Mr.  S.  Sharpe  on  the  Moon  seen  by  the  naked  Eye.       427 
orbital  motion  and  towards  the  tangential  direction,  analogous 
in  some  degree  to  the  shifting  of  the  earth's  axis  by  precession 
and  nutation.  Such  displacements  of  the  axis  of  the  setherial 
movements  would  cause  it  to  take  positions  relative  to  the  earth 
continually  more  westerly. 
47.  I  take  this  opportunity  for  rectifying  an  opinion  respect- 
ing earth-currents  which  I  have  expressed  in  a  note  in  p.  653  of 
my  work  on  the  '  Principles  of  Physics.'  Since  the  note  was 
written  I  have  learnt  from  the  Greenwich  Observations  that 
magnets  and  the  galvanometers  employed  as  indicators  of  earth- 
currents  are  simultaneously  affected  only  by  large  magnetic  dis- 
turbances, and  that  generally  no  correspondences  exist  between 
small  magnetic  variations  and  the  indications  of  the  galvanome- 
ters. It  does  not  appear,  therefore,  that  earth-currents  and 
terrestrial  magnetism  have  any  special  relation  to  each  other, 
the  effect  of  the  large  magnetic  disturbances  on  the  earth-current 
being  only  an  instance  of  the  ordinary  mutual  action  between  a 
galvanic  current  and  a  magnetic  current. 
Having  completed  this  review  of  the  hydrodynamical  theory 
of  magnetism,  I  think  I  may  say  that  it  is  now  supported  by 
arguments  which  should  command  the  attention  of  physicists. 
Considering  the  number  and  variety  of  the  explanations  of  phe- 
nomena it  gives,  which  in  fact  might  have  been  much  ex- 
tended, I  do  not  see  how  the  inference  that  the  facts  of  mag- 
netism are  referable  to  hydrodynamical  laws  can  be  resisted. 
Cambridge,  April  19,  1872. 
LI  I.   On  the  Moon  seen  by  the  naked  Eye. 
By  Samuel  Sharpe,  Esq. 
To  the  Editors  of  the  Philosophical  Magazine  and  Journal. 
Gentlemen, 
I  SHOULD  like  to  be  allowed  to  lay  the  following  conjectures 
before  your  readers.  The  opinions  may  not  be  true,  and 
may  not  be  new  ;  but  if  new  they  seem  worth  consideration. 
1st.  As  to  the  moon's  atmosphere. 
On  looking  at  the  new  moon,  when  about  two  days  old,  with 
the  naked  eye  on  a  clear  evening,  we  see,  first,  the  thin  crescent, 
being  that  portion  of  the  moon  which  enjoys  sunshine;  then  the 
larger  portion,  nearly  circular,  dimly  lightened  up  by  earthshine. 
But  the  edge  of  the  moon  which  is  furthest  from  the  bright 
crescent  is  also  slightly  illuminated.  Now  this  portion  of  the 
moon  ^eems  to  be  in  the  twilight,  illuminated  by  the  sun's  rays 
reflected  from  its  own  possible  atmosphere.  Though  astronomers 
have  not  detected  such  an  atmosphere  by  an  effect  on  a  star  when 
