height of the upper part of the fabric of the ballonets  or upon the degree of their inflations, upon the lift co› efficient of the gas, and upon the  f~bric  weight which  reacts on the air.  Curve in Fig. VII shows the relation between this  difference in pressure and the inflation of the ballonets  at standard lift coefficient.  It will be noted the.t the  difference of pressure is about 0.50 inches at full infla› tion and  nea~ly  zero when the ballonets are fully deflated.  Since the automatic air valves are operated by the  air  pressur~  in the ballonets and not by the gas pressure,  it will be obvious that the system will tend to regulate  for a higher gas pressure when the ballonets are empty than  when they are inflated.  (c) Pressure Regulation  Under normal flight condition a gas pressure of about  1.5 inches of water should be maintained with a maximum vari- ation of 1.25 to 2.00 inches.  CAUTION:  Under no condition should the pressure be allowed  to go lower than 0.5 inch or higher than 3.0 inches  of water, the lower pressure being permissible only  in landing operations or in the hangar when there  is little or no forward speed.  It is particularly essential to maintain an adequate  internal pressure when the ship is operated at high speed  or when moored at the mast during high winds.  -27-