We have now traced the art of balloon guidance and propulsion from its earliest inception to the close of the nineteenth century. It was a period of extravagant hope and chimerical scheming, but withal a period fruitful in devices of fundamental value. The best experiments paid no dividends, but they prepared the way for really useful vessels. The methods of manipulation and control had been sufficiently developed to answer immediate needs. The air ship was at least dirigible, if not practical. It kept its shape, obeyed its rudder, rose and fell according to the operator’s will. It was, however, a fair-weather machine, beautiful in appearance, but helpless in any considerable wind. Speed was now the desideratum, and the attainment of this involved new difficulties. The storm-proof balloon was still a dream. Naturally one inquires what velocity makes a dirigible air ship really practical, assuming all other requirements satisfied. The minimum allowable speed depends largely upon the locality and season. On Long Island an assured velocity of forty to fifty miles an hour would seem desirable; for there the winds are swift and the water near. In Washington, or Berlin, thirty miles an hour is enough, though each additional mile per hour must be regarded as a considerable gain on a small margin of progress in facing a stiff breeze. Colonel Renard has estimated, from a study of the wind records near Paris, that a dirigible is practically useful in that locality if it can maintain a speed of twenty-eight miles an hour for ten or twelve hours; since in that case it can maneuver 81 days in 100. Renard’s own graceful ship attained a speed of but half that much. In order, therefore, to give his vessel the desired usefulness its speed must be doubled. This would require an eightfold[11] increase of motive power without increase of weight. Evidently then the cardinal requisite was a light durable motor of extraordinary output. Such motors fortunately were now coming into the market, owing to the development of gasoline engines for automobile racing.