General Meusnier introduced important special features in the design of dirigibles for preserving their form and poise. He insisted that the bag and boat should be so rigidly connected that one could not swerve from alignment and relative position with the other. He also emphasized the necessity of preserving the vessel from deformation during flight, in order to diminish its resistance. To that end he proposed to provide the hull with a double envelope, the inner one thin and light but impermeable to hydrogen; the outer one strong and air-tight; the space between the two envelopes to be pumped full of air under pressure sufficient to preserve the form of the bag when beating its way swiftly against a buffeting wind. This was an important invention which in later years was adopted in many of the most powerful motor balloons—for all, indeed, except those of the rigid type. He also proposed the use of stabilizing planes to control the poise of the vessel, thus anticipating the Lebaudy brothers by more than a century. Like the Robert brothers he proposed to raise or lower the vessel in search of suitable currents, by altering the quantity of air in the space between the inner and outer envelope, by use of hand bellows. Apparently General Meusnier and his colleagues were endowed with constructive genius sufficient to have developed a practical motor balloon, had they been able to secure a light engine. Lacking this the early aëronauts could do little more than describe their projects, and await the growth of the collateral arts and sciences. Accordingly no substantial advance in motor balloons beyond Meusnier’s designs was effected till after the middle of the nineteenth century; and until then the art of aëronautics remained in the hands of showmen. Hundreds of projects, indeed, were advanced, some exciting considerable interest and expectation, but nevertheless of such paltry value as hardly to deserve comment. One notable exception to these was the invention of Porter in America. In 1820 Rufus Porter, a Yankee inventor, and later the original founder of the _Scientific American_, patented an air ship of very promising appearance for that early day. Its hull was a long, finely tapering symmetrical spindle, suspending a car of similar shape by means of cords, which were vertical at its middle but more and more slanting toward its ends. Midway between the hull and car was a large screw propeller actuated by a steam engine in the car. A model of this dirigible exhibited in Boston and New York, some years later, is reported to have carried its own power, at fair speed, and to have obeyed its helm satisfactorily. [Illustration: FIG. 16.—RUFUS PORTER’S DIRIGIBLE, 1820.]