336 On Atmospheric Deposits 
of the dust, the state of the health of those exposed to 
it, the changes of the wind, the warmth of the ocean, 
and the fact that the east wind does not always bring 
dust, the air being during the L’Este in Madeira clearer 
than usual, confirm the belief of the phenomena having a 
volcanic origin. The common impression is, that this 
dust, so frequent off the Cape de Verde group, comes 
from the desert; but, if so, it is singular that off* the 
coast of the Zaliara, between the latitudes of Cape Verde 
and the Canaries (in wdiich neighbourhood similar dust 
has also been noticed), no ashes or dust have been met 
with : the wind blowing nearly always between Cape 
Bojador and the mouth of the Senegal from the west ,— 
induced, doubtless, by the rush of the cooler ocean air 
to the heated atmosphere over the desert.* The ashes 
appear to fall not where they ought, if their origin be in 
Africa, but only off islands which are known to be 
volcanic, and frequently in active eruption. 
There is another argument also to be derived from the 
variety of colours assumed by this transported matter. 
Sometimes it is greyish brown , brownish , and red ; and, 
in Malcolm’s Travels in South-eastern Asiaf, mention is 
made of a yellowish sand deposited in every part of the 
rigging and ship for a number of days after crossing the 
equator, to the north, in the same latitudes. The date 
is not given ; but the writer says, “ it could be wiped 
from the deck like dust from a table” He remarks, that 
“ this, of course , was from the coast of Africa, above a 
thousand miles distant but he does not say why, “ of 
course” Mr. Malcolm thinks, though not uncommon, 
the phenomenon has not been noticed in books. In this 
he is mistaken. 
Assuming that these ashes are volcanic, they agree in 
* See Bylandt’s Theorie des Volcans, ii. 40. 
f Vol. ii. p. 201, 
