THE CONQUEST OF TIME AND SPACE 



been those in imitation of birds; but during the early 

 part of the eighteenth century the idea of the balloon 

 was developed. This was the result of the numerous 

 important discoveries made about that time as to the 

 qualities of the atmosphere, and also several other 

 "airs," as gases were called, such as their expansion 

 and contraction under different conditions of tempera- 

 ture. 



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 con- 

 structing 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 Mont- 

 golfier, 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 



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