De Rozier had no false notion of the risks he was prepared to run, and he superintended with the greatest care the construction of his balloon. It was of enormous size, with a cage slung underneath the brazier for heating the air. Befors making his free ascent De Rozier made a trial ascent with the balloon held captive by a long rope. At length, in November, 1783, accompanied by the Marquis d'Arlandes as a passenger, he determined to venture. The experiment aroused immense excitement all over France, and a large concourse of people were gathered together on the outskirts of Paris to witness the risky feat. The balloon made a perfect ascent, and quickly reached a height of about half a mile above sea-level. A strong current of air in the upper regions caused the balloon to take an opposite direction from that intended, and the aeronauts drifted right over Paris. It would have gone hard with them if they had been forced to descend in the city, but the craft was driven by the wind to some distance beyond the suburbs and they alighted quite safely about six miles from their starting-point, after having been up in the air for about half an hour. Their voyage, however, had by no means been free from anxiety. We are told that the fabric of the balloon repeatedly caught fire, which it took the aeronauts all their time to extinguish. At times, too, they came down perilously near to the Seine, or to the housetops of Paris, but after the most exciting half-hour of their lives they found themselves once more on Mother Earth. Here we must make a slight digression and speak of the invention of the hydrogen, or gas, balloon. In a previous chapter we read of the discovery of hydrogen gas by Henry Cavendish, and the subsequent experiments with this gas by Dr. Black, of Glasgow. It was soon decided to try to inflate a balloon with this "inflammable air"--as the newly-discovered gas was called--and with this end in view a large public subscription was raised in France to meet the heavy expenses entailed in the venture. The work was entrusted to a French scientist, Professor Charles, and two brothers named Robert. It was quickly seen that paper, such as was used by the Montgolfiers, was of little use in the construction of a gas balloon, for the gas escaped. Accordingly the fabric was made of silk and varnished with a solution of india-rubber and turpentine. The first hydrogen balloon was only about 13 feet in diameter, for in those early days the method of preparing hydrogen was very laborious and costly, and the constructors thought it advisable not to spend too much money over the initial experiments, in case they should be a failure.