MATTER,  FORCE,  AND  MOTION.  17

The  mass  of  one  cubic  decimeter  (1000  cc.)  of  water  constitutes  the  kilo-
gram  (i  kg.).  The  standard  kilogram  was  prepared  by  the  Inter-
national Metric  Committee  and  deposited  with  the  U.  S.  Geodetic  Sur-
vey in  Washington.
The  commercial  unit  is  the  pound  (avoirdupois) ,  which  contains  7000
grains.  The  standard  is  kept  in  the  Exchequer  office  in  London.  The
Troy  pound  contains  5760  grains.
Tables  of  the  metric  system  and  of  English  weights  and  measures  will
be  found  in  the  Appendix.
5.  Motion  and  Force.
If  a  body  retains  its  position  in  space  constant  through  a  period  of
time,  it  is  in  a  state  of  rest;  if  it  is  changing,  the  body  is  said  to  be  in
motion.  We  must  remember  that  these  statements  are  not  to  be  taken
as  absolute.  The  body  is  at  rest  with  reference  to  its  original  position,
or  in  motion  with  reference  to  the  same,  while  both  the  body  and  the
original  fixed  point  may  be  in  motion  with  reference  to  some  other
distant  point.    Motion  and  rest  are  therefore  purely  relative  terms.
