LESS the friction head consumed in reaching that point.  Both  velocity  head  and  friction  represent energy  that  came  from  the  original  static  head. Energy cannot be destroyed. So, the sum of the static  head,  velocity  head,  and  friction  at  any point in the system must add up to the original static  head.  This,  then,  is  Bernoulli’s  principle; more  simply  stated,  if  a  noncompressible  fluid flowing through a tube reaches a constriction, or narrowing  of  the  tube,  the  velocity  of  fluid flowing  through  the  constriction  increases,  and the pressure decreases. When  we  apply  a  force  to  the  end  of  a column  of  confined  liquid,  the  force  is  trans- mitted  not  only  straight  through  to  the  other end but also equally in every direction through- out  the  column.  This  is  why  a  flat  fire  hose takes  on  a  circular  cross  section  when  it  is filled  with  water  under  pressure.  The  outward push  of  the  water  is  equal  in  every  direction. Water  will  leak  from  the  hose  at  the  same velocity  regardless  of  where  the  leaks  are  in the hose. Let  us  now  consider  the  effect  of  Pascal’s law  in  the  systems  shown  in  figure  2-14, views   A   and   B.   If   the   total   force   at   the input  piston  is  100  pounds  and  the  area  of the  piston  is  10  square  inches,  then  each square  inch  of  the  piston  surface  is  exerting 10  pounds  of  force.  This  liquid  pressure  of 10 psi is transmitted to the output piston, which will  be  pushed  upward  with  a  force  of  10  psi. In   this   example,    we  are  merely  considering a  liquid  column  of  equal  cross  section  so  the areas  of  these  pistons  are  equal.  All  we  have done  is  to  carry  a  100-pound  force  around  a bend. However, the principle shown is the basis for  almost  all  mechanical  hydraulics. The same principle may be applied where the area of the input piston is much smaller than the area  of  the  output  piston  or  vice  versa.  In  view B of figure 2-14, the area of the input piston is 2 square inches and the area of the output piston is 20 square inches. If you apply a pressure of 20 pounds to the 2 square-inch piston, the pressure created  in  the  liquid  will  again  be  10  psi.  The upward  force  on  the  larger  piston  will  be  200 pounds—10  pounds  for  each  of  its  20  square inches. Thus, you can see that if two pistons are used  in  a  hydraulic  system,  the  force  acting  on each  piston  will  be  directly  proportional  to  its area. A. EQUAL  INPUT  AND  OUTPUT  AREA B. UNEQUAL  INPUT  AND  OUTPUT  AREA Figure  2-14.—Principle  of  mechanical  hydraulics.  A.  Equal input and output area. B. Unequal input and output area. PRINCIPLES OF PNEUMATICS PNEUMATICS  is  that  branch  of  mechanics that deals with the mechanical properties of gases. Perhaps  the  most  common  application  of  these properties  in  the  Navy  today  is  the  use  of compressed   air.    Compressed   air   is   used   to transmit  pressure  in  a  variety  of  applications.  For example,   in   tires   and   air-cushioned   springs, compressed air acts as a cushion to absorb shock. Air  brakes  on  locomotives  and  large  trucks contribute  greatly  to  the  safety  of  railroad  and truck  transportation.  In  the  Navy,  compressed  air is  used  in  many  ways,  For  example,  tools  such as riveting hammers and pneumatic drills are air operated. Automatic combustion control systems 2-18


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