By uniting with this construction a trapeze formed by ropes hanging vertically from the ends of the balloon and connected parallel to the longi tudinal axis by a bar, and by further suspending to this bar a heavy gon dola, or car, in such a manner that it may be shifted backward and for ward upon it, then, in ascending in a calm, a different direction from the vertical may be taken in consequence of the gondola being shifted into an oblique position in reference to the axis of the balloon, the resultant of the two components—the upward impelling force and the resistance of the air— in this case being in a direction other than the yertical. By combining this motion with an alternate ascent and descent, a horizontal advance in a calm and against the wind becomes possible; it will take place in a zigzag track laid in a vertical plane.
The alternate ascent and descent are effected, respectively, by throwing out ballast and letting out gas, and also by the use of the air-reservoir. This air-reservoir is a second balloon, placed in the interior of the main balloon and charged with air, which, being specifically heavier than that at higher altitudes, enables the aeronaut bv its partial discharge to render the aerostat relatively lighter, thus imparting considerable ascending--power. This contrivance was proposed and experimented with by. _Meunier shortly after the invention of the balloon.
Joidie's System.—The plan proposed by Joulie is, however, more effect ive. A reservoir of g-as attached to the gondola and communicating with the balloon, by a pump and flexible tube is made to receive a quantity of the gas in a compressed state by the action of the pump, or to deliver it to the balloon, as may be determined, the ascending-power of which is de creased in the former case and considerably increased in the latter; the alternate compression and dilatation of the gas thus effect respectively the descent or ascent of the balloon without the discharge of gas or ballast.
Proficllers.—The rudder is effective only in case there is a propelling apparatus operated by some form of motor. An apparatus of this kind will work the more advantageously the less resisting surface it offers to the wind counteracting the motion. Its most suitable form of construction thus far has been that of the propeller, or ship-screw, which, to be at all effective, on account of the comparatively small surface, requires to be given a great velocity of rotation. Fully recognizing- this, modern inventors have adopted for dirigible aerial vessels screws similar to those in Figures t to 3 63), arranged between the balloon and car. The cigar- or fish-shape
has also been preferred for these dirigible balloons, for the reason that though, under otherwise equal circumstances, the ascending-power depends not on the form, but on the volume, the resistance of the air to be over come is the smaller the less the cross-section of the balloon that is pre sented to it in the direction of the motion. The form that presents the least cross-section—that is, the fish- or cigar-form—is therefore the best for the purpose in view.
It is worthy of notice at this point that the earliest correct suggestions for controlling the movement of balloons emanated from the Francis Hop kinson above named. He wrote a letter to his friend Dr. Benjamin Frank lin, dated Philadelphia, May 24, 3784, in which he recommended that the balloon should be made oblong- instead of spherical, and that it should be provided with a larg-e and light wheel at the stern. The letter continues: " This wheel should consist of many vanes, or fans, of canvas, whose planes should be considerably inclined with respect to the plane of its motion, ex actly like the wheel of a smoke-jack. If the navigator turns this wheel swiftly round by means of a winch, there is no doubt but it would (in a calm, at least) give the machine a progressive motion, upon the same prin ciple that a boat is sculled through the water." Nearly seventy years elapsed before these suggestions were reduced to practice.
Giffttran s Dirigible had already experimented in 3852 with a construction of this kind (fiq. 1). Giffard's aerial vessel was pointed at both ends, and was filled with illuminating g,as. Beneath it, and sus pended therefrom by cords, was a longitudinal shaft carrying at one end a triangular sail. This could be turned about a vertical axis, and was de signed to serve the purpose of a rudder. At a distance of about 20 feet below this shaft was suspended a platform of wood, on which was placed a small steam-engine which actuated a screw-propeller. The weight of the balloon and its machinery was nearly fourteen hundred pounds. With this machine Giffard made several vovag,res, aud was able to make percep tible headway against a strong wind, and to guide the vessel in any desired direction. The danp.-,er from sparks from the furnace, however, made these experiments, as may be imagined, extremely hazardous.