538  Royal  Society  : — 
d#    "by   5* 
where  ^j>    sri*    ^  are  the  components   of  the  velocity  of  the 
part  of  the  sheet  to  which  the  electrode  is  applied. 
In  the  case  of  a  sheet  revolving  with  velocity  uj  about  the  axis  of 
z,  this  becomes 
Note  2. — The  velocity  R  for  a  copper  plate  of  best  quality  1  mil- 
limetre in  thickness  is  about  25  metres  per  second.  Hence  it  is 
only  for  very  small  velocities  of  the  apparatus  that  we  can  obtain 
any  approximation  to  the  true  result  by  neglecting  the  mutual  in- 
duction of  the  currents. — Feb.  13. 
Feb.  22. — William  Spottiswoode,  M.A.,  Treasurer  and  Vice-Presi- 
dent, in  the  Chair. 
The  following  communication  was  read  : — 
"On  a  New  Hygrometer."     By  Wildman  Whitehouse,  Esq. 
The  use  of  Mason's  wet-bulb  thermometer  as  a  means  of  hy- 
grometric  measurement,  though  admitted  to  be  the  most  prac- 
tically useful,  and  indeed  the  only  recording  instrument  for  the  pur- 
pose, has  yet  this  serious  inconvenience,  not  to  say  defect,  viz.  that 
its  indications  either  cease  or  are  valueless  at  temperatures  below32°F. 
In  a  conversation  which  the  writer  had  with  the  Director  of  the 
Meteorological  Office  some  months  ago,  the  question  arose  whether 
any  thing  could  be  suggested  to  remedy  this  inconvenience. 
It  was  obviously  inadmissible  to  substitute  any  other  fluid  for, 
or  to  make  any  addition  to,  the  water  employed  for  the  wet  bulb,  as 
then  it  would  cease  to  be  a  test  for  the  purely  hygrometric  capacity 
of  the  air.  It  became  therefore  necessary  to  fall  back  in  another 
direction,  and  to  find  some  hygrometric  body  which  should  readily 
and  rapidly  absorb  moisture  from  the  air,  and  at  the  same  time 
afford  some  means  of  measuring  and  recording  the  amount  of  such 
absorption. 
Fused  chloride  of  zinc  or  of  calcium  seemed  promising,  as  very 
active  agents,  absorbing  rapidly  on  their  surface,  and  allowing  the 
readiest  possible  escape  of  the  fluid  hydrate  for  measurement ;  yet  no 
means  presented  itself  either  of  accurately  measuring,  regulating,  or 
maintaining  the  exact  extent  of  surface  exposed  for  absorption  ;  nor 
could  the  substance  itself  be  easily  renewed  when  required ;  nor,  in- 
deed, could  either  of  these  substances  be  regarded  as  wholly  free  from 
the  interference  of  frost,  as  the  moisture  absorbed  from  the  atmo- 
sphere at  a  temperature  much  below  freezing-point  may  remain  frozen 
on  the  surface,  and  become  incapable  of  continuous  measurement. 
It  seemed  essential  to  the  accuracy  and  practical  utility  of  any  in- 
strument designed  for  this  purpose  : — 
1st.  That  a  fixed  and  invariable  extent  of  surface  should  at  all 
times  be  exposed  for  absorption  of  moisture  ; 
