In 1766 the English philosopher, Henry Cavendish, discovered that hydrogen gas has only about one-seventh the weight of an equal bulk of air, this scientific discovery pointing naturally to balloon construction, since obviously if such a light gas were confined in a suitable receptacle, the device would rise to a certain height through the heavier atmosphere, as a cork rises through water. At the same time the experiments of the chemist, Dr. Joseph Black, and those of his younger contemporary, Doctor Priestly, were directed along the same lines, all of them pointing to the possibility of constructing an aerostat with buoyancy and lifting-power, and Priestly's _Experiments Relating to the Different Kinds of Air_ is said to have been directly responsible for stimulating the efforts of Stephen and Joseph Montgolfier, the French paper manufacturers, who finally invented and sent up the first balloon. Even before Montgolfier's invention, Tiberius Cavallo, an Italian living in England, had demonstrated the possibility of making toy-balloons. But the balloons of Cavallo were small affairs made of bladders or paper bags filled with hydrogen gas. One of these materials being too heavy and the other too porous for successful balloon construction, the performances of these toy-balloons were not conclusively demonstrative. THE BALLOON INVENTED Throughout the entire spring of 1783, all Auvergne, in France, was kept in breathless expectancy by constant rumors that the two Montgolfiers had really solved the problem of aerial flight, and would soon be seen soaring over the country in a strange birdlike machine. Rumor pictured this machine in various forms and sizes, but in point of fact there was really very little secrecy on the part of the inventors themselves, who frankly explained the principle of the balloon they were constructing. It was hardly to be expected, however, that most persons would believe the plain truth that so simple a device as a bag filled with hot air would do what had long been considered impossible. Spring advanced and lapsed into summer, however, and as no flying-machine made its appearance, public clamor became so loud that the Montgolfiers felt they could postpone their demonstration no longer, although the balloon they were working on was not completed to their entire satisfaction. Nevertheless, they fixed on the definite date of June 5, (1783) as the day and Annonay as the place for making the trial, and their faith in their invention was shown by the fact that special invitations were sent to the leading persons in the vicinity, and a general invitation extended to the world at large.