MECHANICS  AND   HYDROSTATICS.  99

The  experimental  truth  of  this  principle  is  a
matter  of  obvious  and  universal  experience.  The
weight  of  a  basket  of  stones  is  not  altered  by  shak-
ing the  stones  into  new  positions.  We  cannot
make  the  direct  burden  of  a  stone  less  by  altering
its  position  in  our  hands ;  and  if  we  try  the  effect
on  a  balance  or  a  machine  of  any  kind,  we  shall
see  still  more  clearly  and  exactly  that  the  altered
position  of  one  weight,  or  the  altered  arrangement
of  several,  produces  no  change  in  their  effect,  so
long  as  their  point  of  support  remains  unchanged.
This  general  fact  is  obvious,  when  we  possess  in
our  minds  the  ideas  which  are  requisite  to  appre-
hend it  clearly.  But  when  we  are  so  prepared,  the
truth  appears  to  be  manifest,  even  independent  of
experience,  and  is  seen  to  be  a  rule  to  which  expe-
rience must  conform.  What  then  is  the  leading  Idea
which  thus  enables  us  to  reason  effectively  upon
mechanical  subjects?  By  attention  to  the  course
of  such  reasonings,  we  perceive  that  it  is  the  Idea
of  Pressure ;  Pressure  being  conceived  as  a  mea-
surable effect  of  heavy  bodies  at  rest,  distinguish-
able from  all  other  effects,  such  as  motion,  change
of  figure,  and  the  like.  It  is  not  here  necessary  to
attempt  to  trace  the  history  of  this  Idea  in  our
minds ;  but  it  is  certain  that  such  an  Idea  may  be
distinctly  formed,  and  that  upon  it  the  whole
science  of  statics  may  be  built.  Pressure,  load,
weight,  are  names  by  which  this  Idea  is  denoted
when  the  effect  tends  directly  downwards;  but  we

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100    PHYSICAL  SCIENCES  IN  ANCIENT  GREECE.
