A balloon is an apparatus with an envelope filled with gas, the specific gravity of which is less than that of the atmosphere near the surface of the earth. It is practically at the mercy of air-currents. The science of balloon aeronautics dates definitely from 1783, when the Montgolfier brothers at Angonay in France constructed their first balloons. These Frenchmen and their successors developed the spherical balloons to a state of efficiency which has scarcely been improved upon to this day. The balloon in time came to be adopted throughout Europe for military uses, mainly for the purpose of spying out the enemy's position and defenses. A dirigible balloon usually has an elongated envelope and is equipped with a motor and a rudder by which it can be steered at will against a moderate wind. Balloon aeronautics became popular in 1898, when Santos-Dumont, a wealthy young Brazilian, performed a series of spectacular feats with his dirigible balloon. Immediately ballooning became the sporting fad in France and the craze spread rapidly over the Continent and to England. Numerous airships of the dirigible type made their appearance and many balloon factories were established. [Illustration: A WRIGHT BIPLANE By Courtesy of Brooks Brothers] In Germany every community has its aero club. In the United States there are about 300,000 club members scattered throughout the land who individually or collectively own over 200 balloons. All of the great nations own one or more aerial warships of the dirigible type, as well as numerous spherical balloons. An aeroplane, as commonly known, is a machine which is sustained in the air by one, two, or three sets of rigid surfaces or planes. Unlike the balloon, it is heavier than air, and it must therefore maintain its position in the air by some form of mechanical propulsion. It must, in other words, fly like a bird. [Illustration: A BLERIOT MONOPLANE] The first aeroplane was invented by Henson, an Englishman, who in 1843 flew his machine, using a two-horse-power steam engine. In 1888 and in 1900 two other practically successful models appeared, one made by a French and the other by an English inventor. Langley, an American, who began experimenting in 1885, managed to fly over the Potomac in 1896. The Wright brothers made their initial flights under motor power in 1903. During the years since 1903 innumerable types of aeroplanes have been developed, all based upon the lines laid down by Langley, Henson, Maxim, and other pioneers. Among the most successful experimenters have been Farman, Delagrange, Bleriot, Curtiss, and the Voisins. The flapping-wing machine is called an orthopter (_orthos_, straight, + _ptera_, wing) and is supposed to copy bird flight. Screw-flyers, called helicopters, lift themselves from the ground by the thrust of varying numbers of rapidly moving propellers, revolving horizontally.