89.  Kinds  of  Barometers. — The  barometer  in  its  simplest
form  consists  of  a  long  inverted  vacuum  tube,  sealed  at  the
upper  end.     The  lower  end  dips  into  a  cup  of  mercury.     A
graduated  scale  on  the  side  of  the  tube  measures  the  rise
and  fall  of  the  mercury.     Such  an  instrument  is  often  used
to  determine  the  height  of  mountains  and  other  high  places,


90


APPLIED  SCIENCE


The  air  becomes  thinner  or  rarified,  the  higher  one  goes  and
the  pressure  becomes  less  and  less  on  the  mercury  in  the  open
cup,  so  that  the  mercury  in  the  long  tube  is  made  to  fall.
If  the  distance  the  mercury  will  fall  for  every  100  ft.  of  alti-
tude is  known,  the  height  of  a  mountain  may  easily  be
ascertained  by  noticing  the  height  of  the  mercury  column
first  at  the  bottom  of  the  mountain  and  then  at  the  top.
90.     Aneroid  Barometer. — The  barometer  most  commonly
made   for   commercial   purposes   is   the   aneroid  barometer

(Fig.  54).  The  word  " aneroid"
comes  from  the  Greek  and  means
"not  wet,"  and  was  selected
because  this  type  of  barometer
operates  without  any  fluids.  It
consists  of  a  round,  metallic,  air-
tight vacuum  case,  somewhat  like
a  watch,  the  lid  of  which,  held  by
metallic  springs  inside,  rises  and
falls  with  the  pressure  of  the  at-
mosphere. By  means  of  levers
and  a  delicate  chain  inside,  this
rise  and  fall  is  made  to  turn  the
pointers  on  the  index.  The  deflection  may  then  be  read
on  the  circular  scale.
FIG.  54. — Aneroid  Barometer.
