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 masthead. 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 finds that this
dust consists in great part of infusoria with siliceous shields,
and of the siliceous tissue of plants. (1/3. I must take this
opportunity of acknowledging the great kindness with which this
illustrious naturalist has examined many of my specimens. I have
sent (June 1845) a full account of the falling of this dust to the
Geological Society.) 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.
Pages:
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47