88 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



be due to ascending aerial currents in the region of the atmosphere in 

 which it finds itself. Such is briefly the theory of aerostation, which 

 was only understood and successfully applied for the first time in 1783 

 by Joseph Montgolfier. In reality, the idea of rising and being sus- 

 pended in the air had a long time previously suggested numerous 

 projects more or less chimerical which mostly existed in the imagina- 

 tion of their authors ; the rare attempts at realization and execution 

 were frustrated on account of insufficient knowledge of mechanical 

 and physical laws. 



FIG. 54. Ascension of soap-bubbles filled with hydrogen. 



Joseph Montgolfier, who doubtless knew of the experiments of 

 Black, Cavendish arid Cavallo, on the ascension of bladders and 

 soap-bubbles filled with hydrogen gas (Fig. 54), formed the idea of 

 imitating these experiments on a large scale, and of making them of 

 use in the exploration of the atmosphere. He first made balloons of 

 silk or paper, which, filled with hydrogen, rose to a certain height, 

 but descended very soon, as he foresaw, because the gas escaped 

 through the permeable envelope. He then substituted warm air for 

 the hydrogen, the density being much greater than that of the gas, 



