till  the  steam  has  become  water  at  temperature  due  to  pressure  of
5  Ibs.,  namely,  162°  Fahr.  The  temperature  of  the  1  Ib.  of  water
falls  still  further  to  32°  Fahr.,  and  the  heat  thus  given  up  by  the
water  is  represented  by  the  area  abed.  The  total  heat  rejected  in
this  case  is  given  by  the  whole  area  below  the  cross-lined  portion  of
the  figure,  namely,  the  area  abcefg.
Case  IV.  Showing  the  work  done  by  steam  at  various  initial  pressures
without  expansion  (Fig.  34).
For  steam  of  50  Ibs.  pressure,  admitted  through  the  whole  length
of  stroke  and  exhausted  against  atmospheric  pressure,  the  work  done


Cut- ft.
FIG.  34.
per  pound  is  shown  by  the  shaded  area  A.  The  value  of  this  area  in
heat-units  may  be  obtained  by  direct  measurement  with  the  plani-
ineter,  or  it  may  be  obtained  from  the  steam  Tables,  thus :  the
external  latent  heat  of  steam  at  50  Ibs.  pressure  is  77'7  ;  the  external
latent  heat  of  (ac  ~  ag)  Ib.  of  steam  at  212°  =  area  tac  =  72'3  X  84
-f-26  =  23-4,  72-3  being  the  external  latent  heat  of  steam  at  212°
Fahr.,  fc  the  constant-volume  line  for  8-4  cub.  ft.,  and  26  the  volume
in  cubic  feet  of  1  Ib.  of  steam  at  212°.
Then  the  shaded  area  A  =  77 '7  -  23 -4  =  54-3  heat  units,  and  the
efficiency  of  the  steam  =  54-3  4-  total  heat  of  steam  at  50  =  54 -3  ~
1167'6  =  4-65  per  cent.,  reckoning  feed-water  at  32°.  The  remainder
passes  away  to  exhaust;  namely,  1167'6  —  54-3  =  1113-3  heat-units.
