COLOUR OF THE OCEAN. 67 
\ 
Mariners are accustomed to observe the appear- 
ances of the sky more carefully than landsmen, and 
among the numerous meteorological rules which 
pilots transmit to each other, several evince great sa- 
gacity. Prognostics are also in general less uncer- 
tain on the ocean, and especially in the equinoctial 
parts of it, than on land, where the inequalities of 
the ground interrupt the regularity of their mani- 
festation. 
Humboldt also applied the cyanometer to mea- 
sure the colour of the sea. In fine calm weather, 
the tint was found to be equal to 33°, 38°, some- 
times even 44° of the instrument, although the 
sky was very pale, and scarcely attained 14° or 
15°. When, instead of directing the apparatus 
to a great extent of open sea, the observer fixes 
his eyes on a small part of its surface viewed 
through a narrow aperture, the water appears of a 
rich ultramarine colour. Towards evening again, 
when the edge of the waves, as the sun shines 
upon them, is of an emerald-green, the surface of 
the shaded side reflects a purple hue. Nothing 
is more striking than the rapid changes which 
the colour of the sea undergoes under a clear sky, 
in the midst of the ocean and in deep water, when 
it may be seen passing from indigo-blue to the 
deepest green, and from this to slate-gray. The 
blue is almost independent of the reflection of the 
atmosphere. The intertropical seas are in general 
of a deeper and purer tint than in high latitudes, and 
the ocean often remains blue, when, in fine weather, 
more than four-fifths of the sky are covered with 
light and scattered clouds of a white colour. 
