The Robert brothers were the first to employ in practice an air bag inside a gas bag. This was held within the balloon by ropes and connected with the outer atmosphere by a tube, the idea being to regulate the internal pressure of the balloon by introducing air into, or withdrawing it from, the smaller bag. But during an ascension with their patron, the Duke de Chartres, they entered a violent eddy which tore away the oars and rudder, at the same time agitating the balloon so violently that the internal air bag broke its sustaining cords and fell upon the bottom of the gas bag, thus throttling the connection with the external atmosphere. The vessel rose swiftly and the gas expanded dangerously near to the bursting pressure. At a height of 16,000 feet the Duke de Chartres, perceiving the imminent danger of an explosion of the envelope, drew his sword and cut a rent ten feet long in its lower part. A part of the gas immediately rushed forth, and the balloon sank rapidly, but after the discharge of the ballast, landed safely without further mishap. The Duke acted wisely enough, but he was afterwards ridiculed for his apparent lack of courage. If he had possessed more bravery and less caution he might have allowed the balloon to burst and descend as a parachute, thus anticipating the spectacular performance of John Wise, in 1838. Simultaneously other inventors were evolving designs of no less importance in the ultimate perfection of the dirigible. In a letter written to Benjamin Franklin on May 24, 1784, Francis Hopkinson of Philadelphia proposed to build a balloon of spindle shape and to drive it by means of a wheel-like propeller at the stern, consisting of vanes set at an angle to the line of progression, like the common smokejack. This proposed craft, the harbinger of the modern screw-driven motor balloon, far antedated the screw-driven boat and the submarine torpedo which it most resembles.[9] While Blanchard and other aëronauts were paddling their globose bags in search of favorable winds, vainly hoping thereby to direct their course in the air, General Meusnier of the French army, and member of the Academy of Sciences, made a systematic study of the requirements for practical air navigation. After some research on forms suitable for aëronautic hulls, he designed a power balloon having a pointed car suspended from a bag of goose-egg form, this latter embodying his idea of the best shape for a balloon that must cleave the air swiftly and resist deformation. The propulsion was to be effected by means of three coaxial screw propellers, supported on the rigging between car and bag, and actuated by eighty men, for lack of a light artificial motor. He thus hoped to obtain a moderate velocity which, combined with skillfully selected air currents, would enable the ship to reach her destination in ordinary weather. [Illustration: FIG. 15.—GEN. MEUSNIER’S PROPOSED DIRIGIBLE, 1784.]