There is another type of craft that is being adapted and improved for naval use, and this is the airship. Although it is costly to build, requires a huge shed in which to house it, and needs also the service of trained crews to handle it when ascending or alighting, the airship is a vastly important machine for the purposes of war. Its ability to make flights lasting several days, and the power of its pilot to manœuvre it at night, render it particularly suitable for naval use. Our Admiralty is buying large machines, aiming at a preliminary fleet of fifteen, and the German Navy is organising a squadron of Zeppelins. The airship, although in use before the aeroplane, has developed more slowly, and is even to-day in a crude form. This has been due to the cost of experiments. Whereas a new aeroplane may be built and tested for £1000 or so, the construction of an airship and the provision of its shed spells an expenditure of many thousands. This money may, as in the case of the aeroplane, be lost as a result of one mishap. In Germany Count Zeppelin spent a fortune upon airships, before the Government and the nation helped him with funds. With very few exceptions, none but Governments--which have long purses to draw upon--can afford to build large airships. [Illustration: FIG. 72.--A modern Balloon.] The airship is a development of the balloon; and the balloon we owe to Stephen and Joseph Montgolfier--two French brothers who, after watching clouds floating in the sky, had the notion that if they could fill a light envelope with “some substance of a cloud-like nature” it would raise itself into the air. Their father was a paper manufacturer, and so they had facilities for making a number of very large paper bags. Under these they lit fires of chopped straw, allowing the hot air and smoke to rush up into the bags, and, when they were released the bags ascended, carried up by the lifting influence of the heated air within them. Thus was invented the hot-air balloon; and we buy paper replicas of it to-day.