In some of the English dirigibles the cars were suspended by network passing over the top of the balloon. Keeping the Keel Horizontal In the _Zeppelin_, a sliding weight could be moved along the keel so as to cause the center of gravity to coincide with the center of upward pressure in spite of variations in weight and position of gas, fuel, and ballast. In the German balloon _Parseval_, the car itself was movable on a longitudinal suspending cable which carried supporting sheaves. This balloon has figured in recent press notices. It was somewhat damaged by a collision with its shed in March: the sixteen passengers escaped unharmed. A few days later, emergency deflation by the rip-strip was made necessary during a severe storm. In the ordinary non-rigid balloon, the pumping of air between the ballonets aids in controlling longitudinal equilibrium. The pump may be arranged for either hand or motor operation: that in the _Clément-Bayard_ had a capacity of 1800 liters per minute against the pressure of a little over three-fifths of an ounce. The _Parseval_ has two ballonets. Into the rear of these air is pumped at starting. This raises the bow and facilitates ascent on the principle of the inclined surface of an aeroplane. After some elevation is attained, the forward ballonet is also filled. [Illustration: CAR OF THE ZEPPELIN (From the _Transactions_ of the American Society of Mechanical Engineers)] Stability Besides proper distribution of the loads, correct vertical location of the propeller is important if the balloon is to travel on a level keel. In some early balloons, two envelopes side by side had the propeller at the height of the axes of the gas bags and midway between them. The modern forms carry the car, motor, and propeller below the balloon proper. The air resistance is mostly that of the bow of the envelope: but there is some resistance due to the car, and the propeller shaft should properly be at the equivalent center of all resistance, which will be between car and axis of gas bag and nearer the latter than the former. With a single envelope and propeller, this arrangement is impracticable. By using four (or even two) propellers, as in the _Zeppelin_ machine (page 68), it can be accomplished. If only one propeller is employed, horizontal rudder planes must be disposed at such angles and in such positions as to compensate for the improper position of the tractive force. Even on the _Zeppelin_, such planes were employed with advantage (pages 66 and 73). Perfect stability also involves freedom from rolling. This is usually inherent in a balloon, because the center of mass is well below the center of buoyancy: but in machines of the non-rigid type the absence of a ballonet might lead to both rolling and pitching when the gas was partially exhausted. [Illustration: STERN VIEW OF THE ZEPPELIN]