systems, and appendix III provides symbols used in  nonaeronautical  mechanical  systems. The remainder of chapter 1 is devoted to the advantages  and  problems  of  fluid  power  appli- cations.  Included  are  brief  sections  on  the  history, development, and applications of hydraulics, the states of matter. ADVANTAGES  OF  FLUID  POWER and The extensive use of hydraulics and pneuma- tics  to  transmit  power  is  due  to  the  fact  that properly constructed fluid power systems possess a   number   of   favorable   characteristics.   They eliminate  the  need  for  complicated  systems  of gears,  cams,  and  levers.  Motion  can  be  trans- mitted  without  the  slack  inherent  in  the  use  of solid  machine  parts.  The  fluids  used  are  not subject to breakage as are mechanical parts, and the mechanisms are not subjected to great wear. The  different  parts  of  a  fluid  power  system can be conveniently located at widely separated points,  since  the  forces  generated  are  rapidly transmitted  over  considerable  distances  with  small loss. These forces can be conveyed up and down or around corners with small loss in efficiency and without  complicated  mechanisms.  Very  large forces  can  be  controlled  by  much  smaller  ones  and can be transmitted through comparatively small lines  and  orifices. If the system is well adapted to the work it is required  to  perform,  and  if  it  is  not  misused,  it can  provide  smooth,  flexible,  uniform  action without vibration, and is unaffected by variation of  load.  In  case  of  an  overload,  an  automatic release  of  pressure  can  be  guaranteed,  so  that  the system is protected against breakdown or strain. Fluid power systems can provide widely variable motions  in  both  rotary  and  straight-line  trans- mission  of  power.  The  need  for  control  by  hand can   be   minimized.   In   addition,   fluid   power systems  are  economical  to  operate. The question may arise as to why hydraulics is  used  in  some  applications  and  pneumatics  in others. Many factors are considered by the user and/or  the  manufacturer  when  determining  which type  of  system  to  use  in  a  specific  application. There  are  no  hard  and  fast  rules  to  follow; however,   past   experience   has   provided   some sound ideas that are usually considered when such decisions  are  made.  If  the  application  requires speed,  a  medium  amount  of  pressure,  and  only fairly accurate control, a pneumatic system may be used. If the application requires only a medium amount of pressure and a more accurate control, a combination of hydraulics and pneumatics may be used. If the application requires a great amount of pressure and/or extremely accurate control, a hydraulic  system  should  be  used. SPECIAL PROBLEMS The  extreme  flexibility  of  fluid  power  elements presents a number of problems. Since fluids have no  shape  of  their  own,  they  must  be  positively confined  throughout  the  entire  system.  Special consideration  must  be  given  to  the  structural integrity  of  the  parts  of  a  fluid  power  system. Strong  pipes  and  containers  must  be  provided. Leaks  must  be  prevented.  This  is  a  serious problem  with  the  high  pressure  obtained  in  many fluid  power  installations. The operation of the system involves constant movement  of  the  fluid  within  the  lines  and components.   This   movement   causes   friction within the fluid itself and against the containing surfaces  which,  if  excessive,  can  lead  to  serious losses  in  efficiency.  Foreign  matter  must  not  be allowed to accumulate in the system, where it will clog small passages or score closely fitted parts. Chemical  action  may  cause  corrosion.  Anyone working  with  fluid  power  systems  must  know  how a  fluid  power  system  and  its  components  operate, both  in  terms  of  the  general  principles  common to all physical mechanisms and of the peculiarities of  the  particular  arrangement  at  hand. HYDRAULICS The word  hydraulics  is  based  on  the  Greek word for water, and originally covered the study of the physical behavior of water at rest and in motion.  Use  has  broadened  its  meaning  to  include the behavior of all liquids, although it is primarily concerned  with  the  motion  of  liquids. Hydraulics   includes   the   manner   in   which liquids  act  in  tanks  and  pipes,  deals  with  their properties, and explores ways to take advantage of  these  properties. DEVELOPMENT  OF  HYDRAULICS Although   the   modern   development   of hydraulics is comparatively recent, the ancients were  familiar  with  many  hydraulic  principles  and their  applications.  The  Egyptians  and  the  ancient people  of  Persia,  India,  and  China  conveyed  water 1-2