624 FALLS OF DUST IN THE ATLANTIC. 



lastly, that the months during which it falls coincide with those when the 

 Harmattan blows from the continent, and when it is known that clouds of 

 dust and sand are raised by it, I think that there can be no doubt that the 

 Dust which falls in the Atlantic does come from Africa. How to explain the 

 enigma of the absence of characteristic African forms, and of the presence 

 of two species from South America, I will not pretend to conjecture. 



Finally, I may remark, that the circumstance of such quantities of Dust 

 being periodically blown, year after year, over so immense an area in the 

 Atlantic Ocean, is interesting, as showing by how apparently inefficient 

 a cause a widely-extended deposit may be in process of formation ; and 

 this deposit, it appears, from the researches of Professor Ehrenberg, will 

 in chief part consist of fresh-water Polygastrica and PhytoUtharia. 



This concludes Professor Darwin's remarks. 



About 30 years later. Dr. Hellmann collected a series of observations 

 from ships' logs, and from his and later examinations there seems but little 

 doubt that the source of this Dust is in the African deserts, and that tne 

 Trade Wind is the means of its dispersal. It has been found nearly as 

 far North as the parallel of Cape Juby, in long. 37° W., and as far South 

 as that of Cape Palmas, in 31^° W. The westernmost observation was 

 in 15° N., 40^° W., about midway between Cape Blanco and Trinidad. 

 It is most frequently observed in the winter months, especially January 

 and February, and the area of its greatest frequency is around the Cape 

 Verde Islands, or between lat. 10° and 20° N., long. 20° and 30° W.* 



Of a merchant-fleet from St. Helena, under convoy, in November, 1813, 

 most of the ships had their sails covered with red sand, and they must 

 have been from 400 to 500 miles from shore, in about 27° and 28° N., 

 after a succession of easterly winds. " I once," says Mr. Luccock. " saw 

 the sails and deck of a vessel covered with it, when 400 miles from the 

 coast, and have heard of the same phenomenon being remarked at a far 

 greater distance. This moving expanse of sand was probably, at some 

 anterior period, a large inland shallow sea, communicating with the 

 Mediterranean by the Syrtes (Gulf of Sydra), &c." 



A similar phenomenon occurred to the brig Parssboro', on her voyage 

 from Barbados to Belfast, and when she was upwards of 900 miles from 

 the mainland of Africa. The wind had been at East, and was interrupted 

 by one of those gales noticed in our description of the Azores. " In lat. 

 30° 50' N., long. 32° 40' W., Cape Verde Islands bearing S.E., distant 590 

 miles, the appearance of a heavy squall rising in the S.E. direction. At 

 6'' 30° p.m., lightning, thunder, and the squall approaching. The sun, 

 about 15° above the western horizon, became overcast with peculiar- 

 looking clouds, and every appearance of an approaching storm. I conse- 

 quently shortened sail, although the barometer did not indicate anything 

 serious. At 8 p.m., the wind became very variable, from N.E. to S.W., 

 every ten or fifteen minutes alternately, for two hours. There was a fall 

 of rain when the heaviest of the squall was on the zenith. At midnight 

 it had all passed to the S.W., and the wind resumed its former place, 



• "DieStaubfiilleimPassatgebiete des Nordatlaatischen Ozcans," von J. von Goerne, 

 in " Globas," Band Iv., 1889, pages 241—244. 



