I 
ATMOSPHERIC DUST 
5 
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 1 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 ascer¬ 
tained 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, 
1 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 account of the falling of this dust to the Geological Society. 
