the resistance is proportional to the two-thirds power of the volume.  TMs leads to a more convenient expression for the resistance of airship  hulls as follows :  where  OD  is called the P randtl shape coefficient after the eminent au› thority, Professor Prandtl.  Values of  0 D  for various speeds are given  in table I.  c.  The offsets for different types of airships are given in table II.  A study  of  the shapes given therein in connection with the Prandtl  coefficients will bring out the relative efficiency of the different stream› lines.  d.  Certain general rules  of  design developed by experience and test  may be summarized as follows:  ( 1) The best form is one of continuous curvature with radius of cur› vature constantly increasing toward rear portion.  (2) T he shape of extreme rear portion of the hull does not seriously  affect the resistance.  ( 3) The introduction of a cylindrical midsection causes an addi› tional resistance equal  to the skin friction on the increased surface  area of the hull.  (4) The major diameter should lie between 33 and 40 percent of total  length from the bow.  11.  Prismatic coefficient.- The ratio of the volume of any hull  form to that of the circumscribing cylinder is called the prismatic  coefficient,  Qv.  The prismatic coefficients for different shapes are given in table I . .  AIR CORPS  TM 1-3’20  10-11  R  =  0 DP  (volume)  •;s  v~.se  Volume  Qv=  Maximum cross-sectional area  X  length  VOl =  Q.t.A.L  24 