io6 OUR PHYSICAL WORLD 



water and a sponge. On one of their voyages when they sailed 

 across the city of Paris, the balloon repeatedly took fire, but they 

 were fortunate in dashing the wet sponge on to the burning spots 

 before the flames had done much damage. In this particular 

 ascent they were in the air twenty-five minutes. 



About this time (1784) Cavallo, in England, discovered what 

 we now know as hydrogen gas, then called inflammable air. 

 This gas is very much lighter than air, and Cavallo at once saw 

 that it would be a good substance with which to fill a balloon. 

 He tried to do this, but he could not get a bag that was suffi- 

 ciently impervious to prevent the escape of the hydrogen. He 

 did, however, blow soap bubbles with this gas, and they arose with 

 celerity. Two French brothers by the name of Roberts and 

 another Frenchman by the name of Charles did succeed that same 

 year in building a balloon and inflating it with hydrogen gas. 

 With such a balloon it was much easier to make prolonged ascents. 

 In 1794 Monsieur Blanchard, accompanied by a Benedictine 

 monk, made an ascent at Paris with a hydrogen balloon, reaching 

 a height of 9,600 feet. In January the next year Blanchard and 

 an American physician by the name of Jeffries undertook to 

 cross the English Channel. They started from Dover and were 

 slowly carried by the wind toward the French coast. It was 

 only after they had thrown out all their ballast and much of their 

 clothing in order to lighten the load that they reached shore 

 safely not far from Calais. 



The Frenchman, De Rozier, determined not to be outdone 

 by any newcomer in the field of aeronautics, also undertook to 

 make the trip across the Channel. To prevent his balloon set- 

 tling into the sea as Blanchard's had so nearly done, he fastened 

 below his hydrogen balloon a hot-air balloon with a fire-basket 

 underneath it to keep the air hot. When out in mid-channel, at 

 a height of 3,000 feet, his balloon was seen to burst into flames, 

 an explosion followed, and De Rozier fell to his death. 



The French military authorities were prompt to see the possi- 

 bilities of the balloon in war time. In 1794 they used a captive 



