We have seen that the latter half of the nineteenth century was a time of invention. It was a time when the great discoveries of many centuries bore fruit in great inventions. It was thought by some scientists that all the great discoveries had been made, and that all that remained was careful work in applying the great principles that had been discovered. So far was this from being true that in the last ten years of the nineteenth century discoveries were made more startling, if possible, than any that had preceded. The nineteenth century not only brought forth many great inventions, but handed down to the twentieth century a series of discoveries that point the way to still greater inventions. Air-Ships For centuries men sailed over the water at the mercy of the wind. The sailing vessel is helpless in a storm. Early in the nineteenth century they learned to use the power of steam for ocean travel, and the wind lost its terrors. Late in the eighteenth century men learned to sail through the air in balloons even more at the mercy of the wind than the sailing vessels on the ocean. More than a hundred years later they learned to propel air-ships in the teeth of the wind. The nineteenth century saw the mastery of the water. The twentieth is witnessing the mastery of the air. The first balloon ascension was made in 1783, two men being carried over Paris by what Benjamin Franklin called a "bag of smoke." The balloon was a bag of oiled silk open at the bottom. In the middle of the opening was a grate in which bundles of fagots and sheaves of straw were burned. The heated air filled the balloon, and as the heated air was lighter than the air around it the balloon could rise and carry a load. Beneath the grate was a wicker car for the men. They were supplied with straw and fagots with which to feed the fire. When they wanted to rise higher they added fuel to heat the air in the balloon. When they wished to descend they allowed the fire to die out, so that the air in the balloon would cool. They could not guide the balloon, but drifted with the wind. That great philosopher Benjamin Franklin, who saw the ascension, said that the time might come when the balloon could be made to move in a calm and guided in a wind. In the second ascension bags of sand were taken as ballast, and the car was suspended from a net which enclosed the balloon. In this second ascension hydrogen gas was used in place of heated air.