510 
THE  ADMIRALTY  ASTRONOmCAL  EXPERBIEXT 
never  lost  sight  of  through  the  whole  extent  of  angular  motion  possible  to  the  eyepiece 
and  its  carrying  bar,  about  130°;  and  the  results  therefore,  for  that  region,  must  be 
negative.  But  for  greater  solar  distances,  reckoning  from  the  equator  of  a sphere 
having  the  sun  in  one  of  its  poles,  or  in  terms  of  “ sky  declination,”  we  have  the  follow- 
ing results  at  the  three  stations  noted. 
Polarimeter. 
Guajara. 
Date. 
No. 
of 
obs. 
Sky 
deck 
Inverse  in- 
tensity of 
sky  polariz". 
1856, 
1 
0 
1 
52-1 
Aug.  14. 
1 
4 
41-0 
1 
6 
42-0 
1 
9 
41-5 
Exceptional  day,  abnormal  local 
cloud  suspended  over  station. 
Aug.  l6. 
10 
3-8 
28-4 
10 
10-1 
25*6 
10 
13*3 
27-7 
10 
18-1 
32-2 
10 
26*1 
36-2 
10 
33-8 
43-0 
10 
40-5 
49*7 
10 
46-7 
54-2 
5 
56-8 
63-9 
Alta  Vista. 
No. 
Sky 
deck 
Inverse  in- 
Date. 
of 
tensity  of 
obs. 
sky  polariz". 
1856. 
10 
O 
1-2 
23-6 
Aug.  23 
10 
3-7 
24-6 
to 
10 
5-7 
26-4 
Sept.  11. 
10 
7-7 
25’6 
10 
10-2 
26-8 
10 
13'8 
28-0 
10 
16-0 
27-6 
10 
19-9 
30-7 
10 
23*5 
32*2 
10 
26-2 
33-3 
10 
28-5 
35-7 
10 
32-8 
40-6 
10 
36-3 
43-0 
10 
40-1 
46-0 
10 
45-0 
50-2 
10 
49-5 
56-4 
10 
53-8 
59-1 
10 
59*6 
63-2 
Edinburgh. 
Date. 
Xo, 
of 
obs. 
Sky 
deck 
Inverse  in- 
tensity of 
sky  polariz”. 
1857. 
4 
0 
0-0 
20-8 
Oct.  30. 
6 
5’0 
20-2 
4 
12-5 
22-9  ' 
2 
22-5 
28-0 
Nov.  19. 
12 
0-0 
33-4 
4 
5*0 
34-8 
4 
100 
37-2 
3 
15-0 
38-9 
5 
25-0 
37*8 
Sky  clear  from  clouds,  air  calm. 
but  filled  with  smoke. 
Nov.  20. 
4 
0 
2M 
4 
5 
21-8 
5 
9 
22-3 
2 
15 
23-2  ! 
1 
Nov.  24. 
2 
0 
21*2 
2 
5 
21-5 
2 
10 
22*2 
1 
15 
23-1  > 
] 
Before  comparing  the  above  observations,  we  must  reject  the  Guajara  series  of 
August  14,  as  vitiated  by  an  unusual  local  cloud ; and  the  Edinburgh  series  of  Novem- 
ber 19,  on  account  of  the  evidently  prejudicial  effect  of  smoke  nearly  uniformly  diffused 
in  all  the  upper  air.  To  test  this  point  still  further,  the  following  series  for  intensity  in 
the  sky  equator  was  taken,  everything  remaining  the  same  but  the  altitude  of  the  prin- 
cipal tube;  and  when  this  was  directed  at  10°  to  the  strongly  smoky  band  on  the  hori- 
zon, there  was  evidently  a sudden  decrease  of  the  intensity  of  the  polarization,  shomi 
as  before  by  an  increase  of  the  instrumental  numbers : — 
O O 
Altitude  45,  inverse  intensity  of  sky  polarization  =31’7 
Altitude  20,  inverse  intensity  of  sky  polarization  =32’3 
Altitude  15,  inverse  intensity  of  sky  polarization  =32*9 
Altitude  10,  inverse  intensity  of  sky  polarization  =41'0 
Comparing  then  the  three  normal  Edinburgh  days  with  the  mean  of  the  whole  Alta 
Vista  period,  the  polarization  w^ould  appear  to  be  rather  stronger  below  than  above,  but 
at  either  station  to  follow  the  law  of  maximum  intensity  at  or  near  90°. 
