With the differential valve setting outlined above,  it is apparent that as long as the ship is operated below  the pressure height of the forward ballonet, air is re› leased only from this ballonet when the altitude of the  ship is increased.  Hence, if upon descending, the pilot  operates only the forward damper, the air which has been  valved out during the ascent will be replaced to the for› ward ballonet, thus restoring the trim of the ship to the  same condition that exis ted at take-off.  During short flights, the opera tion of the system is  automatic, and no manual adjustment needs to be made except  to open the forward damper.  During long flights, occasional  manual adjustments of the air balance may be necessary, but  the system remains largely self-operating.  All valves are adjusted to their specified settings  and t ested before being installed in the airship and re› tested after installation.  The valve settings should not be changed unless diffi› culties arise.  Adjustments should then be made only by a  qualified person.  During emergency, it may be necessary to  adjust the valve in flight.  Instructions below should  clarify any difficulties that may be encountered.  (d) Air Valve Adjustment  -29-