The craft shown in Fig. 75 is of the non-rigid type; its car, that is to say, is hung by ropes from the envelope; and when the envelope is deflated it can be detached from the car and the machine packed away in a relatively small space. But as airships were built larger, and greater speeds were obtained, it became necessary to strengthen the envelopes with some form of keel; and this led to a type which is known as the semi-rigid, and is developed successfully in France. Fig. 77 illustrates an airship of this build. Along the lower side of its envelope is placed a light, rigid framework or keel, and from this is suspended the car which contains engines and crew. [Illustration: FIG. 77.--Semi-rigid Airship. A. Gas-containing envelope; B. Strengthening keel; C.C. Stabilising-planes; D. Rudder; E. Car carrying engines, propeller, and crew.] The car of an airship, showing its construction and the disposition of motors and propellers, is sketched in Fig. 78; while in Fig. 79 may be seen the pilot’s driving platform with wheels, dials, and speaking-tube to the engineers. [Illustration: FIG. 78.--The Car of an Airship. A.A. Propellers; B. Motors; C. Engineer’s platform; D. Pilot’s controlling platform; E. Elevating-planes; F. Fuel tanks; G. Passenger’s platform.] [Illustration: FIG. 79.--Control platform of an Airship. A.A. Wheels operating elevating-planes and rudder; B. Height recorder; C. Speaking-tube to communicate with engineers.] [Illustration: FIG. 80.--Hull of a Zeppelin during construction.] Craft of the semi-rigid type provide a link between small, non-rigid ships and the very large machine which is built with an entirely rigid framework, and has its example in the Zeppelin. The maker forms a skeleton hull of aluminium or some light metal alloy, a method that is shown in Fig. 80. The hull of a Zeppelin, slightly more than 500 feet in length, is sheathed with tightly stretched fabric; and within it are the gas-containers--a row of seventeen separate balloons, each in a compartment by itself, and containing a total of nearly 1,000,000 cubic feet of gas--which give these airships a lifting power of close upon 30 tons. The arrangement of the gas-holders, and the general outline of the machine, may be observed from Fig. 81. The vessel offers comparatively little resistance to the air, despite its size, and this is due to the finely tapering shape of the hull; while its rigidity allows it to be driven at speeds of more than 50 miles an hour. The lifting capacity, also, enables long flights to be made. Taking up crew and petrol, such a craft can remain aloft for several days, and travel distances of more than 1000 miles. [Illustration: FIG. 81.--A Zeppelin Airship.