8.  The  Heat  Unit.  —  Heat  is  measured  by  its  capacity  to
raise  the  temperature  of  a  known  weight  of  water.  The  unit  of
measurement  is  termed  a  calorie  in  the  metric  system,  and  a
British  thermal  unit  (B.  T.  U.)  in  the  English  system.  A  calorie  is
commonly  defined  as  the  heat  required  to  raise  one  kilogram  of
water  from  the  freezing-point  to  one  degree  Centigrade,  and  a
British  thermal  unit  (B.  T.  U)  that  required  to  raise  one  pound  of
water  from  32  to  33  degrees  Fahrenheit.  Because  of  the  variar
tion  in  the  specific  heat  of  water  near  the  freezing-point  Professor
Peabody  in  his  Tables  of  Saturated  Steam  defines  the  thermal
unit  as  that  required  to  raise  the  temperature  from  62  to  63  degrees
Fahrenheit,  or  from  about  15  to  16  degrees  Centigrade.
The  specific  heat  of  water  changes  slightly  for  different  tem-
peratures as  already  noted,  but  it  can  be  considered  as  constant
without  sensible  error  for  all  ordinary  purposes  in  the  measure-
ment of  heat  when  the  temperature  is  maintained  between  the
freezing  and  boiling  points.
It  is  noted  from  the  above  statement  that  heat  is  measured
not  by  the  temperature  alone  but  by  its  ability  to  heat  a  mass  of
water  from  one  temperature  to  another;  the  total  heat  expressed


INTRODUCTION,  DEFINITIONS,  ETC.  15

in  thermal  units  being  equal  to  the  product  of  the  weight  of  water
by  the  change  of  temperature.  Thus  if  20  pounds  of  water  be
heated  25  degrees  Fahrenheit,  the  heat  required  is  the  product
of  20  times  25  =  500  B.  T.  U.
