ON  THE  PEAK  OE  TENEEIFPE. 
489 
At  Guajara. 
Twilight. 
Duration  of,  at  sunrise. 
Twilight. 
Duration  of,  at  sunset. 
h 
m 
h 
m 
s 
July  22nd  .... 
1 
2 
July  17th 
1 
10 
0 
July  21st 
1 
22 
0 
July  23rd 
1 
26 
0 
July  26th 
1 
22 
30 
July  27th 
1 
19 
0 
July  28th  . . 
1 
16 
0 
July  29th 
1 
22 
0 
July  31st 
1 
2 
July  30th  . . . 
1 
21 
0 
August  2nd  .... 
1 
13 
August  1st  . 
1 
21 
0 
August  8th  .... 
1 
17 
August  2nd  . . . 
1 
18 
0 
August  3rd  . 
1 
16 
0 
August  17th. 
1 
19 
0 
August  18th . 
1 
14 
0 
Mean,  July  31st 
1 
8 
Mean,  July  30th 
1 
19 
0 
At  Alta  Vista. 
Twilight. 
Twilight. 
Duration  of,  at  sunrise. 
Duration  of,  at  sunset. 
h 
m 
August  22nd  .... 
1 
9 
No  sunset  visible  at  this  station. 
September  8th  . 
0 
64 
Mean,  August  31st . 
1 
2 
The  difference  of  duration  observable  here  between  the  morning  and  evening  twilight 
is  remarkable,  and  may  be  partly  owing  to  the  difference  of  intensity  of  the  zodiacal 
light,  which  in  the  evening  was  barely  visible,  but  was  in  the  morning  so  bright  as  to 
prevent  the  very  first  trace  of  dawn  being  perceptible. 
The  following  are  a few  of  the  additional  observations  made  respecting  twilight  at 
Guajara : — 
On  the  morning  of  the  8th  of  August,  the  first  symptom  of  colours  was  distinguish- 
able nine  minutes  after  the  first  appearance  of  light ; while  in  the  evening  of  July  17  th, 
and  on  July  26th,  the  colours  were  on  each  occasion  distinguishable  to  within  five 
minutes  of  the  cessation  of  light. 
Further,  on  August  17th  p.m.,  eighteen  minutes  before  the  end  of  twilight,  its  altitude 
to  the  extreme  blue  was  found  to  be  9°.  And  again,  on  August  18th  p.m.,  at  twenty-one 
minutes  before  the  cessation  of  twilight,  the  altitude  of  the  extreme  blue  was  found  9°, 
and  to  the  extreme  red  3°;  while  at  eight  minutes  before  the  cessation,  blue  colours 
were  alone  \isible,  and  reached  3°  high. 
At  the  latter  periods  of  twilight,  1 had  some  confidence  in  attempting  to  measure  the 
3 T 2 
