No sooner had the Montgolfier brothers proved that it was possible to rise off the earth upon a bubble of hot air, than inventors began to devise schemes of propelling the bubble and controlling its course. The first dirigible balloon was built in 1784 and, strangely enough, oars were used to propel it. The balloon had a fish-shaped body, just like that of modern airships, and the bag was kept inflated by means of air as it is to-day. It was realized even at that early date that the envelope must retain its shape if it is to plow through the air with a minimum of friction. As the gas would gradually leak out of the envelope the bag would become flabby and present a crumpled bow to the ocean of the air and, to overcome this, a double envelope was provided and air was introduced between the inner and outer skin. Means were provided for pumping air in as the gas slowly escaped, thus keeping the envelope fully inflated. As the air was pumped in sand ballast was thrown overboard so that the buoyancy of the airship was not impaired. To-day air is used to keep gas envelopes inflated, but instead of placing the air in an outside envelope it is introduced into a small bag inside called a “ballonet.” From time to time other means of propulsion were proposed and tried. In 1852 Henri Giffard, who is known as the Fulton of aerial navigation, built and operated a dirigible driven by a steam engine, which he hung at a considerable distance below the gas bag, so that the fire box would not be liable to ignite the highly inflammable gas. A draft was maintained in the fire box as it is in a locomotive by letting the steam discharge into the smoke stack, but in this case the stack was inverted and the smoke and gases were blown downward and away from the coal gas with which the balloon was inflated. The danger of fire and the difficulty of building a power plant light enough to be carried aloft and powerful enough to give the craft any material headway were the chief obstacles that had to be overcome. At the close of the nineteenth century, Santos Dumont, a Brazilian inventor, built a balloon that was driven by a gasoline engine. Despite the apprehension of other aeronauts this machine proved an unqualified success. Two years later, in 1900, Count Zeppelin introduced the rigid dirigible, which to-day is the standard for large airships. THREE TYPES OF DIRIGIBLES