What is called "route stability" describes the condition of straight flight. The balloon must point directly in its (independent) course. This involves the use of a steering rudder, and, in addition, of fixed vertical planes, which, on the principle of the vertical partitions of Voisin, probably give some automatic steadiness to the course. To avoid the difficulty or impossibility of holding the head up to the wind at high speeds, an _empennage_ or feathering tail is a feature of all present balloons. The empennage of the _Patrie_ (page 77) consisted of pairs of vertical and horizontal planes at the extreme stern. In the _France_, thirty-two feet in maximum diameter and nearly 200 feet long, empennage planes aggregating about 400 square feet were placed somewhat forward of the stern. In the _Clément-Bayard_, the empennage consisted of cylindro-conical ballonets projecting aft from the stern. A rather peculiar grouping of such ballonets was used about the prolonged stern of the _Ville de Paris_. [Illustration: THE "CLÉMENT-BAYARD"] [Illustration: THE "VILLE DE PARIS"] Rudders and Planes The dirigible has thus several air-resisting or gliding surfaces. The approximately "horizontal" (actually somewhat inclined) planes permit of considerable ascent and descent by the expenditure of power rather than gas, and thus somewhat influence the problem of altitude control. Each of the four sets of horizontal rudder planes on the _Zeppelin_, for example, has, at thirty-five miles per hour, with an inclination equal to one-sixth a right angle, a lifting power of nearly a ton; about equal to that of all of the gas in one of the sixteen compartments. [Illustration: CAR OF THE "LIBERTÉ"] Movable rudders may be either hand or motor-operated. The double vertical steering rudder of the _Ville de Paris_ had an area of 150 square feet. The horizontally pivoted rudders for vertical direction had an area of 130 square feet. Arrangement and Accessories The motor in the _Ville de Paris_ was at the front of the car, the operator behind it. This car had the excessive weight of nearly 700 pounds. The _Patrie_ employed a non-combustible shield over the motor, for the protection of the envelope: its steering wheel was in front and the motor about in the middle of the car. The gasoline tank was under the car, compressed air being used to force the fuel up to the motor, which discharged its exhaust downward at the rear through a spark arrester. Motors have battery and magneto ignition and decompression cocks, and are often carried on a spring-supported chassis. The interesting _Parseval_ propeller has four cloth blades which hang limp when not revolving. When the motor is running, these blades, which are weighted with lead at the proper points, assume the desired form.