488 
THE  ADMIEALTT  ASTEOXOMICAL  EXPEEBIEXT 
was  invariably  the  spurious  horizon  of  the  great  stratum  of  X.E.  clouds,  at  a varying 
height  of  perhaps  3000  to  5000  feet,  and  seen  at  Guajara  under  a zenith  distance 
varying  from  91°  5'  to  91°  11'. 
The  cloud  attached  to  the  mountain  was  not  more  than  2500  feet  in  altitude,  and 
seldom  stretched  more  than  two  or  three  miles  away  fi’om  it ; so  that  what  portion  of 
sea  was  visible  between  its  termination  and  the  beginning  of  the  sea-cloud,  was,  from 
our  station,  many  degrees  below  the  horizon,  and  quite  useless  for  altitude  pm'poses.  In 
the  case  of  the  cloud  floating  over  the  sea,  on  the  contrary,  the  extent  from  the  moimtain 
seemed  illimitable ; and  from  what  we  had  seen  for  several  days  before  reaching  Tene- 
riffe,  we  had  every  reason  to  believe,  that  generally  round  about  that  island,  but  chiefly 
towards  its  N,,  N.W.,  and  N.E.,  a cloud  stratum  extends  almost  uninterruptedly  for 
several  hundred  miles,  always  at  the  same  height  above  the  sea,  and  of  the  same  generic 
character,  or  strikingly  like  the  “ cumulonus  ” of  Admu’al  Fitzeot  (see  Plates  XXX. 
and  XXXII.). 
The  form  was  not  improbably  a consequence  of  the  mechanical  action  of  the  trade- 
wind  on  the  cloud-material ; and  though  the  rollers  of  mist,  moving  along  always  rapidly 
from  N.E.  to  S.W.,  often  had  intervals  between  them  when  examined  in  plan,  yet  Gewed 
as  they  were  in  extreme  perspective  on  our  visible  horizon,  no  gaps  were  seen  in  that 
position ; while  their  substance,  condensed  apparently  by  distance,  made  them  form  as 
opake  an  edge  for  the  sun  to  rise  behind,  as  a snowy  mountain. 
Although,  then,  the  absolute  time  of  sunrise  so  observed  would  not  have  been  capable 
of  geographic  truth,  its  duration  could  be  taken  pretty  well.  All  the  more  extraordi- 
nary therefore  is  the  account  by  a celebrated  traveller,  that  the  one  sumise  which  lie 
observed  from  a height  of  11,000  feet  on  the  Peak  of  TenerifFe,  had  the  remarkable 
anomaly  of  occupying  upwards  of  eight  minutes. 
Subsequent  observation  at  the  same  spot  cannot  prove  or  disprove  any  exceptional 
mirage  that  may  have  occurred  to  a former  observer ; but  it  may  mdicate  whether  the 
locality  is  frequently  liable  to  such  extreme  dislocations  of  refraction;  and  uith  this 
view,  the  rough  naked-eye  observations  which  I used  to  take  at  Guajara  for  meteoro- 
logical purposes,  and  which  are  entered  in  vols.  1 and  2,  may  be  examined,  and  will  be 
found  to  show  no  anomaly  of  the  sort  on  any  occasion.  A characteristic  featme  of  moun- 
tain sunrise  and  sunset  used  certainly  to  be  the  very  idsible  flattening  of  the  solar  orb ; 
but  this  was  a constant  and  normal  phenomenon ; and  if  it  reached,  by  sextant  measure, 
so  large  a quantity  as  5',  that  was  but  the  proper  refraction-effect,  due  to  a zenith  distance 
of  over  91°. 
(6.)  Duration  of  Twilight. 
Observations  for  the  duration  of  tivilight  were  found  capable,  on  being  actually  tried, 
of  more  precision  than  might  have  been  expected ; two  minutes  + or  — appearing  to  , 
include  all  uncertainties  as  to  the  vanishing  of  the  last  trace  of  the  sun’s  light  in  the  | 
west,  or  its  appearance  in  the  east,  the  zodiacal  light  alone  excepted ; but  this  quantity  ' 
does  not  include  certain  natural  causes  of  difference  in  one  day  from  another,  which  will  j 
be  found  to  vary  much  more. 
