512 
THE  ADMIEALTY  ASTEONOHICAL  EXPEEDIEXT 
observed  in  the  passage  of  the  black  into  the  white  cross,  both  at  Alta  Vista  and  at 
Edinburgh. 
Alta  Vista. 
Edinburgh. 
Plane. 
No.  of  ob- 
Amount  of 
Plane. 
No.  of  ob- 
Amount  of 
servations. 
rotation. 
servations. 
rotation. 
-82-5 
18 
O 
4-3 
-80 
1 
O 
4 
— 85-0 
18 
2-0 
— 85 
1 
1 
— 87'5 
18 
0-5 
-87 
1 
1 
90 
17 
0-6 
90 
1 
0 
+ 87*5 
19 
1-9 
+ 87 
1 
2 
+ 85-0 
19 
3-9 
+ 85 
1 
3 
+ 82*5 
17 
6-1 
+ 83 
1 
3 
+ 80 
1 
5 
The  point  of  least  rotation  would  seem  at  either  place  to  be  from  1°  to  3°  on  the  — side 
of  90°,  or  to  indicate  that  the  true  plane  passes  through  the  part  of  the  blue  sky  under 
examination, — the  observer’s  eye, — and,  for  the  third  point,  a spot  a little  above  the  sun; 
an  indication  of  the  neutral  points  of  Aeago,  Babinet,  and  Bkewstek. 
CHAPTER  V. 
METEOEOLOaiCAL  OBSEEVATIONS. 
Appended  to  the  Meteorological  Observations  and  their  reductions  in  vols.  2,  3.  4 
and  5,  there  are  such  full  accounts  of  the  instruments,  observers,  and  methods  of  reduc- 
tion, that  nothing  remains  to  be  done  here  but  to  collect  the  results  for  the  three 
stations ; the  first  of  which,  for  the  sea-level,  is  the  Titania  yacht,  in  Santa  Ci'uz  roads ; 
the  second,  at  8903  feet  of  elevation,  is  Guajara;  and  the  third,  at  10,702  feet  of  eleva- 
tion, is  Alta  Vista. 
(1.)  Hourly  Variations. 
From  the  means  of  three  days  of  hourly  observations  m the  ‘ Titania,’  two  days  at 
Guajara,  and  two  at  Alta  Vista,  the  following  Table  has  been  prepared,  showing  the 
corrections  to  reduce  the  barometer,  thermometer,  and  depression  of  the  dew-point, 
observed  at  any^one  hour,  to  the  mean  for  the  whole  twenty-four  hom-s. 
By  reason  of  the  very  short  period  that  these  observations  extend  over,  the  results  arc 
rough,  with  those  accidental  variations  which  the  employment  of  a greater  number  of 
days  would  have  tended  to  clear  away,  and  which  might  have  been  somewhat  smoothed 
down,  quite  legitimately,  in  making  out  a general  table ; but  I thought  it  better,  on  tlic 
whole,  to  give  the  actual  numbers  resulting  from  the  arithmetical  operations,  as  being 
more  adapted  to  serve  all  the  purposes  suggested  for  the  Expedition. 
The  readings  of  the  barometer,  it  may  be  here  remarked  once  for  all,  have  in  evev\ 
case  been  reduced  to  32°,  and  to  the  Greenwich  standard ; and  the  depressions  of  dew- 
point and  elasticities  of  vapour  have  been  calculated  by  Dr.  Apjohn’s  formula  foi' 
observations  of  wet-  and  dry-bulb  thermometers,  whose  index  errors  liave  been  care- 
fully applied  in  every  case. 
