152  Royal  Society  : — 
Academy  of  Sciences  of  Vienna  a  paper  entitled  "  On  the  depend- 
ence of  the  Earth's  Magnetism  on  the  Rotation  of  the  Sun." 
He  shows  that  the  changes  of  each  of  the  three  elements  of  the 
force  of  terrestrial  magnetism  (decimation,  inclination,  and  horizontal 
force)  indicate  a  period  of  26^  days.  The  periodic  change  of  declina- 
tion for  Prague  (1870)  amounts  to  0705  sin  (.z+1900  20'),  where 
(r=0°at  the  commencement  of  18/0,  and  #=360°  at  the  commence- 
ment of  1871.  For  Vienna  the  range  is  a  little  larger.  The  range 
of  inclination  is  nearly  one-third  of  that  of  declination,  that  of  the 
intensity  nearly  24  units  of  the  4th  decimal  (the  intensity  in  Juue 
1870  was  nearly  2-0485). 
Professor  Hornstein  regards  these  changes  of  the  earth's  magne- 
tism as  the  effect  of  the  sun's  rotation,  and  by  a  mean  of  several 
determinations  finds  for  the  duration  of  the  period  26*33  days.  This 
number  may  consequently  be  regarded  as  the  result  of  the  first  at- 
tempt to  determine  the  synodic  period  of  the  sun's  rotation  by 
means  of  the  magnetic  needle.  The  resulting  true  periodic  time 
of  the  sun's  rotation  is  24*55  days,  almost  exactly  agreeing  with 
the  time  of  rotation  of  the  sun-spots  in  the  sun's  equator  deduced 
from  astronomical  observations  (according  to  Sporer  24*541  days). 
"  Corrections  to  the  Computed  Lengths  of  Waves  of  Light  pub- 
lished in  the  Philosophical  Transactions  of  the  year  1868."  By 
George  Biddell  Airy,  C.B.,  Astronomer  Royal. 
The  author,  after  adverting  to  the  process  by  which  in  a  former 
paper  he  had  attempted  the  computation  of  the  Lengths  of  Waves  of 
Light,  for  the  entire  series  measured  in  the  Solar  Spectrum  by  Kirch- 
hoff,  from  a  limited  number  of  measured  wave-lengths,  and  to  the 
discordances  between  the  results  of  these  computations  and  the  ac- 
tual measure  of  numerous  wave-lengths  to  which  he  subsequently 
had  access,  calls  attention  to  his  remark  that  means  existed  for  giving 
accuracy  to  the  whole.  The  object  of  the  present  paper  is  so  to  use 
these  means  as  to  produce  a  table  of  corrections  applicable  througli 
the  entire  range  of  KirchhofFs  lines,  and  actually  to  apply  the  cor- 
rections to  those  computed  wave-lengths  which  relate  to  spectral 
lines  produced  by  the  atmosphere  and  by  many  metals. 
o 
Adopting  as  foundation  the  comparisons  with  Angstrom's  and 
Ditscheiner's  measures  given  in  the  former  paper,  and  laying  these 
down  graphically,  the  author  remarks  that  in  some  parts  of  the  spec- 
trum the  agreement  of  the  two  experimenters  is  very  close,  that  in 
some  parts  they  are  irreconcilable,  and  that  in  one  part  (where  they 
agree)  there  is  a  peculiarity  which  leads  to  the  supposition  that  some 
important  change  was  made  in  KirchhofFs  adjustments.  He  then 
explains  the  considerations  on  which  he  has  drawn  a  correction-curve, 
whose  ordinates  are  to  give  the  corrections  applicable  to  his  former 
computed  numbers.  A  general  table  of  corrections  is  then  given, 
and  this  is  followed  by  tables  of  the  Lengths  of  the  Light- Waves  for 
the  air  and  metals  as  corrected  by  the  quantities  deduced  from  that 
general  table. 
The  author  remarks  that  he  has  not  yet  succeeded  in  discovering 
any  relation  among  the  wave-lengths  for  the  various  lines  given  by 
