94 



TEE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



Nevertheless the construction of hot-air balloons has been much 

 improved by substituting sponges soaked in spirit for the inconvenient 

 combustibles of" straw and wool. An aeronaut, M. E. Godard, has 

 adapted to the fire a chimney surmounted by a metal curtain or 

 screen, which guards against the danger of conflagration. The use of 

 petroleum lamps would perhaps enable one to increase or moderate 

 the temperature, and consequently, to rise or descend at will. 

 M. Joseph Silbermann has made some interesting researches on this 

 subject; his system of fire-balloons certainly deserves to be tried. 1 

 The inflation of balloons with pure hydrogen gas is accomplished in 



FIG. 58. Operation of inflating a balloon with hydrogen gas. 



the following manner. The gas is produced by the reaction of 

 sulphuric acid on water, iron or zinc.' 2 A system of casks inclosing 

 these substances is arranged so that the gas is collected as it is 

 formed, from a bell-jar reversed in a water trough, similar to a 

 gasometer. Then after having been purified by its passage through 



1 In 1874 some experiments were nmle ;tt Woolwich Arsenal with a balloon 

 invented by Messrs. Menier and Simmonds, which was inflated by means of 

 petroleum. 



2 In 1850 MM. Barral and Bixio used the reaction of hydrochloric acid on water 

 and iron. The gas must be carefully washed to prevent the action of the acid on 

 the envelope. 



