Observations of African dust on the Beagle, St. Jago (Santiago), by Charles Darwin 1833

This short note reviews observations by Charles Darwin of deposited dust and haze at sea near the island of St. Jago (now Santiago, Cape Verde), approximately 800 kilometres west of the African coast, on 16th January, 1833. Darwin (1848) describes his 1833 observations and similar observations by other mariners in the early 1800s, and provides an assessment of the probable dispersion and source of the dust. Darwin's 1833 observations are set out below, verbatim (from Darwin, 1848): <br /><br /> "On the 16th of January (1833), when the Beagle was ten miles off the N.W. end of St. Jago, some very fine dust was found adhering to the under side of the horizontal wind-vane at the mast-head; it appeared to have been filtered by the gauze from the air, as the ship lay inclined to the wind. The wind had been for twenty-four hours previously E.N.E., and hence, from the position of the ship, the dust probably came from the coast of Africa. The atmosphere was so hazy that the visible horizon was only one mile distant. During our stay of three weeks at St. Jago (to February 8th) the wind was N.E., as is always the case during this time of the year; the atmosphere was often hazy, and very fine dust was almost constantly falling, so that the astronomical instruments were roughened and a little injured. The dust collected on the Beagle was excessively fine-grained, and of a reddish brown colour; it does not effervesce with acids; it easily fuses under the blowpipe into a black or gray bead."