The  number  772*692,  obtained  as  the  mean  of  forty  experiments
on  the  friction  of  water,  Joule  considered  the  most  trustworthy ;
but  this  he  reduced  to  772,  Because,  even  in  the  friction  of  liquids,
he  found  it  impossible  entirely  to  avoid  vibration  and  sound.  The
deductions  of  Joule  from  these  experiments  are  :

1.  That  the  quantity  of  heat  produced  by  the  friction  of  bodies,
whether  solid  or  liquid,  is  always  proportional  to  the  force  ex-
pended.
-.  '!"•"'.  '  'intity  of  heat  capable  of  increasing  the  temperature
of  one  pound  of  water  (weighed  in  vacuo,  and  between  55°  and  60°)
by  1°  Fah.,  requires  for  its  evolution  the  expenditure  of  a  mechanical
force  represented  by  the  fall  of  1  pound  through  772  feet,  or  772  foot-
pounds.
This  is  the  "  Mechanical  Equivalent  of  Heat,"  or  the  unit  of  heat,
generally  called  in  honor  of  the  illustrious  physicist,  "Joule's
Unit."

In  French  measures,  the  above  heat-unit  is  thus  stated:  The  heat

capable  of  increasing  the  temperature  of  1  kilogramme  of  water  1°  C.,

is  equivalent  to  a  force  represented  by  the  fall  of  423'55  kilogrammes,

<jh  the  space  of  1  metre.     The  descent  or  ascent  of  1  pound

through  1  foot  is  called  afoot-pound,  while  the  descent  or  ascent  of

1  kilogramme  through  1  metre  is  denominated  a  kilogramme-metre.
he  adoption  of  these  terms,  the  expressions  of  the  above  truths

can  be  mere  concisely  enunciated;  thus,  using  French  meU<

we  say,  423  kilogramme-metres  is  equivalent  to  1  kilogramme-degree

centigrade.
us,  Joule  showed  that  the  heat  developed  was  in  proportion  to
Miass  X  the  distance  fallen  through ;  or,  what  is  the  same,  equiva-

10  mass  X  square  of  the  velocity.
