B.  ENVELOPE PRESSURE CONTROL SYSTEM  (1) General Description  The maintenance of a predetermined pressure differ› ential between the gas in the envelope of a non-rigid air› ship and the surrounding atmosphere is the first basic  requirement to the successful operation of this type of  airship.  The functioning of the envelope in assuming the various  flight stresses, the proper suspension of the car, the effic› ient operation of the controls, all depend upon a closely  held pressure differential.  It is the function of the pressure control system to  maintain this pressure differential within a certain range  and to do this with a minimum loss of lifting gas.  This  result is accomplished by inflating the envelope partly with  gas and partly with air, the air being contained in a forward  and an aft ballonet, and by regulating the inflation of the  air by means of an air system,without changes to the amount  of gas in the envelope.  Under extreme conditions, outside  of the range of the air system, gas can be released auto› matically by meatls of two gas valves to prevent the internal  pressure of the envelope from rising above a safe value.  The pressure control system is also used to adjust the  trim of the ship by regulating the relative inflation of  the two ballonets.  -22-