conclusion  which  greatly  simplifies  the  calculation  of  this  impor-
tant factor :  viz.  that  no  sensible  error  is  inyolyed  in  calculating
the  frictional  resistance  **  upon  the  hypothesis  that  the  immersed
**  skin  is  equiyalent  to  that  of  a  rectangular  surface  of  equal
''  area,  and  of  length  (in  the  line  of  motion)  equal  to  that  of  the
"  model,  moving  at  the  same  speed."  Hence,  it  is  only  necessary
to  experiment  with  such  a  plane  surface  as  will  enable  the  proper
coefficient  of  friction  to  be  found,  then  to  measure  the  immersed
surface  of  the  ship,  and  to  apply  the  coefficient,  neglecting  the
variations  in  speed  of  the  particles  at  different  parts  of  the  surface.
