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.