98 THE PHYSICAL GEOGRAPHY OF THE SEA. 



But the air is invisible ; and it is not easily perceived hovv^ either 

 marks or taUies may be put upon it, that it may be traced in its 

 paths through the clouds. 



The skeptic, therefore, who finds it hard to believe that the gen- 

 eral circulation is such as Plate I. represents it to be, might con- 

 sider himself safe in his unbelief were he to declare his willingness 

 to give it up the moment any one should put tallies on the wings 

 of the wind, which would enable him to recognize that air again, 

 and those tallies, when found at other parts of the earth's surface. 



As difficult as this seems to be, it has actually been done. 

 Ehrenberg, with his microscope, has established, almost beyond a 

 doubt, that the air which the southeast trade-winds bring to the 

 equator does rise up there and pass over into the northern hemi- 

 sphere. 



158. The Sirocco, or African dust, which he has been observ- 

 ing so closely, has turned out to be tallies put upon the wind in 

 the other hemisphere ; and this beautiful instrument of his enables 

 us to detect the marks on these little tallies as plainly as though 

 those marks had been written upon labels of w^ood and tied to the 

 wings of the wind. 



This dust, when subjected to microscopic examination, is foynd 

 to consist of infusoria and organisms whose habitat is not Africa, 

 but South America, and in the southeast trade-wind region of 

 South America. Professor Ehrenberg has examined specimens 

 of sea dust from the Cape de Verds and the regions thereabout, 

 from Malta, Genoa, Lyons, and the Tyrol ; and he has found a 

 similarity among them as striking as it would have been had these 

 specimens been all taken from the same pile. South American 

 forms he recognizes in all of them ; indeed, they are the prevail- 

 ing forms in every specimen he has examined. 



It may, I think, be now regarded as an established fact, that 

 there is a perpetual upper current of air from South America to 

 North Africa ; and that the volume of air which flows to the north- 

 ward in these upper currents is nearly equal to the volume which 

 flows to the southward with the northeast trade-winds, there can 

 be no doubt. 



The " rain dust" has been observed most frequently to fall in 

 spring and autumn ; that is, the fall has occurred after the equi- 

 noxes, but at intervals from them varying from thirty to sixty 



