THE VOYAGE OF THE BEAGLE IS 



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 

 masthead. 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 con- 

 sists 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 ascertained no less than sixty-seven dif- 

 ferent organic forms ! The infusoria, with the exception of 

 two marine species, are all inhabitants of fresh-water. 1 

 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 fallen during those months when the har- 

 mattan is known to raise clouds of dust high into the atmos- 

 phere, 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 hun- 

 dred 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, mixed with 

 finer matter. After this fact one need not be surprised 

 at the diffusion of the far lighter and smaller sporules of 

 cryptogamic plants. 



8 1 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) a full account of the falling of this dust to the 

 Geological Society. 



