While balloons were inflated only with pure hydrogen--for the uncertain and dangerous method of filling with hot air was soon almost entirely abandoned--no great strides could be made in the art of sailing the skies. The filling of a large balloon eighty years ago cost no less than £250, and few people could be found willing to provide so much money for such a purpose. Coal gas, however, was by then to be found in every town of any consequence; and it was Green’s suggestion that though this gas might be greatly inferior to pure hydrogen in buoyancy or “lifting power,” it yet contained a sufficient quantity of hydrogen in it for all ordinary aeronautical purposes. The coronation of King George the Fourth was the occasion chosen by Green to put his new scheme to the test and fill a balloon with coal gas. The experiment was entirely successful, and henceforward balloon ascents became much commoner throughout the world, for Green’s discovery reduced the cost of filling tenfold, and the trouble and anxiety a hundredfold. Green himself became one of the most famous men of his day, and lived to make a thousand ascents, some of them of the most daring and exciting description. [Illustration: THE GREAT NASSAU BALLOON.] The most celebrated event in all his career, however, was the voyage of the Great Nassau Balloon, in November 1836. This voyage created a tremendous sensation at the time, and has always been considered one of the most adventurous enterprises in the whole history of aeronautics. How it came about was as follows:-- The managers of the Vauxhall Gardens, London, had made, with Mr. Green’s assistance, a very large and fine balloon of crimson silk, which stood eighty feet high and held 90,000 cubic feet of gas, and which would carry, if needed, more than twenty persons. After it was made the proprietors proposed exhibiting it in Paris, and there was some question of how this valuable and fragile property had best be conveyed so far. Mr. Hollond, a young gentleman of considerable wealth, and a great lover of adventure, at once came forward, and proposed to take the balloon to the Continent by sky. His offer was accepted, and to make the ascent more noteworthy, it was decided to start from London and cross the sea by night, making as long a voyage as possible, although it was already winter time, and such a venture had never before been made. Preparations were at once commenced. The passengers were limited to three--Mr. Green, who was to manage the balloon, Mr. Hollond, and his friend Mr. Monck Mason. A ton of ballast was to be carried, provisions for a whole fortnight were laid in, and, since none could tell to within a thousand miles or more where they might be drifted, passports to every kingdom in Europe were obtained.