating difficulty in keeping the long, light, shallow vessel in shape when irregularly laden, and when steaming at the high speed expected to be obtained when her powerful engine was exerting its maximum effort, he adopted the expedient of stiffening the hull by means of a truss of simple form. This proved thoroughly satisfactory, and the "hog-frame," as it has since been inelegantly but universally called, is still one of the peculiar features of every American river-steamer of any considerable size. It was in the North America, also, that he first introduced the artificial blast for forcing the fires, which is still another detail of now usual practice. [Illustration: FIG. 88.--The North America and Albany, 1827-'30.] Stevens next turned his attention to the engine again, and adopted spring bearings under the paddle-shaft of the New Philadelphia in 1828, and fitted the steam-cylinder with the "double-poppet" valve, which is now universally used on beam-engines. This consists of two disk-valves, connected by the valve-spindle. The disks are of unequal sizes, the smaller passing through the seat of the larger. When seated, the pressure of the steam is, in the steam-valve, taken on the upper side of the larger and the lower side of the smaller disk, thus producing a partial balancing of the valve, and rendering it easy to work the heaviest engine by the hand-gear. The two valve-seats are formed in the top and the bottom, respectively, of the steam-passage leading to the cylinder; and when the valve is raised, the steam enters at the top and the bottom at the same time, and the two currents, uniting, flow together into the steam-cylinder. The same form of valve is used as an exhaust-valve.