CHAP. V ] 



BALLOONS. 



93 



lightness alone of the hot air which, by virtue of the principle of 

 Archimedes, was the real cause. He also favoured the production of 

 the fluid by burning straw cut up with damp wool, and believed that 

 the straw and wool gave off a special gas to which the ascending 

 power was clue. De Saussure had no trouble to prove that the air 

 produced had no other virtue than warm air, and that electricity went 

 for nothing. 



BaUoons filled with hydrogen, although more expensive than hot- 

 air balloons, are generally preferred. The necessity of carrying 

 combustible materials, the danger of fire, and above all the inferiority 



Vic,. 57. Car of the balloon Le Pole nonl. 



of the ascending power (much less with equality of volumes), are 

 reasons for this preference. 1 



1 The weight of a cubic metre under a pressure of 760 millimetres, is 



1,293 grammes at 



1,247 grammes at . 10 



945 grammes at 50 



278 grammes at 100 



Thus the ascending power of hot air, 46 grammes only per cubic metre at 10, 

 348 grammes at 50, rises to 1015 grammes at 100. At pure hydrogen is 1,203 

 grammes, at 10 it is still 1,160 grammes. As it is difficult to preserve the tem- 

 perature of the air of a montgolfiere at such a height, it follows that the ascending 

 power is very much less than that of a balloon filled with pure hydrogen. 



