508 
THE  ADMIEALTT  ASTEOHOMICAL  EXPEEBIEXT 
it  might  be  likened  to  the  effect  that  would  have  taken  place  to  the  eye,  had  the  moun- 
tain been  seen  through  a much  greater  depth  of  atmosphere,  or  had  that  atmosphere 
been  thicker,  as  mth  a diffusion  of  smoke  throughout  it. 
(6.)  Magnetometer. 
For  the  purpose  of  obtaining  observations  of  magnetic  intensity,  a \ibrating  needle 
was  kindly  supplied  by  Mr.  Aiey,  together  with  some  fine  aloe  fibres,  procured  by  him- 
self in  Madeira.  On  opening  the  boxes  at  Guajara  in  the  beginning  of  August,  at 
which  time  most  of  our  instruments  were  suffering  Aom  the  extreme  di’ought,  the  maho- 
gany stand  was  found  to  have  so  shrunk  on  the  thick  plate-glass  cover  of  the  needle 
area,  as  to  fix  it  in  completely.  By  cutting  away  the  wood,  however,  around  the  glass, 
it  was  got  out,  and  on  August  10,  12,  and  13,  observations  were  made  at  that  station, 
8903  feet  high.  On  August  24,  observations  were  made  at  Alta  Vista,  10,702  feet  high, 
and  on  August  27  in  Orotava,  at  a height  of  only  25  feet  above  the  sea-level. 
The  plan  of  observation  pursued,  was  exactly  the  same  at  all  the  stations : the  needle 
was  set  swinging  at  a large  angle,  by  advancing  a knife,  and  was  then  observed  at  ever}' 
tenth  vibration,  through  300  vibrations;  the  mean  angle  of  a series  was  always  near  45"; 
and  a correction  has  been  since  applied  to  reduce  the  readings  exactly  to  that  angle. 
The  times  were  observed  by  a sidereal  chronometer. 
Magnetometer. 
No. 
Date. 
Locality  general. 
Locality  particular. 
Mean  of 
thirty  ob- 
servations 
of  ten  se- 
conds each. 
Mean  for 
each 
station. 
! 1 
2 
1 3 
: 4 
1 ^ 
h m 
Aug.  11,  4 10  p.M. 
Aug.  11,  4 46  p.M. 
Aug.  12,  5 40  A.M. 
Aug.  12,  6 10  A.M. 
Aug.  13,  9 0 A.M. 
Guajara  
Guajara  
f On  a pedestal  of  the  trachyte  stone  j 
< of  the  hill,  and  under  the  shade  > 
f thrown  by  a deal  plank  J 
Ditto  
sec. 
38-02 
38-05 
37-91 
37- 90 
38- 31 
! 
s.  , 
^38-04  ! 
! 
1 
! 
Guajara  
Ditto  
Guajara  
Ditto  
Guajara  
Ditto  
Altitude=  8,903  feet 
; 6 
7 
1 8 
Aug.  24,  10  30  A.M. 
Aug.  24,  11  25  a.m. 
Aug.  24,  3 20  p.M. 
Alta  Vista  
Alta  Vista  
Alta  Vista  
Altitude=  10,702  feet 
fin  shade  of  large  block  of  black "1 
\ lava,  N.E.  of  station  J 
In  shade  of  lava  stream,  to  S.  of  station 
In  empty  room  in  station  
37- 72 
39-05 
38- 58 
>38-45  , 
i 9 
10 
; 11 
Aug.  27,  8 0 A.M. 
Aug.  28,  7 35  A.M. 
Aug.  28,  8 20  A.M. 
Orotava  
Orotava  
Orotava  
Altitude=  25  feet 
Basalt  step  of  Inn  yard 
Basalt  step  of  Inn  yard  
Centre  of  Inn  yard 
37-54 
37-67 
37-99 
-> 
^37-73  ! 
It  is  satisfactory  to  find  the  means  of  all  the  observations  at  each  station  giving  a con- 
tinually increasing  time  of  vibration  with  the  altitude,  but  I do  not  attach  importance 
to  the  result,  because, — 1st,  a difference  far  exceeding  the  whole  effect  of  height  appears 
to  have  been  caused  at  Alta  Vista  by  change  of  position  in  the  instrument  'with  reference 
