HEAD,  VELOCITY—The  equivalent  head through  which  the  liquid  would  have  to  fall  to attain a given velocity. Mathematically it is equal to the square of the velocity (in feet) divided by 64.4  feet  per  second  square. HEAT   EXCHANGER—A   device   that transfers  heat  through  a  conducting  wall  from  one fluid  to  another. HYDRAULICS—Engineering science pertain- ing  to  liquid  pressure  and  flow. HYDROMETER—An   instrument   for   deter- mining  the  specific  gravities  of  liquids. HYDROPNEUMATICS—Pertaining   to   the combination  of  hydraulic  and  pneumatic  fluid power. HYDROSTATICS—Engineering   science pertaining  to  the  energy  of  liquids  at  rest. IMPACT   PRESSURE—The   pressure   of   a moving fluid brought to rest that is in excess of the pressure the fluid has when it does not flow; that is, total pressure less static pressure. Impact pressure is equal to dynamic pressure in incom- pressible  flow;  but  in  compressible  flow,  impact pressure includes the pressure change owing to the compressibility   effect. IMPINGEMENT—The   striking   or   dashing upon  with  a  clash  or  sharp  collision,  as  air impinging upon the rotor of a turbine or motor. IMPULSE  TURBINE—A  turbine  driven  by a   fluid   at   high   velocity   under   relatively   low pressure. INERTIA—The tendency of a body at rest to remain at rest, and a body in motion to continue to move at a constant speed along a straight line, unless the body is acted upon in either case by an unbalanced   force. INHIBITOR—Any  substance  which  slows  or prevents chemical reactions such as corrosion or oxidation. INVERSE PROPORTION—The relation that exists  between  two  quantities  when  an  increase  in one  of  them  produces  a  corresponding  decrease in the other. KELVIN   SCALE—The   temperature   scale using  absolute  zero  as  the  zero  point  and  divisions that are the same size as centigrade degrees. KINETIC  ENERGY—The  energy  that  a substance  has  while  it  is  in  motion. KINETIC THEORY—A theory of matter that assumes  that  the  molecules  of  matter  are  in constant   motion. LINE—A  tube,  pipe,  or  hose  that  is  used  as a  conductor  of  fluid. LIQUID—A   form   of   matter   that   has   a definite   volume   but   takes   the   shape   of   its container. LOAD—The power that is being delivered by any power-producing device. The equipment that uses the power from the power-producing device. LUBRICATOR—A   device   that   adds controlled  or  metered  amounts  of  lubricant  into a  fluid  power  system. MANIFOLD—A  type  of  fluid  conductor  that provides  multiple  connections  ports. MANOMETER—A   differential   pressure gauge in which pressure is indicated by the height of a liquid column of known density. Pressure is equal to the difference in vertical height between two connected columns multiplied by the density of   the   manometer   liquid.   Some   forms   of manometers are U tube, inclined tube, well, and bell types. MATTER—Any   substance   that   occupies space and has weight. MECHANICAL   ADVANTAGE—The   ratio of  the  resisting  weight  to  the  acting  force.  The ratio  of  the  distance  through  which  the  force  is exerted  divided  by  the  distance  the  weight  is raised. METER-IN—To regulate the amount of fluid into  a  system  or  an  actuator. METER-OUT—To  regulate  the  flow  of  fluid from  a  system  or  actuator. MICRON—A  millionth  of  a  meter  or  about 0.00004  inch. AI-5