During the Franco-German war a number of balloons were sent up from Paris when it was besieged by the Germans. These balloons carried no fewer than two and a half million letters into the provinces of France. But the people in the provinces could not answer these letters, as no balloon could be relied upon to reach Paris. It is not surprising, therefore, that after the conclusion of the war, the French people should have made many efforts to construct balloons which could be guided to any particular destination. One such balloon was built in 1872, and another eleven years afterwards, but those balloons were not suitably constructed, and the engines and screw propellers were not powerful enough to take them against the wind. It was only when motor cars were built in great numbers that people acquired sufficient experience to build very light and powerful engines suitable for propelling balloons. This shows how one class of inventors may help another class. A balloon which can be driven in any chosen direction is called an “air-ship.” Many such air-ships have been built within the last few years. They are shaped very much like a cigar. Some of them have cars made of light rods of steel or aluminium, and such air-ships have been navigated from one city to another over distances of sixty miles. One of the most remarkable of these air-ships is that constructed by Count Zeppelin, a German officer (Fig. 76). It is 420 feet long, and 38 feet high. It is cylindrical in shape, and is covered with silk or gold-beater’s skin stretched over a stiff frame of aluminium rods. It is divided into sixteen air-tight compartments, so that if a hole should be cut in it at any point, the whole balloon would come to the ground very slowly. One such balloon travelled down the Rhine in August 1908 for over eleven hours. On its homeward journey it was caught in a storm and burnt up, but it was thought that it was built on a good plan, and the Germans at once set to work to build more ships of the same kind. In 1909 one of these made a trip from the Lake of Constance, in Switzerland, across Germany, to a place almost within sight of Berlin. But while inventors were busy constructing balloons, the solution of the problem of flight was undertaken along an entirely different line. [Illustration: Fig. 76.--Zeppelin II. on its Flight over Cologne.]