THIS  table,  which  has  been  published  in  the  Annuaire  Meteor  ologique  de  Francs
for  1850,  page  86,  and  following,  has  been  calculated  by  Mr.  Haeghens,  using
Regnault's  Tables  of  Elastic  Forces  of  Vapor.  It  gives  directly  the  relative  humidity,
when  the  hygrometrical  observations  have  been  made  by  means  of  dew  point  instru-
ments like  those  of  Daniell,  Regnault,  Bache,  and  others.
These  hygrometers  are  destined  to  find  out  the  temperature  of  the  dew  point,  that
is  the  temperature  to  which  it  would  be  necessary  to  lower  the  temperature  of  the
air,  in  order  that  this  air  be  completely  saturated  by  the  aqueous  vapor  which  it  con-
tained at  the  time  of  the  observation.
The  force  of  vapor  contained  in  the  air,  or  its  absolute  humidity,  is  thus  the  maxi-
mum of  force  of  vapor  which  corresponds  to  the  temperature  of  the  dew  point ;  it  is
given  directly  in  the  Table  I.  of  the  Elastic  Forces  of  Vapor,  by  Regnault.
The  ratio  of  that  maximum  of  force  of  vapor  at  the  temperature  of  the  dew  point
to  the  force  of  vapor  which  corresponds,  in  the  same  table,  to  the  temperature  of  the
surrounding  air  at  the  time  of  the  observation,  is  the  relative  humidity.  This  ratio  is
given  in  hundredths  in  the  following  table,  which  relieves  the  observer  of  the  trouble
of  calculating  it.
Let  t  —  temperature  of  the  air  surrounding  the  instrument.
t'  =  temperature  of  the  dew  point.
t  —  t1  =  the  difference  between  these  two  temperatures.
The  first  column,  on  the  left,  contains  the  temperature  of  the  air  t,  in  centigrade
degrees.  The  following  ones,  headed  with  the  differences,  t  —  t1,  between  the
temperatures  of  the  air  and  of  the  dew  point,  give  the  relative  humidity  correspond-
ing to  the  two  elements.
