that  one  is  primary,  another  derivative.  Fifthly,  that
one  is  uniform,  another  not.  Sixthly,  that  one  is  more
durable,  another  less  durable.  Of  all  which  distinc
tions  the  members  may  be  sub-distinguished  into  parts
distinguishable  almost  infinitely.  For  the  variety  of
sounds  seems  to  be  not  much  less  than  that  of  colours.
As  vision,  so  hearing  is  generated  by  the  motion  of
the  medium,  but  not  in  the  same  manner.  For  sight  is
from  pressure,  that  is,  from  an  endeavour;  in  which
there  is  no  perceptible  progression  of  any  of  the  parts
of  the  medium ;  but  one  part  urging  or  thrusting  on
another  propagateth  that  action  successively  to  any
distance  whatsoever;  whereas  the  motion  of  the  me
dium,  by  which  sound  is  made,  is  a  stroke.  For  when
we  hear,  the  drum  of  the  ear,  which  is  the  first  organ
of  hearing,  is  stricken ;  and  the  drum  being  stricken,
the  pia  mater  is  also  shaken,  and  with  it  the  arteries
which  are  inserted  into  it ;  by  which  the  action  being
propagated  to  the  heart  itself,  by  the  reaction  of  the
heart  a  phantasm  is  made  which  we  call  sound ;  and
because  the  reaction  tendeth  outwards,  we  think  it  is
without.
