pressure in this space is figured by subtracting the measured  vacuum  (10  in.Hg)  from  the  nearly perfect   vacuum   (29.92   in.Hg).   The   absolute pressure  then  will  be  19.92  or  about  20  in.Hg absolute. Note that the amount of pressure in a space  under  vacuum  can  only  be  expressed  in terms  of  absolute  pressure. You may have noticed that sometimes we use the  letters  psig  to  indicate  gauge  pressure  and other  times  we  merely  use  psi.  By  common convention,  gauge  pressure  is  always  assumed when pressure is given in pounds per square inch, pounds  per  square  foot,  or  similar  units.  The  g (for  gauge)  is  added  only  when  there  is  some possibility of confusion. Absolute pressure, on the other  hand,  is  always  expressed  as  pounds  per square  inch  absolute  (psia),  pounds  per  square foot  absolute  (psfa),  and  so  forth.  It  is  always necessary  to  establish  clearly  just  what  kind  of pressure we are talking about, unless this is very clear  from  the  nature  of  the  discussion. To  this  point,  we  have  considered  only  the most  basic  and  most  common  units  of  measure- ment.  Remember  that  hundreds  of  other  units  can be derived from these units; remember also that specialized   fields   require   specialized   units   of measurement.  Additional  units  of  measurement are introduced in appropriate places throughout the remainder of this training manual. When you have  more  complicated  units  of  measurement,  you may find it helpful to review the basic informa- tion given here first. PRINCIPLES  OF  HYDRAULICS The word  hydraulics is  derived  from  the  Greek word for water  (hydor)  plus the Greek word for a reed instrument like an oboe (aulos). The term hydraulics   originally  covered  the  study  of  the physical behavior of water at rest and in motion. However,  the  meaning  of  hydraulics  has  been broadened  to  cover  the  physical  behavior  of  all liquids,  including  the  oils  that  are  used  in  modern hydraulic  systems.  The  foundation  of  modern hydraulics   began   with   the   discovery   of   the following  law  and  principle: .  Pascal’s  law—This  law  was  discovered  by Blaise  Pascal,  a  French  philosopher  and mathematician who lived from 1623 to 1662 A.D. His law, simply stated, is interpreted as pressure exerted  at  any  point  upon  an  enclosed  liquid  is transmitted   undiminished   in   all   directions. Pascal’s law governs the BEHAVIOR of the static factors  concerning  noncompressible  fluids  when taken by themselves. .  Bernoulli’s  principle—This  principle  was discovered  by  Jacques  (or  Jakob)  Bernoulli,  a Swiss philosopher and mathematician who lived from  1654  to  1705  A.D.  He  worked  extensively with  hydraulics  and  the  pressure-temperature relationship.   Bernoulli’s   principle   governs   the RELATIONSHIP   of   the   static   and   dynamic factors  concerning  noncompressible  fluids.  Figure 2-13  shows  the  effect  of  Bernoulli’s  principle. Chamber A is under pressure and is connected by a  tube  to  chamber  B,  also  under  pressure. Chamber  A  is  under  static  pressure  of  100 psi.  The  pressure  at  some  point,  X,  along  the connecting tube consists of a velocity pressure of 10 psi. This is exerted in a direction parallel to the   line   of   flow,   Added   is   the   unused   static pressure of 90 psi, which obeys Pascal’s law and operates  equally  in  all  directions.  As  the  fluid enters  chamber  B  from  the  constricted  space,  it slows  down.  In  so  doing,  its  velocity  head  is changed  back  to  pressure  head.  The  force  required to   absorb   the   fluid’s   inertia   equals   the   force required  to  start  the  fluid  moving  originally. Therefore,  the  static  pressure  in  chamber  B  is again  equal  to  that  in  chamber  A.  It  was  lower at  intermediate  point  X. Figure 2-13 disregards friction, and it is not encountered in actual practice. Force or head is also  required  to  overcome  friction.  But,  unlike inertia  effect,  this  force  cannot  be  recovered  again although  the  energy  represented  still  exists somewhere as heat. Therefore, in an actual system the pressure in chamber B would be less than in chamber A. This is a result of the pressure used in  overcoming  friction  along  the  way. At all points in a system, the static pressure is always the original static pressure LESS any velocity head at the point in question. It is also Figure 2-13.—Relationship of static and dynamic factors— Bernoulli’s  principle. 2-17