those necessary on a small one traveling at the same speed. An un› stable airship requires closer attention from the pilot than does one which is stable, but it is not necessarily either difficult or dangerous •to operate and has the advantage of being more easily maneuverable than the more stable types. 

30. Directional stability.-a. Directional stability is maintained in part by use of vertical fixed fins and rudder. When the rudder is set in neutral it acts as additional fin surface, but the total fin sur› face is never large. enough to provide complete directional stability Since there is no statical restoring moment to overcome a horizontal deviation from the flight path, maintenance of directional stability devolves upon the pilot who must correct any deviations as soon as they appear. Otherwise a deviation once started will tend to in› . crease until the airship is traveling in a circle of so small a radius that the d~mping moment balances the turning moment due to pres› sure on the nose. This is quite different from the condition of longitu› dinal stability where the elevator can be left locked in any particular position and the airship will return to its original attitude if atmos› pheric disturbances have momentarily changed that attitude. 

b. As soon as there is any deviation from the straight line of flight the a.ir strikes on the side of the envelope and sets up a moment tend› ing to turn the airship farther from its original course. This moment corresponds exactly to the upsetting moment, Me, which opposes lon› gitudinal stability. There is then an unbalanced moment which tends to give the airship an angular acceleration and so to turn her more and more rapidly. At the same time the lateral force on the envelope, which corresponds to the dynamic lift, is increasing and .furnishes the necessary centripetal force to keep the airship traveling in a circular path. It is quite true that a force resisting this circling is exerted by the vertical surfaces, but, as mentione.d above, the vertical fin surfaces are never large enough to provide full stability, and the rudder must be used to assist them. Use of the rudder will be more fully discussed in sec6on V. 

31. Lateral stability.-a. Stability in roll, which is a very diffi› cult problem in airplanes, is taken care of almost automatically in air› ships; since the same statical restoring moment acts with regard to roll as with regard to pitch and there is no dynamic upsetting moment to oppose it. The only rolling motions are those due to side gusts against the car and bag and those due to centrifugal force when turning. The• moments of these forces are overcome immediately by the large re› storing moment due to the low position of the center of gravity. Roll- 

