Then by degrees came larger craft, more powerfully engined, and built to attain greater speed. Hydrogen, far more buoyant than coal-gas, was used to inflate their envelopes, and so they obtained a greater “lift.” Speed with the airship was recognised as vital. If it could not fly fast it was at the mercy of the wind, gusts striking powerfully against its envelope, and driving it off its course. [Illustration: FIG. 75.--Early-type Airship. A. Gas envelope; B. Car suspended below envelope; C. Motor, which drives propeller (D) through a shaft; E. Small horizontal plane for rising or descending; F. Fixed fin, or keel plane, to give stability; G. Rudder.] [Illustration: FIG. 76.--Airship “ballonette.” A. Gas envelope; B. The interior ballonette, or collapsible chamber, which is filled with air through the pipe C.] A typical craft, representing the first of those navigated with any certainty, is shown in Fig. 75. A gas-containing envelope, made of a light, strong, varnished fabric, is kept taut by the pressure of the gas within; the car, constructed of wood or metal tubing, is suspended by ropes from the envelope, and contains engine and crew, with a two-bladed propeller revolving astern. Such a machine, in its control, had an elevating-plane and rudder, upon the same principle as those of the aeroplane. One of the difficulties to be overcome was the expansion and contraction of gas in the envelope owing to differences in altitude and temperature. When the craft ascended, its envelope completely inflated, the gas began to dilate owing to the outer air becoming less dense; and some had to be allowed to escape through automatic valves. Then, should the machine descend to a lower level, there was not sufficient gas in the envelope to keep it tightly stretched, and it tended to sag at the bow as it was driven through the air. To prevent this kinking, which would have reduced the speed of the airship, and made it difficult to control, an interior chamber, called the “ballonette,” was fitted to the envelope, as shown in Fig. 76. When the gas-container was tightly filled, this ballonette lay empty upon its lower surface; but, should the envelope tend to become flaccid, through a loss of gas, a fan pumped air into the ballonette; and, swelling out within the balloon, it compensated for the gas which had escaped, and prevented the envelope from losing its shape.