CHAP, v.] A ERIA L NA VIGA TION. 101 



on \vhieh aerial currents liave great hold. Besides the difficulty of 

 loading the car of a balloon with the weight of a motor sufficient 

 to drive a mechanism either with paddle-wheels or screw, there is 

 the danger of fire, if this motor is furnished with a steam generator 

 and consequently with a fire : hydrogen gas is, Under these circum- 

 stances, too dangerous. Taking then their model in the ascending 

 motive power of birds, these experimenters turned their efforts towards 

 the discovery of a contrivance to raise and move machines heavier 

 than air, thus reducing the resistance which aerial currents oppose to 

 a large surface, and at the same time to avoid all danger of explosion 

 and fire. Theoretically speaking, the solution of the problem is 

 possible : the difficulty is in the practical realization. 



On the other hand, instead of trying to find the complete solution 

 of the direction of balloons, some savants, amongst whom we must 

 mention first a French engineer, M. Giffard, have confined themselves 

 only to obtain a sensible effect of deviation from the line of wind. 

 This effect obtained, they have only to tack about, as seamen do, to 

 cause the balloon to take the course nearest to the desired direction. 

 Several trials were made by M. Giffard which did not give satisfactory 

 results. They were repeated, twenty years later, in January 1872, by 

 M. Dupuy de Lome, who constructed an aerostat, having calculated 

 the form, arrangement, and mechanism to the end we have just 

 defined. 



M. Dupuy de Lome's balloon has an oval or oblong form, offering 

 an axis with least resistance in the direction of motion. The pro- 

 pelling power is obtained by the movement of a screw with two 

 or four branches, with taffetas or silken stuff sails, worked by a 

 number of men, alternately replacing each other. The balloon is 

 filled with ordinary gas. In its interior it carries a small balloon 

 with a volume equal to the tenth of the volume of the large balloon 

 and which can be filled with air by means of a ventilator carried and 

 worked in the car. The purpose of this little balloon is to preserve a 

 permanent form to the large one, whatever the variation of atmo- 

 spheric pressure may be : it thus allows a descent from a .height of 

 86H metres, when the dimensions are those of the machine made by 

 the inventor, that is, with a total volume of 3,454 cubic metres for the 

 large balloon, and consequently 345 m *4, for the small interior one. A 

 rudder, formed by a triangular sail placed under the balloon, at the 



