Mr.  W.  Whitehouse  on  a  New  Hygrometer.  539 
2nd.  That  the  apparatus  should  be  simple,  inexpensive,  and  not 
inconvenient  in  use  ; 
3rd.  That  the  hygrometric  substance  should  be  continuously  and 
steadily  reuewable  ;  and  above  all,  if  it  were  possible, 
4th.  That  the  measurement  should  be  effected  thermometrically. 
No  solid  hygrometric  substance  seemed  capable  of  meeting  these 
requirements  ;  but  all  the  conditions  seemed  likely  to  be  fulfilled  by 
the  use  of  concentrated  sulphuric  acid.  This  would  admit  of  being 
spread  in  an  exquisitely  fine  film  over  the  surface  of  the  bulb  of  a 
thermometer  by  means  of  a  glass  capillary  siphon,  of  which  one  end 
should  rest  on  the  upper  part  of  the  bulb,  while  the  other  end  dipped 
into  a  reservoir  of  the  acid.  A  continuous  supply  could  be  maintained 
for  any  required  length  of  time  by  suitable  arrangements.  The  ab- 
sorption of  moisture  would  necessarily  be  attended  by  a  rise  in  tem- 
perature, and  this  would  be  proportioned  to  the  amount  of  hygro- 
metric moisture  absorbed ;  while  the  hydrated  acid,  having  fulfilled 
its  office,  would  fall  in  drops  from  the  bulb  into  any  tube  or  reservoir 
placed  for  the  purpose. 
An  instrument  has  been  constructed  by  the  writer  to  test  this  prin- 
ciple, which  has,  by  the  courtesy  of  the  Director  of  the  Meteoro- 
logical Office,  been  under  observation  for  some  weeks. 
It  consists  essentially  of  three  thermometers  of  similar  construc- 
tion, and  used  as  a  "  wet  bulb,"  a  "  dry  bulb,"  and  an  "  acid  bulb," 
respectively,  placed  side  by  side  on  a  suitable  frame,  and  read  together 
for  comparison. 
The  experience  already  gained  in  the  use  of  this  instrument  has 
shown  that,  with  a  reservoir  of  proper  construction,  the  supply  of  acid 
may  be  made  continuous  for  any  required  length  of  time,  and  that, 
from  the  very  slight  variations  of  flow  which  occur  in  its  action,  the 
supply  to  the  thermometer  will  be  sensibly  equable. 
The  length  of  the  siphon,  and  the  size  of  the  capillary  bore, 
together  with  the  difference  of  level  between  the  surface  of  the  fluid 
in  the  trough  of  the  reservoir  and  the  point  of  delivery  on  the  bulb, 
will  determine  the  rate  of  supply  of  the  acid. 
It  is  clear  that  either  a  too  rapid  and  continuous  stream  of  acid 
at  the  temperature  of  the  air,  or  a  too  scanty  supply,  would  diminish 
the  readings ;  yet  it  is  found  that  practically  there  may  be  a  pretty 
wide  range  of  variation  in  the  supply  of  acid,  within  which  no  essen- 
tial change  in  the  sensibility  of  the  instrument  is  noticed. 
For  a  bulb  having  one  square  inch  of  surface  one  drop  per  mi- 
nute is  sufficient,  though  the  time  may  range  from  40  to  100  seconds 
without  inconvenience,  the  time  being  noted  as  the  hydrated  acid, 
after  having  fulfilled  its  office,  falls  drop  by  drop  from  the  bulb. 
The  quantity  of  acid  required  at  this  rate  is  about  3  fluid  ozs.  per 
diem,  or  one  imperial  pint  per  week,  which  is  procurable  of  uniform 
density,  sufficiently  pure  and  free  from  lead,  at  a  cost  of  about  2hd. 
The  temperature  of  the  acid  in  the  reservoir  is  of  course  that  of 
the  surrounding  air ;  the  elevation  of  temperature  shown  by  the 
acid-bulb  thermometer  is  due  to,  and  seems  to  be  strictly  a  measure 
of,  the  amount  of  moisture  absorbed  by  the  film  of  acid  spread  on 
