510  Royal  Society  : — 
the  surface  of  the  bulb,  say  one  square  inch,  continuously  supplied  iu 
its  concentrated  state,  and  as  constantly  passing  off  hydrated. 
While,  therefore,  this  instrument  is,  like  Mason's,  intended  to 
measure  the  amount  of  hygrometric  moisture  in  the  air,  and  to  do 
so  thermometrically,  it  yet  is,  in  its  principle  and  in  its  operation, 
essentially  of  an  opposite  character. 
The  ordinary  wet-bulb  thermometer  is  at  the  zero  of  its  scale  in 
an  atmosphere  of  perfect  saturation,  and  its  action  depends  upon  the 
amount  of  sensible  heat  absorbed  and  rendered  latent  by  evaporation 
of  the  water  from  its  surface. 
The  acid-bulb  thermometer  is  at  its  zero  in  a  perfectly  dry  atmo- 
sphere ;  and  its  action  depends  upon  the  amount  of  latent  heat  ren- 
dered sensible  by  the  condensation  of  vapour  into  water  on  the  sur- 
face of  the  bulb,  and  by  the  combination  of  this  water  with  the  con- 
centrated acid. 
It  would  appear  that  an  hygrometer  on  this  principle  is  entirely 
free  from  the  action  of  frost ;  while  its  sensibility  is  so  great  as  to 
be  at  first  almost  embarrassing. 
This  may,  however,  be  easily  regulated  and  toned  down,  if  neces- 
sary, to  any  required  range  by  the  dilution  of  the  acid  with  glycerine, 
a  fluid  which  is  also  of  itself  hygrometric,  though  its  thermal  effects 
are  far  less  marked  than  those  of  sulphuric  acid. 
The  following  series  of  observations,  made  hourly  and  otherwise, 
at  intervals  during  the  past  few  weeks,  at  the  Meteorological  Office, 
by  the  kindness  of  the  Director,  will  suffice  to  show  approximately 
the  relations  of  the  "  acid  "  and  the  "  wet  bulb  "  respectively. 
They  have  been  chiefly  actual  out-door  observations,  and  have  ex- 
tended over  a  considerable  range  of  temperature  and  atmospheric 
variations. 
It  will  require  a  most  careful  series  of  observations  to  elicit  all  the 
points  noteworthy  in  the  new  instrument,  and  to  determine  the  rela- 
tive values  of  the  wet-  and  acid-bulb  readings,  noting  the  behaviour 
of  each  at  every  part  of  the  scale,  from  absolute  dryness  to  satu- 
ration, and  at  temperatures  ranging  from  75°  or  80°  down  to  0°. 
This  will  be  necessary  before  the  instrument  can  aspire  to  take  its 
place  among  the  recognized  standards  of  meteorological  science  ; 
but  in  the  mean  time  the  writer  has  been  advised  to  offer,  at  the 
earliest  time,  a  brief  description  of  it  to  the  notice  of  the  Royal 
Society. 
