Encouraged by the success of this model, Tissandier undertook the con struction of a large aerostat on the same plan, and eventually succeeded in building one 92 feet long and 3o feet in its greatest diameter, with a capacity of 3S,000 cubic feet. This, when inflated with hydrogen, had an ascensive force of 2Soo pounds. The aerostat (fii. 61, fig. 4) was provided with a two-bladed propeller 9 feet in diameter, while a triangular sail, placed at one end of the balloon and close beneath it, was designed to be operated as a rudder. With this apparatus Tissandier made an ascension on October S, 1883. At the height of 1600 feet the wind was blowing at the rate of six miles per hour, and lie was enabled, by turning the head of his balloon against the wind, to keep it motionless for some minutes, its propeller mak ing three revolutions per second. The rudder, however, proved insuffi cient. By turning the aerostat with head pointed with the wind, a marked acceleration of speed was attained, and a considerable deviation from the direction of the•wind could be obtarned with the aid of the rudder. The use of electricity as the motive-power of a dirigible balloon was by this experiment demonstrated to be decidedly superior to that of hand- or steam-power.
Renard and Krebs' s by the success of Tissandier, two officers of the French army, MM. Renard and Krebs, assisted by a government appropriation of one hundred thousand francs, constructed a navigable aerostat embodving the same general features of construction, though inodified in a number of points and employing- elec tricity as the motive-power. The success achieved by these experimenters in directing- the motion of their aerostat at will was so decided, and so greatly in advance of anything previously achieved, that their results mark an era in the history of the science of aerostation.
Professor W. le Conte Stevens, in an instructive paper entitled " Recent Progress in Aerial Navigation," which appeared (July, 1885) in the Pop ular Science Monthly, gives the following 'account of these remarkable results: " The pecuniary resources at their command gave them a great advantage over Tissandier in the ability to construct a balloon much larger than that with which Tissandier's success had been achieved, and this per mitted the application of a motor nearly seven times as powerful as the one previously employed. Their balloon (pl. 61, fig. 3) is 166 feet long, 28 feet in greatest diameter, its capacity 67,000 ciibic feet and ascensional power nearly 5000 pounds The details of the battery and motor have not been given to the public by Captain Renard. The rudder is almost a par allelogram in form and thickest in the middle, the cloth being tightly stretched over a light framework, so as to present a rigid surface to the air. The propeller is fixed to the extremity of a long shaft and placed at the front instead of rear of the balloon. The front end of the machine is thicker than the rear end The balloon is filled with hydrogen, but within it is a subsidiary balloon connected by a tube with the cage, where air can be pumped in or out at pleasure, thus varying slightly the specific gravity of the mass as a whole, and enabling the aeronauts to vary their elevation at will.
" On August 9, 1884, an ascent was accomplished with this balloon, the atmosphere being ahnost perfectly calm. A journey of nearly two miles was made in a southerly direction, then over a mile westward, after which the balloon was turned northward and eastward. Very slight mo tion of the rudder was needed to execute these curves. Twenty-three min ntes after their flight was begun the aeronauts were immediately over their starting-point, having made a trip of not quite five miles. In descending it was necessary to move backward and forward several times in succession, alternately reversing the direction of the propeller. The return to the ground was at the very spot from which the departure had been made. This remarkable feat was thus accomplished almost exactly one hundred and one years after the ascent of the first hydrogen balloon, sent tip by Charles from a point but a few miles distant (p. 366) On the 8th of November two successive journeys were taken, the balloon returning- each time to its point of departure, and attaining a speed of nearly fifteen miles an hour independently of the wind, which was blowing at the rate of five miles an hour." From this interesting account it would appear that to France, which was the birthplace of the balloon, the world is indebted for its highest de velopment. Nevertheless, though we may reasonably expect continual improvement in its manageability along the lines followed by Tissandier and Renard and Krebs, it is scarcely within the bounds of possibility that it should ever attain to any commercial importance as a means of locomo tion. I3y modifications of construction which shall still further decrease its resistance, and by increasing the efficiency of the motor—that is, by increasing its power in relation to its weight—it is possible that a balloon may be built with which a speed of twenty-five or thirty miles in a calm may be attained; but the restriction of its use to fair weather and a still atmosphere, its enormous volume, and its very limited carrying capacity must always render the thoug,lit of competition with the railway or the steamship too visionary' to be entertained seriously. For one useful pur pose, however, the balloon has already fully demonstrated its value, and in the future will, in all probability, be found indispensable—that is, in warfare. The ability to direct the motions of a balloon at an elevation sufficiently great to insure its safety from an enemy on the ground must render it a most valuable adjunct as a means of obtaining information of the strength, disposition, and movements by an army, and of conveying aid and information to and from the forces within a beleaguered city or fortress shut off from other means of communication with the outside world.