Six other similar voyages were made within the two years following, and we have as a result, that in five out of the seven trials, the balloon returned to its point of departure. Its failure to return in the other two trials was due, in the one case, to the breaking down of the motor; in the other, to the resistance of a strong wind which made it necessary to land at a distance from the starting point. The last of these remarkable voyages was performed in presence of the Minister of War, on September 23, 1885. The balloon started from Calais and sailed against the wind directly to Paris, passed over the fortifications, described a graceful curve and returned to its place of departure, recording an average speed of 14.5 miles an hour.
The torpedo form of hull, chosen by Renard and Krebs, has two important advantages; one is projectile stability, the other is economy of propulsive power. Owing to the blunt bow and long tapering stern, the center of mass is well forward, while the center of side wind pressure is more to the rear. As a consequence, if the vessel should encounter a quartering wind-gust, or have her nose slightly turned from the course, she would promptly right herself like a dart or an arrow. If on the contrary, the hull were a symmetrical spindle, the vessel would move forward in unstable equilibrium, and, once slightly diverted from her course, would tend to deviate further, like an arrow with unloaded head.
The second advantage mentioned is also worth attention, viz.: that at ordinary transportation speeds a longish spindle has less resistance with a blunt bow than with a very sharp one. Renard and Krebs did not account for this fact; but the present writer, by determining separately the skin friction and the impactual resistance of the air, proved that in sharpening the bow beyond a certain best form, its friction increases faster than its head resistance diminishes, the most suitable shape being that of a torpedo whose nose has a radius of curvature of about two diameters, and its stern a radius of about twelve diameters.
While the successors of Giffard in France were thus engaged in developing dirigibles driven by muscular or electric power, a few German experimenters were applying gas and benzine engines to such vessels, with better promise of ultimate practical success and usefulness. The first of these was Hänlein, who in 1872 advanced the meritorious project of driving a well shaped balloon by means of a gas engine taking its fuel from inside the balloon, and making good the loss by pumping air into the ballonet. This balloon was of far better design for swiftness and kinetic stability than the contemporary one of Dupuy de Lome. Its hull was a well pointed cylinder 164 feet long, 30 feet in diameter and of 85,000 cubic feet capacity, made air-tight by a thick coating of rubber inside, and a thin one outside. The car was rigidly suspended near the envelope and carried a 6 horse-power Lenoir gas engine actuating a large screw. Notwithstanding that the buoyancy was small, owing to the use of coal gas, this air ship attained a speed of 15 feet per second. By employing hydrogen, a much larger engine could have been carried, entailing a much swifter speed. During its trial the balloon was kept near the earth’s surface, held loosely by ropes in the hands of soldiers. The air ship was remarkably successful for that early date, and had the potency of greater achievement than its contemporaries in France; but owing to lack of funds its capabilities were not fully developed. If it had been inflated with hydrogen, and propelled by use of gas and petrol, so that the loss of weight would compensate for the loss of buoyancy, it might have anticipated the speed and endurance of the best air ships built toward the close of the nineteenth century, or later.
PLATE II.
HAENLEIN’S GAS-DRIVEN DIRIGIBLE.
WÖLFERT’S BENZINE-DRIVEN DIRIGIBLE.