The Hygrometer Gave A Difference
Of 29.6 Degs., Between The Temperature Of The Air, And The
Point At Which Dew Was Precipitated.
This difference was
nearly double that which I had observed on the previous
mornings.
This unusual degree of atmospheric dryness was
accompanied by continual flashes of lightning. Is it not an
uncommon case, thus to find a remarkable degree of aerial
transparency with such a state of weather?
Generally the atmosphere is hazy; and this is caused by
the falling of impalpably fine dust, which was found to have
slightly injured the astronomical instruments. The morning
before we anchored at Porto Praya, I collected a little packet
of this brown-coloured fine dust, which appeared to have
been filtered from the wind by the gauze of the vane at the
mast-head. Mr. Lyell has also given me four packets of dust
which fell on a vessel a few hundred miles northward of
these islands. Professor Ehrenberg [3] finds that this dust
consists in great part of infusoria with siliceous shields, and
of the siliceous tissue of plants. In five little packets which
I sent him, he has ascertained no less than sixty-seven
different organic forms! The infusoria, with the exception of
two marine species, are all inhabitants of fresh-water. I
have found no less than fifteen different accounts of dust
having fallen on vessels when far out in the Atlantic. From
the direction of the wind whenever it has fallen, and from
its having always fallen during those months when the harmattan
is known to raise clouds of dust high into the atmosphere,
we may feel sure that it all comes from Africa. It
is, however, a very singular fact, that, although Professor
Ehrenberg knows many species of infusoria peculiar to
Africa, he finds none of these in the dust which I sent him.
On the other hand, he finds in it two species which hitherto
he knows as living only in South America. The dust falls
in such quantities as to dirty everything on board, and to
hurt people's eyes; vessels even have run on shore owing to
the obscurity of the atmosphere. It has often fallen on
ships when several hundred, and even more than a thousand
miles from the coast of Africa, and at points sixteen hundred
miles distant in a north and south direction. In some
dust which was collected on a vessel three hundred miles
from the land, I was much surprised to find particles of
stone above the thousandth of an inch square, mixed with
finer matter. After this fact one need not be surprised
at the diffusion of the far lighter and smaller sporules of
cryptogamic plants.
The geology of this island is the most interesting part of
its natural history. On entering the harbour, a perfectly
horizontal white band, in the face of the sea cliff, may be seen
running for some miles along the coast, and at the height of
about forty-five feet above the water.
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