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^ 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 iMlen 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, 



^ 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) ^ ^'■•^^ account of the falling of this dust to the Geological Society. 



