(2

from dry to low & wooded southward and of the mouth of the Senagal.

(The dust is of a pale reddish-brown colour, it fuses under the blowpipe into a black grey or black bead. it is generally excessively fine, & adheres to the sails & every object on board.) (When the Beagle was ten miles off in W end of St. Jago (with the wind from the east of Africa, & the visible horizon only about a mile distant) I found a considerable quantity adhering to the under side of the horizontal wind-vane; it appearing to have been filtered from the air by the gauze, as the ship lay by inclined to the wind: the wind had been for 24 hours previously from ENE. During one which stay at St. Jago. Lieut. James *(4) on four successive days collected four packets of the dust (which together with a written communication, I owe to the kindness of Mr. Lyell) but the dust was so fine, that he was unable to collect any, except by means of a moistened moistened well fresh sponge but he despaired of being able to do so, until he thought of moistening a sponge washing it, & filtering which he afterwards washed & then filtered the water. Near the continent the dust appears to fall in large quantities: Baron Roussin states that his survey was impaired by the thick fog or haze "that prevails almost all the year." Several vessels have been ship-wrecked owing to this cause; & Horsburgh (*5) recommends all ships to avoid the passage between the Cape Verd Isld & the mainland, from the haziness of the atmosphere. during the three weeks we remained at St. Jago, in the Beagle, the astronomical instruments were found to be injured except from by the dust which was almost constantly falling fine dust with a South & North wind of Cape Horn Noon, Lieut Arlett described the water as so discoloured & thick, that the back of a ship is visible for a long time, & he attributes in part to the fine sand blown from the deserts, "with which everything