Yet  although  we  thus  know  nothing  of  matter,  space,  time  and
force  in  themselves,  we  can  and  do  investigate  them  in  their  measur-
able relations,  and  such  investigation  is  the  object  of  all  physical
science.
Mechanics — Kinematics  and  Dynamics — Statics  and  Kinetics. —
That  branch  of  physical  science  which  treats  of  the  measurable
relations  of  space  alone  is  called  geometry.
That  which  deals  with  the  measurable  relations  of  space  and
time  only,  that  is  with  pure  motion,  is  called  kinematics  {Kivrffi  >,
motion).     To  the  ideas  of  geometry  it  adds  the  idea  of  motion.
That  which  deals  with  the  measurable  relations  of  space,  time
and  matter,  involved  in  the  study  of  the  motion  of  material  bodies
under  the  action  of  force,  is  called  dynamics  {8vvai.ni^  force).  To
the  ideas  of  kinematics  it  adds  the  idea  of  force.
We  divide  dynamics  into  two  parts  :  statics,  which  treats  of
material  bodies  at  rest  under  the  action  of  force,  and  kinetics,
which  treats  of  material  bodies  in  motion  under  the  action  of
force.  Statics  is  thus  a  special  case  of  dynamics  which  it  is  con-
venient to  consider  separately.
In  the  study  of  machines,  or  of  moving  bodies  generally,  under
the  action  of  force,  we  have  to  make  use  both  of  the  principles  of


2  INTRODUCTION".  [CHAP.    I.
kinematics  and  of  dynamics.  The  term  mechanics  is  therefore  used
to  include  the  general  principles  of  both  kinematics  and  dynamics,
while  their  special  application  to  machines  is  called  applied  me-
chanics or  mechanism.  We  treat  in  this  work  of  mechanics  as  thus
defined,  or  the  general  principles  of  kinematics  and  dynamics.
We  have  then  as  the  scheme  of  the  present  work  :

C  Vol.      I,  Kinematics ;
Mechanics:  -s  Vol.    II,  Statics;      / -n^^ ««„;«»
(Vol.  m;  Kinetics;  [  I>y^amics.
